[syzbot] [kernel?] possible deadlock in bh_worker

[syzbot] [kernel?] possible deadlock in bh_worker

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    dcb68831eac7 Merge tag 'block-7.2-20260815' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11c2d949580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4c9eb8e593cf8c5c
dashboard link: https://syzkaller.appspot.com/bug?extid=1bd20115328f8254ed62
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/1174a1aaa938/disk-dcb68831.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c14ec1120394/vmlinux-dcb68831.xz
kernel image: https://storage.googleapis.com/syzbot-assets/5971a3555672/bzImage-dcb68831.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+1bd20115328f8254ed62@syzkaller.appspotmail.com

============================================
WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
ktimers/0/16 is trying to acquire lock:
ffff8880b873a990 (&pool->cb_lock){+...}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:45 [inline]
ffff8880b873a990 (&pool->cb_lock){+...}-{3:3}, at: worker_lock_callback kernel/workqueue.c:3144 [inline]
ffff8880b873a990 (&pool->cb_lock){+...}-{3:3}, at: bh_worker+0x7d/0x880 kernel/workqueue.c:3688

but task is already holding lock:
ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:45 [inline]
ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: worker_lock_callback kernel/workqueue.c:3144 [inline]
ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: bh_worker+0x7d/0x880 kernel/workqueue.c:3688

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&pool->cb_lock);
  lock(&pool->cb_lock);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

6 locks held by ktimers/0/16:
 #0: ffffffff8e059220 (local_bh){.+.+}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
 #1: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
 #2: ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:45 [inline]
 #2: ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: worker_lock_callback kernel/workqueue.c:3144 [inline]
 #2: ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: bh_worker+0x7d/0x880 kernel/workqueue.c:3688
 #3: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
 #3: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:840 [inline]
 #3: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: __rt_spin_lock kernel/locking/spinlock_rt.c:50 [inline]
 #3: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1e0/0x400 kernel/locking/spinlock_rt.c:57
 #4: ffff88813ff1dd38 ((wq_completion)events_bh){+...}-{0:0}, at: process_one_work kernel/workqueue.c:3297 [inline]
 #4: ffff88813ff1dd38 ((wq_completion)events_bh){+...}-{0:0}, at: process_scheduled_works+0xa20/0x14e0 kernel/workqueue.c:3405
 #5: ffffc90000157ba0 ((work_completion)(&dead_work.work)){+...}-{0:0}, at: process_one_work kernel/workqueue.c:3297 [inline]
 #5: ffffc90000157ba0 ((work_completion)(&dead_work.work)){+...}-{0:0}, at: process_scheduled_works+0xa20/0x14e0 kernel/workqueue.c:3405

stack backtrace:
CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_deadlock_bug+0x279/0x290 kernel/locking/lockdep.c:3041
 check_deadlock kernel/locking/lockdep.c:3093 [inline]
 validate_chain kernel/locking/lockdep.c:3895 [inline]
 __lock_acquire+0x24df/0x2ce0 kernel/locking/lockdep.c:5237
 lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
 rt_spin_lock+0x83/0x400 kernel/locking/spinlock_rt.c:56
 spin_lock include/linux/spinlock_rt.h:45 [inline]
 worker_lock_callback kernel/workqueue.c:3144 [inline]
 bh_worker+0x7d/0x880 kernel/workqueue.c:3688
 drain_dead_softirq_workfn+0x95/0x220 kernel/workqueue.c:3763
 process_one_work kernel/workqueue.c:3322 [inline]
 process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
 bh_worker+0x46a/0x880 kernel/workqueue.c:3708
 tasklet_action+0xc/0x70 kernel/softirq.c:965
 handle_softirqs+0x1d9/0x6c0 kernel/softirq.c:622
 __do_softirq kernel/softirq.c:656 [inline]
 run_ktimerd+0x69/0x100 kernel/softirq.c:1151
 smpboot_thread_fn+0x57c/0xa80 kernel/smpboot.c:160
 kthread+0x388/0x470 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
BUG: workqueue leaked atomic, lock or RCU: ktimers/0[16]
     preempt=0x00000000 lock=4->0 RCU=2->2 workfn=drain_dead_softirq_workfn
INFO: lockdep is turned off.
CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 process_one_work kernel/workqueue.c:3343 [inline]
 process_scheduled_works+0x1161/0x14e0 kernel/workqueue.c:3405
 bh_worker+0x46a/0x880 kernel/workqueue.c:3708
 tasklet_action+0xc/0x70 kernel/softirq.c:965
 handle_softirqs+0x1d9/0x6c0 kernel/softirq.c:622
 __do_softirq kernel/softirq.c:656 [inline]
 run_ktimerd+0x69/0x100 kernel/softirq.c:1151
 smpboot_thread_fn+0x57c/0xa80 kernel/smpboot.c:160
 kthread+0x388/0x470 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>


---
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#syz undup

Re: [syzbot] [kernel?] possible deadlock in bh_worker

[Thomas Gleixner]

On Tue, Aug 18 2026 at 05:48, syzbot wrote:

