| 0/0 |
2026/08/09 04:45 |
flow |
assessment-kcsan |
43m
Results: map[Benign:false Explanation:### Race Summary
- **Functions:** `__rb_reserve_next` vs `ring_buffer_iter_empty` (inlined in
`ring_buffer_iter_peek`)
- **Variable/Field:** `tail_page->page->time_stamp` (written in
`__rb_reserve_next`) vs `commit_page->page->time_stamp` (read in
`ring_buffer_iter_empty`)
---
### Synchronization Context
- **Writer (`__rb_reserve_next`):** Operates locklessly to reserve space in the
ring buffer. When reserving the first event on a page (`tail == 0`), it updates
the page's timestamp.
- **Reader (`ring_buffer_iter_empty`):** Operates locklessly to check if the
iterator has more data to read. It reads the commit page's timestamp twice
(before and after a memory barrier) to detect concurrent writer activity.
- **Lack of Mutual Exclusion:** Because both threads operate locklessly on the
same page structure, they can concurrently access the `time_stamp` field without
holding any shared locks.
---
### Final Classification
**HARMFUL** (specifically, **Marking Required for Correctness**)
---
### Failure Reasoning
1. **Torn Accesses:** The `time_stamp` field is a plain 64-bit (`u64`) variable.
On 32-bit architectures, plain 64-bit reads and writes are not atomic and are
split into two 32-bit instructions. Without proper annotations, the compiler can
perform store tearing on the write and load tearing on the read, resulting in a
mixed/torn garbage value.
2. **Inconsistent Annotations:** In `ring_buffer_iter_empty()`, the second read
of the timestamp (`curr_commit_ts`) is correctly marked with `READ_ONCE()`, but
the first read (`commit_ts`) is a plain load. Similarly, the write in
`__rb_reserve_next()` is a plain store. This allows the compiler to assume the
value is stable and optimize the plain accesses in unexpected ways, violating
the Linux Kernel Memory Model (LKMM).
3. **Data Flow Safety:** While the algorithm is logically designed to handle
races (by returning `0` / "not empty" if the timestamps do not match, which is a
safe fallback), the lack of annotations can lead to compiler-induced bugs or
undefined behavior.
---
### Failure Sequencing (Torn Read on 32-bit Architecture)
```text
CPU0 (Writer) CPU1 (Reader)
__rb_reserve_next() ring_buffer_iter_empty()
=================== ========================
// Read commit_page
commit_page =
READ_ONCE(cpu_buffer->commit_page);
// Plain read of 64-bit time_stamp
(split on 32-bit)
// Reads low 32-bits of old
timestamp
commit_ts_low =
commit_page->page->time_stamp_low;
// Write new 64-bit timestamp (split on 32-bit)
tail_page->page->time_stamp_low = ts_low;
tail_page->page->time_stamp_high = ts_high;
// Reads high 32-bits of new
timestamp (TORN READ!)
commit_ts_high =
commit_page->page->time_stamp_high;
smp_rmb();
// Read curr_commit_ts (using
READ_ONCE)
curr_commit_ts =
READ_ONCE(curr_commit_page->page->time_stamp);
// Comparison fails due to torn
read
if (curr_commit_ts != commit_ts)
return 0; // Safe fallback,
but incorrect state
representation
```
---
### Recommended Fix
To ensure compiler safety, prevent load/store tearing, and satisfy KCSAN, the
accesses must be marked:
1. In `__rb_reserve_next()` (line ~4851), use `WRITE_ONCE` to write the
timestamp:
```c
if (unlikely(!tail))
WRITE_ONCE(tail_page->page->time_stamp, info->ts);
```
2. In `ring_buffer_iter_empty()` (line ~5647), use `READ_ONCE` to read the first
timestamp:
```c
commit_page = READ_ONCE(cpu_buffer->commit_page);
commit_ts = READ_ONCE(commit_page->page->time_stamp);
```]
|
| 1/1 |
2026/08/09 04:45 |
action |
kernel-checkouter |
1m
Results: map[KernelSrc:/app/workdir/cache/src/2c647d04ba64204b0de04a5458fe5db9d3049774]
|
| 2/1 |
2026/08/09 04:47 |
action |
kernel-builder |
20m
Results: map[KernelObj:/app/workdir/cache/build/9e525aa50817175c6f0ab058b51598bf6e388668]
|
| 3/1 |
2026/08/09 05:07 |
action |
codesearch-prepare |
15m
Results: map[Index:codesearch-index]
|
| 4/1 |
2026/08/09 05:23 |
agent |
expert |
5m
Model:
Results:
Instruction: # KCSAN Data Race Severity Analysis Protocol
You are an expert Linux kernel concurrency engineer analyzing a Kernel
Concurrency Sanitizer (KCSAN) data race report to determine if it is
**BENIGN** or **HARMFUL**.
## 1. CLASSIFICATIONS
### **BENIGN (Truly Benign)**
The logic is sound and structurally tolerant to compiler optimizations or
stale/torn reads.
- **Diagnostics/Stats:** Reads used only for `/proc`, `/sys`, counters, or
`pr_info`.
- **Heuristic Hints:** A "hint" flag where an old value only causes a
slightly delayed update or a sub-optimal but safe fast-path.
- **Single-Writer Flag Updates:** A single writer updating flags where the
concurrent read is a simple bitwise check (e.g., `flags & MASK`). These are
historically tolerated, assuming neither "Fused Accesses" nor "Ordering
Violations" are relevant in this context.
- **Marked Reloads:** A load feeding into a `cmpxchg()` loop or checked
against a later `READ_ONCE()` reload.
- **Safe Overwrites:** Writing the same value already present.
### **HARMFUL (Logic Bug or Marking Required)**
The race causes incorrect behavior due to a synchronization failure or
because missing annotations allow the compiler to break the algorithm.
**Marking Required for Correctness:**
The algorithm is logically sound but requires annotations (`READ_ONCE()`,
`WRITE_ONCE()`, `smp_load_acquire()`, `smp_store_release()`, etc.) to be safe.
- **Fused Accesses:** The compiler might merge accesses or hoist a load out
of a loop, breaking polling/wait loops (livelocks).
- **Torn Accesses:** A large access (e.g., 64-bit on 32-bit arch) might be
split into multiple non-atomic accesses. Note that `READ_ONCE()` does **not**
guarantee atomicity for 64-bit variables on 32-bit architectures.
- **Ordering Violations:** The race breaks a "happens-before" relationship
(requires primitives with implied or explicit memory barriers).
**Logic Bugs:**
A fundamental synchronization failure. Marking accesses will **not** fix it;
the logic itself must change.
- **Pointers/Lifecycle:** The racing variable is a pointer being dereferenced
or a refcount governing object lifecycle (Use-After-Free risk).
- **Control Flow:** The variable guards a critical section, memory allocation,
or hardware command.
- **Bitfields:** Concurrent writes to different bits in the same word.
Compilers often use non-atomic read-modify-write sequences, meaning a
write to `bit_A` can "clobber" a concurrent write to `bit_B`. However,
do not blindly assume all bitfield accesses are harmful; you must prove
that a concurrent write actually clobbers another in a way that breaks
logic.
- **Complex Structures:** Races on shared lists, trees, or hashmaps.
- **Lossy Updates:** Concurrent plain RMW operations (e.g., `var++`) on
non-diagnostic variables where every increment must be preserved.
- **State Machines:** Races allowing a state machine to bypass transitions
or enter an invalid state.
- **Adjacent Unsynchronized Operations:** Consider races happening at the
same time. For example, if both threads execute `struct->has_elements = true;
list_add(node, &struct->list);`, the race on `has_elements` implies an
adjacent race on `list_head`, which is HARMFUL.
## 2. RESEARCH & ANALYSIS WORKFLOW
1. **Locate the Race:** Find the exact variables and functions in the stack
traces. Do not speculate about hypothetical compiler
behaviors or theoretical dangers (e.g., dismissing something as
"fundamentally unsafe") without tracing the actual data flow to a crash.
2. **Contextualize:** Identify held locks, RCU sections, or interrupt
contexts. Explain why the race is possible (e.g., "Thread A holds `lock`,
but Thread B is a lockless reader").
3. **Data Flow:** Follow the racing variable into its subsequent uses. If
the reader sees a stale or torn value, what branch is taken? Does it lead
to a pointer dereference or `BUG_ON()`? **You must prove harm via this
data flow.**
4. **Failure Sequencing (if HARMFUL):** Construct a two-column execution trace
demonstrating the failure.
*Example format:*
```text
CPU0 CPU1
function_a()
// lockless read
ptr = global_ptr
<PREEMPT>
function_b()
lock(&my_lock)
global_ptr = NULL
unlock(&my_lock)
kfree(ptr)
if (ptr)
*ptr = 1; // -> Use-After-Free!
```
## 3. OUTPUT FORMAT
- **Race Summary:** `[Function A]` vs `[Function B]` on `[Variable/Field]`.
- **Synchronization context:** List held locks and explain the lack of mutual
exclusion.
- **Final Classification:** **BENIGN** or **HARMFUL**.
**If BENIGN:**
- **Reasoning:** Briefly explain why the race is structurally tolerant to stale
or torn reads.
- **Recommended Annotations:** Suggest appropriate annotations (e.g.,
`READ_ONCE()`, `data_race()`).
**If HARMFUL:**
- **Failure Reasoning:** Explain and provide a two-column interleaving showing
exactly how the race leads to a failure such as corruption or kernel crash.
- **Recommended Fix (ONLY if fix is trivial):** Suggest a structural fix (e.g.,
"Hold `mapping->i_pages` lock", "Convert to `atomic_t`") or required memory
ordering annotations (e.g., "Wrap in `READ_ONCE()`", "Use
`smp_load_acquire()`").
Don't make assumptions about the kernel source code (it may be different from what you assume it is).
Extensively use the provided code access tools (codesearch-*, git-*, grepper, etc)
to examine the actual source code, and confirm any assumptions.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt:
The data race report is:
==================================================================
BUG: KCSAN: data-race in __rb_reserve_next / ring_buffer_iter_peek
write to 0xffff8881331f4000 of 8 bytes by task 12741 on cpu 0:
__rb_reserve_next+0x5f1/0x6c0 kernel/trace/ring_buffer.c:4851
rb_reserve_next_event kernel/trace/ring_buffer.c:4925 [inline]
ring_buffer_lock_reserve+0x68c/0x830 kernel/trace/ring_buffer.c:4984
__trace_buffer_lock_reserve kernel/trace/trace.h:1676 [inline]
trace_event_buffer_lock_reserve+0x148/0x300 kernel/trace/trace.c:1924
trace_event_buffer_reserve+0x120/0x180 kernel/trace/trace_events.c:688
do_trace_event_raw_event_kmem_cache_free include/trace/events/kmem.h:117 [inline]
trace_event_raw_event_kmem_cache_free+0xa4/0x140 include/trace/events/kmem.h:117
__do_trace_kmem_cache_free include/trace/events/kmem.h:117 [inline]
trace_kmem_cache_free include/trace/events/kmem.h:117 [inline]
kmem_cache_free+0x31d/0x380 mm/slub.c:6503
anon_vma_chain_free mm/rmap.c:147 [inline]
unlink_anon_vmas+0x1b8/0x360 mm/rmap.c:514
free_pgtables+0x50e/0x630 mm/memory.c:414
exit_mmap+0x367/0x5e0 mm/mmap.c:1312
__mmput+0x28/0x1c0 kernel/fork.c:1187
mmput+0x40/0x50 kernel/fork.c:1210
exec_mm_put_old fs/exec.c:916 [inline]
setup_new_exec+0x20a/0x230 fs/exec.c:1353
load_elf_binary+0x77a/0x1a30 fs/binfmt_elf.c:1024
search_binary_handler fs/exec.c:1694 [inline]
exec_binprm fs/exec.c:1726 [inline]
bprm_execve+0x45c/0x9b0 fs/exec.c:1778
do_execveat_common+0x7a4/0x8a0 fs/exec.c:1876
__do_sys_execve fs/exec.c:1949 [inline]
__se_sys_execve fs/exec.c:1943 [inline]
__x64_sys_execve+0x5f/0x80 fs/exec.c:1943
x64_sys_call+0x2752/0x3020 arch/x86/include/generated/asm/syscalls_64.h:60
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
read to 0xffff8881331f4000 of 8 bytes by task 11658 on cpu 1:
ring_buffer_iter_empty kernel/trace/ring_buffer.c:5647 [inline]
rb_iter_peek kernel/trace/ring_buffer.c:6143 [inline]
ring_buffer_iter_peek+0x265/0xb10 kernel/trace/ring_buffer.c:6305
peek_next_entry kernel/trace/trace.c:2349 [inline]
__find_next_entry+0x1a9/0x420 kernel/trace/trace.c:2398
trace_find_next_entry_inc kernel/trace/trace.c:2672 [inline]
s_next+0x10e/0x370 kernel/trace/trace.c:2711
traverse+0x1c1/0x3b0 fs/seq_file.c:121
seq_read_iter+0x7dc/0x8f0 fs/seq_file.c:196
do_iter_readv_writev+0x4f6/0x5a0 fs/read_write.c:-1
vfs_readv+0x1e8/0x6d0 fs/read_write.c:1019
do_preadv fs/read_write.c:1133 [inline]
__do_sys_preadv2 fs/read_write.c:1192 [inline]
__se_sys_preadv2+0xfc/0x1c0 fs/read_write.c:1183
__x64_sys_preadv2+0x67/0x80 fs/read_write.c:1183
x64_sys_call+0x2b3b/0x3020 arch/x86/include/generated/asm/syscalls_64.h:328
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
value changed: 0x000001502ab37190 -> 0x000001505958442b
Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 11658 Comm: syz.6.11885 Tainted: G W syzkaller #0 PREEMPT(lazy)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
==================================================================
Reply: ### Race Summary
- **Functions:** `__rb_reserve_next` vs `ring_buffer_iter_empty` (inlined in `ring_buffer_iter_peek`)
- **Variable/Field:** `tail_page->page->time_stamp` (written in `__rb_reserve_next`) vs `commit_page->page->time_stamp` (read in `ring_buffer_iter_empty`)
---
### Synchronization Context
- **Writer (`__rb_reserve_next`):** Operates locklessly to reserve space in the ring buffer. When reserving the first event on a page (`tail == 0`), it updates the page's timestamp.
- **Reader (`ring_buffer_iter_empty`):** Operates locklessly to check if the iterator has more data to read. It reads the commit page's timestamp twice (before and after a memory barrier) to detect concurrent writer activity.
- **Lack of Mutual Exclusion:** Because both threads operate locklessly on the same page structure, they can concurrently access the `time_stamp` field without holding any shared locks.
---
### Final Classification
**HARMFUL** (specifically, **Marking Required for Correctness**)
---
### Failure Reasoning
1. **Torn Accesses:** The `time_stamp` field is a plain 64-bit (`u64`) variable. On 32-bit architectures, plain 64-bit reads and writes are not atomic and are split into two 32-bit instructions. Without proper annotations, the compiler can perform store tearing on the write and load tearing on the read, resulting in a mixed/torn garbage value.
2. **Inconsistent Annotations:** In `ring_buffer_iter_empty()`, the second read of the timestamp (`curr_commit_ts`) is correctly marked with `READ_ONCE()`, but the first read (`commit_ts`) is a plain load. Similarly, the write in `__rb_reserve_next()` is a plain store. This allows the compiler to assume the value is stable and optimize the plain accesses in unexpected ways, violating the Linux Kernel Memory Model (LKMM).
3. **Data Flow Safety:** While the algorithm is logically designed to handle races (by returning `0` / "not empty" if the timestamps do not match, which is a safe fallback), the lack of annotations can lead to compiler-induced bugs or undefined behavior.
---
### Failure Sequencing (Torn Read on 32-bit Architecture)
```text
CPU0 (Writer) CPU1 (Reader)
__rb_reserve_next() ring_buffer_iter_empty()
=================== ========================
// Read commit_page
commit_page = READ_ONCE(cpu_buffer->commit_page);
// Plain read of 64-bit time_stamp (split on 32-bit)
// Reads low 32-bits of old timestamp
commit_ts_low = commit_page->page->time_stamp_low;
// Write new 64-bit timestamp (split on 32-bit)
tail_page->page->time_stamp_low = ts_low;
tail_page->page->time_stamp_high = ts_high;
// Reads high 32-bits of new timestamp (TORN READ!)
commit_ts_high = commit_page->page->time_stamp_high;
smp_rmb();
// Read curr_commit_ts (using READ_ONCE)
curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
// Comparison fails due to torn read
if (curr_commit_ts != commit_ts)
return 0; // Safe fallback, but incorrect state representation
```
---
### Recommended Fix
To ensure compiler safety, prevent load/store tearing, and satisfy KCSAN, the accesses must be marked:
1. In `__rb_reserve_next()` (line ~4851), use `WRITE_ONCE` to write the timestamp:
```c
if (unlikely(!tail))
WRITE_ONCE(tail_page->page->time_stamp, info->ts);
```
2. In `ring_buffer_iter_empty()` (line ~5647), use `READ_ONCE` to read the first timestamp:
```c
commit_page = READ_ONCE(cpu_buffer->commit_page);
commit_ts = READ_ONCE(commit_page->page->time_stamp);
```
|
| 5/2 |
2026/08/09 05:23 |
llm |
expert |
0m
Model:
Tokens:
input: 6790
output: 79
thoughts: 283
Thoughts: **Analyzing a KCSAN Data Race Report**
Okay, let's dive into this KCSAN data race report. The core of the problem seems to be a conflict between `__rb_reserve_next` and `ring_buffer_iter_peek`. Specifically, we've got a write happening to memory address `0xffff8881331f4000` of 8 bytes from task 12741 on CPU 0, originating from `__rb_reserve_next` at line 4851 of `kernel/trace/ring_buffer.c`. Simultaneously, we see a read from that *same* memory location of 8 bytes by task 11658 on CPU 1, this time coming from `ring_buffer_iter_empty` (which is inlined within `rb_iter_peek` and, therefore, `ring_buffer_iter_peek`), found at line 5647 in the same file.
My initial instinct is to go directly to those lines in the code using `codesearch-definition-source` or maybe even just a quick `read-file` command. But, before I get too deep into that, let's take a slight step back for clarity. Let's start with a focused search for these two functions, `__rb_reserve_next` and `ring_buffer_iter_empty`, within `kernel/trace/ring_buffer.c` to re-orient myself in the code and confirm that my understanding of the report is accurate. Gotta start with a good baseline.
|
| 6/2 |
2026/08/09 05:23 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:__rb_reserve_next]
Results: map[SourceCode:4716: static struct ring_buffer_event *
4717: __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
4718: struct rb_event_info *info)
4719: {
4720: struct ring_buffer_event *event;
4721: struct buffer_page *tail_page;
4722: unsigned long tail, write, w;
4723:
4724: /* Don't let the compiler play games with cpu_buffer->tail_page */
4725: tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
4726:
4727: /*A*/ w = local_read(&tail_page->write) & RB_WRITE_MASK;
4728: barrier();
4729: rb_time_read(&cpu_buffer->before_stamp, &info->before);
4730: rb_time_read(&cpu_buffer->write_stamp, &info->after);
4731: barrier();
4732: info->ts = rb_time_stamp(cpu_buffer->buffer);
4733:
4734: if ((info->add_timestamp & RB_ADD_STAMP_ABSOLUTE)) {
4735: info->delta = info->ts;
4736: } else {
4737: /*
4738: * If interrupting an event time update, we may need an
4739: * absolute timestamp.
4740: * Don't bother if this is the start of a new page (w == 0).
4741: */
4742: if (!w) {
4743: /* Use the sub-buffer timestamp */
4744: info->delta = 0;
4745: } else if (unlikely(info->before != info->after)) {
4746: info->add_timestamp |= RB_ADD_STAMP_FORCE | RB_ADD_STAMP_EXTEND;
4747: info->length += RB_LEN_TIME_EXTEND;
4748: } else {
4749: info->delta = info->ts - info->after;
4750: if (unlikely(test_time_stamp(info->delta))) {
4751: info->add_timestamp |= RB_ADD_STAMP_EXTEND;
4752: info->length += RB_LEN_TIME_EXTEND;
4753: }
4754: }
4755: }
4756:
4757: /*B*/ rb_time_set(&cpu_buffer->before_stamp, info->ts);
4758:
4759: /*C*/ write = local_add_return(info->length, &tail_page->write);
4760:
4761: /* set write to only the index of the write */
4762: write &= RB_WRITE_MASK;
4763:
4764: tail = write - info->length;
4765:
4766: /* See if we shot pass the end of this buffer page */
4767: if (unlikely(write > cpu_buffer->buffer->subbuf_size)) {
4768: check_buffer(cpu_buffer, info, CHECK_FULL_PAGE);
4769: return rb_move_tail(cpu_buffer, tail, info);
4770: }
4771:
4772: if (likely(tail == w)) {
4773: /* Nothing interrupted us between A and C */
4774: /*D*/ rb_time_set(&cpu_buffer->write_stamp, info->ts);
4775: /*
4776: * If something came in between C and D, the write stamp
4777: * may now not be in sync. But that's fine as the before_stamp
4778: * will be different and then next event will just be forced
4779: * to use an absolute timestamp.
4780: */
4781: if (likely(!(info->add_timestamp &
4782: (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE))))
4783: /* This did not interrupt any time update */
4784: info->delta = info->ts - info->after;
4785: else
4786: /* Just use full timestamp for interrupting event */
4787: info->delta = info->ts;
4788: check_buffer(cpu_buffer, info, tail);
4789: } else {
4790: u64 ts;
4791: /* SLOW PATH - Interrupted between A and C */
4792:
4793: /* Save the old before_stamp */
4794: rb_time_read(&cpu_buffer->before_stamp, &info->before);
4795:
4796: /*
4797: * Read a new timestamp and update the before_stamp to make
4798: * the next event after this one force using an absolute
4799: * timestamp. This is in case an interrupt were to come in
4800: * between E and F.
4801: */
4802: ts = rb_time_stamp(cpu_buffer->buffer);
4803: rb_time_set(&cpu_buffer->before_stamp, ts);
4804:
4805: barrier();
4806: /*E*/ rb_time_read(&cpu_buffer->write_stamp, &info->after);
4807: barrier();
4808: /*F*/ if (write == (local_read(&tail_page->write) & RB_WRITE_MASK) &&
4809: info->after == info->before && info->after < ts) {
4810: /*
4811: * Nothing came after this event between C and F, it is
4812: * safe to use info->after for the delta as it
4813: * matched info->before and is still valid.
4814: */
4815: info->delta = ts - info->after;
4816: } else {
4817: /*
4818: * Interrupted between C and F:
4819: * Lost the previous events time stamp. Just set the
4820: * delta to zero, and this will be the same time as
4821: * the event this event interrupted. And the events that
4822: * came after this will still be correct (as they would
4823: * have built their delta on the previous event.
4824: */
4825: info->delta = 0;
4826: }
4827: info->ts = ts;
4828: info->add_timestamp &= ~RB_ADD_STAMP_FORCE;
4829: }
4830:
4831: /*
4832: * If this is the first commit on the page, then it has the same
4833: * timestamp as the page itself.
4834: */
4835: if (unlikely(!tail && !(info->add_timestamp &
4836: (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE))))
4837: info->delta = 0;
4838:
4839: /* We reserved something on the buffer */
4840:
4841: event = __rb_page_index(tail_page, tail);
4842: rb_update_event(cpu_buffer, event, info);
4843:
4844: local_inc(&tail_page->entries);
4845:
4846: /*
4847: * If this is the first commit on the page, then update
4848: * its timestamp.
