sched: Make dl_task_time() use task_rq_lock()

Kirill reported that a dl task can 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;

This invalidates the assumption from commit 0f397f2c90ce ("sched/dl:
Fix race in dl_task_timer()"):

  "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".

And therefore we have to use the full task_rq_lock() here.

This further amends the fact that we forgot to update the rq lock loop
for TASK_ON_RQ_MIGRATE, from commit cca26e8009d1 ("sched: Teach
scheduler to understand TASK_ON_RQ_MIGRATING state").

Reported-by: Kirill Tkhai <ktkhai@parallels.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@arm.com>
Link: http://lkml.kernel.org/r/20150217123139.GN5029@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 3 insertions, 9 deletions

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index a027799ae130..e88847d9fc6a 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -511,16 +511,10 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 						     struct sched_dl_entity,
 						     dl_timer);
 	struct task_struct *p = dl_task_of(dl_se);
+	unsigned long flags;
 	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;
-	}
+	rq = task_rq_lock(current, &flags);
 
 	/*
 	 * We need to take care of several possible races here:
@@ -555,7 +549,7 @@ again:
 		push_dl_task(rq);
 #endif
 unlock:
-	raw_spin_unlock(&rq->lock);
+	task_rq_unlock(rq, current, &flags);
 
 	return HRTIMER_NORESTART;
 }