workqueue: make rescuer_thread() empty wq->maydays list before exiting (4d595b86) · Commits · 戴 / test

After a @pwq is scheduled for emergency execution, other workers may consume the affectd work items before the rescuer gets to them. This means that a workqueue many have pwqs queued on @wq->maydays list while not having any work item pending or in-flight. If destroy_workqueue() executes in such condition, the rescuer may exit without emptying @wq->maydays. This currently doesn't cause any actual harm. destroy_workqueue() can safely destroy all the involved data structures whether @wq->maydays is populated or not as nobody access the list once the rescuer exits. However, this is nasty and makes future development difficult. Let's update rescuer_thread() so that it empties @wq->maydays after seeing should_stop to guarantee that the list is empty on rescuer exit. tj: Updated comment and patch description. Signed-off-by:

Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by:

Tejun Heo <tj@kernel.org> Cc: stable@vger.kernel.org # v3.10+

kernel/workqueue.c

+16 −5

Original line number	Diff line number	Diff line
		@@ -2398,6 +2398,7 @@ static int rescuer_thread(void *__rescuer)
		struct worker *rescuer = __rescuer;
		struct workqueue_struct *wq = rescuer->rescue_wq;
		struct list_head *scheduled = &rescuer->scheduled;
		bool should_stop;

		set_user_nice(current, RESCUER_NICE_LEVEL);

		@@ -2409,11 +2410,15 @@ static int rescuer_thread(void *__rescuer)
		repeat:
		set_current_state(TASK_INTERRUPTIBLE);

		if (kthread_should_stop()) {
		__set_current_state(TASK_RUNNING);
		rescuer->task->flags &= ~PF_WQ_WORKER;
		return 0;
		}
		/*
		* By the time the rescuer is requested to stop, the workqueue
		* shouldn't have any work pending, but @wq->maydays may still have
		* pwq(s) queued. This can happen by non-rescuer workers consuming
		* all the work items before the rescuer got to them. Go through
		* @wq->maydays processing before acting on should_stop so that the
		* list is always empty on exit.
		*/
		should_stop = kthread_should_stop();

		/* see whether any pwq is asking for help */
		spin_lock_irq(&wq_mayday_lock);
		@@ -2459,6 +2464,12 @@ repeat:

		spin_unlock_irq(&wq_mayday_lock);

		if (should_stop) {
		__set_current_state(TASK_RUNNING);
		rescuer->task->flags &= ~PF_WQ_WORKER;
		return 0;
		}

		/* rescuers should never participate in concurrency management */
		WARN_ON_ONCE(!(rescuer->flags & WORKER_NOT_RUNNING));
		schedule();

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