[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+1bd201...@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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[Thomas Gleixner]

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

CC+: Workqueue folks