Commit aee38ea9 authored Oct 24, 2014 by Juri Lelli Committed by Ingo Molnar Oct 28, 2014

sched/deadline: Fix races between rt_mutex_setprio() and dl_task_timer()



dl_task_timer() is racy against several paths. Daniel noticed that
the replenishment timer may experience a race condition against an
enqueue_dl_entity() called from rt_mutex_setprio(). With his own
words:

 rt_mutex_setprio() resets p->dl.dl_throttled. So the pattern is:
 start_dl_timer() throttled = 1, rt_mutex_setprio() throlled = 0,
 sched_switch() -> enqueue_task(), dl_task_timer-> enqueue_task()
 throttled is 0

=> BUG_ON(on_dl_rq(dl_se)) fires as the scheduling entity is already
enqueued on the -deadline runqueue.

As we do for the other races, we just bail out in the replenishment
timer code.

Reported-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Tested-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: vincent@legout.info
Cc: Dario Faggioli <raistlin@linux.it>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Cc: Fabio Checconi <fchecconi@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1414142198-18552-5-git-send-email-juri.lelli@arm.com


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

parent 64be6f1f

kernel/sched/deadline.c

+13 −5

Original line number	Diff line number	Diff line
		@@ -518,12 +518,20 @@ again:
		}

		/*
		* We need to take care of a possible races here. In fact, the
		* task might have changed its scheduling policy to something
		* different from SCHED_DEADLINE or changed its reservation
		* parameters (through sched_setattr()).
		* We need to take care of several possible races here:
		*
		* - the task might have changed its scheduling policy
		* to something different than SCHED_DEADLINE
		* - the task might have changed its reservation parameters
		* (through sched_setattr())
		* - the task might have been boosted by someone else and
		* might be in the boosting/deboosting path
		*
		* In all this cases we bail out, as the task is already
		* in the runqueue or is going to be enqueued back anyway.
		*/
		if (!dl_task(p) \|\| dl_se->dl_new)
		if (!dl_task(p) \|\| dl_se->dl_new \|\|
		dl_se->dl_boosted \|\| !dl_se->dl_throttled)
		goto unlock;

		sched_clock_tick();

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