kthread: Unpark only parked kthread

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.

Reported-and-tested-by [email protected]

Suggested-by Thomas Gleixner <[email protected]>

Signed-off-by Frederic Weisbecker <[email protected]>

---
 kernel/kthread.c | 2 ++
 1 file changed, 2 insertions(+)

kernel/kthread.c +2 -0

index f7be976ff88a..5e2ba556aba8 100644
@@ -623,6 +623,8 @@ void kthread_unpark(struct task_struct *k)
 {
 	struct kthread *kthread = to_kthread(k);
 
+	if (!test_bit(KTHREAD_SHOULD_PARK, &kthread->flags))
+		return;
 	/*
 	 * Newly created kthread was parked when the CPU was offline.
 	 * The binding was lost and we need to set it again.

Signature · 1 line

2.46.0

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.

Quoted 28 lines · On Fri, 13 Sep 2024 23:46:34 +0200 Frederic Weisbecker <[email protected]>…

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>

Quoted 1 line

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?

Quoted 30 lines · Andrew Morton

Quoted 28 lines · On Fri, 13 Sep 2024 23:46:34 +0200 Frederic Weisbecker <[email protected]>…

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>

Quoted 3 lines

Quoted 1 line

Fix this with skipping unecessary unparking while stopping a kthread.

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

I guess it takes some module unload performing a destroy workqueue to trigger this. And it's an issue in current mainline.

Quoted 6 lines

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?

It's hard to tell. The core of the issue is there for a long while but the conditions for it to really happen in practice is probably since:

5c25b5ff89f0 (workqueue: Tag bound workers with KTHREAD_IS_PER_CPU)

So it might deserve a Fixes: actually.