syzbot

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.1.1127/13688 is trying to acquire lock:
ffff888061a45eb0 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:601

but task is already holding lock:
ffff888019c4f238 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_fop_readdir+0x275/0x8d0 fs/kernfs/dir.c:1898

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (&root->kernfs_rwsem){++++}-{4:4}:
       down_write+0x3a/0x50 kernel/locking/rwsem.c:1590
       kernfs_add_one+0x41/0x5c0 fs/kernfs/dir.c:796
       kernfs_create_dir_ns+0xde/0x130 fs/kernfs/dir.c:1098
       sysfs_create_dir_ns+0x12f/0x2a0 fs/sysfs/dir.c:59
       create_dir lib/kobject.c:73 [inline]
       kobject_add_internal+0x631/0xd10 lib/kobject.c:240
       kobject_add_varg lib/kobject.c:374 [inline]
       kobject_add+0x163/0x240 lib/kobject.c:426
       btrfs_sysfs_add_block_group_type+0x243/0x6c0 fs/btrfs/sysfs.c:1866
       btrfs_create_pending_block_groups+0xd77/0x1b40 fs/btrfs/block-group.c:2902
       __btrfs_end_transaction+0x140/0x640 fs/btrfs/transaction.c:1080
       btrfs_inc_block_group_ro+0x647/0x7c0 fs/btrfs/block-group.c:3181
       btrfs_relocate_block_group+0x276/0xa60 fs/btrfs/relocation.c:5367
       btrfs_relocate_chunk+0x115/0x830 fs/btrfs/volumes.c:3590
       __btrfs_balance+0x1dbf/0x2af0 fs/btrfs/volumes.c:4501
       btrfs_balance+0xafa/0x11f0 fs/btrfs/volumes.c:4888
       btrfs_ioctl_balance+0x3d6/0x610 fs/btrfs/ioctl.c:3452
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #2 (btrfs_trans_num_extwriters){++++}-{0:0}:
       join_transaction+0x1a4/0xc90 fs/btrfs/transaction.c:323
       start_transaction+0x6b9/0x1650 fs/btrfs/transaction.c:708
       btrfs_rebuild_free_space_tree+0xbb/0x6e0 fs/btrfs/free-space-tree.c:1335
       btrfs_start_pre_rw_mount+0x1529/0x1f20 fs/btrfs/disk-io.c:3071
       open_ctree+0x2e07/0x4220 fs/btrfs/disk-io.c:3668
       btrfs_fill_super fs/btrfs/super.c:981 [inline]
       btrfs_get_tree_super fs/btrfs/super.c:1944 [inline]
       btrfs_get_tree_subvol fs/btrfs/super.c:2087 [inline]
       btrfs_get_tree+0x1061/0x1930 fs/btrfs/super.c:2121
       vfs_get_tree+0x92/0x2a0 fs/super.c:1754
       fc_mount fs/namespace.c:1193 [inline]
       do_new_mount_fc fs/namespace.c:3763 [inline]
       do_new_mount+0x341/0xd30 fs/namespace.c:3839
       do_mount fs/namespace.c:4172 [inline]
       __do_sys_mount fs/namespace.c:4361 [inline]
       __se_sys_mount+0x31d/0x420 fs/namespace.c:4338
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (btrfs_trans_num_writers){++++}-{0:0}:
       __lock_release kernel/locking/lockdep.c:5574 [inline]
       lock_release+0x199/0x3d0 kernel/locking/lockdep.c:5889
       percpu_up_read include/linux/percpu-rwsem.h:112 [inline]
       __sb_end_write include/linux/fs/super.h:14 [inline]
       sb_end_intwrite+0x26/0x1c0 include/linux/fs/super.h:101
       __btrfs_end_transaction+0x248/0x640 fs/btrfs/transaction.c:1085
       btrfs_dirty_inode+0x14c/0x190 fs/btrfs/inode.c:6457
       touch_atime+0x2f8/0x6b0 fs/inode.c:2271
       file_accessed include/linux/fs.h:2263 [inline]
       btrfs_file_mmap_prepare+0x189/0x200 fs/btrfs/file.c:2051
       vfs_mmap_prepare include/linux/fs.h:2075 [inline]
       call_mmap_prepare mm/vma.c:2644 [inline]
       __mmap_region mm/vma.c:2743 [inline]
       mmap_region+0xddc/0x2230 mm/vma.c:2837
       do_mmap+0xc2f/0x10c0 mm/mmap.c:559
       vm_mmap_pgoff+0x2cc/0x4f0 mm/util.c:581
       ksys_mmap_pgoff+0x4e8/0x720 mm/mmap.c:605
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&mm->mmap_lock){++++}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3165 [inline]
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain kernel/locking/lockdep.c:3908 [inline]
       __lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
       lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
       down_read_killable+0xa4/0x220 kernel/locking/rwsem.c:1560
       mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:601
       get_mmap_lock_carefully mm/mmap_lock.c:450 [inline]
       lock_mm_and_find_vma+0x2d7/0x340 mm/mmap_lock.c:501
       do_user_addr_fault+0x330/0x1340 arch/x86/mm/fault.c:1357
       handle_page_fault arch/x86/mm/fault.c:1474 [inline]
       exc_page_fault+0x6a/0xc0 arch/x86/mm/fault.c:1527
       asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
       filldir+0x2b5/0x690 fs/readdir.c:295
       dir_emit include/linux/fs.h:3566 [inline]
       kernfs_fop_readdir+0x57e/0x8d0 fs/kernfs/dir.c:1915
       iterate_dir+0x3a5/0x580 fs/readdir.c:108
       __do_sys_getdents fs/readdir.c:327 [inline]
       __se_sys_getdents+0xf1/0x270 fs/readdir.c:312
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  &mm->mmap_lock --> btrfs_trans_num_extwriters --> &root->kernfs_rwsem

