INFO: task hung in synchronize_rcu (3)

Status: premoderation: reported on 2026/07/10 14:29
Reported-by: syzbot+032923af5bc587515ae5@syzkaller.appspotmail.com
First crash: 27d, last: 1h28m
Similar bugs (16)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
upstream INFO: task hung in synchronize_rcu (3) 1 syz, C done done 374 1268d 2206d 22/29 fixed on 2023/02/24 13:50
linux-6.1 INFO: task hung in synchronize_rcu origin:lts-only 1 syz, C unreliable 57 2d20h 1223d 0/3 upstream: reported C repro on 2023/04/01 04:49
android-49 INFO: task hung in synchronize_rcu 1 1 2442d 2442d 0/3 auto-closed as invalid on 2020/03/28 16:57
linux-4.14 INFO: task hung in synchronize_rcu vfs 1 syz, C error 162 1283d 2464d 0/1 upstream: reported C repro on 2019/11/07 04:32
linux-4.19 INFO: task hung in synchronize_rcu 1 syz, C error 70 1272d 2463d 0/1 upstream: reported C repro on 2019/11/08 21:27
linux-5.15 INFO: task hung in synchronize_rcu 1 1 1208d 1208d 0/3 auto-obsoleted due to no activity on 2023/08/14 02:05
android-6-12 INFO: task hung in synchronize_rcu 1 41 267d 391d 0/1 auto-obsoleted due to no activity on 2026/01/01 15:51
linux-5.15 INFO: task hung in synchronize_rcu (3) origin:lts-only 1 syz, C error 56 13d 741d 0/3 upstream: reported C repro on 2024/07/26 03:55
upstream INFO: task hung in synchronize_rcu 1 syz done 1206 2442d 2468d 14/29 fixed on 2019/11/29 15:48
upstream INFO: task hung in synchronize_rcu (4) prio:high fs net 1 syz, C done error 4245 7h45m 1190d 0/29 upstream: reported C repro on 2023/05/04 02:01
linux-5.15 INFO: task hung in synchronize_rcu (2) 1 1 897d 897d 0/3 auto-obsoleted due to no activity on 2024/05/31 13:53
android-414 INFO: task hung in synchronize_rcu 1 1 2460d 2460d 0/1 auto-closed as invalid on 2020/03/10 10:42
upstream INFO: task hung in synchronize_rcu (2) trace 1 8 2441d 2442d 0/29 closed as invalid on 2019/11/30 16:54
linux-6.6 INFO: task hung in synchronize_rcu 1 11 1h25m 280d 0/2 upstream: reported on 2025/10/30 17:16
android-6-12 INFO: task hung in synchronize_rcu (2) 1 5 132d 173d 0/1 auto-obsoleted due to no activity on 2026/06/25 10:28
upstream INFO: task hung in rtnetlink_rcv_msg net 1 syz, C inconclusive inconclusive 1970 758d 2722d 26/29 fixed on 2024/07/09 19:14

Sample crash report:
INFO: task syz-executor:1905 blocked for more than 123 seconds.
      Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz-executor    state:D stack:0     pid:1905  tgid:1905  ppid:1      flags:0x00004004
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:6060 [inline]
 __schedule+0x1496/0x20b0 kernel/sched/core.c:7880
 __schedule_loop kernel/sched/core.c:7957 [inline]
 schedule+0xc5/0x240 kernel/sched/core.c:7972
 schedule_timeout+0xc7/0x3b0 kernel/time/timer.c:2596
 do_wait_for_common kernel/sched/completion.c:95 [inline]
 __wait_for_common kernel/sched/completion.c:116 [inline]
 wait_for_common+0x368/0x650 kernel/sched/completion.c:127
 wait_for_completion_state+0x19/0x40 kernel/sched/completion.c:264
 __wait_rcu_gp+0x1b5/0x290 kernel/rcu/update.c:516
 synchronize_rcu_normal+0x14c/0x1d0 kernel/rcu/tree.c:4043
 synchronize_rcu_expedited+0xda/0x960 kernel/rcu/tree_exp.h:972
 bdi_remove_from_list mm/backing-dev.c:1151 [inline]
 bdi_unregister+0x174/0x8c0 mm/backing-dev.c:1159
 generic_shutdown_super+0x269/0x410 fs/super.c:688
 kill_anon_super+0x4b/0x1d0 fs/super.c:1243
 fuse_kill_sb_anon+0x228/0x290 fs/fuse/inode.c:2144
 deactivate_locked_super+0xd5/0x270 fs/super.c:478
 deactivate_super+0xb8/0xe0 fs/super.c:511
 cleanup_mnt+0x438/0x4d0 fs/namespace.c:1380
 __cleanup_mnt+0x1d/0x40 fs/namespace.c:1387
 task_work_run+0x1e8/0x260 kernel/task_work.c:246
 resume_user_mode_work+0x35/0x50 include/linux/resume_user_mode.h:50
 exit_to_user_mode_loop kernel/entry/common.c:114 [inline]
 exit_to_user_mode_prepare include/linux/entry-common.h:328 [inline]
 __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
 syscall_exit_to_user_mode+0x63/0xb0 kernel/entry/common.c:218
 do_syscall_64+0x63/0xf0 arch/x86/entry/common.c:84
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7f76a679f317
RSP: 002b:00007ffc52170ab8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
RAX: 0000000000000000 RBX: 00007f76a6834474 RCX: 00007f76a679f317
RDX: 0000000000000000 RSI: 0000000000000009 RDI: 00007ffc52170b70
RBP: 00007ffc52170b70 R08: 00007ffc52171b70 R09: 00000000ffffffff
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffc52171c00
R13: 00007f76a6834474 R14: 000000000001cc17 R15: 00007ffc52171c40
 </TASK>
INFO: task syz.0.548:2028 blocked for more than 125 seconds.
      Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.0.548       state:D stack:0     pid:2028  tgid:2025  ppid:1181   flags:0x00004006
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:6060 [inline]
 __schedule+0x1496/0x20b0 kernel/sched/core.c:7880
 __schedule_loop kernel/sched/core.c:7957 [inline]
 schedule+0xc5/0x240 kernel/sched/core.c:7972
 schedule_timeout+0xc7/0x3b0 kernel/time/timer.c:2596
 do_wait_for_common kernel/sched/completion.c:95 [inline]
 __wait_for_common kernel/sched/completion.c:116 [inline]
 wait_for_common+0x368/0x650 kernel/sched/completion.c:127
 wait_for_completion_state+0x19/0x40 kernel/sched/completion.c:264
 __wait_rcu_gp+0x1b5/0x290 kernel/rcu/update.c:516
 synchronize_rcu_normal+0x14c/0x1d0 kernel/rcu/tree.c:4043
 synchronize_rcu_expedited+0xda/0x960 kernel/rcu/tree_exp.h:972
 namespace_unlock+0x248/0x450 fs/namespace.c:1720
 drop_collected_mounts fs/namespace.c:2248 [inline]
 put_mnt_ns+0xad/0xf0 fs/namespace.c:5655
 free_nsproxy+0x52/0x290 kernel/nsproxy.c:193
 put_nsproxy include/linux/nsproxy.h:118 [inline]
 switch_task_namespaces kernel/nsproxy.c:251 [inline]
 exit_task_namespaces+0x99/0xe0 kernel/nsproxy.c:256
 do_exit+0xa41/0x2850 kernel/exit.c:954
 do_group_exit+0x21f/0x2f0 kernel/exit.c:1098
 get_signal+0x134c/0x1490 kernel/signal.c:2933
 arch_do_signal_or_restart+0xbc/0x700 arch/x86/kernel/signal.c:337
 exit_to_user_mode_loop kernel/entry/common.c:111 [inline]
 exit_to_user_mode_prepare include/linux/entry-common.h:328 [inline]
 __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
 syscall_exit_to_user_mode+0x57/0xb0 kernel/entry/common.c:218
 do_syscall_64+0x63/0xf0 arch/x86/entry/common.c:84
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7f5f31b9e0d9
RSP: 002b:00007f5f305f70e8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 00007f5f31e26188 RCX: 00007f5f31b9e0d9
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 00007f5f31e26188
RBP: 00007f5f31e26180 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f5f31e26218 R14: 00007ffd50b14fd0 R15: 00007ffd50b150b8
 </TASK>
NMI backtrace for cpu 0
CPU: 0 UID: 0 PID: 37 Comm: khungtaskd Not tainted syzkaller #0 67f5df147bfe8fe71cdb2f404f013e40b3f12e05
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
 <TASK>
 __dump_stack+0x21/0x30 lib/dump_stack.c:94
 dump_stack_lvl+0x140/0x1c0 lib/dump_stack.c:120
 dump_stack+0x19/0x20 lib/dump_stack.c:129
 nmi_cpu_backtrace+0x2ba/0x2d0 lib/nmi_backtrace.c:113
 nmi_trigger_cpumask_backtrace+0x142/0x2c0 lib/nmi_backtrace.c:62
 arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:41
 trigger_all_cpu_backtrace include/linux/nmi.h:158 [inline]
 check_hung_uninterruptible_tasks kernel/hung_task.c:267 [inline]
 watchdog+0xe45/0xf90 kernel/hung_task.c:423
 kthread+0x2c7/0x360 kernel/kthread.c:389
 ret_from_fork+0x67/0xa0 arch/x86/kernel/process.c:153
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
 </TASK>
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 33 Comm: rcuop/1 Not tainted syzkaller #0 67f5df147bfe8fe71cdb2f404f013e40b3f12e05
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:rhashtable_jhash2+0x0/0x50 lib/rhashtable.c:981
