Re: [PATCH] kthread: Unpark only parked kthread

[Andrew Morton]

On Fri, 13 Sep 2024 23:46:34 +0200 Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:

> Calling into kthread unparking unconditionally is mostly harmless when
> the kthread is already unparked. The wake up is then simply ignored
> because the target is not in TASK_PARKED state.
> 
> However if the kthread is per CPU, the wake up is preceded by a call
> to kthread_bind() which expects the task to be inactive and in
> TASK_PARKED state, which obviously isn't the case if it is unparked.
> 
> As a result, calling kthread_stop() on an unparked per-cpu kthread
> triggers such a warning:
> 
> 	WARNING: CPU: 0 PID: 11 at kernel/kthread.c:525 __kthread_bind_mask kernel/kthread.c:525
> 	 <TASK>
> 	 kthread_stop+0x17a/0x630 kernel/kthread.c:707
> 	 destroy_workqueue+0x136/0xc40 kernel/workqueue.c:5810
> 	 wg_destruct+0x1e2/0x2e0 drivers/net/wireguard/device.c:257
> 	 netdev_run_todo+0xe1a/0x1000 net/core/dev.c:10693
> 	 default_device_exit_batch+0xa14/0xa90 net/core/dev.c:11769
> 	 ops_exit_list net/core/net_namespace.c:178 [inline]
> 	 cleanup_net+0x89d/0xcc0 net/core/net_namespace.c:640
> 	 process_one_work kernel/workqueue.c:3231 [inline]
> 	 process_scheduled_works+0xa2c/0x1830 kernel/workqueue.c:3312
> 	 worker_thread+0x86d/0xd70 kernel/workqueue.c:3393
> 	 kthread+0x2f0/0x390 kernel/kthread.c:389
> 	 ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
> 	 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
> 	 </TASK>
> 
> Fix this with skipping unecessary unparking while stopping a kthread.

How does userspace trigger this?  Is it an issue in current mainline?

Should we backport the fix into -stable kernels (depends on the answers
to the above questions).

It looks like the issue is old, so a Fixes: probably isn't needed.  But
as the issue is old, why did it come to light now?