Re: [syzbot] [fs?] [usb?] [input?] INFO: rcu detected stall in __fsnotify_parent (3)

[Jan Kara]

Hello!

On Tue 19-05-26 13:54:37, syzbot wrote:
> syzbot found the following issue on:
> 
> HEAD commit:    6916d5703ddf Merge tag 'drm-fixes-2026-05-16' of https://g..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=13437cc8580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=59da38148f3a3d24
> dashboard link: https://syzkaller.appspot.com/bug?extid=27ce6cc06c1311c152f5
> compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=123e8596580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/637181391a5c/disk-6916d570.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/bfbefc8a3671/vmlinux-6916d570.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/a419831b32a6/bzImage-6916d570.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+27ce6cc06c1311c152f5@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> rcu: 	Tasks blocked on level-0 rcu_node (CPUs 0-1): P4963/1:b..l
> 
> rcu: 	(detected by 1, t=10506 jiffies, g=15565, q=1900 ncpus=2)
> task:acpid           state:R
>   running task     stack:22856 pid:4963  tgid:4963  ppid:1      task_flags:0x400040 flags:0x00080001

FWIW all the reproducer does is some games with USB, no signs of fsnotify.
Bisection has failed, the problem was hit only once more than a week ago.
I'm going to sign this off to some random fluctuation.