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  rlock(&root->kernfs_rwsem);
                               lock(btrfs_trans_num_extwriters);
                               lock(&root->kernfs_rwsem);
  rlock(&mm->mmap_lock);

 *** DEADLOCK ***

3 locks held by syz.1.1127/13688:
 #0: ffff8880540eaf28 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x252/0x320 fs/file.c:1261
 #1: ffff888064ac1778 (&type->i_mutex_dir_key#4){++++}-{4:4}, at: iterate_dir+0x29e/0x580 fs/readdir.c:101
 #2: ffff888019c4f238 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_fop_readdir+0x275/0x8d0 fs/kernfs/dir.c:1898

stack backtrace:
CPU: 1 UID: 0 PID: 13688 Comm: syz.1.1127 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_circular_bug+0x2e1/0x300 kernel/locking/lockdep.c:2043
 check_noncircular+0x12e/0x150 kernel/locking/lockdep.c:2175
 check_prev_add kernel/locking/lockdep.c:3165 [inline]
 check_prevs_add kernel/locking/lockdep.c:3284 [inline]
 validate_chain kernel/locking/lockdep.c:3908 [inline]
 __lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
 lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
 down_read_killable+0xa4/0x220 kernel/locking/rwsem.c:1560
 mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:601
 get_mmap_lock_carefully mm/mmap_lock.c:450 [inline]
 lock_mm_and_find_vma+0x2d7/0x340 mm/mmap_lock.c:501
 do_user_addr_fault+0x330/0x1340 arch/x86/mm/fault.c:1357
 handle_page_fault arch/x86/mm/fault.c:1474 [inline]
 exc_page_fault+0x6a/0xc0 arch/x86/mm/fault.c:1527
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
RIP: 0010:filldir+0x2b5/0x690 fs/readdir.c:296
Code: 87 73 02 00 00 0f 01 cb 0f ae e8 48 8b 44 24 50 49 89 44 24 08 48 8b 4c 24 08 48 8b 44 24 58 48 89 01 48 8b 04 24 8b 54 24 1c <66> 89 41 10 80 e2 0f 41 89 c7 42 88 54 39 ff 49 63 ee c6 44 29 12
RSP: 0018:ffffc9000e617c40 EFLAGS: 00050283
RAX: 0000000000000020 RBX: ffffc9000e617e38 RCX: 0000200000001ff0
RDX: 0000000000000004 RSI: 0000200000001fd8 RDI: 0000200000002010
RBP: 00007ffffffff000 R08: 0000000000000000 R09: 0000000000000000
R10: dffffc0000000000 R11: ffffed1005f2d7a1 R12: 0000200000001fd8
R13: ffffffff8b4bef40 R14: 0000000000000006 R15: 0000200000002010
 dir_emit include/linux/fs.h:3566 [inline]
 kernfs_fop_readdir+0x57e/0x8d0 fs/kernfs/dir.c:1915
 iterate_dir+0x3a5/0x580 fs/readdir.c:108
 __do_sys_getdents fs/readdir.c:327 [inline]
 __se_sys_getdents+0xf1/0x270 fs/readdir.c:312
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fa82429c799
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fa8224f6028 EFLAGS: 00000246 ORIG_RAX: 000000000000004e
RAX: ffffffffffffffda RBX: 00007fa824515fa0 RCX: 00007fa82429c799
RDX: 00000000000000b8 RSI: 0000200000001fc0 RDI: 0000000000000005
RBP: 00007fa824332c99 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fa824516038 R14: 00007fa824515fa0 R15: 00007ffd8cae3158
 </TASK>
----------------
Code disassembly (best guess):
   0:	87 73 02             	xchg   %esi,0x2(%rbx)
   3:	00 00                	add    %al,(%rax)
   5:	0f 01 cb             	stac
   8:	0f ae e8             	lfence
   b:	48 8b 44 24 50       	mov    0x50(%rsp),%rax
  10:	49 89 44 24 08       	mov    %rax,0x8(%r12)
  15:	48 8b 4c 24 08       	mov    0x8(%rsp),%rcx
  1a:	48 8b 44 24 58       	mov    0x58(%rsp),%rax
  1f:	48 89 01             	mov    %rax,(%rcx)
  22:	48 8b 04 24          	mov    (%rsp),%rax
  26:	8b 54 24 1c          	mov    0x1c(%rsp),%edx
* 2a:	66 89 41 10          	mov    %ax,0x10(%rcx) <-- trapping instruction
  2e:	80 e2 0f             	and    $0xf,%dl
  31:	41 89 c7             	mov    %eax,%r15d
  34:	42 88 54 39 ff       	mov    %dl,-0x1(%rcx,%r15,1)
  39:	49 63 ee             	movslq %r14d,%rbp
  3c:	c6                   	.byte 0xc6
  3d:	44 29 12             	sub    %r10d,(%rdx)