4849: */
4850: if (unlikely(!tail))
4851: tail_page->page->time_stamp = info->ts;
4852:
4853: /* account for these added bytes */
4854: local_add(info->length, &cpu_buffer->entries_bytes);
4855:
4856: return event;
4857: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 7/2 |
2026/08/09 05:23 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:ring_buffer_iter_empty]
Results: map[SourceCode:5628: /**
5629: * ring_buffer_iter_empty - check if an iterator has no more to read
5630: * @iter: The iterator to check
5631: */
5632: int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
5633: {
5634: struct ring_buffer_per_cpu *cpu_buffer;
5635: struct buffer_page *reader;
5636: struct buffer_page *head_page;
5637: struct buffer_page *commit_page;
5638: struct buffer_page *curr_commit_page;
5639: unsigned commit;
5640: u64 curr_commit_ts;
5641: u64 commit_ts;
5642:
5643: cpu_buffer = iter->cpu_buffer;
5644: reader = cpu_buffer->reader_page;
5645: head_page = cpu_buffer->head_page;
5646: commit_page = READ_ONCE(cpu_buffer->commit_page);
5647: commit_ts = commit_page->page->time_stamp;
5648:
5649: /*
5650: * When the writer goes across pages, it issues a cmpxchg which
5651: * is a mb(), which will synchronize with the rmb here.
5652: * (see rb_tail_page_update())
5653: */
5654: smp_rmb();
5655: commit = rb_page_size(commit_page);
5656: /* We want to make sure that the commit page doesn't change */
5657: smp_rmb();
5658:
5659: /* Make sure commit page didn't change */
5660: curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
5661: curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
5662:
5663: /* If the commit page changed, then there's more data */
5664: if (curr_commit_page != commit_page ||
5665: curr_commit_ts != commit_ts)
5666: return 0;
5667:
5668: /* Still racy, as it may return a false positive, but that's OK */
5669: return ((iter->head_page == commit_page && iter->head >= commit) ||
5670: (iter->head_page == reader && commit_page == head_page &&
5671: head_page->read == commit &&
5672: iter->head == rb_page_size(cpu_buffer->reader_page)));
5673: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 8/2 |
2026/08/09 05:23 |
llm |
expert |
0m
Model:
Tokens:
input: 10532
output: 58
thoughts: 273
Thoughts: **Investigating the Ring Buffer Time Stamp Access**
Okay, here we go. I'm looking at this code snippet, and immediately something jumps out at me. Specifically, I'm noticing how the `time_stamp` field of a page within the ring buffer is being handled. In `__rb_reserve_next`, at line 4851, we have a write to `tail_page->page->time_stamp`. This is a straightforward write operation, as expected.
Now, let's contrast that with the read operations in `ring_buffer_iter_empty`. On line 5647, we have a simple read: `commit_page->page->time_stamp`. But then, at line 5661, we see `READ_ONCE(curr_commit_page->page->time_stamp)`. This is where my brain starts to ping. Why the difference? Why is one a plain read and the other uses `READ_ONCE`? This has got to be some kind of memory barrier/ordering consideration.
I need to understand the implications of these different access methods. Are we dealing with potential race conditions, cache coherency issues, or perhaps a difference in how the compiler optimizes these reads? I need a much clearer picture of what's happening.
Okay, let's dig deeper. The first step is to locate all references to `time_stamp` within `kernel/trace/ring_buffer.c`. I need to review all reads and writes to this variable to understand the access patterns and what's driving this distinction between a regular read and `READ_ONCE`. Gotta trace the data flow and understand the intent.
|
| 9/2 |
2026/08/09 05:23 |
tool |
codesearch-find-references |
0m
Error:
Args: map[ContextFile:kernel/trace/ring_buffer.c IncludeSnippetLines:5 Name:time_stamp SourceTreePrefix:kernel/trace/ring_buffer.c]
Results: map[References:<nil> TruncatedOutput:false]
|
| 10/2 |
2026/08/09 05:23 |
llm |
expert |
0m
Model:
Tokens:
input: 11239
output: 36
thoughts: 86
Thoughts: Okay, here's my thought process as I'm looking at this code, thinking out loud to myself:
**Tracing Down the Time Stamp**
Alright, so `time_stamp` is a field, that much is clear. But it's nested within some structure. First thing's first, I need to figure out where that `time_stamp` field lives. The structure name is the key. Let's see... it's `tail_page->page->time_stamp` or `commit_page->page->time_stamp`. Okay, so `page` must be a member of `tail_page` or `commit_page`, which are likely different. The important piece here is the `page` member, and I need to find its definition. Now, I'm pretty sure `page` is either a `struct buffer_page` or directly a `struct page`, or something similar. Time to pull up the definitions and quickly confirm what's going on. Once I have the layout of `buffer_page` (or whatever `page` resolves to), I'll see where `time_stamp` finally resides.
|
| 11/2 |
2026/08/09 05:23 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:buffer_page]
Results: map[Fields:[map[Name:list OffsetBits:0 SizeBits:128] map[Name:write OffsetBits:128 SizeBits:64] map[Name:read OffsetBits:192 SizeBits:32] map[Name:entries OffsetBits:256 SizeBits:64] map[Name:real_end OffsetBits:320 SizeBits:64] map[Name:order OffsetBits:384 SizeBits:32] map[Name:id OffsetBits:416 SizeBits:30] map[Name:range OffsetBits:446 SizeBits:1] map[Name:page OffsetBits:448 SizeBits:64]]]
|
| 12/2 |
2026/08/09 05:23 |
llm |
expert |
0m
Model:
Tokens:
input: 11788
output: 60
thoughts: 74
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience:
**Debugging a Pointer in a Memory Structure**
Alright, so `page` is clearly a pointer to *something* within this kernel memory layout. My first thought is to understand the underlying structure it points to. Given the context, I suspect it's likely a `struct buffer_page`, `struct ring_buffer_page`, or something along those lines β definitely a page-oriented data structure used for buffering. Now, to track down the exact definition of the structure, I'll need to locate where `page` is actually *defined*. I can use `grepper` or `codesearch-find-references` to efficiently search the codebase for usages of a field like `buffer_page::page`, or `ring_buffer_page::page` -- or simply find the structs themselves. That should quickly zero in on the relevant header files and reveal the structure's members, allowing me to fully understand what `page` is actually pointing at, and how it's being used. This will hopefully provide the context I need to resolve whatever issue I'm looking at.
|
| 13/2 |
2026/08/09 05:23 |
tool |
codesearch-find-references |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c IncludeSnippetLines:5 Name:buffer_page::page SourceTreePrefix:kernel/trace/ring_buffer.c]
Results: map[References:[map[ReferenceKind:writes ReferencingEntityKind:function ReferencingEntityName:__rb_allocate_pages SourceFile:kernel/trace/ring_buffer.c SourceLine:2416 SourceSnippet:2411: */
2412: list_add_tail(&bpage->list, pages);
2413:
2414: if (meta) {
2415: /* A range was given. Use that for the buffer page */
2416: bpage->page = rb_range_buffer(cpu_buffer, i + 1);
2417: if (!bpage->page)
2418: goto free_pages;
2419: /* If this is valid from a previous boot */
2420: if (meta->head_buffer)
2421: rb_meta_buffer_update(cpu_buffer, bpage);
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:__rb_allocate_pages SourceFile:kernel/trace/ring_buffer.c SourceLine:2417 SourceSnippet:2412: list_add_tail(&bpage->list, pages);
2413:
2414: if (meta) {
2415: /* A range was given. Use that for the buffer page */
2416: bpage->page = rb_range_buffer(cpu_buffer, i + 1);
2417: if (!bpage->page)
2418: goto free_pages;
2419: /* If this is valid from a previous boot */
2420: if (meta->head_buffer)
2421: rb_meta_buffer_update(cpu_buffer, bpage);
2422: bpage->range = 1;
] map[ReferenceKind:writes ReferencingEntityKind:function ReferencingEntityName:__rb_allocate_pages SourceFile:kernel/trace/ring_buffer.c SourceLine:2430 SourceSnippet:2425: void *p = ring_buffer_desc_page(desc, i + 1);
2426:
2427: if (WARN_ON(!p))
2428: goto free_pages;
2429:
2430: bpage->page = p;
2431: bpage->range = 1; /* bpage->page can't be freed */
2432: bpage->id = i + 1;
2433: cpu_buffer->subbuf_ids[i + 1] = bpage;
2434: } else {
2435: int order = cpu_buffer->buffer->subbuf_order;
] map[ReferenceKind:writes ReferencingEntityKind:function ReferencingEntityName:__rb_allocate_pages SourceFile:kernel/trace/ring_buffer.c SourceLine:2436 SourceSnippet:2431: bpage->range = 1; /* bpage->page can't be freed */
2432: bpage->id = i + 1;
2433: cpu_buffer->subbuf_ids[i + 1] = bpage;
2434: } else {
2435: int order = cpu_buffer->buffer->subbuf_order;
2436: bpage->page = alloc_cpu_data(cpu_buffer->cpu, order);
2437: if (!bpage->page)
2438: goto free_pages;
2439: }
2440: bpage->order = cpu_buffer->buffer->subbuf_order;
2441:
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:__rb_allocate_pages SourceFile:kernel/trace/ring_buffer.c SourceLine:2437 SourceSnippet:2432: bpage->id = i + 1;
2433: cpu_buffer->subbuf_ids[i + 1] = bpage;
2434: } else {
2435: int order = cpu_buffer->buffer->subbuf_order;
2436: bpage->page = alloc_cpu_data(cpu_buffer->cpu, order);
2437: if (!bpage->page)
2438: goto free_pages;
2439: }
2440: bpage->order = cpu_buffer->buffer->subbuf_order;
2441:
2442: if (user_thread && fatal_signal_pending(current))
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:__rb_get_reader_page SourceFile:kernel/trace/ring_buffer.c SourceLine:5850 SourceSnippet:5845: /*
5846: * Reset the reader page to size zero.
5847: */
5848: local_set(&cpu_buffer->reader_page->write, 0);
5849: local_set(&cpu_buffer->reader_page->entries, 0);
5850: rb_init_data_page(cpu_buffer->reader_page->page);
5851: cpu_buffer->reader_page->real_end = 0;
5852:
5853: spin:
5854: /*
5855: * Splice the empty reader page into the list around the head.
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:__rb_get_reader_page SourceFile:kernel/trace/ring_buffer.c SourceLine:5935 SourceSnippet:5930: goto again;
5931:
5932: out:
5933: /* Update the read_stamp on the first event */
5934: if (reader && reader->read == 0)
5935: cpu_buffer->read_stamp = reader->page->time_stamp;
5936:
5937: arch_spin_unlock(&cpu_buffer->lock);
5938: local_irq_restore(flags);
5939:
5940: /*
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:__rb_get_reader_page_from_remote SourceFile:kernel/trace/ring_buffer.c SourceLine:5747 SourceSnippet:5742: return NULL;
5743:
5744: /* More to read on the reader page */
5745: if (cpu_buffer->reader_page->read < rb_page_size(cpu_buffer->reader_page)) {
5746: if (!cpu_buffer->reader_page->read)
5747: cpu_buffer->read_stamp = cpu_buffer->reader_page->page->time_stamp;
5748: return cpu_buffer->reader_page;
5749: }
5750:
5751: prev_reader = cpu_buffer->subbuf_ids[cpu_buffer->meta_page->reader.id];
5752:
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:__rb_get_reader_page_from_remote SourceFile:kernel/trace/ring_buffer.c SourceLine:5789 SourceSnippet:5784:
5785: cpu_buffer->head_page = new_head;
5786: cpu_buffer->reader_page = new_reader;
5787: cpu_buffer->reader_page->read = 0;
5788: cpu_buffer->pages = &new_head->list;
5789: cpu_buffer->read_stamp = new_reader->page->time_stamp;
5790: cpu_buffer->lost_events = cpu_buffer->meta_page->reader.lost_events;
5791:
5792: return rb_page_size(cpu_buffer->reader_page) ? cpu_buffer->reader_page : NULL;
5793: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:__rb_map_vma SourceFile:kernel/trace/ring_buffer.c SourceLine:7691 SourceSnippet:7686:
7687: if (WARN_ON_ONCE(s >= nr_subbufs))
7688: return -EINVAL;
7689:
7690: subbuf = cpu_buffer->subbuf_ids[s];
7691: page = virt_to_page((void *)subbuf->page);
7692:
7693: for (; off < (1 << (subbuf_order)); off++, page++) {
7694: if (p >= nr_pages)
7695: break;
7696:
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:__rb_page_index SourceFile:kernel/trace/ring_buffer.c SourceLine:3433 SourceSnippet:3431: static __always_inline void *__rb_page_index(struct buffer_page *bpage, unsigned index)
3432: {
3433: return bpage->page->data + index;
3434: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:__rb_reserve_next SourceFile:kernel/trace/ring_buffer.c SourceLine:4851 SourceSnippet:4846: /*
4847: * If this is the first commit on the page, then update
4848: * its timestamp.
4849: */
4850: if (unlikely(!tail))
4851: tail_page->page->time_stamp = info->ts;
4852:
4853: /* account for these added bytes */
4854: local_add(info->length, &cpu_buffer->entries_bytes);
4855:
4856: return event;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:__rb_validate_buffer SourceFile:kernel/trace/ring_buffer.c SourceLine:1907 SourceSnippet:1903: static int __rb_validate_buffer(struct buffer_page *bpage, int cpu,
1904: struct ring_buffer_cpu_meta *meta,
1905: u64 prev_ts, u64 next_ts)
1906: {
1907: struct buffer_data_page *dpage = bpage->page;
1908: unsigned long long ts;
1909: unsigned long tail;
1910: u64 delta;
1911: int ret;
1912:
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:free_buffer_page SourceFile:kernel/trace/ring_buffer.c SourceLine:398 SourceSnippet: 394: static void free_buffer_page(struct buffer_page *bpage)
395: {
396: /* Range pages are not to be freed */
397: if (!bpage->range)
398: free_pages((unsigned long)bpage->page, bpage->order);
399: kfree(bpage);
400: }
] map[ReferenceKind:writes ReferencingEntityKind:function ReferencingEntityName:rb_allocate_cpu_buffer SourceFile:kernel/trace/ring_buffer.c SourceLine:2525 SourceSnippet:2520: * Range mapped buffers have the same restrictions as memory
2521: * mapped ones do.
2522: */
2523: cpu_buffer->mapped = 1;
2524: cpu_buffer->ring_meta = rb_range_meta(buffer, nr_pages, cpu);
2525: bpage->page = rb_range_buffer(cpu_buffer, 0);
2526: if (!bpage->page)
2527: goto fail_free_reader;
2528: if (cpu_buffer->ring_meta->head_buffer)
2529: rb_meta_buffer_update(cpu_buffer, bpage);
2530: bpage->range = 1;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_allocate_cpu_buffer SourceFile:kernel/trace/ring_buffer.c SourceLine:2526 SourceSnippet:2521: * mapped ones do.
2522: */
2523: cpu_buffer->mapped = 1;
2524: cpu_buffer->ring_meta = rb_range_meta(buffer, nr_pages, cpu);
2525: bpage->page = rb_range_buffer(cpu_buffer, 0);
2526: if (!bpage->page)
2527: goto fail_free_reader;
2528: if (cpu_buffer->ring_meta->head_buffer)
2529: rb_meta_buffer_update(cpu_buffer, bpage);
2530: bpage->range = 1;
2531: } else if (buffer->remote) {
] map[ReferenceKind:writes ReferencingEntityKind:function ReferencingEntityName:rb_allocate_cpu_buffer SourceFile:kernel/trace/ring_buffer.c SourceLine:2549 SourceSnippet:2544:
2545: /* Remote buffers are read-only and immutable */
2546: atomic_inc(&cpu_buffer->record_disabled);
2547: atomic_inc(&cpu_buffer->resize_disabled);
2548:
2549: bpage->page = ring_buffer_desc_page(desc, cpu_buffer->meta_page->reader.id);
2550: if (!bpage->page)
2551: goto fail_free_reader;
2552:
2553: bpage->range = 1;
2554: cpu_buffer->subbuf_ids[0] = bpage;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_allocate_cpu_buffer SourceFile:kernel/trace/ring_buffer.c SourceLine:2550 SourceSnippet:2545: /* Remote buffers are read-only and immutable */
2546: atomic_inc(&cpu_buffer->record_disabled);
2547: atomic_inc(&cpu_buffer->resize_disabled);
2548:
2549: bpage->page = ring_buffer_desc_page(desc, cpu_buffer->meta_page->reader.id);
2550: if (!bpage->page)
2551: goto fail_free_reader;
2552:
2553: bpage->range = 1;
2554: cpu_buffer->subbuf_ids[0] = bpage;
2555: } else {
] map[ReferenceKind:writes ReferencingEntityKind:function ReferencingEntityName:rb_allocate_cpu_buffer SourceFile:kernel/trace/ring_buffer.c SourceLine:2557 SourceSnippet:2552:
2553: bpage->range = 1;
2554: cpu_buffer->subbuf_ids[0] = bpage;
2555: } else {
2556: int order = cpu_buffer->buffer->subbuf_order;
2557: bpage->page = alloc_cpu_data(cpu, order);
2558: if (!bpage->page)
2559: goto fail_free_reader;
2560: }
2561:
2562: INIT_LIST_HEAD(&cpu_buffer->reader_page->list);
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_allocate_cpu_buffer SourceFile:kernel/trace/ring_buffer.c SourceLine:2558 SourceSnippet:2553: bpage->range = 1;
2554: cpu_buffer->subbuf_ids[0] = bpage;
2555: } else {
2556: int order = cpu_buffer->buffer->subbuf_order;
2557: bpage->page = alloc_cpu_data(cpu, order);
2558: if (!bpage->page)
2559: goto fail_free_reader;
2560: }
2561:
2562: INIT_LIST_HEAD(&cpu_buffer->reader_page->list);
2563: INIT_LIST_HEAD(&cpu_buffer->new_pages);
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_buffer_peek SourceFile:kernel/trace/ring_buffer.c SourceLine:6090 SourceSnippet:6085: goto again;
6086:
6087: case RINGBUF_TYPE_TIME_STAMP:
6088: if (ts) {
6089: *ts = rb_event_time_stamp(event);
6090: *ts = rb_fix_abs_ts(*ts, reader->page->time_stamp);
6091: ring_buffer_normalize_time_stamp(cpu_buffer->buffer,
6092: cpu_buffer->cpu, ts);
6093: }
6094: /* Internal data, OK to advance */
6095: rb_advance_reader(cpu_buffer);
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_clear_buffer_page SourceFile:kernel/trace/ring_buffer.c SourceLine:6486 SourceSnippet:6482: static void rb_clear_buffer_page(struct buffer_page *page)
6483: {
6484: local_set(&page->write, 0);
6485: local_set(&page->entries, 0);
6486: rb_init_data_page(page->page);
6487: page->read = 0;
6488: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_decrement_entry SourceFile:kernel/trace/ring_buffer.c SourceLine:5015 SourceSnippet:5010: struct buffer_page *start;
5011:
5012: addr &= ~((PAGE_SIZE << cpu_buffer->buffer->subbuf_order) - 1);
5013:
5014: /* Do the likely case first */
5015: if (likely(bpage->page == (void *)addr)) {
5016: local_dec(&bpage->entries);
5017: return;
5018: }
5019:
5020: /*
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_decrement_entry SourceFile:kernel/trace/ring_buffer.c SourceLine:5027 SourceSnippet:5022: * start with the next page and check the end loop there.
5023: */
5024: rb_inc_page(&bpage);
5025: start = bpage;
5026: do {
5027: if (bpage->page == (void *)addr) {
5028: local_dec(&bpage->entries);
5029: return;
5030: }
5031: rb_inc_page(&bpage);
5032: } while (bpage != start);
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_head_page_activate SourceFile:kernel/trace/ring_buffer.c SourceLine:1309 SourceSnippet:1304: */
1305: rb_set_list_to_head(head->list.prev);
1306:
1307: if (cpu_buffer->ring_meta) {
1308: struct ring_buffer_cpu_meta *meta = cpu_buffer->ring_meta;
1309: meta->head_buffer = (unsigned long)head->page;
1310: }
1311: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_inc_iter SourceFile:kernel/trace/ring_buffer.c SourceLine:3536 SourceSnippet:3531: rb_inc_page(&iter->head_page);
3532:
3533: if (rb_page_commit(iter->head_page) & RB_MISSED_EVENTS)
3534: iter->missed_events = -1;
3535:
3536: iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
3537: iter->head = 0;
3538: iter->next_event = 0;
3539: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_iter_head_event SourceFile:kernel/trace/ring_buffer.c SourceLine:3487 SourceSnippet:3482: * event was safely copied without the writer entering the page.
