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: 1019a359d3 ("sched/deadline: Fix stale yield state")
Link: http://lkml.kernel.org/r/1374601424090314@web4j.yandex.ru
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/deadline.c

@@ -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);