kthread_worker: prevent queuing delayed work from timer_fn when it is being canceled

There is a small race window when a delayed work is being canceled and
the work still might be queued from the timer_fn:

CPU0 CPU1
kthread_cancel_delayed_work_sync()
__kthread_cancel_work_sync()
__kthread_cancel_work()
work->canceling++;
kthread_delayed_work_timer_fn()
kthread_insert_work();

BUG: kthread_insert_work() should not get called when work->canceling is
set.

Signed-off-by: Zqiang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Reviewed-by: Petr Mladek <[e-mail]>
Acked-by: Tejun Heo <[e-mail]>
Cc: <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/kthread.c

···
 	/* Move the work from worker->delayed_work_list. */
 	WARN_ON_ONCE(list_empty(&work->node));
 	list_del_init(&work->node);
-	kthread_insert_work(worker, work, &worker->work_list);
+	if (!work->canceling)
+		kthread_insert_work(worker, work, &worker->work_list);
 
 	raw_spin_unlock_irqrestore(&worker->lock, flags);
 }