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Commit 4d595b86 authored by Lai Jiangshan's avatar Lai Jiangshan Committed by Tejun Heo
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workqueue: make rescuer_thread() empty wq->maydays list before exiting 



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: default avatarLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org # v3.10+
parent 77f300b1

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 @@ static int rescuer_thread(void *__rescuer) 


	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();