Commit a79ec89f authored Feb 16, 2015 by Kirill Tkhai Committed by Ingo Molnar Feb 18, 2015

sched/dl: Prevent enqueue of a sleeping task in dl_task_timer()



A deadline task may be throttled and dequeued at the same time.
This happens, when it becomes throttled in schedule(), which
is called to go to sleep:

current->state = TASK_INTERRUPTIBLE;
schedule()
    deactivate_task()
        dequeue_task_dl()
            update_curr_dl()
                start_dl_timer()
            __dequeue_task_dl()
    prev->on_rq = 0;

Later the timer fires, but the task is still dequeued:

dl_task_timer()
    enqueue_task_dl() /* queues on dl_rq; on_rq remains 0 */

Someone wakes it up:

try_to_wake_up()

    enqueue_dl_entity()
        BUG_ON(on_dl_rq())

Patch fixes this problem, it prevents queueing !on_rq tasks
on dl_rq.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
[ Wrote comment. ]
Cc: Juri Lelli <juri.lelli@arm.com>
Fixes: 1019a359 ("sched/deadline: Fix stale yield state")
Link: http://lkml.kernel.org/r/1374601424090314@web4j.yandex.ru


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 3960c8c0

kernel/sched/deadline.c

+20 −0

Original line number	Diff line number	Diff line
		@@ -535,6 +535,26 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)

		sched_clock_tick();
		update_rq_clock(rq);

		/*
		* If the throttle happened during sched-out; like:
		*
		* schedule()
		* deactivate_task()
		* dequeue_task_dl()
		* update_curr_dl()
		* start_dl_timer()
		* __dequeue_task_dl()
		* prev->on_rq = 0;
		*
		* We can be both throttled and !queued. Replenish the counter
		* but do not enqueue -- wait for our wakeup to do that.
		*/
		if (!task_on_rq_queued(p)) {
		replenish_dl_entity(dl_se, dl_se);
		goto unlock;
		}

		enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
		if (dl_task(rq->curr))
		check_preempt_curr_dl(rq, p, 0);