3483: */
3484: smp_rmb();
3485:
3486: /* Make sure the page didn't change since we read this */
3487: if (iter->page_stamp != iter_head_page->page->time_stamp ||
3488: commit > rb_page_size(iter_head_page))
3489: goto reset;
3490:
3491: iter->next_event = iter->head + length;
3492: return iter->event;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_iter_head_event SourceFile:kernel/trace/ring_buffer.c SourceLine:3495 SourceSnippet:3490:
3491: iter->next_event = iter->head + length;
3492: return iter->event;
3493: reset:
3494: /* Reset to the beginning */
3495: iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
3496: iter->head = 0;
3497: iter->next_event = 0;
3498: iter->missed_events = 1;
3499: return NULL;
3500: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_iter_peek SourceFile:kernel/trace/ring_buffer.c SourceLine:6185 SourceSnippet:6180: goto again;
6181:
6182: case RINGBUF_TYPE_TIME_STAMP:
6183: if (ts) {
6184: *ts = rb_event_time_stamp(event);
6185: *ts = rb_fix_abs_ts(*ts, iter->head_page->page->time_stamp);
6186: ring_buffer_normalize_time_stamp(cpu_buffer->buffer,
6187: cpu_buffer->cpu, ts);
6188: }
6189: /* Internal data, OK to advance */
6190: rb_advance_iter(iter);
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_iter_reset SourceFile:kernel/trace/ring_buffer.c SourceLine:5598 SourceSnippet:5593: iter->cache_read = cpu_buffer->read;
5594: iter->cache_pages_removed = cpu_buffer->pages_removed;
5595:
5596: if (iter->head) {
5597: iter->read_stamp = cpu_buffer->read_stamp;
5598: iter->page_stamp = cpu_buffer->reader_page->page->time_stamp;
5599: } else {
5600: iter->read_stamp = iter->head_page->page->time_stamp;
5601: iter->page_stamp = iter->read_stamp;
5602: }
5603: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_iter_reset SourceFile:kernel/trace/ring_buffer.c SourceLine:5600 SourceSnippet:5595:
5596: if (iter->head) {
5597: iter->read_stamp = cpu_buffer->read_stamp;
5598: iter->page_stamp = cpu_buffer->reader_page->page->time_stamp;
5599: } else {
5600: iter->read_stamp = iter->head_page->page->time_stamp;
5601: iter->page_stamp = iter->read_stamp;
5602: }
5603: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_meta_buffer_update SourceFile:kernel/trace/ring_buffer.c SourceLine:2318 SourceSnippet:2313: static void rb_meta_buffer_update(struct ring_buffer_per_cpu *cpu_buffer,
2314: struct buffer_page *bpage)
2315: {
2316: struct ring_buffer_cpu_meta *meta = cpu_buffer->ring_meta;
2317:
2318: if (meta->head_buffer == (unsigned long)bpage->page)
2319: cpu_buffer->head_page = bpage;
2320:
2321: if (meta->commit_buffer == (unsigned long)bpage->page) {
2322: cpu_buffer->commit_page = bpage;
2323: cpu_buffer->tail_page = bpage;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_meta_buffer_update SourceFile:kernel/trace/ring_buffer.c SourceLine:2321 SourceSnippet:2316: struct ring_buffer_cpu_meta *meta = cpu_buffer->ring_meta;
2317:
2318: if (meta->head_buffer == (unsigned long)bpage->page)
2319: cpu_buffer->head_page = bpage;
2320:
2321: if (meta->commit_buffer == (unsigned long)bpage->page) {
2322: cpu_buffer->commit_page = bpage;
2323: cpu_buffer->tail_page = bpage;
2324: }
2325: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_meta_inject_reader_page SourceFile:kernel/trace/ring_buffer.c SourceLine:2016 SourceSnippet:2011: bpage->list.next = &head_page->list;
2012: rb_set_list_to_head(&bpage->list);
2013: cpu_buffer->pages = &head_page->list;
2014:
2015: cpu_buffer->head_page = head_page;
2016: meta->head_buffer = (unsigned long)head_page->page;
2017:
2018: /* Reset all the indexes */
2019: bpage = cpu_buffer->reader_page;
2020: meta->buffers[0] = rb_meta_subbuf_idx(meta, bpage->page);
2021: bpage->id = 0;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_meta_inject_reader_page SourceFile:kernel/trace/ring_buffer.c SourceLine:2020 SourceSnippet:2015: cpu_buffer->head_page = head_page;
2016: meta->head_buffer = (unsigned long)head_page->page;
2017:
2018: /* Reset all the indexes */
2019: bpage = cpu_buffer->reader_page;
2020: meta->buffers[0] = rb_meta_subbuf_idx(meta, bpage->page);
2021: bpage->id = 0;
2022:
2023: for (i = 1, bpage = head_page; i < meta->nr_subbufs;
2024: i++, rb_inc_page(&bpage)) {
2025: meta->buffers[i] = rb_meta_subbuf_idx(meta, bpage->page);
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_meta_inject_reader_page SourceFile:kernel/trace/ring_buffer.c SourceLine:2025 SourceSnippet:2020: meta->buffers[0] = rb_meta_subbuf_idx(meta, bpage->page);
2021: bpage->id = 0;
2022:
2023: for (i = 1, bpage = head_page; i < meta->nr_subbufs;
2024: i++, rb_inc_page(&bpage)) {
2025: meta->buffers[i] = rb_meta_subbuf_idx(meta, bpage->page);
2026: bpage->id = i;
2027: }
2028: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_meta_validate_events SourceFile:kernel/trace/ring_buffer.c SourceLine:2055 SourceSnippet:2050: cpu_buffer->cpu);
2051: /* Don't bother rewinding */
2052: skip = true;
2053: state.ts = 0;
2054: } else {
2055: state.ts = head_page->page->time_stamp;
2056: }
2057:
2058: /* Do the reader page - reader must be previous to head. */
2059: rb_validate_buffer(orig_reader, cpu_buffer, meta, &state, 0, state.ts);
2060:
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_meta_validate_events SourceFile:kernel/trace/ring_buffer.c SourceLine:2079 SourceSnippet:2074: /* Rewind until tail (writer) page. */
2075: if (head_page == cpu_buffer->tail_page)
2076: break;
2077:
2078: /* Rewind until unused page (no timestamp, no commit). */
2079: if (!head_page->page->time_stamp && rb_page_commit(head_page) == 0)
2080: break;
2081:
2082: /*
2083: * Skip if the page is invalid, or its timestamp is newer than the
2084: * previous valid page.
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_meta_validate_events SourceFile:kernel/trace/ring_buffer.c SourceLine:2107 SourceSnippet:2102: head_page = orig_head;
2103: }
2104:
2105: skip_rewind:
2106: /* If the commit_buffer is the reader page, update the commit page */
2107: if (meta->commit_buffer == (unsigned long)cpu_buffer->reader_page->page) {
2108: cpu_buffer->commit_page = cpu_buffer->reader_page;
2109: /* Nothing more to do, the only page is the reader page */
2110: goto done;
2111: }
2112: state.ts = head_page->page->time_stamp;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_meta_validate_events SourceFile:kernel/trace/ring_buffer.c SourceLine:2112 SourceSnippet:2107: if (meta->commit_buffer == (unsigned long)cpu_buffer->reader_page->page) {
2108: cpu_buffer->commit_page = cpu_buffer->reader_page;
2109: /* Nothing more to do, the only page is the reader page */
2110: goto done;
2111: }
2112: state.ts = head_page->page->time_stamp;
2113:
2114: /* Iterate until finding the commit page */
2115: for (i = 0; i < meta->nr_subbufs + 1; i++, rb_inc_page(&head_page)) {
2116:
2117: /* The original reader page has already been checked/counted. */
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_meta_validate_events SourceFile:kernel/trace/ring_buffer.c SourceLine:2164 SourceSnippet:2159: meta->head_buffer = 0;
2160: meta->commit_buffer = 0;
2161:
2162: /* Reset the reader page */
2163: local_set(&cpu_buffer->reader_page->entries, 0);
2164: rb_init_data_page(cpu_buffer->reader_page->page);
2165:
2166: /* Reset all the subbuffers */
2167: for (i = 0; i < meta->nr_subbufs - 1; i++, rb_inc_page(&head_page)) {
2168: local_set(&head_page->entries, 0);
2169: rb_init_data_page(head_page->page);
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_meta_validate_events SourceFile:kernel/trace/ring_buffer.c SourceLine:2169 SourceSnippet:2164: rb_init_data_page(cpu_buffer->reader_page->page);
2165:
2166: /* Reset all the subbuffers */
2167: for (i = 0; i < meta->nr_subbufs - 1; i++, rb_inc_page(&head_page)) {
2168: local_set(&head_page->entries, 0);
2169: rb_init_data_page(head_page->page);
2170: }
2171: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_page_commit SourceFile:kernel/trace/ring_buffer.c SourceLine:386 SourceSnippet: 384: static __always_inline unsigned int rb_page_commit(struct buffer_page *bpage)
385: {
386: return rb_data_page_commit(bpage->page);
387: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_page_id SourceFile:kernel/trace/ring_buffer.c SourceLine:6516 SourceSnippet:6511: /*
6512: * For boot buffers, the id is the index,
6513: * otherwise, set the buffer page with this id
6514: */
6515: if (cpu_buffer->ring_meta)
6516: id = rb_meta_subbuf_idx(cpu_buffer->ring_meta, bpage->page);
6517: else
6518: bpage->id = id;
6519:
6520: return id;
6521: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_page_size SourceFile:kernel/trace/ring_buffer.c SourceLine:391 SourceSnippet: 389: static __always_inline unsigned int rb_page_size(struct buffer_page *bpage)
390: {
391: return rb_data_page_size(bpage->page);
392: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_set_commit_to_write SourceFile:kernel/trace/ring_buffer.c SourceLine:4189 SourceSnippet:4184: return;
4185: /*
4186: * No need for a memory barrier here, as the update
4187: * of the tail_page did it for this page.
4188: */
4189: local_set(&cpu_buffer->commit_page->page->commit,
4190: rb_page_write(cpu_buffer->commit_page));
4191: rb_inc_page(&cpu_buffer->commit_page);
4192: if (cpu_buffer->ring_meta) {
4193: struct ring_buffer_cpu_meta *meta = cpu_buffer->ring_meta;
4194: meta->commit_buffer = (unsigned long)cpu_buffer->commit_page->page;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_set_commit_to_write SourceFile:kernel/trace/ring_buffer.c SourceLine:4194 SourceSnippet:4189: local_set(&cpu_buffer->commit_page->page->commit,
4190: rb_page_write(cpu_buffer->commit_page));
4191: rb_inc_page(&cpu_buffer->commit_page);
4192: if (cpu_buffer->ring_meta) {
4193: struct ring_buffer_cpu_meta *meta = cpu_buffer->ring_meta;
4194: meta->commit_buffer = (unsigned long)cpu_buffer->commit_page->page;
4195: }
4196: /* add barrier to keep gcc from optimizing too much */
4197: barrier();
4198: }
4199: while (rb_commit_index(cpu_buffer) !=
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_set_commit_to_write SourceFile:kernel/trace/ring_buffer.c SourceLine:4204 SourceSnippet:4199: while (rb_commit_index(cpu_buffer) !=
4200: rb_page_write(cpu_buffer->commit_page)) {
4201:
4202: /* Make sure the readers see the content of what is committed. */
4203: smp_wmb();
4204: local_set(&cpu_buffer->commit_page->page->commit,
4205: rb_page_write(cpu_buffer->commit_page));
4206: RB_WARN_ON(cpu_buffer,
4207: rb_page_commit(cpu_buffer->commit_page) & ~RB_WRITE_MASK);
4208: barrier();
4209: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_tail_page_update SourceFile:kernel/trace/ring_buffer.c SourceLine:1505 SourceSnippet:1500: /*
1501: * No need to worry about races with clearing out the commit.
1502: * it only can increment when a commit takes place. But that
1503: * only happens in the outer most nested commit.
1504: */
1505: local_set(&next_page->page->commit, 0);
1506:
1507: /* Either we update tail_page or an interrupt does */
1508: if (try_cmpxchg(&cpu_buffer->tail_page, &tail_page, next_page))
1509: local_inc(&cpu_buffer->pages_touched);
1510: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_try_to_discard SourceFile:kernel/trace/ring_buffer.c SourceLine:4110 SourceSnippet:4105:
4106: /*
4107: * Make sure the tail_page is still the same and
4108: * the next write location is the end of this event
4109: */
4110: if (bpage->page == (void *)addr && rb_page_write(bpage) == old_index) {
4111: unsigned long write_mask =
4112: local_read(&bpage->write) & ~RB_WRITE_MASK;
4113: unsigned long event_length = rb_event_length(event);
4114:
4115: /*
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_update_meta_head SourceFile:kernel/trace/ring_buffer.c SourceLine:3555 SourceSnippet:3551: static void rb_update_meta_head(struct ring_buffer_per_cpu *cpu_buffer,
3552: struct buffer_page *next_page)
3553: {
3554: struct ring_buffer_cpu_meta *meta = cpu_buffer->ring_meta;
3555: unsigned long old_head = (unsigned long)next_page->page;
3556: unsigned long new_head;
3557:
3558: rb_inc_page(&next_page);
3559: new_head = (unsigned long)next_page->page;
3560:
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_update_meta_head SourceFile:kernel/trace/ring_buffer.c SourceLine:3559 SourceSnippet:3554: struct ring_buffer_cpu_meta *meta = cpu_buffer->ring_meta;
3555: unsigned long old_head = (unsigned long)next_page->page;
3556: unsigned long new_head;
3557:
3558: rb_inc_page(&next_page);
3559: new_head = (unsigned long)next_page->page;
3560:
3561: /*
3562: * Only move it forward once, if something else came in and
3563: * moved it forward, then we don't want to touch it.
3564: */
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_update_meta_reader SourceFile:kernel/trace/ring_buffer.c SourceLine:3572 SourceSnippet:3568: static void rb_update_meta_reader(struct ring_buffer_per_cpu *cpu_buffer,
3569: struct buffer_page *reader)
3570: {
3571: struct ring_buffer_cpu_meta *meta = cpu_buffer->ring_meta;
3572: void *old_reader = cpu_buffer->reader_page->page;
3573: void *new_reader = reader->page;
3574: int id;
3575:
3576: id = reader->id;
3577: cpu_buffer->reader_page->id = id;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_update_meta_reader SourceFile:kernel/trace/ring_buffer.c SourceLine:3573 SourceSnippet:3568: static void rb_update_meta_reader(struct ring_buffer_per_cpu *cpu_buffer,
3569: struct buffer_page *reader)
3570: {
3571: struct ring_buffer_cpu_meta *meta = cpu_buffer->ring_meta;
3572: void *old_reader = cpu_buffer->reader_page->page;
3573: void *new_reader = reader->page;
3574: int id;
3575:
3576: id = reader->id;
3577: cpu_buffer->reader_page->id = id;
3578: reader->id = 0;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_update_remote_head SourceFile:kernel/trace/ring_buffer.c SourceLine:5550 SourceSnippet:5545:
5546: orig = next = cpu_buffer->head_page;
5547: rb_inc_page(&next);
5548:
5549: /* Run after the writer */
5550: while (cpu_buffer->head_page->page->time_stamp > next->page->time_stamp) {
5551: rb_inc_page(&next);
5552:
5553: rb_list_head_clear(cpu_buffer->head_page->list.prev);
5554: rb_inc_page(&cpu_buffer->head_page);
5555: rb_set_list_to_head(cpu_buffer->head_page->list.prev);
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_update_remote_head SourceFile:kernel/trace/ring_buffer.c SourceLine:5550 SourceSnippet:5545:
5546: orig = next = cpu_buffer->head_page;
5547: rb_inc_page(&next);
5548:
5549: /* Run after the writer */
5550: while (cpu_buffer->head_page->page->time_stamp > next->page->time_stamp) {
5551: rb_inc_page(&next);
5552:
5553: rb_list_head_clear(cpu_buffer->head_page->list.prev);
5554: rb_inc_page(&cpu_buffer->head_page);
5555: rb_set_list_to_head(cpu_buffer->head_page->list.prev);
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_update_remote_head SourceFile:kernel/trace/ring_buffer.c SourceLine:5566 SourceSnippet:5561: }
5562:
5563: orig = cpu_buffer->commit_page = cpu_buffer->head_page;
5564: retry = 3;
5565:
5566: while (cpu_buffer->commit_page->page->time_stamp < next->page->time_stamp) {
5567: rb_inc_page(&next);
5568: rb_inc_page(&cpu_buffer->commit_page);
5569:
5570: if (cpu_buffer->commit_page == orig) {
5571: if (WARN_ON_ONCE(!(--retry)))
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_update_remote_head SourceFile:kernel/trace/ring_buffer.c SourceLine:5566 SourceSnippet:5561: }
5562:
5563: orig = cpu_buffer->commit_page = cpu_buffer->head_page;
5564: retry = 3;
5565:
5566: while (cpu_buffer->commit_page->page->time_stamp < next->page->time_stamp) {
5567: rb_inc_page(&next);
5568: rb_inc_page(&cpu_buffer->commit_page);
5569:
5570: if (cpu_buffer->commit_page == orig) {
5571: if (WARN_ON_ONCE(!(--retry)))
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:rb_validate_buffer SourceFile:kernel/trace/ring_buffer.c SourceLine:1982 SourceSnippet:1977: if (ret)
1978: local_inc(&cpu_buffer->pages_touched);
1979:
1980: state->entries += ret;
1981: state->entry_bytes += rb_page_size(bpage);
1982: state->ts = bpage->page->time_stamp;
1983: }
1984: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_event_time_stamp SourceFile:kernel/trace/ring_buffer.c SourceLine:738 SourceSnippet: 733: u64 ts;
734:
735: /* If the event includes an absolute time, then just use that */
736: if (event->type_len == RINGBUF_TYPE_TIME_STAMP) {
737: ts = rb_event_time_stamp(event);
738: return rb_fix_abs_ts(ts, cpu_buffer->tail_page->page->time_stamp);
739: }
740:
741: nest = local_read(&cpu_buffer->committing);
742: verify_event(cpu_buffer, event);
743: if (WARN_ON_ONCE(!nest))
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_iter_empty SourceFile:kernel/trace/ring_buffer.c SourceLine:5647 SourceSnippet:5642:
5643: cpu_buffer = iter->cpu_buffer;
5644: reader = cpu_buffer->reader_page;
5645: head_page = cpu_buffer->head_page;
5646: commit_page = READ_ONCE(cpu_buffer->commit_page);
5647: commit_ts = commit_page->page->time_stamp;
5648:
5649: /*
5650: * When the writer goes across pages, it issues a cmpxchg which
5651: * is a mb(), which will synchronize with the rmb here.
5652: * (see rb_tail_page_update())
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_iter_empty SourceFile:kernel/trace/ring_buffer.c SourceLine:5661 SourceSnippet:5656: /* We want to make sure that the commit page doesn't change */
5657: smp_rmb();
5658:
5659: /* Make sure commit page didn't change */
5660: curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
5661: curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
5662:
5663: /* If the commit page changed, then there's more data */
5664: if (curr_commit_page != commit_page ||
5665: curr_commit_ts != commit_ts)
5666: return 0;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_iter_empty SourceFile:kernel/trace/ring_buffer.c SourceLine:5661 SourceSnippet:5656: /* We want to make sure that the commit page doesn't change */
5657: smp_rmb();
5658:
5659: /* Make sure commit page didn't change */
5660: curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
5661: curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
5662:
5663: /* If the commit page changed, then there's more data */
5664: if (curr_commit_page != commit_page ||
5665: curr_commit_ts != commit_ts)
5666: return 0;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_iter_empty SourceFile:kernel/trace/ring_buffer.c SourceLine:5661 SourceSnippet:5656: /* We want to make sure that the commit page doesn't change */
5657: smp_rmb();
5658:
5659: /* Make sure commit page didn't change */
5660: curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
5661: curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
5662:
5663: /* If the commit page changed, then there's more data */
5664: if (curr_commit_page != commit_page ||
5665: curr_commit_ts != commit_ts)
5666: return 0;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_iter_empty SourceFile:kernel/trace/ring_buffer.c SourceLine:5661 SourceSnippet:5656: /* We want to make sure that the commit page doesn't change */
5657: smp_rmb();
5658:
5659: /* Make sure commit page didn't change */
5660: curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
5661: curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
5662:
5663: /* If the commit page changed, then there's more data */
5664: if (curr_commit_page != commit_page ||
5665: curr_commit_ts != commit_ts)
5666: return 0;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_iter_empty SourceFile:kernel/trace/ring_buffer.c SourceLine:5661 SourceSnippet:5656: /* We want to make sure that the commit page doesn't change */
5657: smp_rmb();
5658:
5659: /* Make sure commit page didn't change */
5660: curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
5661: curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
5662:
5663: /* If the commit page changed, then there's more data */
5664: if (curr_commit_page != commit_page ||
5665: curr_commit_ts != commit_ts)
5666: return 0;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_iter_empty SourceFile:kernel/trace/ring_buffer.c SourceLine:5661 SourceSnippet:5656: /* We want to make sure that the commit page doesn't change */
5657: smp_rmb();
5658:
5659: /* Make sure commit page didn't change */
5660: curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
5661: curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
5662:
5663: /* If the commit page changed, then there's more data */
5664: if (curr_commit_page != commit_page ||
5665: curr_commit_ts != commit_ts)
5666: return 0;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_iter_empty SourceFile:kernel/trace/ring_buffer.c SourceLine:5661 SourceSnippet:5656: /* We want to make sure that the commit page doesn't change */
5657: smp_rmb();
5658:
5659: /* Make sure commit page didn't change */
5660: curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
5661: curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
5662:
5663: /* If the commit page changed, then there's more data */
5664: if (curr_commit_page != commit_page ||
5665: curr_commit_ts != commit_ts)
5666: return 0;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_map_get_reader SourceFile:kernel/trace/ring_buffer.c SourceLine:7880 SourceSnippet:7875: /* Check if any events were dropped */
7876: missed_events = cpu_buffer->lost_events;
7877:
7878: if (missed_events) {
7879: if (cpu_buffer->reader_page != cpu_buffer->commit_page) {
7880: struct buffer_data_page *dpage = reader->page;
7881: unsigned int commit;
7882: /*
7883: * Use the real_end for the data size,
7884: * This gives us a chance to store the lost events
7885: * on the page.
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_map_get_reader SourceFile:kernel/trace/ring_buffer.c SourceLine:7914 SourceSnippet:7909: * with new events). In this case it's not an
7910: * error, but it should still be reported.
7911: *
7912: * TODO: Add missed events to the page for user space to know.
7913: */
7914: pr_info("Ring buffer [%d] commit overrun lost %ld events at timestamp:%lld\n",
7915: cpu, missed_events, cpu_buffer->reader_page->page->time_stamp);
7916: }
7917: }
7918:
7919: cpu_buffer->lost_events = 0;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_map_get_reader SourceFile:kernel/trace/ring_buffer.c SourceLine:7925 SourceSnippet:7920:
7921: goto consume;
7922:
7923: out:
7924: /* Some archs do not have data cache coherency between kernel and user-space */
7925: flush_kernel_vmap_range(cpu_buffer->reader_page->page,
7926: buffer->subbuf_size + BUF_PAGE_HDR_SIZE);
7927:
7928: rb_update_meta_page(cpu_buffer);
7929:
7930: raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_oldest_event_ts SourceFile:kernel/trace/ring_buffer.c SourceLine:5356 SourceSnippet:5351: if (cpu_buffer->tail_page == cpu_buffer->reader_page)
5352: bpage = cpu_buffer->reader_page;
5353: else
5354: bpage = rb_set_head_page(cpu_buffer);
5355: if (bpage)
5356: ret = bpage->page->time_stamp;
5357: raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
5358:
5359: return ret;
5360: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_read_page SourceFile:kernel/trace/ring_buffer.c SourceLine:7128 SourceSnippet:7123: * Otherwise, we can simply swap the page with the one passed in.
7124: */
7125: if (read || (len < (size - read)) ||
7126: cpu_buffer->reader_page == cpu_buffer->commit_page ||
7127: force_memcpy) {
7128: struct buffer_data_page *rpage = cpu_buffer->reader_page->page;
7129: unsigned int rpos = read;
7130: unsigned int pos = 0;
7131: unsigned int event_size;
7132: unsigned int flags = 0;
7133:
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:ring_buffer_read_page SourceFile:kernel/trace/ring_buffer.c SourceLine:7198 SourceSnippet:7193: cpu_buffer->read += rb_page_entries(reader);
7194: cpu_buffer->read_bytes += rb_page_size(reader);
7195:
7196: /* swap the pages */
7197: rb_init_data_page(dpage);
7198: dpage = reader->page;
7199: reader->page = data_page->data;
7200: local_set(&reader->write, 0);
7201: local_set(&reader->entries, 0);
7202: reader->read = 0;
7203: data_page->data = dpage;
] map[ReferenceKind:writes ReferencingEntityKind:function ReferencingEntityName:ring_buffer_read_page SourceFile:kernel/trace/ring_buffer.c SourceLine:7199 SourceSnippet:7194: cpu_buffer->read_bytes += rb_page_size(reader);
7195:
7196: /* swap the pages */
7197: rb_init_data_page(dpage);
7198: dpage = reader->page;
7199: reader->page = data_page->data;
7200: local_set(&reader->write, 0);
7201: local_set(&reader->entries, 0);
7202: reader->read = 0;
7203: data_page->data = dpage;
7204: if (!missed_events && rb_data_page_commit(dpage) & RB_MISSED_EVENTS)
]] TruncatedOutput:false]
|
| 14/2 |
2026/08/09 05:23 |
llm |
expert |
0m
Model:
Tokens:
input: 29266
output: 38
thoughts: 54
Thoughts: Here's my thought process, as it unfolds:
**Examining `buffer_data_page` Structure**
Okay, let's dive in. I need to get a clear picture of the `buffer_data_page` structure. The first step is, obviously, to track down its definition. Knowing the structure will inform my approach to whatever I'm working on. Now, rather than manually digging through the code, I know there are tools specifically designed for this sort of structural analysis. I'll reach for either `codesearch-struct-layout` or `codesearch-definition-source`. Either of those should get me the exact definition quickly. That will allow me to understand this `buffer_data_page` and get on with the task at hand.
|
| 15/2 |
2026/08/09 05:23 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:buffer_data_page]
Results: map[Fields:[map[Name:time_stamp OffsetBits:0 SizeBits:64] map[Name:commit OffsetBits:64 SizeBits:64] map[Name:data OffsetBits:128 SizeBits:0]]]
|
| 16/2 |
2026/08/09 05:23 |
llm |
expert |
0m
Model:
Tokens:
input: 29588
output: 39
thoughts: 687
Thoughts: **Analyzing a Potential Data Race in the Ring Buffer**
Okay, so I'm looking at this `struct buffer_data_page`. It has a timestamp, a commit field, and then a data field. The data field is zero bits, but I know it's really an array of chars. Now, I'm re-examining this data race report. I see a write to an 8-byte region β looks like the timestamp field β by one task, and a read of that same location by another task. The write is happening in `__rb_reserve_next`, specifically setting the `time_stamp` of a `tail_page`. The read, meanwhile, is in `ring_buffer_iter_empty`, and it's reading the `time_stamp` of a `commit_page`.
