sched/dl: Fix race in dl_task_timer()

Throttled task is still on rq, and it may be moved to other cpu if user is playing with sched_setaffinity(). Therefore, unlocked task_rq() access makes the race. Juri Lelli reports he got this race when dl_bandwidth_enabled() was not set. Other thing, pointed by Peter Zijlstra: "Now I suppose the problem can still actually happen when you change the root domain and trigger a effective affinity change that way". To fix that we do the same as made in __task_rq_lock(). We do not use __task_rq_lock() itself, because it has a useful lockdep check, which is not correct in case of dl_task_timer(). We do not need pi_lock locked here. This case is an exception (PeterZ): "The only reason we don't strictly need ->pi_lock now is because we're guaranteed to have p->state == TASK_RUNNING here and are thus free of ttwu races". Signed-off-by: Kirill Tkhai <tkhai@yandex.ru> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: <stable@vger.kernel.org> # v3.14+ Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/3056991400578422@web14g.yandex.ru Signed-off-by: Ingo Molnar <mingo@kernel.org>

sched/dl: Fix race in dl_task_timer()
Throttled task is still on rq, and it may be moved to other cpu if user is playing with sched_setaffinity(). Therefore, unlocked task_rq() access makes the race. Juri Lelli reports he got this race when dl_bandwidth_enabled() was not set. Other thing, pointed by Peter Zijlstra: "Now I suppose the problem can still actually happen when you change the root domain and trigger a effective affinity change that way". To fix that we do the same as made in __task_rq_lock(). We do not use __task_rq_lock() itself, because it has a useful lockdep check, which is not correct in case of dl_task_timer(). We do not need pi_lock locked here. This case is an exception (PeterZ): "The only reason we don't strictly need ->pi_lock now is because we're guaranteed to have p->state == TASK_RUNNING here and are thus free of ttwu races". Signed-off-by: Kirill Tkhai <tkhai@yandex.ru> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: <stable@vger.kernel.org> # v3.14+ Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/3056991400578422@web14g.yandex.ru Signed-off-by: Ingo Molnar <mingo@kernel.org>
Kirill Tkhai · Ingo Molnar · Eric Lee · Eric Lee · Eric Lee · Eric Lee
1 parent b14ed2c273
Showing 1 changed file with 9 additions and 1 deletions Side-by-side Diff
kernel/sched/deadline.c
@@ -513,8 +513,16 @@
 						     struct sched_dl_entity,
 						     dl_timer);
 	struct task_struct *p = dl_task_of(dl_se);
-	struct rq *rq = task_rq(p);
+	struct rq *rq;
+again:
+	rq = task_rq(p);
 	raw_spin_lock(&rq->lock);
+
+	if (rq != task_rq(p)) {
+		/* Task was moved, retrying. */
+		raw_spin_unlock(&rq->lock);
+		goto again;
+	}
  
 	/*
 	 * We need to take care of a possible races here. In fact, the
...	...	@@ -513,8 +513,16 @@
513	513	struct sched_dl_entity,
514	514	dl_timer);
515	515	struct task_struct *p = dl_task_of(dl_se);
516		- struct rq *rq = task_rq(p);
	516	+ struct rq *rq;
	517	+again:
	518	+ rq = task_rq(p);
517	519	raw_spin_lock(&rq->lock);
	520	+
	521	+ if (rq != task_rq(p)) {
	522	+ /* Task was moved, retrying. */
	523	+ raw_spin_unlock(&rq->lock);
	524	+ goto again;
	525	+ }
518	526
519	527	/*
520	528	* We need to take care of a possible races here. In fact, the