Code: 48 ba 00 00 00 00 00 fc ff df e9 ec fb ff ff 66 2e 0f 1f 84 00 00 00 00 00 90 b8 77 52 9e 4c 90 90 90 90 90 90 90 90 90 90 90 <f3> 0f 1e fa 55 48 89 e5 41 57 41 56 53 89 d3 41 89 f6 49 89 ff e8
RSP: 0018:ffffc900002304b8 EFLAGS: 00000257
RAX: ffff888103fd0e20 RBX: ffff888109b44008 RCX: ffff888103678000
RDX: 00000000ed8deb37 RSI: 0000000000000009 RDI: ffffc900002306a0
RBP: ffffc90000230628 R08: 0001000000000000 R09: 0000001800240000
R10: 0000000000000000 R11: ffffffff82ae9900 R12: ffff888175249bf8
R13: ffff888103fd0e2e R14: 00000000ed8deb37 R15: dffffc0000000000
FS:  0000000000000000(0000) GS:ffff8881f6f00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fff74105cbc CR3: 0000000165658000 CR4: 00000000003526b0
Call Trace:
 <IRQ>
 br_mdb_ip_get_rcu net/bridge/br_multicast.c:102 [inline]
 br_mdb_entry_skb_get+0x614/0x850 net/bridge/br_multicast.c:198
 br_handle_frame_finish+0x9f8/0x1620 net/bridge/br_input.c:183
 nf_hook_bridge_pre net/bridge/br_input.c:301 [inline]
 br_handle_frame+0x5a5/0xba0 net/bridge/br_input.c:424
 __netif_receive_skb_core+0xfc4/0x3a70 net/core/dev.c:5695
 __netif_receive_skb_one_core net/core/dev.c:5799 [inline]
 __netif_receive_skb net/core/dev.c:5914 [inline]
 process_backlog+0x40b/0xaf0 net/core/dev.c:6250
 __napi_poll+0xcd/0x600 net/core/dev.c:6901
 napi_poll net/core/dev.c:6970 [inline]
 net_rx_action+0x5dc/0xd70 net/core/dev.c:7092
 handle_softirqs+0x1aa/0x620 kernel/softirq.c:621
 __do_softirq+0xf/0x16 kernel/softirq.c:659
 do_softirq+0xb8/0x110 kernel/softirq.c:503
 </IRQ>
 <TASK>
 __local_bh_enable_ip+0x74/0x80 kernel/softirq.c:430
 local_bh_enable include/linux/bottom_half.h:33 [inline]
 rcu_do_batch+0x590/0xcb0 kernel/rcu/tree.c:2594
 nocb_cb_wait kernel/rcu/tree_nocb.h:923 [inline]
 rcu_nocb_cb_kthread+0x533/0xb60 kernel/rcu/tree_nocb.h:957
 kthread+0x2c7/0x360 kernel/kthread.c:389
 ret_from_fork+0x67/0xa0 arch/x86/kernel/process.c:153
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
 </TASK>
net_ratelimit: 94416 callbacks suppressed
bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
bridge0: received packet on veth0_to_bridge with own address as source address (addr:5a:04:31:68:18:00, vlan:0)
bridge0: received packet on veth0_to_bridge with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:1b, vlan:0)
bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:1b, vlan:0)
bridge0: received packet on veth0_to_bridge with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
bridge0: received packet on veth0_to_bridge with own address as source address (addr:5a:04:31:68:18:00, vlan:0)
bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
bridge0: received packet on veth0_to_bridge with own address as source address (addr:5a:04:31:68:18:00, vlan:0)

Crashes (3):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/08/06 22:39 android16-6.12 749a6f9d41aa c38879ea .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-6-12-rust INFO: task hung in synchronize_rcu
2026/07/22 16:51 android16-6.12 b739cd4ffbe5 4a865b71 .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-6-12-rust INFO: task hung in synchronize_rcu
2026/07/10 14:28 android16-6.12 0e0925f37543 99e64375 .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-6-12-rust INFO: task hung in synchronize_rcu
* Struck through repros no longer work on HEAD.