CC+: Workqueue folks

> syzbot found the following issue on:
>
> HEAD commit:    dcb68831eac7 Merge tag 'block-7.2-20260815' of git://git.k..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11c2d949580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=4c9eb8e593cf8c5c
> dashboard link: https://syzkaller.appspot.com/bug?extid=1bd20115328f8254ed62
> compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/1174a1aaa938/disk-dcb68831.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/c14ec1120394/vmlinux-dcb68831.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/5971a3555672/bzImage-dcb68831.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+1bd20115328f8254ed62@syzkaller.appspotmail.com
>
> ============================================
> WARNING: possible recursive locking detected
> syzkaller #0 Not tainted
> --------------------------------------------
> ktimers/0/16 is trying to acquire lock:
> ffff8880b873a990 (&pool->cb_lock){+...}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:45 [inline]
> ffff8880b873a990 (&pool->cb_lock){+...}-{3:3}, at: worker_lock_callback kernel/workqueue.c:3144 [inline]
> ffff8880b873a990 (&pool->cb_lock){+...}-{3:3}, at: bh_worker+0x7d/0x880 kernel/workqueue.c:3688
>
> but task is already holding lock:
> ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:45 [inline]
> ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: worker_lock_callback kernel/workqueue.c:3144 [inline]
> ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: bh_worker+0x7d/0x880 kernel/workqueue.c:3688
>
> other info that might help us debug this:
>  Possible unsafe locking scenario:
>
>        CPU0
>        ----
>   lock(&pool->cb_lock);
>   lock(&pool->cb_lock);
>
>  *** DEADLOCK ***
>
>  May be due to missing lock nesting notation
>
> 6 locks held by ktimers/0/16:
>  #0: ffffffff8e059220 (local_bh){.+.+}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
>  #1: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
>  #2: ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:45 [inline]
>  #2: ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: worker_lock_callback kernel/workqueue.c:3144 [inline]
>  #2: ffff8880b863a990 (&pool->cb_lock){+...}-{3:3}, at: bh_worker+0x7d/0x880 kernel/workqueue.c:3688
>  #3: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
>  #3: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:840 [inline]
>  #3: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: __rt_spin_lock kernel/locking/spinlock_rt.c:50 [inline]
>  #3: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1e0/0x400 kernel/locking/spinlock_rt.c:57
>  #4: ffff88813ff1dd38 ((wq_completion)events_bh){+...}-{0:0}, at: process_one_work kernel/workqueue.c:3297 [inline]
>  #4: ffff88813ff1dd38 ((wq_completion)events_bh){+...}-{0:0}, at: process_scheduled_works+0xa20/0x14e0 kernel/workqueue.c:3405
>  #5: ffffc90000157ba0 ((work_completion)(&dead_work.work)){+...}-{0:0}, at: process_one_work kernel/workqueue.c:3297 [inline]
>  #5: ffffc90000157ba0 ((work_completion)(&dead_work.work)){+...}-{0:0}, at: process_scheduled_works+0xa20/0x14e0 kernel/workqueue.c:3405
>
> stack backtrace:
> CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
>  print_deadlock_bug+0x279/0x290 kernel/locking/lockdep.c:3041
>  check_deadlock kernel/locking/lockdep.c:3093 [inline]
>  validate_chain kernel/locking/lockdep.c:3895 [inline]
>  __lock_acquire+0x24df/0x2ce0 kernel/locking/lockdep.c:5237
>  lock_acquire+0x106/0x350 kernel/locking/lockdep.c:5868
>  rt_spin_lock+0x83/0x400 kernel/locking/spinlock_rt.c:56
>  spin_lock include/linux/spinlock_rt.h:45 [inline]
>  worker_lock_callback kernel/workqueue.c:3144 [inline]
>  bh_worker+0x7d/0x880 kernel/workqueue.c:3688
>  drain_dead_softirq_workfn+0x95/0x220 kernel/workqueue.c:3763
>  process_one_work kernel/workqueue.c:3322 [inline]
>  process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405
>  bh_worker+0x46a/0x880 kernel/workqueue.c:3708
>  tasklet_action+0xc/0x70 kernel/softirq.c:965
>  handle_softirqs+0x1d9/0x6c0 kernel/softirq.c:622
>  __do_softirq kernel/softirq.c:656 [inline]
>  run_ktimerd+0x69/0x100 kernel/softirq.c:1151
>  smpboot_thread_fn+0x57c/0xa80 kernel/smpboot.c:160
>  kthread+0x388/0x470 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
> BUG: workqueue leaked atomic, lock or RCU: ktimers/0[16]
>      preempt=0x00000000 lock=4->0 RCU=2->2 workfn=drain_dead_softirq_workfn
> INFO: lockdep is turned off.
> CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
>  process_one_work kernel/workqueue.c:3343 [inline]
>  process_scheduled_works+0x1161/0x14e0 kernel/workqueue.c:3405
>  bh_worker+0x46a/0x880 kernel/workqueue.c:3708
>  tasklet_action+0xc/0x70 kernel/softirq.c:965
>  handle_softirqs+0x1d9/0x6c0 kernel/softirq.c:622
>  __do_softirq kernel/softirq.c:656 [inline]
>  run_ktimerd+0x69/0x100 kernel/softirq.c:1151
>  smpboot_thread_fn+0x57c/0xa80 kernel/smpboot.c:160
>  kthread+0x388/0x470 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@googlegroups.com.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
>
> If you want to overwrite report's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
>
> If the report is a duplicate of another one, reply with:
> #syz dup: exact-subject-of-another-report
>
> If you want to undo deduplication, reply with:
> #syz undup