Hmm, this looks like a textbook data race. The writer and reader are accessing the same memory location, the timestamp, without any synchronization. The question now is, are `tail_page` and `commit_page` the *same* page? Yes, if the writer is currently writing to a page that's also the current commit page. And the writer is writing without a lock that the reader holds.
Let's check the readerβs lock usage in `ring_buffer_iter_empty`. Ah, it's called locklessly! Good, let's see if there are other reads of `time_stamp` in `ring_buffer_iter_empty`. Yes, line 5661 looks interesting. I see a read to `curr_commit_page->page->time_stamp`, but it uses `READ_ONCE`! Thatβs different from the original read in line 5647. Why are we using `READ_ONCE` in one place and not the other? Is this an oversight? Let's trace it and check when the code at these lines was added or modified. `git-blame` will hopefully shed some light on this.
|
| 17/2 |
2026/08/09 05:23 |
tool |
git-blame |
0m
Args: map[End:5670 File:kernel/trace/ring_buffer.c Start:5640]
Results: map[Output:ead6ecfddea54 5640) u64 curr_commit_ts;
ead6ecfddea54 5641) u64 commit_ts;
7a8e76a3829f1 5642)
7a8e76a3829f1 5643) cpu_buffer = iter->cpu_buffer;
78f7a45dac2a2 5644) reader = cpu_buffer->reader_page;
78f7a45dac2a2 5645) head_page = cpu_buffer->head_page;
f1e30cb636925 5646) commit_page = READ_ONCE(cpu_buffer->commit_page);
ead6ecfddea54 5647) commit_ts = commit_page->page->time_stamp;
ead6ecfddea54 5648)
ead6ecfddea54 5649) /*
ead6ecfddea54 5650) * When the writer goes across pages, it issues a cmpxchg which
ead6ecfddea54 5651) * is a mb(), which will synchronize with the rmb here.
ead6ecfddea54 5652) * (see rb_tail_page_update())
ead6ecfddea54 5653) */
ead6ecfddea54 5654) smp_rmb();
8913e2a48b8d3 5655) commit = rb_page_size(commit_page);
ead6ecfddea54 5656) /* We want to make sure that the commit page doesn't change */
ead6ecfddea54 5657) smp_rmb();
ead6ecfddea54 5658)
ead6ecfddea54 5659) /* Make sure commit page didn't change */
ead6ecfddea54 5660) curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
ead6ecfddea54 5661) curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
ead6ecfddea54 5662)
ead6ecfddea54 5663) /* If the commit page changed, then there's more data */
ead6ecfddea54 5664) if (curr_commit_page != commit_page ||
ead6ecfddea54 5665) curr_commit_ts != commit_ts)
ead6ecfddea54 5666) return 0;
78f7a45dac2a2 5667)
ead6ecfddea54 5668) /* Still racy, as it may return a false positive, but that's OK */
785888c544e04 5669) return ((iter->head_page == commit_page && iter->head >= commit) ||
78f7a45dac2a2 5670) (iter->head_page == reader && commit_page == head_page &&
]
|
| 18/2 |
2026/08/09 05:23 |
llm |
expert |
0m
Model:
Tokens:
input: 31598
output: 22
thoughts: 21
Thoughts: Okay, let's dive into commit `ead6ecfddea54`. I need to understand the impact of this change. My initial approach is always to start with a broad overview, and then go deeper into any areas that look particularly interesting or potentially problematic, given my expertise.
|
| 19/2 |
2026/08/09 05:23 |
tool |
git-show |
0m
Args: map[Commit:ead6ecfddea54]
Results: map[Output:commit ead6ecfddea54f4754c97f64ab7198cc1d8c0daa
Author: Steven Rostedt (VMware) <rostedt@goodmis.org>
Date: Tue Mar 17 17:32:24 2020 -0400
ring-buffer: Have ring_buffer_empty() not depend on tracing stopped
It was complained about that when the trace file is read, that the tracing
is disabled, as the iterator expects writing to the buffer it reads is not
updated. Several steps are needed to make the iterator handle a writer,
by testing if things have changed as it reads.
This step is to make ring_buffer_empty() expect the buffer to be changing.
Note if the current location of the iterator is overwritten, then it will
return false as new data is being added. Note, that this means that data
will be skipped.
Link: http://lkml.kernel.org/r/20200317213415.870741809@goodmis.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 61f0e92ace99..1718520a2809 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3590,16 +3590,37 @@ int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
struct buffer_page *reader;
struct buffer_page *head_page;
struct buffer_page *commit_page;
+ struct buffer_page *curr_commit_page;
unsigned commit;
+ u64 curr_commit_ts;
+ u64 commit_ts;
cpu_buffer = iter->cpu_buffer;
-
- /* Remember, trace recording is off when iterator is in use */
reader = cpu_buffer->reader_page;
head_page = cpu_buffer->head_page;
commit_page = cpu_buffer->commit_page;
+ commit_ts = commit_page->page->time_stamp;
+
+ /*
+ * When the writer goes across pages, it issues a cmpxchg which
+ * is a mb(), which will synchronize with the rmb here.
+ * (see rb_tail_page_update())
+ */
+ smp_rmb();
commit = rb_page_commit(commit_page);
+ /* We want to make sure that the commit page doesn't change */
+ smp_rmb();
+
+ /* Make sure commit page didn't change */
+ curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
+ curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
+
+ /* If the commit page changed, then there's more data */
+ if (curr_commit_page != commit_page ||
+ curr_commit_ts != commit_ts)
+ return 0;
+ /* Still racy, as it may return a false positive, but that's OK */
return ((iter->head_page == commit_page && iter->head == commit) ||
(iter->head_page == reader && commit_page == head_page &&
head_page->read == commit &&
]
|
| 20/2 |
2026/08/09 05:23 |
llm |
expert |
0m
Model:
Tokens:
input: 32606
output: 40
thoughts: 1316
Thoughts: **Analyzing a Potential Data Race in the Ring Buffer Implementation**
Okay, so the commit message highlights a change in `ring_buffer_empty()` that anticipates buffer modifications during iteration, potentially leading to data being skipped if the iterator's position is overwritten. Let's dig deeper into the code and the implications of this change.
Initially, I see this:
```c
commit_page = READ_ONCE(cpu_buffer->commit_page);
commit_ts = commit_page->page->time_stamp;
```
My first thought is, why isn't the `commit_ts` read using `READ_ONCE`? Then looking further down the code I see this:
```c
curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
```
Why is `curr_commit_ts` using `READ_ONCE`? If `commit_ts` doesn't, is it a problem? I'm immediately considering compiler optimizations here. `commit_ts` read without `READ_ONCE` might be optimized by the compiler. However, because there's an `smp_rmb()` between the read of `commit_ts` and `curr_commit_ts`, compiler reordering or merging shouldn't be a problem.
Now, what about the data race itself? Is it benign or something worse? I'll look at the KCSAN report.
```
write to 0xffff8881331f4000 of 8 bytes by task 12741 on cpu 0:
__rb_reserve_next+0x5f1/0x6c0 kernel/trace/ring_buffer.c:4851
read to 0xffff8881331f4000 of 8 bytes by task 11658 on cpu 1:
ring_buffer_iter_empty kernel/trace/ring_buffer.c:5647
```
Okay, line 5647 confirms the race:
```c
commit_ts = commit_page->page->time_stamp;
```
The value changed: `0x000001502ab37190 -> 0x000001505958442b`. Is this a torn read or a stale read? Since `time_stamp` is a 64-bit value, and on x86_64, which we are on, an 8-byte read/write is atomic. There can't be a torn read on this architecture!
But what about other architectures? On 32-bit architectures, a 64-bit read or write can be torn. We need to consider how `time_stamp` is updated:
```c
if (unlikely(!tail))
tail_page->page->time_stamp = info->ts;
```
This is a plain 64-bit write. On 32-bit, this is split into two 32-bit writes. Likewise, the read is a plain 64-bit read. If concurrent, the reader could see a torn value (a mix of old and new values).
What happens if the reader sees a torn value? Let's trace `commit_ts` and `curr_commit_ts`:
```c
/* If the commit page changed, then there's more data */
if (curr_commit_page != commit_page ||
curr_commit_ts != commit_ts)
return 0;
```
If `curr_commit_ts != commit_ts`, the function returns 0 (not empty). If `commit_ts` is a torn value, `curr_commit_ts` is likely to be the new timestamp (or another torn value, or the old timestamp, depending on what happened before the `smp_rmb()` call)
The result is that it's likely that `curr_commit_ts != commit_ts` is true, and it returns 0 (not empty). Is this safe? The comment says "false positive" which is returning 1 (empty) when it isn't. But in this case, returning 0 is safe because if it's 0 the caller will then attempt to read the data. If it doesn't exist, it will then find out at read time. I need to check how `ring_buffer_iter_peek` behaves with this return value. Let's look at `rb_iter_peek` or `ring_
|
| 21/2 |
2026/08/09 05:23 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:ring_buffer_iter_peek]
Results: map[SourceCode:6288: /**
6289: * ring_buffer_iter_peek - peek at the next event to be read
6290: * @iter: The ring buffer iterator
6291: * @ts: The timestamp counter of this event.
6292: *
6293: * This will return the event that will be read next, but does
6294: * not increment the iterator.
6295: */
6296: struct ring_buffer_event *
6297: ring_buffer_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
6298: {
6299: struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
6300: struct ring_buffer_event *event;
6301: unsigned long flags;
6302:
6303: again:
6304: raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
6305: event = rb_iter_peek(iter, ts);
6306: raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
6307:
6308: if (event && event->type_len == RINGBUF_TYPE_PADDING)
6309: goto again;
6310:
6311: return event;
6312: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 22/2 |
2026/08/09 05:23 |
llm |
expert |
0m
Model:
Tokens:
input: 35394
output: 38
thoughts: 12
Thoughts: Okay, let's dive into this. Here's my take on it, considering my expertise:
**Diving into `rb_iter_peek`**
Alright, so the task at hand is to understand `rb_iter_peek`. This function is likely part of the implementation for iterating through something β probably a Red-Black tree or some similar balanced data structure, given the `rb_` prefix. Knowing the specific context, like the programming language or library we're in, would be immensely helpful, but I can make some educated assumptions.
My immediate thought is that `peek` suggests a non-destructive read. It's not supposed to modify the iterator's internal state. It's probably retrieving the "next" element without actually advancing the iterator. This is a common pattern for inspecting what's coming up in a sequence without actually consuming it.
The function's signature and implementation details are, of course, critical. I'd want to examine how it handles edge cases like an empty iterator or if the current position is at the end. Does it return a special value (like `NULL` or a specific error code), throw an exception, or something else entirely? These are key considerations for robust code.
Further, I'd want to understand the time complexity. `rb_iter_peek` should ideally be a constant-time operation if it's operating on a well-balanced structure. If it's iterating or searching in a worst-case scenario, that would be a significant point to note.
Finally, I'd need to consider concurrency. Is this function thread-safe? Does it need synchronization mechanisms if it's operating in a multi-threaded environment? If so, what are the implications on performance? I'd be looking for things like mutexes or atomic operations, and how they might affect overall throughput.
|
| 23/2 |
2026/08/09 05:23 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:rb_iter_peek]
Results: map[SourceCode:6116: static struct ring_buffer_event *
6117: rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
6118: {
6119: struct trace_buffer *buffer;
6120: struct ring_buffer_per_cpu *cpu_buffer;
6121: struct ring_buffer_event *event;
6122: int nr_loops = 0;
6123: int max_loops;
6124:
6125: if (ts)
6126: *ts = 0;
6127:
6128: cpu_buffer = iter->cpu_buffer;
6129: buffer = cpu_buffer->buffer;
6130: max_loops = cpu_buffer->ring_meta ? cpu_buffer->nr_pages : 3;
6131:
6132: /*
6133: * Check if someone performed a consuming read to the buffer
6134: * or removed some pages from the buffer. In these cases,
6135: * iterator was invalidated and we need to reset it.
6136: */
6137: if (unlikely(iter->cache_read != cpu_buffer->read ||
6138: iter->cache_reader_page != cpu_buffer->reader_page ||
6139: iter->cache_pages_removed != cpu_buffer->pages_removed))
6140: rb_iter_reset(iter);
6141:
6142: again:
6143: if (ring_buffer_iter_empty(iter))
6144: return NULL;
6145:
6146: /*
6147: * As the writer can mess with what the iterator is trying
6148: * to read, just give up if we fail to get an event after
6149: * three tries. The iterator is not as reliable when reading
6150: * the ring buffer with an active write as the consumer is.
6151: * Do not warn if the three failures is reached.
6152: */
6153: if (++nr_loops > max_loops)
6154: return NULL;
6155:
6156: if (rb_per_cpu_empty(cpu_buffer))
6157: return NULL;
6158:
6159: if (iter->head >= rb_page_size(iter->head_page)) {
6160: rb_inc_iter(iter);
6161: goto again;
6162: }
6163:
6164: event = rb_iter_head_event(iter);
6165: if (!event)
6166: goto again;
6167:
6168: switch (event->type_len) {
6169: case RINGBUF_TYPE_PADDING:
6170: if (rb_null_event(event)) {
6171: rb_inc_iter(iter);
6172: goto again;
6173: }
6174: rb_advance_iter(iter);
6175: return event;
6176:
6177: case RINGBUF_TYPE_TIME_EXTEND:
6178: /* Internal data, OK to advance */
6179: rb_advance_iter(iter);
6180: goto again;
6181:
6182: case RINGBUF_TYPE_TIME_STAMP:
6183: if (ts) {
6184: *ts = rb_event_time_stamp(event);
6185: *ts = rb_fix_abs_ts(*ts, iter->head_page->page->time_stamp);
6186: ring_buffer_normalize_time_stamp(cpu_buffer->buffer,
6187: cpu_buffer->cpu, ts);
6188: }
6189: /* Internal data, OK to advance */
6190: rb_advance_iter(iter);
6191: goto again;
6192:
6193: case RINGBUF_TYPE_DATA:
6194: if (ts && !(*ts)) {
6195: *ts = iter->read_stamp + event->time_delta;
6196: ring_buffer_normalize_time_stamp(buffer,
6197: cpu_buffer->cpu, ts);
6198: }
6199: return event;
6200:
6201: default:
6202: RB_WARN_ON(cpu_buffer, 1);
6203: }
6204:
6205: return NULL;
6206: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 24/2 |
2026/08/09 05:23 |
llm |
expert |
0m
Model:
Tokens:
input: 37136
output: 40
thoughts: 2515
Thoughts: **Analyzing Potential Races in Ring Buffer Iteration**
Okay, so I'm diving back into this `rb_iter_peek` function. I need to make sure I understand the intricacies of how this lockless ring buffer works, especially in edge cases. My initial thought is about what happens when `ring_buffer_iter_empty` might return a false positive, indicating emptiness when there are actually events. That's a known race and handled by the design. It leads to a potential missed event but is okay.
My mind then shifts to potential compiler optimization pitfalls, specifically, the use (or lack thereof) of `READ_ONCE`. KCSAN and the Linux kernel memory model are key here. I need to make certain that concurrent conflicting accesses, particularly writes, are synchronized, and so the use of `READ_ONCE` becomes critical. Without it, the compiler might erroneously assume that values don't change, potentially leading to incorrect code generation.
Looking at `ring_buffer_iter_empty`, I see reads of `cpu_buffer->commit_page->page->time_stamp`. The question is whether the compiler could optimize away the second read of the time stamp. Since the second read *is* marked with `READ_ONCE`, that's safe, and the compiler can't eliminate it. But what about the first read of the commit page? The presence of `smp_rmb()` between the reads provides a compiler barrier, meaning the reads can't be reordered.
Now, I'm thinking about torn reads, especially on 32-bit architectures where 64-bit accesses are not atomic. Is there a possibility that a read of `commit_page->page->time_stamp` could yield a torn value? If it does, a later read may return an earlier or later torn value.
I realize that `time_stamp` is 8-byte aligned because of the struct layout, and this ensures atomicity on 64-bit. But on 32-bit architectures, this could be a problem, and the ring buffer *does* support those.
The question then becomes: If we have a torn read, is that harmful? If `curr_commit_ts` and `commit_ts` are the same, even torn values, the function might return "empty" incorrectly. I break down the possibility of torn reads, which can happen if the two 32-bit writes that are necessary to construct the 64-bit value do not complete atomically. I work through the example of a scenario where the writes can get out of order.
However, I realize the timestamp is written *during the reserve*, before commit. So, even if the iterator incorrectly identifies a page as empty due to a torn read, it's not a *real* problem because the data is *not* committed yet.
I then consider another aspect: is there a race if, at the time the new page timestamp is written, the old page is stable? My conclusion is that any race on the timestamp only affects data that is *not yet* committed, meaning the read should not show that data.
Finally, I have the good sense to go back and check the kernel logs for other related fixes and data race reports. I search for `time_stamp` or related functions to search for fixes.
|
| 25/2 |
2026/08/09 05:24 |
tool |
git-log |
0m
Args: map[CodeRegexp:time_stamp Count:10 PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:7cf02d0aa6bd ring-buffer: Cleanup buffer_data_page related code
ea250b7ef9ff ring-buffer: Cleanup persistent ring buffer validation
c8a7d4b4723a ring-buffer: Skip invalid sub-buffers when rewinding persistent ring buffer
93ae1b76fff9 ring-buffer: Export buffer_data_page and macros
fbd1743ecba1 ring-buffer: Add non-consuming read for ring-buffer remotes
2e67fabd8b77 ring-buffer: Introduce ring-buffer remotes
ca296d32ece3 tracing: ring_buffer: Rewind persistent ring buffer on reboot
4fc78a7c9ca9 ring-buffer: Do not trigger WARN_ON() due to a commit_overrun
5f3b6e839f3c ring-buffer: Validate boot range memory events
3cb3091138ca ring-buffer: Use subbuf_order for buffer page masking
]
|
| 26/2 |
2026/08/09 05:24 |
llm |
expert |
0m
Model:
Tokens:
input: 40641
output: 34
thoughts: 32
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm already quite familiar with kernel tracing and concurrency issues:
**Investigating Potential Data Races in Ring Buffer Tracing**
Alright, I need to dig into the ring buffer implementation within the kernel trace code, specifically the `kernel/trace/ring_buffer.c` file. Given the mention of "data race" and "KCSAN," my immediate thought is to check the commit history for this particular file. I'm going to leverage `git log` to zero in on relevant commits. The search terms "data race" and "KCSAN" will be crucial; they'll help filter the commits, allowing me to focus on those that addressed or investigated potential concurrency issues within the ring buffer's operation. This direct approach seems the most efficient way to quickly find any past fixes or ongoing discussions about race conditions in this area, which could inform my next steps in either finding a source of a bug, or determining if the system has known issues.
|
| 27/2 |
2026/08/09 05:24 |
tool |
git-log |
0m
Args: map[MessageRegexps:[data race] PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:8843e06f67b1 ring-buffer: Handle race between rb_move_tail and rb_check_pages
]
|
| 28/2 |
2026/08/09 05:24 |
llm |
expert |
0m
Model:
Tokens:
input: 40963
output: 35
thoughts: 26
Thoughts: Okay, here's my attempt at summarizing that thought process, tailored for an expert audience:
**Investigating Potential KCSAN Issues in `kernel/trace/ring_buffer.c`**
Alright, time to get to work. First thing's first: I need to investigate any potential KCSAN-related issues within the `kernel/trace/ring_buffer.c` file. Given the sensitivity of ring buffers and their use for tracing, this seems like a logical place to start looking for potential data races or memory corruption issues that KCSAN might flag. "KCSAN" is my starting point, because it will focus my search for issues associated with that tool. That's the best way to determine if there were prior issues in the past, or if changes in the code may have introduced new ones. I'm going to leverage the power of the git log to efficiently trace any commits mentioning "KCSAN" specifically within this particular file. This should help me quickly identify any past fixes, related discussions, or potentially, areas that might still need further scrutiny in the present.
|
| 29/2 |
2026/08/09 05:24 |
tool |
git-log |
0m
Args: map[MessageRegexps:[KCSAN] PathPrefix:kernel/trace/ring_buffer.c]
Results:
|
| 30/2 |
2026/08/09 05:24 |
llm |
expert |
0m
Model:
Tokens:
input: 41265
output: 36
thoughts: 55
Thoughts: Okay, here's my take on those thoughts, framed as my own:
**Investigating Potential Data Race Issues in `kernel/trace/ring_buffer.c`**
Alright, so the initial check for "KCSAN" commits in `kernel/trace/ring_buffer.c` turned up empty. That's good, but it doesn't mean we're completely in the clear, especially given the nature of ring buffers and concurrent access. My next line of inquiry has to be about potential data races. Since explicit KCSAN labeling isn't present, the likely approach to data race prevention here would be careful use of atomic operations or memory barriers.
Therefore, the next step is to examine the git history for this file, specifically looking for uses of `READ_ONCE` or `WRITE_ONCE`. Those macros are often a strong signal of attempts to deal with memory ordering issues and concurrent access within the kernel. If they're present, it would highlight specific areas where the developers were consciously addressing potential race conditions, and could provide insights into the overall strategy in use there. If they're *absent*, it raises a red flag and suggests a deeper dive is necessary, potentially involving analysis of other concurrency primitives or synchronization techniques employed in this section of code.
|
| 31/2 |
2026/08/09 05:24 |
tool |
git-log |
0m
Args: map[MessageRegexps:[READ_ONCE] PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:f1e30cb63692 ring-buffer: use READ_ONCE() to read cpu_buffer->commit_page in concurrent environment
6aa7de059173 locking/atomics: COCCINELLE/treewide: Convert trivial ACCESS_ONCE() patterns to READ_ONCE()/WRITE_ONCE()
8573636ea794 ring-buffer: Use READ_ONCE() for most tail_page access
]
|
| 32/2 |
2026/08/09 05:24 |
llm |
expert |
0m
Model:
Tokens:
input: 41743
output: 26
thoughts: 22
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Analyzing Commit f1e30cb63692**
Right, let's dive into commit `f1e30cb63692`. My initial thought is to quickly ascertain the *scope* of the changes. I'll need to immediately grasp the surface-level modifications: files affected, lines added/deleted, and the general gist of the commit message. A quick `git show --stat f1e30cb63692` or, depending on the tool I'm using, an equivalent visualization, will be my first move.
Next, I'll need to understand the *motivation* behind this commit. The message itself might be sufficient, but I'll be looking for context. Did it address a specific issue, implement a new feature, or refactor existing code? Understanding the "why" is crucial for assessing its long-term impact and potential for introducing regressions. I might need to cross-reference this commit with related issues or pull requests.
Once I've established the basic overview, I'll start assessing the *implementation details*. The specific code changes will be the focal point. I'll be particularly interested in: the logic introduced, any new dependencies, whether there's sufficient unit/integration testing, and adherence to established coding standards and project conventions. Are there any potential performance bottlenecks or subtle bugs that jump out? This requires a deep understanding of the relevant codebase sections.
Finally, I'll consider the *broader implications* of these changes. How does this commit fit into the overall project architecture? Does it affect any other modules or systems? Are there any potential conflicts or merge issues with other ongoing development efforts? Long term maintainability and scalability are always in the back of my mind. The goal is to decide if this is a "good" change, a potentially problematic one, or something in between.
|
| 33/2 |
2026/08/09 05:24 |
tool |
git-show |
0m
Args:
Results: map[Output:commit f1e30cb6369251c03f63c564006f96a54197dcc4
Author: linke li <lilinke99@qq.com>
Date: Sat Mar 2 12:42:21 2024 +0800
ring-buffer: use READ_ONCE() to read cpu_buffer->commit_page in concurrent environment
In function ring_buffer_iter_empty(), cpu_buffer->commit_page is read
while other threads may change it. It may cause the time_stamp that read
in the next line come from a different page. Use READ_ONCE() to avoid
having to reason about compiler optimizations now and in future.
Link: https://lore.kernel.org/linux-trace-kernel/tencent_DFF7D3561A0686B5E8FC079150A02505180A@qq.com
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: linke li <lilinke99@qq.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 67d32af48640..788d321036bd 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4382,7 +4382,7 @@ int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
cpu_buffer = iter->cpu_buffer;
reader = cpu_buffer->reader_page;
head_page = cpu_buffer->head_page;
- commit_page = cpu_buffer->commit_page;
+ commit_page = READ_ONCE(cpu_buffer->commit_page);
commit_ts = commit_page->page->time_stamp;
/*
]
|
| 34/2 |
2026/08/09 05:24 |
llm |
expert |
0m
Model:
Tokens:
input: 42756
output: 169
thoughts: 5388
Thoughts: **Analysis of Data Race in `ring_buffer_iter_empty()`**
Alright, let's break this down. KCSAN flagged a potential data race in `ring_buffer_iter_empty()` because the `commit_page->page->time_stamp` is read without `READ_ONCE`, while the writer is concurrently updating it. The commit points to the page we're reading from.
The commit introduces a change to use `READ_ONCE()` on the pointer itself (`cpu_buffer->commit_page`), which makes sense to address compiler optimizations, but it *didn't* touch the dereferenced `time_stamp` read. This is a bit odd, and KCSAN is understandably complaining.
My initial thought was, "Why didn't they apply `READ_ONCE()` to both?". The writer also plain-writes this timestamp. The question then becomes whether this data race is benign or harmful.
I looked at the code again and saw the commit and current timestamp reads and comparisons:
```c
commit_ts = commit_page->page->time_stamp;
/* ... */
curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
/* If the commit page changed, then there's more data */
if (curr_commit_page != commit_page ||
curr_commit_ts != commit_ts)
return 0;
```
It seems `commit_ts` is a heuristic. If `curr_commit_ts` and `commit_ts` are different, it returns 0 (not empty). This suggests that any stale or torn value for `commit_ts` will *still* lead to returning 0 (not empty). Is this a "hint" flag? Yes, this is exactly a heuristic that might lead to a safe false positive (declaring not empty when it *is* empty). Then the code will check that it's actually empty. But if they match, we continue with the read.
I then went through the classification. Is it a "Marking Required for Correctness" scenario? I examined possible compiler optimizations like reordering, merging, or torn reads. I reasoned that `smp_rmb()` protects against reordering. Torn reads could occur, but a torn read would only result in `curr_commit_ts != commit_ts`, safely returning 0, or equal meaning not updated in between, still safe.
I had to look at all the other places `time_stamp` is read and written within the ring buffer. Most are protected by the `reader_lock`, but these reads within the loop are not protected by a lock because the read from the commit is an edge case to detect the end of the buffer.
The key observation is: In the context of `ring_buffer_iter_empty()`, the timestamp *is* used as a "hint." A stale value will lead to a slightly delayed update (returning 0 (not empty) to the iterator, which is acceptable). However, the lack of `READ_ONCE` could cause undefined behavior because the compiler is allowed to assume no concurrent writes.
Ultimately, although the compiler optimizations *can* be problematic in theory because of torn reads, `commit_ts` is acting as a heuristic hint, and a stale or torn value *safely* results in a "not empty" indication. This is what makes it tolerant to compiler optimizations. The fact that the code already explicitly reads the current timestamp with `READ_ONCE` confirms it is used for that heuristic.
Now, I'm confident that this is a truly benign data race, and the code is structurally tolerant of potential compiler optimizations, even though it could be improved with `READ_ONCE` for clarity. It would be good to add `READ_ONCE` to avoid KCSAN complaints. I should also check with `codeexpert` to see if the kernel community has a specific opinion on this.
|
| 35/2 |
2026/08/09 05:25 |
tool |
codeexpert |
3m
Args: map[Question:In kernel/trace/ring_buffer.c, there is a KCSAN data race report between __rb_reserve_next() writing to `tail_page->page->time_stamp` and ring_buffer_iter_empty() reading `commit_page->page->time_stamp`.
The write is:
```
tail_page->page->time_stamp = info->ts;
```
The read is:
```
commit_ts = commit_page->page->time_stamp;
```
Is this data race considered benign or harmful? If harmful, does it require READ_ONCE/WRITE_ONCE annotations, or is there a logic bug? Please analyze the concurrency and compiler safety of this race.]
Results: map[Answer:### Concurrency and Compiler Safety Analysis of the KCSAN Data Race in `kernel/trace/ring_buffer.c`
The KCSAN data race report identifies a concurrent write to `tail_page->page->time_stamp` in `__rb_reserve_next()` and a concurrent read of `commit_page->page->time_stamp` in `ring_buffer_iter_empty()`.
---
### 1. Concurrency Logic Analysis (High-Level): **Benign**
From a high-level concurrency logic perspective, this data race is **benign** because the ring buffer's design naturally tolerates any outcome of the race without causing functional incorrectness, crashes, or data corruption.
* **The Writer's Action**: In `__rb_reserve_next()`, when a writer reserves the first event on a page (`tail == 0`), it writes the current timestamp to `tail_page->page->time_stamp`. At this point, the event is only reserved and **not yet committed**.
* **The Reader's Action**: In `ring_buffer_iter_empty()`, the reader checks if the iterator is empty. It reads `commit_page->page->time_stamp` twice (into `commit_ts` and `curr_commit_ts`) with memory barriers (`smp_rmb()`) in between to detect if the writer has moved to a new page or updated the timestamp.
* **Race Outcomes**:
1. **If the reads return different values** (`commit_ts != curr_commit_ts`):
`ring_buffer_iter_empty()` returns `0` (not empty). The reader will then attempt to read an event. Since the writer has not yet committed the reserved event, the reader's read/peek attempt will find no committed events and return `NULL`/empty. This is a safe "false positive" (as noted in the code comment: `/* Still racy, as it may return a false positive, but that's OK */`).
2. **If the reads return the same value** (`commit_ts == curr_commit_ts`):
The function may return `1` (empty). Since the event is not committed yet, the buffer is indeed empty from the reader's perspective, which is correct and safe.
---
### 2. Compiler Safety and LKMM Analysis (Low-Level): **Harmful**
From a low-level compiler safety and Linux Kernel Memory Model (LKMM) perspective, the race is **harmful** because the lack of proper annotations can lead to compiler-induced bugs and undefined behavior:
* **Load/Store Tearing**: The `time_stamp` field is a 64-bit (`u64`) value. On 32-bit architectures, 64-bit loads and writes are not atomic and are split into two 32-bit instructions. Without `READ_ONCE` and `WRITE_ONCE`, the compiler can perform store tearing on the write and load tearing on the read, resulting in a mixed/torn garbage value. Even on 64-bit architectures, the compiler is technically permitted to tear plain 64-bit accesses.
* **Compiler Optimizations**: Plain reads allow the compiler to assume that the memory location is not modified concurrently. Although the `smp_rmb()` calls (which act as compiler barriers) prevent the compiler from reordering or merging the two reads across the barriers, the lack of `READ_ONCE`/`WRITE_ONCE` still violates the LKMM and is flagged by KCSAN.
* **Inconsistency**: In `ring_buffer_iter_empty()`, `curr_commit_ts` is read using `READ_ONCE()`, but `commit_ts` is read using a plain load. This is inconsistent and leaves `commit_ts` vulnerable to compiler optimizations and tearing.
---
### Conclusion & Recommended Fix
This data race **does not indicate a logic bug**, but it **requires `READ_ONCE` and `WRITE_ONCE` annotations** to ensure compiler safety, prevent load/store tearing, and satisfy KCSAN.
#### Recommended Fix:
1. In `__rb_reserve_next()` (line ~4851), use `WRITE_ONCE` to write the timestamp:
```c
if (unlikely(!tail))
WRITE_ONCE(tail_page->page->time_stamp, info->ts);
```
2. In `ring_buffer_iter_empty()` (line ~5647), use `READ_ONCE` to read the first timestamp:
```c
commit_page = READ_ONCE(cpu_buffer->commit_page);
commit_ts = READ_ONCE(commit_page->page->time_stamp);
```]
|
| 36/3 |
2026/08/09 05:25 |
agent |
codeexpert |
3m
Model:
Results: map[Answer:### Concurrency and Compiler Safety Analysis of the KCSAN Data Race in `kernel/trace/ring_buffer.c`
The KCSAN data race report identifies a concurrent write to `tail_page->page->time_stamp` in `__rb_reserve_next()` and a concurrent read of `commit_page->page->time_stamp` in `ring_buffer_iter_empty()`.
---
### 1. Concurrency Logic Analysis (High-Level): **Benign**
From a high-level concurrency logic perspective, this data race is **benign** because the ring buffer's design naturally tolerates any outcome of the race without causing functional incorrectness, crashes, or data corruption.
* **The Writer's Action**: In `__rb_reserve_next()`, when a writer reserves the first event on a page (`tail == 0`), it writes the current timestamp to `tail_page->page->time_stamp`. At this point, the event is only reserved and **not yet committed**.
* **The Reader's Action**: In `ring_buffer_iter_empty()`, the reader checks if the iterator is empty. It reads `commit_page->page->time_stamp` twice (into `commit_ts` and `curr_commit_ts`) with memory barriers (`smp_rmb()`) in between to detect if the writer has moved to a new page or updated the timestamp.
* **Race Outcomes**:
1. **If the reads return different values** (`commit_ts != curr_commit_ts`):
`ring_buffer_iter_empty()` returns `0` (not empty). The reader will then attempt to read an event. Since the writer has not yet committed the reserved event, the reader's read/peek attempt will find no committed events and return `NULL`/empty. This is a safe "false positive" (as noted in the code comment: `/* Still racy, as it may return a false positive, but that's OK */`).
2. **If the reads return the same value** (`commit_ts == curr_commit_ts`):
The function may return `1` (empty). Since the event is not committed yet, the buffer is indeed empty from the reader's perspective, which is correct and safe.
---
### 2. Compiler Safety and LKMM Analysis (Low-Level): **Harmful**
From a low-level compiler safety and Linux Kernel Memory Model (LKMM) perspective, the race is **harmful** because the lack of proper annotations can lead to compiler-induced bugs and undefined behavior:
* **Load/Store Tearing**: The `time_stamp` field is a 64-bit (`u64`) value. On 32-bit architectures, 64-bit loads and writes are not atomic and are split into two 32-bit instructions. Without `READ_ONCE` and `WRITE_ONCE`, the compiler can perform store tearing on the write and load tearing on the read, resulting in a mixed/torn garbage value. Even on 64-bit architectures, the compiler is technically permitted to tear plain 64-bit accesses.
* **Compiler Optimizations**: Plain reads allow the compiler to assume that the memory location is not modified concurrently. Although the `smp_rmb()` calls (which act as compiler barriers) prevent the compiler from reordering or merging the two reads across the barriers, the lack of `READ_ONCE`/`WRITE_ONCE` still violates the LKMM and is flagged by KCSAN.
* **Inconsistency**: In `ring_buffer_iter_empty()`, `curr_commit_ts` is read using `READ_ONCE()`, but `commit_ts` is read using a plain load. This is inconsistent and leaves `commit_ts` vulnerable to compiler optimizations and tearing.
---
### Conclusion & Recommended Fix
This data race **does not indicate a logic bug**, but it **requires `READ_ONCE` and `WRITE_ONCE` annotations** to ensure compiler safety, prevent load/store tearing, and satisfy KCSAN.
#### Recommended Fix:
1. In `__rb_reserve_next()` (line ~4851), use `WRITE_ONCE` to write the timestamp:
```c
if (unlikely(!tail))
WRITE_ONCE(tail_page->page->time_stamp, info->ts);
```
2. In `ring_buffer_iter_empty()` (line ~5647), use `READ_ONCE` to read the first timestamp:
```c
commit_page = READ_ONCE(cpu_buffer->commit_page);
commit_ts = READ_ONCE(commit_page->page->time_stamp);
```]
Instruction:
You are a capable Linux kernel developer tasked with researching complex questions
about kernel source code. You will be given a concrete question, and need to provide
a concrete answer.
Use tools extensively while researching the question. Don't make assumptions,
or rely on your previous knowledge about the kernel source code, use available tools
to access the actual source code.
Use all available sources of information:
- kernel source code
- documentation in the Documentation dir in the source tree
- git commits descriptions, git blame
Do not guess file names or file paths and attempt to read them without
verifying their existence first using content search or directory listing tools.
If a file, symbol, or directory is not found via content search (grepper) or
directory listing (codesearch-dir-index), treat it as completely absent.
Do not attempt to guess alternative names, extensions, or directories.
Tool Selection Guidelines:
1. For C functions, structs, and variables: Prefer codesearch-definition-source
or codesearch-struct-layout FIRST to retrieve clean, exact definitions
without line-number guessing.
2. If symbol lookup fails (e.g., preprocessor macros, macro-generated code, or
disabled #ifdef branches), fall back to read-file or grepper.
3. For file headers, #include directives, preprocessor macro definitions, and
non-C files (Kconfig, Makefiles, docs): Use read-file or grepper directly.
Do NOT use git-log to search for the presence or existence of files in the
repository. git-log is only for tracing commit history of files that are
already present in the current checkout. If a file does not exist in the
current checkout, it cannot be used for reproduction.
Avoid running broad git-log queries (such as searches on the entire repo)
without a specific 'PathPrefix' to restrict the scope, as these are very
expensive and will time out.
If a git-log tool call times out, do not retry the query with the same broad
scope. You must specify a tighter, narrower 'PathPrefix' for subsequent queries.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt: In kernel/trace/ring_buffer.c, there is a KCSAN data race report between __rb_reserve_next() writing to `tail_page->page->time_stamp` and ring_buffer_iter_empty() reading `commit_page->page->time_stamp`.
The write is:
```
tail_page->page->time_stamp = info->ts;
```
The read is:
```
commit_ts = commit_page->page->time_stamp;
```
Is this data race considered benign or harmful? If harmful, does it require READ_ONCE/WRITE_ONCE annotations, or is there a logic bug? Please analyze the concurrency and compiler safety of this race.
|
| 37/4 |
2026/08/09 05:25 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 4022
output: 79
thoughts: 168
Thoughts: **Analyzing a KCSAN Data Race in Ring Buffer Code**
Okay, so the user's looking into a KCSAN data race report, specifically within `kernel/trace/ring_buffer.c`. The report flags a potential issue where `__rb_reserve_next()` writes to `tail_page->page->time_stamp` and `ring_buffer_iter_empty()` reads from `commit_page->page->time_stamp`. The suspicion is that these accesses might not be properly synchronized, leading to a race condition. Let's dig into the details. The write itself is: `tail_page->page->time_stamp = info->ts;` and the read is: `commit_ts = commit_page->page->time_stamp;`. Now, I need to pinpoint these specific locations within the source code to understand the context and confirm the race. I'll need to use `codesearch-definition-source` to locate the definitions of `__rb_reserve_next` and `ring_buffer_iter_empty` within that file, `kernel/trace/ring_buffer.c`. That should give me a good starting point for analysis.
|
| 38/4 |
2026/08/09 05:25 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:__rb_reserve_next]
Results: map[SourceCode:4716: static struct ring_buffer_event *
4717: __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
4718: struct rb_event_info *info)
4719: {
4720: struct ring_buffer_event *event;
4721: struct buffer_page *tail_page;
4722: unsigned long tail, write, w;
4723:
4724: /* Don't let the compiler play games with cpu_buffer->tail_page */
4725: tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
4726:
4727: /*A*/ w = local_read(&tail_page->write) & RB_WRITE_MASK;
4728: barrier();
4729: rb_time_read(&cpu_buffer->before_stamp, &info->before);
4730: rb_time_read(&cpu_buffer->write_stamp, &info->after);
4731: barrier();
4732: info->ts = rb_time_stamp(cpu_buffer->buffer);
4733:
4734: if ((info->add_timestamp & RB_ADD_STAMP_ABSOLUTE)) {
4735: info->delta = info->ts;
4736: } else {
4737: /*
4738: * If interrupting an event time update, we may need an
4739: * absolute timestamp.
4740: * Don't bother if this is the start of a new page (w == 0).
4741: */
4742: if (!w) {
4743: /* Use the sub-buffer timestamp */
4744: info->delta = 0;
4745: } else if (unlikely(info->before != info->after)) {
4746: info->add_timestamp |= RB_ADD_STAMP_FORCE | RB_ADD_STAMP_EXTEND;
4747: info->length += RB_LEN_TIME_EXTEND;
4748: } else {
4749: info->delta = info->ts - info->after;
4750: if (unlikely(test_time_stamp(info->delta))) {
4751: info->add_timestamp |= RB_ADD_STAMP_EXTEND;
4752: info->length += RB_LEN_TIME_EXTEND;
4753: }
4754: }
4755: }
4756:
4757: /*B*/ rb_time_set(&cpu_buffer->before_stamp, info->ts);
4758:
4759: /*C*/ write = local_add_return(info->length, &tail_page->write);
4760:
4761: /* set write to only the index of the write */
4762: write &= RB_WRITE_MASK;
4763:
4764: tail = write - info->length;
4765:
4766: /* See if we shot pass the end of this buffer page */
4767: if (unlikely(write > cpu_buffer->buffer->subbuf_size)) {
4768: check_buffer(cpu_buffer, info, CHECK_FULL_PAGE);
4769: return rb_move_tail(cpu_buffer, tail, info);
4770: }
4771:
4772: if (likely(tail == w)) {
4773: /* Nothing interrupted us between A and C */
4774: /*D*/ rb_time_set(&cpu_buffer->write_stamp, info->ts);
4775: /*
4776: * If something came in between C and D, the write stamp
4777: * may now not be in sync. But that's fine as the before_stamp
4778: * will be different and then next event will just be forced
4779: * to use an absolute timestamp.
4780: */
4781: if (likely(!(info->add_timestamp &
4782: (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE))))
4783: /* This did not interrupt any time update */
4784: info->delta = info->ts - info->after;
4785: else
4786: /* Just use full timestamp for interrupting event */
4787: info->delta = info->ts;
4788: check_buffer(cpu_buffer, info, tail);
4789: } else {
4790: u64 ts;
4791: /* SLOW PATH - Interrupted between A and C */
4792:
4793: /* Save the old before_stamp */
4794: rb_time_read(&cpu_buffer->before_stamp, &info->before);
4795:
4796: /*
4797: * Read a new timestamp and update the before_stamp to make
4798: * the next event after this one force using an absolute
4799: * timestamp. This is in case an interrupt were to come in
4800: * between E and F.
4801: */
4802: ts = rb_time_stamp(cpu_buffer->buffer);
4803: rb_time_set(&cpu_buffer->before_stamp, ts);
4804:
4805: barrier();
4806: /*E*/ rb_time_read(&cpu_buffer->write_stamp, &info->after);
4807: barrier();
4808: /*F*/ if (write == (local_read(&tail_page->write) & RB_WRITE_MASK) &&
4809: info->after == info->before && info->after < ts) {
4810: /*
4811: * Nothing came after this event between C and F, it is
4812: * safe to use info->after for the delta as it
4813: * matched info->before and is still valid.
4814: */
4815: info->delta = ts - info->after;
4816: } else {
4817: /*
4818: * Interrupted between C and F:
4819: * Lost the previous events time stamp. Just set the
4820: * delta to zero, and this will be the same time as
4821: * the event this event interrupted. And the events that
4822: * came after this will still be correct (as they would
4823: * have built their delta on the previous event.
4824: */
4825: info->delta = 0;
4826: }
4827: info->ts = ts;
4828: info->add_timestamp &= ~RB_ADD_STAMP_FORCE;
4829: }
4830:
4831: /*
4832: * If this is the first commit on the page, then it has the same
4833: * timestamp as the page itself.
4834: */
4835: if (unlikely(!tail && !(info->add_timestamp &
4836: (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE))))
4837: info->delta = 0;
4838:
4839: /* We reserved something on the buffer */
4840:
4841: event = __rb_page_index(tail_page, tail);
4842: rb_update_event(cpu_buffer, event, info);
4843:
4844: local_inc(&tail_page->entries);
4845:
4846: /*
4847: * If this is the first commit on the page, then update
4848: * its timestamp.
4849: */
4850: if (unlikely(!tail))
4851: tail_page->page->time_stamp = info->ts;
4852:
4853: /* account for these added bytes */
4854: local_add(info->length, &cpu_buffer->entries_bytes);
4855:
4856: return event;
4857: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 39/4 |
2026/08/09 05:25 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:ring_buffer_iter_empty]
Results: map[SourceCode:5628: /**
5629: * ring_buffer_iter_empty - check if an iterator has no more to read
5630: * @iter: The iterator to check
5631: */
5632: int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
5633: {
5634: struct ring_buffer_per_cpu *cpu_buffer;
5635: struct buffer_page *reader;
5636: struct buffer_page *head_page;
5637: struct buffer_page *commit_page;
5638: struct buffer_page *curr_commit_page;
5639: unsigned commit;
5640: u64 curr_commit_ts;
5641: u64 commit_ts;
5642:
5643: cpu_buffer = iter->cpu_buffer;
5644: reader = cpu_buffer->reader_page;
5645: head_page = cpu_buffer->head_page;
5646: commit_page = READ_ONCE(cpu_buffer->commit_page);
5647: commit_ts = commit_page->page->time_stamp;
5648:
5649: /*
5650: * When the writer goes across pages, it issues a cmpxchg which
5651: * is a mb(), which will synchronize with the rmb here.
5652: * (see rb_tail_page_update())
5653: */
5654: smp_rmb();
5655: commit = rb_page_size(commit_page);
5656: /* We want to make sure that the commit page doesn't change */
5657: smp_rmb();
5658:
5659: /* Make sure commit page didn't change */
5660: curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
5661: curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
5662:
5663: /* If the commit page changed, then there's more data */
5664: if (curr_commit_page != commit_page ||
5665: curr_commit_ts != commit_ts)
5666: return 0;
5667:
5668: /* Still racy, as it may return a false positive, but that's OK */
5669: return ((iter->head_page == commit_page && iter->head >= commit) ||
5670: (iter->head_page == reader && commit_page == head_page &&
5671: head_page->read == commit &&
5672: iter->head == rb_page_size(cpu_buffer->reader_page)));
5673: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 40/4 |
2026/08/09 05:25 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 7557
output: 37
thoughts: 399
Thoughts: **Analyzing Potential Memory Ordering Issues**
Okay, let's dive into this code for `ring_buffer_iter_empty` and `__rb_reserve_next`. I'm focusing on the interaction with that `time_stamp` field. In `__rb_reserve_next`, I see a write: `tail_page->page->time_stamp = info->ts;`. `tail_page` is, of course, `cpu_buffer->tail_page`, which makes sense. No surprises there.
But now, looking at `ring_buffer_iter_empty`, I'm seeing something that's making my internal alarms go off. We have `commit_page = READ_ONCE(cpu_buffer->commit_page);` and then, later, `commit_ts = commit_page->page->time_stamp;`. *Without* `READ_ONCE`! That's a potential data race right there if another CPU can write the timestamp.
And here's where it gets even more peculiar: we *do* use `READ_ONCE` for `curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);` later in the function. Why the inconsistency? `curr_commit_ts` and `commit_ts` are reading the exact same data from the same location! Shouldn't there always be `READ_ONCE`? This inconsistency feels like a ticking time bomb for memory ordering issues.
I really need to figure out the reasoning behind this design choice. Is it intended? Is there some implicit memory barrier I'm missing? I'm going to quickly check the git history. Let's see if there are any commits or discussions related to `ring_buffer_iter_empty`, `time_stamp`, or even specifically `curr_commit_ts`. Hopefully the `git log` will give me some clues.
|
| 41/4 |
2026/08/09 05:25 |
tool |
git-log |
0m
Args: map[CodeRegexp:curr_commit_ts PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:ead6ecfddea5 ring-buffer: Have ring_buffer_empty() not depend on tracing stopped
]
|
| 42/4 |
2026/08/09 05:25 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 8439
output: 21
thoughts: 20
Thoughts: Here's my summary, as if I were thinking these thoughts:
**Examining a Specific Commit with Git-Show**
Okay, let's get down to brass tacks. I need to take a really close look at the changes introduced by the commit identified by its SHA, which is `ead6ecfddea5`. My go-to tool for this level of detail, of course, is `git-show`. It's not just a matter of glancing at a diff; I need the full context: the commit message, the author, the date, and most importantly, *exactly* what files were modified and how. I need to see the differences clearly, potentially with added context lines. Let's fire up `git show` and get to it. Time to dissect this commit and understand its impact.
|
| 43/4 |
2026/08/09 05:25 |
tool |
git-show |
0m
Args:
Results: map[Output:commit ead6ecfddea54f4754c97f64ab7198cc1d8c0daa
Author: Steven Rostedt (VMware) <rostedt@goodmis.org>
Date: Tue Mar 17 17:32:24 2020 -0400
ring-buffer: Have ring_buffer_empty() not depend on tracing stopped
It was complained about that when the trace file is read, that the tracing
is disabled, as the iterator expects writing to the buffer it reads is not
updated. Several steps are needed to make the iterator handle a writer,
by testing if things have changed as it reads.
This step is to make ring_buffer_empty() expect the buffer to be changing.
Note if the current location of the iterator is overwritten, then it will
return false as new data is being added. Note, that this means that data
will be skipped.
Link: http://lkml.kernel.org/r/20200317213415.870741809@goodmis.org
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 61f0e92ace99..1718520a2809 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3590,16 +3590,37 @@ int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
struct buffer_page *reader;
struct buffer_page *head_page;
struct buffer_page *commit_page;
+ struct buffer_page *curr_commit_page;
unsigned commit;
+ u64 curr_commit_ts;
+ u64 commit_ts;
cpu_buffer = iter->cpu_buffer;
-
- /* Remember, trace recording is off when iterator is in use */
reader = cpu_buffer->reader_page;
head_page = cpu_buffer->head_page;
commit_page = cpu_buffer->commit_page;
+ commit_ts = commit_page->page->time_stamp;
+
+ /*
+ * When the writer goes across pages, it issues a cmpxchg which
+ * is a mb(), which will synchronize with the rmb here.
+ * (see rb_tail_page_update())
+ */
+ smp_rmb();
commit = rb_page_commit(commit_page);
+ /* We want to make sure that the commit page doesn't change */
+ smp_rmb();
+
+ /* Make sure commit page didn't change */
+ curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
+ curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
+
+ /* If the commit page changed, then there's more data */
+ if (curr_commit_page != commit_page ||
+ curr_commit_ts != commit_ts)
+ return 0;
+ /* Still racy, as it may return a false positive, but that's OK */
return ((iter->head_page == commit_page && iter->head == commit) ||
(iter->head_page == reader && commit_page == head_page &&
head_page->read == commit &&
]
|
| 44/4 |
2026/08/09 05:25 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 9557
output: 39
thoughts: 82
Thoughts: **Investigating Potential Data Races in Ring Buffer Tracing**
Okay, let's dig into this. I need to see if there's any existing work or discussions surrounding potential data races or issues with the `time_stamp` field, particularly within the `kernel/trace/ring_buffer.c` module. Given my experience, a thorough search is the best approach. I'll leverage `git-log` for this. The goal is to identify any relevant commits, discussions, or patches that touch upon `time_stamp`, `KCSAN` (Kernel Concurrency Sanitizer, obviously relevant here), or the phrase "data race," specifically within that `ring_buffer.c` file. I'll craft a `MessageRegexp` search, feeding those keywords into the `git-log` command to pinpoint any historical context or resolution around this area. It's the most efficient way to see if we have prior art to lean on.
|
| 45/4 |
2026/08/09 05:25 |
tool |
git-log |
0m
Args: map[MessageRegexps:[time_stamp race] PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:f1e30cb63692 ring-buffer: use READ_ONCE() to read cpu_buffer->commit_page in concurrent environment
b3ae7b67b87f ring-buffer: Fix writing to the buffer with max_data_size
6455b6163d8c ring-buffer: Fix race while reader and writer are on the same page
097350d1c6e1 ring-buffer: Zero out time extend if it is nested and not absolute
d6097c9e4454 trace: Fix preempt_enable_no_resched() abuse
00b4145298ae ring-buffer: Add interface for setting absolute time stamps
2c2d7329d8af tracing/ftrace: use preempt_enable_no_resched_notrace in ring_buffer_time_stamp()
e2862c9470be trace: fix printk warning for u64
7a8e76a3829f tracing: unified trace buffer
]
|
| 46/4 |
2026/08/09 05:25 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 10134
output: 26
thoughts: 24
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Analyzing Commit f1e30cb63692 with git-show**
Okay, so I need to dive into this commit, `f1e30cb63692`. My initial thought is to grab `git-show` and see what's changed. There's no need to belabor the point, a quick check with `git-show` will provide the diff and commit metadata I need. I'm looking for the changes, specifically the code diff. I expect to be able to immediately glean the essential modifications and their context, and I'll focus on the specifics of the changed files and the line-by-line alterations to understand the logic.
|
| 47/4 |
2026/08/09 05:25 |
tool |
git-show |
0m
Args:
Results: map[Output:commit f1e30cb6369251c03f63c564006f96a54197dcc4
Author: linke li <lilinke99@qq.com>
Date: Sat Mar 2 12:42:21 2024 +0800
ring-buffer: use READ_ONCE() to read cpu_buffer->commit_page in concurrent environment
In function ring_buffer_iter_empty(), cpu_buffer->commit_page is read
while other threads may change it. It may cause the time_stamp that read
in the next line come from a different page. Use READ_ONCE() to avoid
having to reason about compiler optimizations now and in future.
Link: https://lore.kernel.org/linux-trace-kernel/tencent_DFF7D3561A0686B5E8FC079150A02505180A@qq.com
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: linke li <lilinke99@qq.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 67d32af48640..788d321036bd 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4382,7 +4382,7 @@ int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
cpu_buffer = iter->cpu_buffer;
reader = cpu_buffer->reader_page;
head_page = cpu_buffer->head_page;
- commit_page = cpu_buffer->commit_page;
+ commit_page = READ_ONCE(cpu_buffer->commit_page);
commit_ts = commit_page->page->time_stamp;
/*
]
|
| 48/4 |
2026/08/09 05:25 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 10908
output: 69
thoughts: 53
Thoughts: **Investigating Potential Concurrency Issues in `ring_buffer.c`**
Okay, so I need to dig deeper into `kernel/trace/ring_buffer.c`. My gut tells me there might be some lingering KCSAN or data race issues lurking in there that haven't been fully addressed. To that end, I'm going to leverage `git log` β it's the most effective way to sift through the commit history and identify any related changes. My plan is to utilize `MessageRegexps` within `git log`. I'll craft them to specifically look for commits that mention either "KCSAN" or "data race" in their commit messages. This should help me quickly pinpoint any commits that touched upon these concurrency-related concerns within that specific file. Let's see what we uncover.
|
| 49/4 |
2026/08/09 05:26 |
tool |
git-log |
0m
Args: map[MessageRegexps:[KCSAN] PathPrefix:kernel/trace/ring_buffer.c]
Results:
|
| 50/4 |
2026/08/09 05:26 |
tool |
git-log |
0m
Args: map[MessageRegexps:[data race] PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:8843e06f67b1 ring-buffer: Handle race between rb_move_tail and rb_check_pages
]
|
| 51/4 |
2026/08/09 05:26 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 11256
output: 35
thoughts: 66
Thoughts: Here's my thought process, as it stands:
**Troubleshooting a Ring Buffer Timestamp Issue**
Okay, so I'm trying to track down something related to timestamps and ring buffers, likely an issue with either `time_stamp` or the iterator functions. My initial instinct is to dig into the usual suspects: the project's lore, mailing list archives, or, of course, the git logs. I'd typically start by searching for combinations of keywords like `time_stamp` alongside terms related to the ring buffer iterator, perhaps `ring_buffer_iter_empty` or that internal reserve function, `__rb_reserve_next`. It's possible someone has seen this before.
Wait, hold on a second. Given the context, it might be more efficient to narrow things down. Since the timestamp logic seems ring buffer related, let's broaden the search in the git logs. Specifically, let's zero in on `kernel/trace/ring_buffer.c`, focusing on entries where `time_stamp` appears. This is probably where the relevant timestamping code resides, and this could be quicker than a broader search. I need to be efficient here.
|
| 52/4 |
2026/08/09 05:26 |
tool |
git-log |
0m
Args: map[CodeRegexp:time_stamp PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:7cf02d0aa6bd ring-buffer: Cleanup buffer_data_page related code
ea250b7ef9ff ring-buffer: Cleanup persistent ring buffer validation
c8a7d4b4723a ring-buffer: Skip invalid sub-buffers when rewinding persistent ring buffer
93ae1b76fff9 ring-buffer: Export buffer_data_page and macros
fbd1743ecba1 ring-buffer: Add non-consuming read for ring-buffer remotes
2e67fabd8b77 ring-buffer: Introduce ring-buffer remotes
ca296d32ece3 tracing: ring_buffer: Rewind persistent ring buffer on reboot
4fc78a7c9ca9 ring-buffer: Do not trigger WARN_ON() due to a commit_overrun
5f3b6e839f3c ring-buffer: Validate boot range memory events
3cb3091138ca ring-buffer: Use subbuf_order for buffer page masking
]
|
| 53/4 |
2026/08/09 05:26 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 11852
output: 32
thoughts: 1990
Thoughts: **Concurrency and Compiler Safety Analysis of Ring Buffer Iteration**
Okay, let's break this down. My initial focus is on the concurrency implications and compiler safety of this ring buffer implementation, specifically around the `ring_buffer_iter_empty` function and the `__rb_reserve_next` function where timestamps are being updated.
I'm zeroing in on this section:
`tail_page->page->time_stamp = info->ts;` in `__rb_reserve_next`
and
`commit_ts = commit_page->page->time_stamp;` in `ring_buffer_iter_empty`
My first concern, a big one, is the use of `READ_ONCE` on `commit_page`, but not on the subsequent read of `commit_page->page->time_stamp`. Aha! `time_stamp` is a 64-bit value, which introduces the possibility of non-atomic reads/writes on 32-bit architectures, and even on 64-bit platforms, non-atomic accesses are subject to optimization. This is a critical point. Without the appropriate memory barriers, we can be vulnerable to load tearing, store tearing, load/store merging, and other compiler optimizations that can be disastrous in a concurrent setting.
The question then becomes: is this data race benign or harmful? Let's trace the logic in `ring_buffer_iter_empty`: The function checks if the iterator has reached the end. It reads `commit_page` and `commit_ts`, performs some memory barrier operations, and then reads `curr_commit_page` and `curr_commit_ts` to ensure consistency. If either the page or the timestamp has changed, it indicates that there's more data and the function returns 0 (not empty).
If the pages are the same, but the timestamps are different, the buffer is not empty. If the timestamps are the same, either there was no write, the value written was identical, or (here it comes!) a torn read has occurred. A torn read is where the reader sees a partially updated value of `time_stamp`. If the values happen to match, the function *might* return 1 (empty), when it is not actually empty, if the writer is in the process of updating the timestamp, which means that the buffer *is* not empty!
The code comments acknowledge this as a "false positive" scenario. This means that if it returns empty, even when there's a writer active, it's not catastrophic. The reader might wait, but will eventually be woken up.
Now, from the compiler's perspective, without the appropriate synchronization, memory accesses to `time_stamp` can be optimized in ways that lead to unpredictable results. The Linux kernel memory model (LKMM) requires synchronization for concurrent access to shared variables when one of the accesses is a write. Data races are undefined behavior in C11 and LKMM, or at least open the door to dangerous compiler optimizations.
Specifically, there are several ways a compiler could "break" this code:
1. **Load/Store Tearing**: The compiler might split a 64-bit load/store into two 32-bit operations. This could lead to a torn read of `commit_ts` if it gets garbage values, which the function tries to account for by rereading and rechecking.
If the garbage values match between the two reads, there is a problem. If the garbage value is different, that is, `curr_commit_ts != commit_ts` will be true, and the function will return 0 (not empty).
2. **Compiler Optimizations (Load Merging / Load Elimination)**: The compiler *might* decide to cache `commit_page->page->time_stamp` because it was read earlier and since no barrier exists, optimize away the subsequent read of the `time_stamp`. But there is a compiler barrier, two `smp_rmb()` calls. These contain compiler barriers, which prevent merging or reordering. So it can't merge.
`tail_page->page->time_stamp = info->ts;` is a plain write. So there can be tearing.
The comment mentions that a "false positive" (returning empty when there is still data) is acceptable because the reader will eventually wake up. But it is still unsafe to not mark the read and write with some sort of memory barrier.
Overall, the read of `time_stamp` needs to use `READ_ONCE`. The write also needs to use `WRITE_ONCE`.
|
| 54/4 |
2026/08/09 05:27 |
tool |
grepper |
0m
Args: map[Expression:time_stamp PathPrefix:kernel/trace/ring_buffer.c]
Results: map[Output:kernel/trace/ring_buffer.c=77=int ring_buffer_print_entry_header(struct trace_seq *s)
--
kernel/trace/ring_buffer.c-87- RINGBUF_TYPE_TIME_EXTEND);
kernel/trace/ring_buffer.c:88: trace_seq_printf(s, "\ttime_stamp : type == %d\n",
kernel/trace/ring_buffer.c-89- RINGBUF_TYPE_TIME_STAMP);
--
kernel/trace/ring_buffer.c=302=EXPORT_SYMBOL_GPL(ring_buffer_event_data);
--
kernel/trace/ring_buffer.c-309-
kernel/trace/ring_buffer.c:310:static u64 rb_event_time_stamp(struct ring_buffer_event *event)
kernel/trace/ring_buffer.c-311-{
--
kernel/trace/ring_buffer.c=368=static void rb_init_data_page(struct buffer_data_page *bpage)
--
kernel/trace/ring_buffer.c-370- local_set(&bpage->commit, 0);
kernel/trace/ring_buffer.c:371: bpage->time_stamp = 0;
kernel/trace/ring_buffer.c-372-}
--
kernel/trace/ring_buffer.c=562=struct trace_buffer {
--
kernel/trace/ring_buffer.c-580- struct rb_irq_work irq_work;
kernel/trace/ring_buffer.c:581: bool time_stamp_abs;
kernel/trace/ring_buffer.c-582-
--
kernel/trace/ring_buffer.c=609=int ring_buffer_print_page_header(struct trace_buffer *buffer, struct trace_seq *s)
--
kernel/trace/ring_buffer.c-614- "offset:0;\tsize:%u;\tsigned:%u;\n",
kernel/trace/ring_buffer.c:615: (unsigned int)sizeof(field.time_stamp),
kernel/trace/ring_buffer.c-616- (unsigned int)is_signed_type(u64));
--
kernel/trace/ring_buffer.c=644=static void rb_time_set(rb_time_t *t, u64 val)
--
kernel/trace/ring_buffer.c-650- * Enable this to make sure that the event passed to
kernel/trace/ring_buffer.c:651: * ring_buffer_event_time_stamp() is not committed and also
kernel/trace/ring_buffer.c-652- * is on the buffer that it passed in.
--
kernel/trace/ring_buffer.c=698=static inline u64 rb_fix_abs_ts(u64 abs, u64 save_ts)
--
kernel/trace/ring_buffer.c-708-
kernel/trace/ring_buffer.c:709:static inline u64 rb_time_stamp(struct trace_buffer *buffer);
kernel/trace/ring_buffer.c-710-
kernel/trace/ring_buffer.c-711-/**
kernel/trace/ring_buffer.c:712: * ring_buffer_event_time_stamp - return the event's current time stamp
kernel/trace/ring_buffer.c-713- * @buffer: The buffer that the event is on
--
kernel/trace/ring_buffer.c-727- */
kernel/trace/ring_buffer.c:728:u64 ring_buffer_event_time_stamp(struct trace_buffer *buffer,
kernel/trace/ring_buffer.c-729- struct ring_buffer_event *event)
--
kernel/trace/ring_buffer.c-736- if (event->type_len == RINGBUF_TYPE_TIME_STAMP) {
kernel/trace/ring_buffer.c:737: ts = rb_event_time_stamp(event);
kernel/trace/ring_buffer.c:738: return rb_fix_abs_ts(ts, cpu_buffer->tail_page->page->time_stamp);
kernel/trace/ring_buffer.c-739- }
--
kernel/trace/ring_buffer.c=1039=__poll_t ring_buffer_poll_wait(struct trace_buffer *buffer, int cpu,
--
kernel/trace/ring_buffer.c-1120-
kernel/trace/ring_buffer.c:1121:static inline u64 rb_time_stamp(struct trace_buffer *buffer)
kernel/trace/ring_buffer.c-1122-{
--
kernel/trace/ring_buffer.c-1134-
kernel/trace/ring_buffer.c:1135:u64 ring_buffer_time_stamp(struct trace_buffer *buffer)
kernel/trace/ring_buffer.c-1136-{
--
kernel/trace/ring_buffer.c-1139- preempt_disable_notrace();
kernel/trace/ring_buffer.c:1140: time = rb_time_stamp(buffer);
kernel/trace/ring_buffer.c-1141- preempt_enable_notrace();
--
kernel/trace/ring_buffer.c-1144-}
kernel/trace/ring_buffer.c:1145:EXPORT_SYMBOL_GPL(ring_buffer_time_stamp);
kernel/trace/ring_buffer.c-1146-
kernel/trace/ring_buffer.c:1147:void ring_buffer_normalize_time_stamp(struct trace_buffer *buffer,
kernel/trace/ring_buffer.c-1148- int cpu, u64 *ts)
--
kernel/trace/ring_buffer.c-1152-}
kernel/trace/ring_buffer.c:1153:EXPORT_SYMBOL_GPL(ring_buffer_normalize_time_stamp);
kernel/trace/ring_buffer.c-1154-
--
kernel/trace/ring_buffer.c=1840=static int rb_read_data_buffer(struct buffer_data_page *dpage, int tail, int cpu,
--
kernel/trace/ring_buffer.c-1851-
kernel/trace/ring_buffer.c:1852: ts = dpage->time_stamp;
kernel/trace/ring_buffer.c-1853-
--
kernel/trace/ring_buffer.c-1863- case RINGBUF_TYPE_TIME_EXTEND:
kernel/trace/ring_buffer.c:1864: delta = rb_event_time_stamp(event);
kernel/trace/ring_buffer.c-1865- ts += delta;
--
kernel/trace/ring_buffer.c-1868- case RINGBUF_TYPE_TIME_STAMP:
kernel/trace/ring_buffer.c:1869: delta = rb_event_time_stamp(event);
kernel/trace/ring_buffer.c-1870- delta = rb_fix_abs_ts(delta, ts);
--
kernel/trace/ring_buffer.c=1903=static int __rb_validate_buffer(struct buffer_page *bpage, int cpu,
--
kernel/trace/ring_buffer.c-1939- local_set(&dpage->commit, RB_MISSED_EVENTS);
kernel/trace/ring_buffer.c:1940: dpage->time_stamp = prev_ts ? prev_ts : next_ts;
kernel/trace/ring_buffer.c-1941- ret = -1;
--
kernel/trace/ring_buffer.c=1961=static void rb_validate_buffer(struct buffer_page *bpage,
--
kernel/trace/ring_buffer.c-1981- state->entry_bytes += rb_page_size(bpage);
kernel/trace/ring_buffer.c:1982: state->ts = bpage->page->time_stamp;
kernel/trace/ring_buffer.c-1983- }
--
kernel/trace/ring_buffer.c=2031=static void rb_meta_validate_events(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-2054- } else {
kernel/trace/ring_buffer.c:2055: state.ts = head_page->page->time_stamp;
kernel/trace/ring_buffer.c-2056- }
--
kernel/trace/ring_buffer.c-2078- /* Rewind until unused page (no timestamp, no commit). */
kernel/trace/ring_buffer.c:2079: if (!head_page->page->time_stamp && rb_page_commit(head_page) == 0)
kernel/trace/ring_buffer.c-2080- break;
--
kernel/trace/ring_buffer.c-2111- }
kernel/trace/ring_buffer.c:2112: state.ts = head_page->page->time_stamp;
kernel/trace/ring_buffer.c-2113-
--
kernel/trace/ring_buffer.c=2957=void ring_buffer_set_clock(struct trace_buffer *buffer,
--
kernel/trace/ring_buffer.c-2962-
kernel/trace/ring_buffer.c:2963:void ring_buffer_set_time_stamp_abs(struct trace_buffer *buffer, bool abs)
kernel/trace/ring_buffer.c-2964-{
kernel/trace/ring_buffer.c:2965: buffer->time_stamp_abs = abs;
kernel/trace/ring_buffer.c-2966-}
kernel/trace/ring_buffer.c-2967-
kernel/trace/ring_buffer.c:2968:bool ring_buffer_time_stamp_abs(struct trace_buffer *buffer)
kernel/trace/ring_buffer.c-2969-{
kernel/trace/ring_buffer.c:2970: return buffer->time_stamp_abs;
kernel/trace/ring_buffer.c-2971-}
--
kernel/trace/ring_buffer.c=3444=rb_iter_head_event(struct ring_buffer_iter *iter)
--
kernel/trace/ring_buffer.c-3486- /* Make sure the page didn't change since we read this */
kernel/trace/ring_buffer.c:3487: if (iter->page_stamp != iter_head_page->page->time_stamp ||
kernel/trace/ring_buffer.c-3488- commit > rb_page_size(iter_head_page))
--
kernel/trace/ring_buffer.c-3494- /* Reset to the beginning */
kernel/trace/ring_buffer.c:3495: iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
kernel/trace/ring_buffer.c-3496- iter->head = 0;
--
kernel/trace/ring_buffer.c=3518=static void rb_inc_iter(struct ring_buffer_iter *iter)
--
kernel/trace/ring_buffer.c-3535-
kernel/trace/ring_buffer.c:3536: iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
kernel/trace/ring_buffer.c-3537- iter->head = 0;
--
kernel/trace/ring_buffer.c=3938=static struct ring_buffer_event *
kernel/trace/ring_buffer.c:3939:rb_add_time_stamp(struct ring_buffer_per_cpu *cpu_buffer,
kernel/trace/ring_buffer.c-3940- struct ring_buffer_event *event, u64 delta, bool abs)
--
kernel/trace/ring_buffer.c=3979=static void rb_add_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
--
kernel/trace/ring_buffer.c-4014- }
kernel/trace/ring_buffer.c:4015: *event = rb_add_time_stamp(cpu_buffer, *event, info->delta, abs);
kernel/trace/ring_buffer.c-4016- *length -= RB_LEN_TIME_EXTEND;
--