								Honza

> Call Trace:
>  <TASK>
>  context_switch kernel/sched/core.c:5388 [inline]
>  __schedule+0x1295/0x67a0 kernel/sched/core.c:7189
>  preempt_schedule_irq+0x50/0x90 kernel/sched/core.c:7513
>  irqentry_exit_to_kernel_mode_preempt include/linux/irq-entry-common.h:476 [inline]
>  irqentry_exit_to_kernel_mode include/linux/irq-entry-common.h:547 [inline]
>  irqentry_exit+0x205/0x7e0 kernel/entry/common.c:164
>  asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
> RIP: 0010:lock_acquire+0x5e/0x370 kernel/locking/lockdep.c:5872
> Code: 05 3b a2 26 12 83 f8 07 0f 87 d9 02 00 00 48 0f a3 05 46 41 f3 0e 0f 82 a4 02 00 00 8b 35 ce 74 f3 0e 85 f6 0f 85 bf 00 00 00 <48> 8b 44 24 30 65 48 2b 05 dd a1 26 12 0f 85 ed 02 00 00 48 83 c4
> RSP: 0018:ffffc9000342fb30 EFLAGS: 00000206
> 
> RAX: 0000000000000046 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: ffffffff8defcf06 RDI: ffffffff8c1c3a00
> RBP: ffffffff8e7e57a0 R08: 0000000086db7919 R09: 0000000000000007
> R10: 0000000000000200 R11: 0000000000000000 R12: 0000000000000002
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>  rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
>  rcu_read_lock include/linux/rcupdate.h:838 [inline]
>  dput.part.0+0x4c/0x570 fs/dcache.c:971
>  dput+0x1f/0x30 fs/dcache.c:968
>  __fsnotify_parent+0x694/0xca0 fs/notify/fsnotify.c:242
>  fsnotify_parent include/linux/fsnotify.h:96 [inline]
>  fsnotify_path include/linux/fsnotify.h:113 [inline]
>  fsnotify_file include/linux/fsnotify.h:127 [inline]
>  fsnotify_file include/linux/fsnotify.h:116 [inline]
>  fsnotify_access include/linux/fsnotify.h:425 [inline]
>  vfs_read+0x486/0xb30 fs/read_write.c:578
>  ksys_read+0x1f8/0x250 fs/read_write.c:717
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0x10b/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7feac7fb3407
> RSP: 002b:00007ffc6dd51e00 EFLAGS: 00000202
>  ORIG_RAX: 0000000000000000
> RAX: ffffffffffffffda RBX: 00007feac7f29780 RCX: 00007feac7fb3407
> RDX: 0000000000000018 RSI: 00007ffc6dd51e50 RDI: 000000000000000b
> RBP: 00007ffc6dd51e50 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000202 R12: 0000556e2f8031e4
> R13: 0000556e4a9f2380 R14: 0000000000000007 R15: 000000000000000b
>  </TASK>
> rcu: rcu_preempt kthread starved for 4298 jiffies! g15565 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
> rcu: 	Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
> rcu: RCU grace-period kthread stack dump:
> task:rcu_preempt     state:R
>   running task     stack:27992 pid:16    tgid:16    ppid:2      task_flags:0x208040 flags:0x00080000
> Call Trace:
>  <TASK>
>  context_switch kernel/sched/core.c:5388 [inline]
>  __schedule+0x1295/0x67a0 kernel/sched/core.c:7189
>  __schedule_loop kernel/sched/core.c:7268 [inline]
>  schedule+0xdd/0x390 kernel/sched/core.c:7283
>  schedule_timeout+0x127/0x280 kernel/time/sleep_timeout.c:99
>  rcu_gp_fqs_loop+0x1a9/0x900 kernel/rcu/tree.c:2095
>  rcu_gp_kthread+0x179/0x230 kernel/rcu/tree.c:2297
>  kthread+0x370/0x450 kernel/kthread.c:436
>  ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
> rcu: Stack dump where RCU GP kthread last ran:
> CPU: 1 UID: 0 PID: 5978 Comm: syz.1.18 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
> RIP: 0010:write_comp_data+0x40/0x90 kernel/kcov.c:246
> Code: 02 12 a9 00 01 ff 00 74 1b f6 c4 01 74 07 a9 00 00 ff 00 74 05 c3 cc cc cc cc 8b 87 d4 16 00 00 85 c0 74 f1 8b 87 b0 16 00 00 <83> f8 03 75 e6 48 8b 87 b8 16 00 00 8b bf b4 16 00 00 48 8b 30 48
> RSP: 0018:ffffc900024af648 EFLAGS: 00000246
> 
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff81fc2bcd
> RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff88805a5e0000
> RBP: ffff8880b8443320 R08: 0000000000000005 R09: 0000000000000000
> R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000003
> R13: ffffed1017088665 R14: 0000000000000001 R15: ffff8880b853c5c0
> FS:  0000000000000000(0000) GS:ffff88812446d000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f8c6e1b56e8 CR3: 0000000059a89000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  csd_lock_wait kernel/smp.c:342 [inline]
>  smp_call_function_many_cond+0x5ad/0x1700 kernel/smp.c:892
>  on_each_cpu_cond_mask+0x40/0x90 kernel/smp.c:1057
>  __flush_tlb_multi arch/x86/include/asm/paravirt.h:46 [inline]
>  flush_tlb_multi arch/x86/mm/tlb.c:1361 [inline]
>  flush_tlb_mm_range+0x45f/0x16f0 arch/x86/mm/tlb.c:1451
>  tlb_flush arch/x86/include/asm/tlb.h:23 [inline]
>  tlb_flush_mmu_tlbonly include/asm-generic/tlb.h:509 [inline]
>  tlb_flush_mmu_tlbonly include/asm-generic/tlb.h:499 [inline]
>  tlb_flush_mmu mm/mmu_gather.c:423 [inline]
>  tlb_finish_mmu+0x3fe/0x810 mm/mmu_gather.c:549
>  exit_mmap+0x454/0xa10 mm/mmap.c:1313
>  __mmput+0x12a/0x410 kernel/fork.c:1178
>  mmput+0x67/0x80 kernel/fork.c:1201
>  exit_mm kernel/exit.c:582 [inline]
>  do_exit+0x8b2/0x2af0 kernel/exit.c:964
>  do_group_exit+0xd5/0x2a0 kernel/exit.c:1119
>  get_signal+0x20ff/0x2210 kernel/signal.c:3037
>  arch_do_signal_or_restart+0x91/0x7e0 arch/x86/kernel/signal.c:337
>  __exit_to_user_mode_loop kernel/entry/common.c:64 [inline]
>  exit_to_user_mode_loop+0x8b/0x4f0 kernel/entry/common.c:98
>  __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
>  syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:238 [inline]
>  syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
>  do_syscall_64+0x706/0xf80 arch/x86/entry/syscall_64.c:100
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f7a9eb5d68e
> Code: Unable to access opcode bytes at 0x7f7a9eb5d664.
> RSP: 002b:00007fffdcda5708 EFLAGS: 00000246
>  ORIG_RAX: 00000000000000e6
> RAX: 0000000000000000 RBX: 000055558fc36500 RCX: 00007f7a9eb5d68e
> RDX: 00007fffdcda5760 RSI: 0000000000000000 RDI: 0000000000000000
> RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007fffdcda58b0
> R13: 00007f7a9ee15fac R14: 000000000002bbe6 R15: 00007f7a9ee15fa0
>  </TASK>
> 
> 
> ---
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> 
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> 
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> 
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-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR