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>

1 files changed, 9 insertions, 1 deletions

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 800e99b99075..14bc348ba3b4 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -513,9 +513,17 @@ 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);
-        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
          * task might have changed its scheduling policy to something