kernel/trace/ring_buffer.c=4579=static void dump_buffer_page(struct buffer_data_page *dpage,
--
kernel/trace/ring_buffer.c-4586-
kernel/trace/ring_buffer.c:4587: ts = dpage->time_stamp;
kernel/trace/ring_buffer.c-4588- pr_warn(" [%lld] PAGE TIME STAMP\n", ts);
--
kernel/trace/ring_buffer.c-4596- case RINGBUF_TYPE_TIME_EXTEND:
kernel/trace/ring_buffer.c:4597: delta = rb_event_time_stamp(event);
kernel/trace/ring_buffer.c-4598- ts += delta;
--
kernel/trace/ring_buffer.c-4603- case RINGBUF_TYPE_TIME_STAMP:
kernel/trace/ring_buffer.c:4604: delta = rb_event_time_stamp(event);
kernel/trace/ring_buffer.c-4605- ts = rb_fix_abs_ts(delta, ts);
--
kernel/trace/ring_buffer.c=4717=__rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
--
kernel/trace/ring_buffer.c-4731- barrier();
kernel/trace/ring_buffer.c:4732: info->ts = rb_time_stamp(cpu_buffer->buffer);
kernel/trace/ring_buffer.c-4733-
--
kernel/trace/ring_buffer.c-4749- info->delta = info->ts - info->after;
kernel/trace/ring_buffer.c:4750: if (unlikely(test_time_stamp(info->delta))) {
kernel/trace/ring_buffer.c-4751- info->add_timestamp |= RB_ADD_STAMP_EXTEND;
--
kernel/trace/ring_buffer.c-4801- */
kernel/trace/ring_buffer.c:4802: ts = rb_time_stamp(cpu_buffer->buffer);
kernel/trace/ring_buffer.c-4803- rb_time_set(&cpu_buffer->before_stamp, ts);
--
kernel/trace/ring_buffer.c-4850- if (unlikely(!tail))
kernel/trace/ring_buffer.c:4851: tail_page->page->time_stamp = info->ts;
kernel/trace/ring_buffer.c-4852-
--
kernel/trace/ring_buffer.c=4860=rb_reserve_next_event(struct trace_buffer *buffer,
--
kernel/trace/ring_buffer.c-4899-
kernel/trace/ring_buffer.c:4900: if (ring_buffer_time_stamp_abs(cpu_buffer->buffer)) {
kernel/trace/ring_buffer.c-4901- add_ts_default = RB_ADD_STAMP_ABSOLUTE;
--
kernel/trace/ring_buffer.c=5335=u64 ring_buffer_oldest_event_ts(struct trace_buffer *buffer, int cpu)
--
kernel/trace/ring_buffer.c-5355- if (bpage)
kernel/trace/ring_buffer.c:5356: ret = bpage->page->time_stamp;
kernel/trace/ring_buffer.c-5357- raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
--
kernel/trace/ring_buffer.c=5541=static void rb_update_remote_head(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-5549- /* Run after the writer */
kernel/trace/ring_buffer.c:5550: while (cpu_buffer->head_page->page->time_stamp > next->page->time_stamp) {
kernel/trace/ring_buffer.c-5551- rb_inc_page(&next);
--
kernel/trace/ring_buffer.c-5565-
kernel/trace/ring_buffer.c:5566: while (cpu_buffer->commit_page->page->time_stamp < next->page->time_stamp) {
kernel/trace/ring_buffer.c-5567- rb_inc_page(&next);
--
kernel/trace/ring_buffer.c=5577=static void rb_iter_reset(struct ring_buffer_iter *iter)
--
kernel/trace/ring_buffer.c-5597- iter->read_stamp = cpu_buffer->read_stamp;
kernel/trace/ring_buffer.c:5598: iter->page_stamp = cpu_buffer->reader_page->page->time_stamp;
kernel/trace/ring_buffer.c-5599- } else {
kernel/trace/ring_buffer.c:5600: iter->read_stamp = iter->head_page->page->time_stamp;
kernel/trace/ring_buffer.c-5601- iter->page_stamp = iter->read_stamp;
--
kernel/trace/ring_buffer.c=5632=int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
--
kernel/trace/ring_buffer.c-5646- commit_page = READ_ONCE(cpu_buffer->commit_page);
kernel/trace/ring_buffer.c:5647: commit_ts = commit_page->page->time_stamp;
kernel/trace/ring_buffer.c-5648-
--
kernel/trace/ring_buffer.c-5660- curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
kernel/trace/ring_buffer.c:5661: curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
kernel/trace/ring_buffer.c-5662-
--
kernel/trace/ring_buffer.c=5677=rb_update_read_stamp(struct ring_buffer_per_cpu *cpu_buffer,
--
kernel/trace/ring_buffer.c-5686- case RINGBUF_TYPE_TIME_EXTEND:
kernel/trace/ring_buffer.c:5687: delta = rb_event_time_stamp(event);
kernel/trace/ring_buffer.c-5688- cpu_buffer->read_stamp += delta;
--
kernel/trace/ring_buffer.c-5691- case RINGBUF_TYPE_TIME_STAMP:
kernel/trace/ring_buffer.c:5692: delta = rb_event_time_stamp(event);
kernel/trace/ring_buffer.c-5693- delta = rb_fix_abs_ts(delta, cpu_buffer->read_stamp);
--
kernel/trace/ring_buffer.c=5707=rb_update_iter_read_stamp(struct ring_buffer_iter *iter,
--
kernel/trace/ring_buffer.c-5716- case RINGBUF_TYPE_TIME_EXTEND:
kernel/trace/ring_buffer.c:5717: delta = rb_event_time_stamp(event);
kernel/trace/ring_buffer.c-5718- iter->read_stamp += delta;
--
kernel/trace/ring_buffer.c-5721- case RINGBUF_TYPE_TIME_STAMP:
kernel/trace/ring_buffer.c:5722: delta = rb_event_time_stamp(event);
kernel/trace/ring_buffer.c-5723- delta = rb_fix_abs_ts(delta, iter->read_stamp);
--
kernel/trace/ring_buffer.c=5737=__rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-5746- if (!cpu_buffer->reader_page->read)
kernel/trace/ring_buffer.c:5747: cpu_buffer->read_stamp = cpu_buffer->reader_page->page->time_stamp;
kernel/trace/ring_buffer.c-5748- return cpu_buffer->reader_page;
--
kernel/trace/ring_buffer.c-5788- cpu_buffer->pages = &new_head->list;
kernel/trace/ring_buffer.c:5789: cpu_buffer->read_stamp = new_reader->page->time_stamp;
kernel/trace/ring_buffer.c-5790- cpu_buffer->lost_events = cpu_buffer->meta_page->reader.lost_events;
--
kernel/trace/ring_buffer.c=5796=__rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
--
kernel/trace/ring_buffer.c-5934- if (reader && reader->read == 0)
kernel/trace/ring_buffer.c:5935: cpu_buffer->read_stamp = reader->page->time_stamp;
kernel/trace/ring_buffer.c-5936-
--
kernel/trace/ring_buffer.c=6043=rb_buffer_peek(struct ring_buffer_per_cpu *cpu_buffer, u64 *ts,
--
kernel/trace/ring_buffer.c-6088- if (ts) {
kernel/trace/ring_buffer.c:6089: *ts = rb_event_time_stamp(event);
kernel/trace/ring_buffer.c:6090: *ts = rb_fix_abs_ts(*ts, reader->page->time_stamp);
kernel/trace/ring_buffer.c:6091: ring_buffer_normalize_time_stamp(cpu_buffer->buffer,
kernel/trace/ring_buffer.c-6092- cpu_buffer->cpu, ts);
--
kernel/trace/ring_buffer.c-6100- *ts = cpu_buffer->read_stamp + event->time_delta;
kernel/trace/ring_buffer.c:6101: ring_buffer_normalize_time_stamp(cpu_buffer->buffer,
kernel/trace/ring_buffer.c-6102- cpu_buffer->cpu, ts);
--
kernel/trace/ring_buffer.c=6117=rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
--
kernel/trace/ring_buffer.c-6183- if (ts) {
kernel/trace/ring_buffer.c:6184: *ts = rb_event_time_stamp(event);
kernel/trace/ring_buffer.c:6185: *ts = rb_fix_abs_ts(*ts, iter->head_page->page->time_stamp);
kernel/trace/ring_buffer.c:6186: ring_buffer_normalize_time_stamp(cpu_buffer->buffer,
kernel/trace/ring_buffer.c-6187- cpu_buffer->cpu, ts);
--
kernel/trace/ring_buffer.c-6195- *ts = iter->read_stamp + event->time_delta;
kernel/trace/ring_buffer.c:6196: ring_buffer_normalize_time_stamp(buffer,
kernel/trace/ring_buffer.c-6197- cpu_buffer->cpu, ts);
--
kernel/trace/ring_buffer.c=6473=unsigned long ring_buffer_max_event_size(struct trace_buffer *buffer)
--
kernel/trace/ring_buffer.c-6475- /* If abs timestamp is requested, events have a timestamp too */
kernel/trace/ring_buffer.c:6476: if (ring_buffer_time_stamp_abs(buffer))
kernel/trace/ring_buffer.c-6477- return buffer->max_data_size - RB_LEN_TIME_EXTEND;
--
kernel/trace/ring_buffer.c=7064=int ring_buffer_read_page(struct trace_buffer *buffer,
--
kernel/trace/ring_buffer.c-7186- local_set(&dpage->commit, pos | flags);
kernel/trace/ring_buffer.c:7187: dpage->time_stamp = save_timestamp;
kernel/trace/ring_buffer.c-7188-
--
kernel/trace/ring_buffer.c=7836=int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu)
--
kernel/trace/ring_buffer.c-7914- pr_info("Ring buffer [%d] commit overrun lost %ld events at timestamp:%lld\n",
kernel/trace/ring_buffer.c:7915: cpu, missed_events, cpu_buffer->reader_page->page->time_stamp);
kernel/trace/ring_buffer.c-7916- }
]
|
| 55/4 |
2026/08/09 05:27 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 21266
output: 40
thoughts: 2189
Thoughts: **Analyzing `ring_buffer_iter_empty` for Potential Data Races**
Okay, let's dissect this `ring_buffer_iter_empty` code. I need to understand why it's structured this way, specifically concerning the reads of the `time_stamp`. First, we read `commit_page` and its associated `time_stamp` into `commit_ts`. Then, we have the memory barriers, a critical part of the puzzle. After the barriers, we re-read the `commit_page` and `time_stamp` into `curr_commit_page` and `curr_commit_ts` using `READ_ONCE`. Now, the crucial question: why is only one of these using `READ_ONCE`?
It clicks: If `commit_page` and `curr_commit_page` point to the same page, we're essentially reading the *same* memory location. But that location, `commit_page->page->time_stamp`, is potentially being written to concurrently by a writer (when it's the `tail_page`). That write is a plain write. So we have a potential data race on the `time_stamp` field. Both the initial read and the second read are racing with the write to `tail_page->page->time_stamp`. This immediately triggers concerns: why no `WRITE_ONCE` on the write, and why no `READ_ONCE` on the initial read?
Let's think through the implications of a data race on `time_stamp`: reads might return old, new, or even torn/garbage values. If a read returns the old `time_stamp` and the buffer is empty, it's correct from the reader's point of view. But, what is happening during the reservation of new space, where the timestamp is set when the first event on a page is written? If the writer is currently on the same page as the reader's `commit_page`, the writer has reserved space on `tail_page` but hasn't updated the commit pointer. The event is not yet "committed" from the reader's perspective, so reporting empty in `ring_buffer_iter_empty` is fine, even if a slightly incorrect old timestamp is read. The reader will just retry later.
What if the reader gets a garbage value? If `commit_ts` is torn and `curr_commit_ts` is also read (either torn or new), the comparison `curr_commit_ts != commit_ts` will almost certainly be true, meaning the function correctly returns 0 (not empty). The reader will then try to read the "uncommitted" event and get NULL or empty anyway, extra work but harmless. If we have the same torn/garbage value for both, `curr_commit_ts == commit_ts` is true, so returning "empty" is still correct, since the event isn't committed yet.
Now, if one reads the old value and the other the new value, we correctly return "not empty." And it's also true that `curr_commit_ts` will not read the old value *after* the initial `commit_ts` read has read the new value, due to the barriers.
So, in all these cases, we seem to be in the clear, at least for now. But we still need to consider compiler optimizations: could the compiler reorder or eliminate instructions in an unsafe way? Specifically, since `commit_ts` is a plain read, could the compiler assume it doesn't change and optimize away the `curr_commit_ts != commit_ts` check? However, the `smp_rmb()` *must* prevent reordering. The compiler has to generate a load instruction for both, because of the barrier and `READ_ONCE`. But can the write be torn, though? Yes, although usually not for aligned 64-bit writes. This is a point to be extra careful about.
Finally, we also need to consider other concurrent reads and writes. I'll need to check the call sites of `rb_update_remote_head`.
|
| 56/4 |
2026/08/09 05:27 |
tool |
codesearch-find-references |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:rb_update_remote_head]
Results: map[References:[map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:rb_iter_reset SourceFile:kernel/trace/ring_buffer.c SourceLine:5583]] TruncatedOutput:false]
|
| 57/4 |
2026/08/09 05:27 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 24444
output: 38
thoughts: 21
Thoughts: Here's my summary, as though I'm thinking through this process:
**Diving into `rb_iter_reset`**
Okay, time to dissect `rb_iter_reset`. My first move is to leverage `codesearch-definition-source`. That's the most direct way to get to the core of this function and see the original implementation, the surrounding context, and any comments that might illuminate its purpose. I'm expecting this to either confirm my existing understanding or reveal some subtleties I might have missed in the past. I'm anticipating this is a standard idiom, so I expect to see it used often, and I'll want to see how that usage shapes the design of the function itself.
|
| 58/4 |
2026/08/09 05:27 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:rb_iter_reset]
Results: map[SourceCode:5577: static void rb_iter_reset(struct ring_buffer_iter *iter)
5578: {
5579: struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
5580:
5581: if (cpu_buffer->remote) {
5582: rb_read_remote_meta_page(cpu_buffer);
5583: rb_update_remote_head(cpu_buffer);
5584: }
5585:
5586: /* Iterator usage is expected to have record disabled */
5587: iter->head_page = cpu_buffer->reader_page;
5588: iter->head = cpu_buffer->reader_page->read;
5589: iter->next_event = iter->head;
5590: iter->missed_events = 0;
5591:
5592: iter->cache_reader_page = iter->head_page;
5593: iter->cache_read = cpu_buffer->read;
5594: iter->cache_pages_removed = cpu_buffer->pages_removed;
5595:
5596: if (iter->head) {
5597: iter->read_stamp = cpu_buffer->read_stamp;
5598: iter->page_stamp = cpu_buffer->reader_page->page->time_stamp;
5599: } else {
5600: iter->read_stamp = iter->head_page->page->time_stamp;
5601: iter->page_stamp = iter->read_stamp;
5602: }
5603: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 59/4 |
2026/08/09 05:27 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 25124
output: 38
thoughts: 4006
Thoughts: **Data Race Analysis in `kernel/trace/ring_buffer.c`**
Okay, so the question is about a potential data race between the writer (in `__rb_reserve_next()`) and the reader (in `ring_buffer_iter_empty()`) of the `time_stamp` field of a ring buffer page. Specifically, `__rb_reserve_next()` writes `tail_page->page->time_stamp = info->ts;` when reserving the first event on a new page, and `ring_buffer_iter_empty()` reads `commit_page->page->time_stamp` to check if a commit has changed. Since the user asked about concurrency and compiler safety, I'll need to look at both the logic and the compiler's view.
First, the concurrency logic. In `__rb_reserve_next()`, the timestamp write happens when the buffer *reserves* space. This means the writer "owns" that memory location. In `ring_buffer_iter_empty()`, the reader is checking if there is a new commit. The reader takes `READ_ONCE(cpu_buffer->commit_page)` to get `commit_page`. This is correct, as this pointer can be updated concurrently by the writer. Then, the reader *also* reads the `time_stamp` of this commit_page. The reader reads `commit_ts = commit_page->page->time_stamp;` which is a plain load, immediately followed by a barrier, `smp_rmb()`. After the barrier, the reader again reads the timestamp from the current commit page, `curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);`. The intent is to make sure it did not change while executing.
The logic seems sound. If the timestamps don't match after the barrier, the reader knows something changed, and they check again. If the timestamps do match, then the reader can be certain that no new data has been committed. However, this check is racy, as it may return a false positive, but that's safe in this case. It seems the intent is that it is safe to return "not empty", even if it is not. This means, the higher-level concurrency *appears* benign.
However, the problem is not about functional correctness. It's about how the compiler treats plain reads and writes of memory locations in the Linux kernel memory model (LKMM). KCSAN is flagging this for a reason.
The key issue is the *lack* of `READ_ONCE` and `WRITE_ONCE`. The write in `__rb_reserve_next()` needs `WRITE_ONCE` to ensure that, on 32-bit architectures, the compiler generates atomic loads/stores for 64-bit `time_stamp` values, and does not interleave reads and writes. Without `WRITE_ONCE`, the compiler could perform "store tearing", where the 64-bit write is split into two 32-bit writes that can interleave with the read. Similarly, the read `commit_ts = commit_page->page->time_stamp;` needs `READ_ONCE`. Without `READ_ONCE`, on 32-bit architectures, the compiler could perform "load tearing", where it reads a mixed value from different 32 bit reads. On 64-bit architectures, the compiler *might* be smart enough to do an atomic 64 bit load. But, relying on the compiler to be that smart is a bug waiting to happen. More importantly, the compiler *assumes* that plain reads and writes are data-race-free, and it *can* make optimizations assuming so. It can optimize away the second read of the timestamp. This also causes issues. These kinds of optimizations could violate the intended memory ordering and cause undefined behavior or subtle bugs.
The history shows the original commit had `READ_ONCE` around the `commit_page` variable, but not around `commit_ts`. It appears a later patch added the `READ_ONCE` to `commit_page` to prevent the `commit_page` pointer from changing, but did not fix the concurrent read of `time_stamp`. The LKMM requires that the reads and writes of shared variables be protected by explicit synchronization mechanisms like `READ_ONCE` and `WRITE_ONCE`.
Thus, this data race is **harmful** because of the compiler safety implications. The plain read in `ring_buffer_iter_empty()` and plain write in `__rb_reserve_next()` of `time_stamp` without `READ_ONCE` and `WRITE_ONCE` are not sufficient under LKMM. They are susceptible to compiler optimizations like load/store tearing and potential reordering
|
| 60/4 |
2026/08/09 05:27 |
tool |
read-file |
0m
Args: map[File:kernel/trace/ring_buffer.c FirstLine:5630 LineCount:50]
Results: map[Contents:5630: * @iter: The iterator to check
5631: */
5632: int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
5633: {
5634: struct ring_buffer_per_cpu *cpu_buffer;
5635: struct buffer_page *reader;
5636: struct buffer_page *head_page;
5637: struct buffer_page *commit_page;
5638: struct buffer_page *curr_commit_page;
5639: unsigned commit;
5640: u64 curr_commit_ts;
5641: u64 commit_ts;
5642:
5643: cpu_buffer = iter->cpu_buffer;
5644: reader = cpu_buffer->reader_page;
5645: head_page = cpu_buffer->head_page;
5646: commit_page = READ_ONCE(cpu_buffer->commit_page);
5647: commit_ts = commit_page->page->time_stamp;
5648:
5649: /*
5650: * When the writer goes across pages, it issues a cmpxchg which
5651: * is a mb(), which will synchronize with the rmb here.
5652: * (see rb_tail_page_update())
5653: */
5654: smp_rmb();
5655: commit = rb_page_size(commit_page);
5656: /* We want to make sure that the commit page doesn't change */
5657: smp_rmb();
5658:
5659: /* Make sure commit page didn't change */
5660: curr_commit_page = READ_ONCE(cpu_buffer->commit_page);
5661: curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
5662:
5663: /* If the commit page changed, then there's more data */
5664: if (curr_commit_page != commit_page ||
5665: curr_commit_ts != commit_ts)
5666: return 0;
5667:
5668: /* Still racy, as it may return a false positive, but that's OK */
5669: return ((iter->head_page == commit_page && iter->head >= commit) ||
5670: (iter->head_page == reader && commit_page == head_page &&
5671: head_page->read == commit &&
5672: iter->head == rb_page_size(cpu_buffer->reader_page)));
5673: }
5674: EXPORT_SYMBOL_GPL(ring_buffer_iter_empty);
5675:
5676: static void
5677: rb_update_read_stamp(struct ring_buffer_per_cpu *cpu_buffer,
5678: struct ring_buffer_event *event)
5679: {
]
|
| 61/4 |
2026/08/09 05:27 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 31021
output: 38
thoughts: 137
Thoughts: **Analyzing Potential Concurrency Issues**
Okay, so I'm looking at this code and confirming that `commit_ts = commit_page->page->time_stamp;` is, as I suspected, a straightforward, regular read. And similarly, `curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);` is also a `READ_ONCE` read, which should offer some level of memory barrier guarantees. But, that's not enough; I need to be *absolutely* certain there aren't any sneaky concurrent accesses that could lead to data races.
So, I'm going to take another look at the code, specifically to scrutinize how `time_stamp` is being accessed. Let's delve back into `kernel/trace/ring_buffer.c`, particularly around the `__rb_reserve_next` function. I remember we were looking at that earlier, and that area is a prime suspect for concurrent operations. I'm going to carefully re-examine lines 4830 through 4865, just to be thorough and verify that there aren't any hidden writes or reads to `time_stamp` that might be interacting in an unsafe way. This is crucial for maintaining data integrity in this particular part of the ring buffer implementation.
|
| 62/4 |
2026/08/09 05:28 |
tool |
read-file |
0m
Args: map[File:kernel/trace/ring_buffer.c FirstLine:4830 LineCount:35]
Results: map[Contents:4830:
4831: /*
4832: * If this is the first commit on the page, then it has the same
4833: * timestamp as the page itself.
4834: */
4835: if (unlikely(!tail && !(info->add_timestamp &
4836: (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE))))
4837: info->delta = 0;
4838:
4839: /* We reserved something on the buffer */
4840:
4841: event = __rb_page_index(tail_page, tail);
4842: rb_update_event(cpu_buffer, event, info);
4843:
4844: local_inc(&tail_page->entries);
4845:
4846: /*
4847: * If this is the first commit on the page, then update
4848: * its timestamp.
4849: */
4850: if (unlikely(!tail))
4851: tail_page->page->time_stamp = info->ts;
4852:
4853: /* account for these added bytes */
4854: local_add(info->length, &cpu_buffer->entries_bytes);
4855:
4856: return event;
4857: }
4858:
4859: static __always_inline struct ring_buffer_event *
4860: rb_reserve_next_event(struct trace_buffer *buffer,
4861: struct ring_buffer_per_cpu *cpu_buffer,
4862: unsigned long length)
4863: {
4864: struct ring_buffer_event *event;
]
|
| 63/4 |
2026/08/09 05:28 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 31971
output: 1057
thoughts: 1660
Thoughts: **KCSAN Race Analysis: Benign Race, Critical Annotations**
Alright, let's break this down. KCSAN flagged a data race, so first, my internal alarms go off. This is about `__rb_reserve_next()` and `ring_buffer_iter_empty()`, specifically around the `time_stamp` field of a ring buffer page. From a high-level concurrency standpoint, this race is **benign**. It's not leading to any functional bugs, crashes, or data corruption. The code *intends* for this race to happen, in a way. The reader is checking for buffer emptiness by comparing timestamps β essentially asking, "has the writer updated this page?" Any result from the race - reading an old, new, or torn timestamp β leads to safe behavior. If the reader sees different timestamps (`commit_ts` and `curr_commit_ts`), it correctly assumes data might be present (a "false positive", as the comment says), and keeps reading. If it sees the same timestamps, the buffer is essentially empty (for now), and the iterator will attempt to read from the buffer later, or be woken up. Either way, no harm, no foul.
However, from a low-level, compiler perspective, we have a problem. The 64-bit `time_stamp` is causing load/store tearing on 32-bit architectures, and even on 64-bit, the compiler's still technically permitted to do this. Compilers, being the clever things they are, can also make unwarranted assumptions because `commit_ts` is a plain read β even though there's an `smp_rmb()` compiler barrier, the lack of `READ_ONCE`/`WRITE_ONCE` still violates the LKMM. That's why KCSAN is catching it. There's inconsistency here, as `curr_commit_ts` uses `READ_ONCE`, but `commit_ts` does not, leaving the latter vulnerable. This is precisely the kind of thing that can bite you, manifesting as subtle, difficult-to-reproduce issues.
The fix is straightforward, thankfully. Itβs not a functional bug, but a matter of proper annotation. In `__rb_reserve_next()`, we need `WRITE_ONCE(tail_page->page->time_stamp, info->ts)`. And, in `ring_buffer_iter_empty()`, it's `commit_ts = READ_ONCE(commit_page->page->time_stamp)`. We need to consistently annotate concurrent accesses to `time_stamp` like that, and this should cover it. No need to go chasing down every `time_stamp` reference right now, as we can focus on this race specifically. I'm going to set-results now to document this analysis.
|
| 64/4 |
2026/08/09 05:28 |
tool |
set-results |
0m
Args: map[Answer:### Concurrency and Compiler Safety Analysis of the KCSAN Data Race in `kernel/trace/ring_buffer.c`
The KCSAN data race report identifies a concurrent write to `tail_page->page->time_stamp` in `__rb_reserve_next()` and a concurrent read of `commit_page->page->time_stamp` in `ring_buffer_iter_empty()`.
---
### 1. Concurrency Logic Analysis (High-Level): **Benign**
From a high-level concurrency logic perspective, this data race is **benign** because the ring buffer's design naturally tolerates any outcome of the race without causing functional incorrectness, crashes, or data corruption.
* **The Writer's Action**: In `__rb_reserve_next()`, when a writer reserves the first event on a page (`tail == 0`), it writes the current timestamp to `tail_page->page->time_stamp`. At this point, the event is only reserved and **not yet committed**.
* **The Reader's Action**: In `ring_buffer_iter_empty()`, the reader checks if the iterator is empty. It reads `commit_page->page->time_stamp` twice (into `commit_ts` and `curr_commit_ts`) with memory barriers (`smp_rmb()`) in between to detect if the writer has moved to a new page or updated the timestamp.
* **Race Outcomes**:
1. **If the reads return different values** (`commit_ts != curr_commit_ts`):
`ring_buffer_iter_empty()` returns `0` (not empty). The reader will then attempt to read an event. Since the writer has not yet committed the reserved event, the reader's read/peek attempt will find no committed events and return `NULL`/empty. This is a safe "false positive" (as noted in the code comment: `/* Still racy, as it may return a false positive, but that's OK */`).
2. **If the reads return the same value** (`commit_ts == curr_commit_ts`):
The function may return `1` (empty). Since the event is not committed yet, the buffer is indeed empty from the reader's perspective, which is correct and safe.
---
### 2. Compiler Safety and LKMM Analysis (Low-Level): **Harmful**
From a low-level compiler safety and Linux Kernel Memory Model (LKMM) perspective, the race is **harmful** because the lack of proper annotations can lead to compiler-induced bugs and undefined behavior:
* **Load/Store Tearing**: The `time_stamp` field is a 64-bit (`u64`) value. On 32-bit architectures, 64-bit loads and writes are not atomic and are split into two 32-bit instructions. Without `READ_ONCE` and `WRITE_ONCE`, the compiler can perform store tearing on the write and load tearing on the read, resulting in a mixed/torn garbage value. Even on 64-bit architectures, the compiler is technically permitted to tear plain 64-bit accesses.
* **Compiler Optimizations**: Plain reads allow the compiler to assume that the memory location is not modified concurrently. Although the `smp_rmb()` calls (which act as compiler barriers) prevent the compiler from reordering or merging the two reads across the barriers, the lack of `READ_ONCE`/`WRITE_ONCE` still violates the LKMM and is flagged by KCSAN.
* **Inconsistency**: In `ring_buffer_iter_empty()`, `curr_commit_ts` is read using `READ_ONCE()`, but `commit_ts` is read using a plain load. This is inconsistent and leaves `commit_ts` vulnerable to compiler optimizations and tearing.
---
### Conclusion & Recommended Fix
This data race **does not indicate a logic bug**, but it **requires `READ_ONCE` and `WRITE_ONCE` annotations** to ensure compiler safety, prevent load/store tearing, and satisfy KCSAN.
#### Recommended Fix:
1. In `__rb_reserve_next()` (line ~4851), use `WRITE_ONCE` to write the timestamp:
```c
if (unlikely(!tail))
WRITE_ONCE(tail_page->page->time_stamp, info->ts);
```
2. In `ring_buffer_iter_empty()` (line ~5647), use `READ_ONCE` to read the first timestamp:
```c
commit_page = READ_ONCE(cpu_buffer->commit_page);
commit_ts = READ_ONCE(commit_page->page->time_stamp);
```]
Results: map[Answer:### Concurrency and Compiler Safety Analysis of the KCSAN Data Race in `kernel/trace/ring_buffer.c`
The KCSAN data race report identifies a concurrent write to `tail_page->page->time_stamp` in `__rb_reserve_next()` and a concurrent read of `commit_page->page->time_stamp` in `ring_buffer_iter_empty()`.
---
### 1. Concurrency Logic Analysis (High-Level): **Benign**
From a high-level concurrency logic perspective, this data race is **benign** because the ring buffer's design naturally tolerates any outcome of the race without causing functional incorrectness, crashes, or data corruption.
* **The Writer's Action**: In `__rb_reserve_next()`, when a writer reserves the first event on a page (`tail == 0`), it writes the current timestamp to `tail_page->page->time_stamp`. At this point, the event is only reserved and **not yet committed**.
* **The Reader's Action**: In `ring_buffer_iter_empty()`, the reader checks if the iterator is empty. It reads `commit_page->page->time_stamp` twice (into `commit_ts` and `curr_commit_ts`) with memory barriers (`smp_rmb()`) in between to detect if the writer has moved to a new page or updated the timestamp.
* **Race Outcomes**:
1. **If the reads return different values** (`commit_ts != curr_commit_ts`):
`ring_buffer_iter_empty()` returns `0` (not empty). The reader will then attempt to read an event. Since the writer has not yet committed the reserved event, the reader's read/peek attempt will find no committed events and return `NULL`/empty. This is a safe "false positive" (as noted in the code comment: `/* Still racy, as it may return a false positive, but that's OK */`).
2. **If the reads return the same value** (`commit_ts == curr_commit_ts`):
The function may return `1` (empty). Since the event is not committed yet, the buffer is indeed empty from the reader's perspective, which is correct and safe.
---
### 2. Compiler Safety and LKMM Analysis (Low-Level): **Harmful**
From a low-level compiler safety and Linux Kernel Memory Model (LKMM) perspective, the race is **harmful** because the lack of proper annotations can lead to compiler-induced bugs and undefined behavior:
* **Load/Store Tearing**: The `time_stamp` field is a 64-bit (`u64`) value. On 32-bit architectures, 64-bit loads and writes are not atomic and are split into two 32-bit instructions. Without `READ_ONCE` and `WRITE_ONCE`, the compiler can perform store tearing on the write and load tearing on the read, resulting in a mixed/torn garbage value. Even on 64-bit architectures, the compiler is technically permitted to tear plain 64-bit accesses.
* **Compiler Optimizations**: Plain reads allow the compiler to assume that the memory location is not modified concurrently. Although the `smp_rmb()` calls (which act as compiler barriers) prevent the compiler from reordering or merging the two reads across the barriers, the lack of `READ_ONCE`/`WRITE_ONCE` still violates the LKMM and is flagged by KCSAN.
* **Inconsistency**: In `ring_buffer_iter_empty()`, `curr_commit_ts` is read using `READ_ONCE()`, but `commit_ts` is read using a plain load. This is inconsistent and leaves `commit_ts` vulnerable to compiler optimizations and tearing.
---
### Conclusion & Recommended Fix
This data race **does not indicate a logic bug**, but it **requires `READ_ONCE` and `WRITE_ONCE` annotations** to ensure compiler safety, prevent load/store tearing, and satisfy KCSAN.
#### Recommended Fix:
1. In `__rb_reserve_next()` (line ~4851), use `WRITE_ONCE` to write the timestamp:
```c
if (unlikely(!tail))
WRITE_ONCE(tail_page->page->time_stamp, info->ts);
```
2. In `ring_buffer_iter_empty()` (line ~5647), use `READ_ONCE` to read the first timestamp:
```c
commit_page = READ_ONCE(cpu_buffer->commit_page);
commit_ts = READ_ONCE(commit_page->page->time_stamp);
```]
|
| 65/2 |
2026/08/09 05:28 |
llm |
expert |
0m
Model:
Tokens:
input: 50235
output: 38
thoughts: 591
Thoughts: **Analyzing the Atomicity of Timestamp Accesses**
Okay, this analysis is sharp, really well done! Before I sign off on this, I need to make *absolutely* sure we haven't missed any edge cases related to this race condition. Let's re-examine the classification rules we're using... Ah, here they are. Okay, the important part here is: "HARMFUL (Logic Bug or Marking Required)". I see, "Marking Required for Correctness." The algorithm is sound *but* requires annotations. And there it is: "...`READ_ONCE()` does **not** guarantee atomicity for 64-bit variables on 32-bit architectures."
Wait, hold on a second! If `READ_ONCE()` *doesn't* guarantee atomicity for 64-bit variables on 32-bit architectures, then how are 64-bit variables safely read and written on these systems? Typically, in a ring buffer implementation, `time_stamp` is a 64-bit value. So, is there some *other* mechanism being used in the ring buffer to handle atomic 64-bit reads and writes on 32-bit architectures? I need to dig into this.
Let me see... perhaps there's a helper function or a dedicated struct within the ring buffer code for atomically handling 64-bit values. Okay, let's start by looking for `rb_time_t`, that sounds like a good candidate. I'll search for `rb_time_t`, then `rb_time_read`, and `rb_time_set` in `kernel/trace/ring_buffer.c`.
Ah-ha! Bingo. In `__rb_reserve_next`, I can see:
```c
rb_time_read(&cpu_buffer->before_stamp, &info->before);
rb_time_read(&cpu_buffer->write_stamp, &info->after);
```
and also:
```c
rb_time_set(&cpu_buffer->before_stamp, info->ts);
```
So, there it is! `rb_time_t` *is* a special structure designed precisely to hold 64-bit timestamps atomically, even on 32-bit architectures. I need to examine its definition to understand the implementation. Now I have a clear path to classify and mitigate this risk!
|
| 66/2 |
2026/08/09 05:28 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:rb_time_t]
Results: map[SourceCode: 495: typedef struct rb_time_struct rb_time_t;
SourceFile:kernel/trace/ring_buffer.c]
|
| 67/2 |
2026/08/09 05:28 |
llm |
expert |
0m
Model:
Tokens:
input: 51432
output: 38
thoughts: 14
Thoughts: Here's my thought process, as if I were considering this myself:
**Investigating `struct rb_time_struct`**
Okay, so I need to get my hands on the definition of `struct rb_time_struct`. It's likely a critical piece of the puzzle I'm currently working on, and I need to understand its structure. Where would I find this? My first instinct is to check the relevant kernel headers, obviously. I'll need to remember the context of where I encountered this structure - was it in networking code, scheduler code, or maybe filesystem-related? That'll give me a good starting point for my search. I'll also try a quick `grep` through the relevant source tree directories if I have a hunch where it might be. Once I find it, I'll need to carefully examine each member to understand its purpose and how it's used within the surrounding code. I need to pay particular attention to any associated functions or macros that operate on this struct. Understanding its relationships to other structures and its role in any algorithms will be the next step. Then, I can integrate that understanding into the larger issue I'm solving.
|
| 68/2 |
2026/08/09 05:28 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:rb_time_struct]
Results: map[SourceCode: 492: struct rb_time_struct {
493: local64_t time;
494: };
SourceFile:kernel/trace/ring_buffer.c]
|
| 69/2 |
2026/08/09 05:28 |
llm |
expert |
0m
Model:
Tokens:
input: 51798
output: 76
thoughts: 45
Thoughts: Okay, here's my interpretation of those thoughts, tailored for an expert audience:
**Investigating Ruby Time Structs and Their Internal Representation**
Right, so I'm digging into the Ruby Time structure, specifically looking at the `rb_time_struct`. My immediate thought is, "Aha! I see it contains a `local64_t` field." This immediately piques my interest. That's how they're storing the internal time value, then. Likely, this is a 64-bit integer, probably representing the time since the epoch in some unit β probably nanoseconds if it's following common practices.
Now, before jumping to conclusions, I need to understand how the value in that field is being used. Let's delve into the `rb_time_read` and `rb_time_set` definitions. I need to see how the `local64_t` is being read from and written to, so I can understand any precision issues, any conversions being done, how time zones are handled at this level, and how the value is used internally. This might reveal insights into potential performance bottlenecks or subtle behavior related to large time values or specific platforms. Understanding the getter and setter methods will allow me to piece together how `rb_time_struct` is actually being used in Ruby.
|
| 70/2 |
2026/08/09 05:28 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:rb_time_read]
Results: map[SourceCode: 640: static inline void rb_time_read(rb_time_t *t, u64 *ret)
641: {
642: *ret = local64_read(&t->time);
643: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 71/2 |
2026/08/09 05:28 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/trace/ring_buffer.c Name:rb_time_set]
Results: map[SourceCode: 644: static void rb_time_set(rb_time_t *t, u64 val)
645: {
646: local64_set(&t->time, val);
647: }
SourceFile:kernel/trace/ring_buffer.c]
|
| 72/2 |
2026/08/09 05:28 |
llm |
expert |
0m
Model:
Tokens:
input: 52388
output: 15
thoughts: 649
Thoughts: **Analyzing the Ring Buffer Time Stamp Read/Write**
Okay, so I'm looking at this code and immediately I see that `rb_time_read` and `rb_time_set` are just thin layers around `local64_read` and `local64_set`. My first thought is: "Is `local64_t` atomic on 32-bit architectures?" And I quickly recall that it's atomic only with respect to *local* interrupts, but not necessarily SMP-safe. Then I realize the `tail_page->page->time_stamp` is *not* an `rb_time_t`, it's a plain `u64`!
I double-check the `struct buffer_data_page` definition to be sure. Yep, `time_stamp` is indeed a plain `u64`. Then the question hits me: "Why isn't it an `rb_time_t` or `local64_t`?" And the answer is obvious: `buffer_data_page` is the raw data page, so it *has* to be a plain `u64`.
The next concern, which jumps right into my head because of the 32-bit architecture aspect, is "Is a plain `u64` read/write atomic?" The answer is no, it's not. But then I have to think through the implications of a torn read. Okay, even if a torn read happens, it only impacts the comparison `curr_commit_ts != commit_ts`. If the read is torn, the comparison will evaluate to true. In that case, `ring_buffer_iter_empty` will return 0 (not empty). That's totally safe as a fallback; it just means the reader won't find anything.
But then, I need to consider compiler optimizations. What if the compiler decides to optimize the plain read/write in some unexpected way? That's when I realize the need for `READ_ONCE` and `WRITE_ONCE`. This isn't just a potential race; it's a classic case where annotations are *required* for correctness.
Okay, let's re-read the classification rules to make sure. I'm looking at the rules, and it screams "**HARMFUL (Logic Bug or Marking Required)**". The critical point is: "**Marking Required for Correctness:** The algorithm is logically sound but requires annotations (`READ_ONCE()`, `WRITE_ONCE()`, `smp_load_acquire()`, `smp_store_release()`, etc.) to be safe." Bingo! That's it, this is exactly "Marking Required for Correctness."
So, the classification is **HARMFUL**. Which means it's not a benign issue.
Now, let's call `set-results` and ensure the `Benign` flag is set to `false`. That's the next step.
|
| 73/2 |
2026/08/09 05:28 |
tool |
set-results |
0m
Args:
Results:
|
| 74/2 |
2026/08/09 05:28 |
llm |
expert |
0m
Model:
Tokens:
input: 53695
output: 1029
thoughts: 958
Thoughts: **Analysis of Timestamp Access in Ring Buffer Iteration**
Okay, here's the breakdown. I need to document this vulnerability in a clear, concise, and professional manner, so let's get organized.
First, let's start with a clear definition: We're looking at a race condition between `__rb_reserve_next` (the writer) and `ring_buffer_iter_empty` (the reader) on the `time_stamp` field, specifically residing in either `tail_page->page->time_stamp` or `commit_page->page->time_stamp`.
The synchronization context is crucial: both the writer and reader operate locklessly. The writer updates the timestamp when reserving the first event on a page, and the reader checks for an empty iterator using a timestamp comparison, incorporating a memory barrier. The concurrent, lockless access to the same `time_stamp` field on the shared page structure is where we'll find our issue.
Based on this, my assessment is: **HARMFUL**, specifically, a case of **Marking Required for Correctness**.
The root cause is straightforward: on 32-bit architectures, plain 64-bit reads and writes are not atomic. They can be "torn," meaning split into two 32-bit operations. This creates a data race. The problem is compounded by a lack of proper memory barrier annotations. `ring_buffer_iter_empty()` uses `READ_ONCE()` for the second timestamp read (`curr_commit_ts`), but the first read (`commit_ts`) is a plain load. Similarly, the write in `__rb_reserve_next()` is a plain store, which violates the Linux Kernel Memory Model (LKMM).
While the algorithm includes a safe fallback mechanism β returning `0` (not empty) if the timestamps don't match β that won't necessarily prevent subtle bugs.
Let's illustrate the failure sequence: A two-column execution trace will be helpful. We'll show how a torn read can occur on a 32-bit architecture and how the algorithm behaves, as a result.
```text
CPU0 (Writer) CPU1 (Reader)
__rb_reserve_next() ring_buffer_iter_empty()
=================== ========================
// Read commit_page
commit_page = READ_ONCE(cpu_buffer->commit_page);
// Plain read of 64-bit time_stamp (split on 32-bit)
// Reads low 32-bits of old timestamp
commit_ts_low = commit_page->page->time_stamp_low;
// Write new 64-bit timestamp (split on 32-bit)
tail_page->page->time_stamp_low = ts_low;
tail_page->page->time_stamp_high = ts_high;
// Reads high 32-bits of new timestamp (TORN READ!)
commit_ts_high = commit_page->page->time_stamp_high;
smp_rmb();
// Read curr_commit_ts (using READ_ONCE)
curr_commit_ts = READ_ONCE(curr_commit_page->page->time_stamp);
// Comparison fails due to torn read
if (curr_commit_ts != commit_ts)
return 0; // Safe fallback, but incorrect state representation
```
The fix is simple:
* In `ring_buffer_iter_empty()`, wrap the first read of the timestamp with `READ_ONCE()`: `commit_ts = READ_ONCE(commit_page->page->time_stamp);`
* In `__rb_reserve_next()`, wrap the write of the timestamp with `WRITE_ONCE()`: `WRITE_ONCE(tail_page->page->time_stamp, info->ts);`
This is well-defined, and fully satisfies the requirements. Excellent.
|
| 75/1 |
2026/08/09 05:29 |
action |
format-explanation |
0m
Results: map[Explanation:### Race Summary
- **Functions:** `__rb_reserve_next` vs `ring_buffer_iter_empty` (inlined in
`ring_buffer_iter_peek`)
- **Variable/Field:** `tail_page->page->time_stamp` (written in
`__rb_reserve_next`) vs `commit_page->page->time_stamp` (read in
`ring_buffer_iter_empty`)
---
### Synchronization Context
- **Writer (`__rb_reserve_next`):** Operates locklessly to reserve space in the
ring buffer. When reserving the first event on a page (`tail == 0`), it updates
the page's timestamp.
- **Reader (`ring_buffer_iter_empty`):** Operates locklessly to check if the
iterator has more data to read. It reads the commit page's timestamp twice
(before and after a memory barrier) to detect concurrent writer activity.
- **Lack of Mutual Exclusion:** Because both threads operate locklessly on the
same page structure, they can concurrently access the `time_stamp` field without
holding any shared locks.
---
### Final Classification
**HARMFUL** (specifically, **Marking Required for Correctness**)
---
### Failure Reasoning
1. **Torn Accesses:** The `time_stamp` field is a plain 64-bit (`u64`) variable.
On 32-bit architectures, plain 64-bit reads and writes are not atomic and are
split into two 32-bit instructions. Without proper annotations, the compiler can
perform store tearing on the write and load tearing on the read, resulting in a
mixed/torn garbage value.
2. **Inconsistent Annotations:** In `ring_buffer_iter_empty()`, the second read
of the timestamp (`curr_commit_ts`) is correctly marked with `READ_ONCE()`, but
the first read (`commit_ts`) is a plain load. Similarly, the write in
`__rb_reserve_next()` is a plain store. This allows the compiler to assume the
value is stable and optimize the plain accesses in unexpected ways, violating
the Linux Kernel Memory Model (LKMM).
3. **Data Flow Safety:** While the algorithm is logically designed to handle
races (by returning `0` / "not empty" if the timestamps do not match, which is a
safe fallback), the lack of annotations can lead to compiler-induced bugs or
undefined behavior.
---
### Failure Sequencing (Torn Read on 32-bit Architecture)
```text
CPU0 (Writer) CPU1 (Reader)
__rb_reserve_next() ring_buffer_iter_empty()
=================== ========================
// Read commit_page
commit_page =
READ_ONCE(cpu_buffer->commit_page);
// Plain read of 64-bit time_stamp
(split on 32-bit)
// Reads low 32-bits of old
timestamp
commit_ts_low =
commit_page->page->time_stamp_low;
// Write new 64-bit timestamp (split on 32-bit)
tail_page->page->time_stamp_low = ts_low;
tail_page->page->time_stamp_high = ts_high;
// Reads high 32-bits of new
timestamp (TORN READ!)
commit_ts_high =
commit_page->page->time_stamp_high;
smp_rmb();
// Read curr_commit_ts (using
READ_ONCE)
curr_commit_ts =
READ_ONCE(curr_commit_page->page->time_stamp);
// Comparison fails due to torn
read
if (curr_commit_ts != commit_ts)
return 0; // Safe fallback,
but incorrect state
representation
```
---
### Recommended Fix
To ensure compiler safety, prevent load/store tearing, and satisfy KCSAN, the
accesses must be marked:
1. In `__rb_reserve_next()` (line ~4851), use `WRITE_ONCE` to write the
timestamp:
```c
if (unlikely(!tail))
WRITE_ONCE(tail_page->page->time_stamp, info->ts);
```
2. In `ring_buffer_iter_empty()` (line ~5647), use `READ_ONCE` to read the first
timestamp:
```c
commit_page = READ_ONCE(cpu_buffer->commit_page);
commit_ts = READ_ONCE(commit_page->page->time_stamp);
```]
|