[PATCH] sched/fair: do not announce throttled next buddy in dequeue_task_fair

[Konstantin Khlebnikov]

Hierarchy could be already throttled at this point. Throttled next
buddy could trigger null pointer dereference in pick_next_task_fair().

Signed-off-by: Konstantin Khlebnikov <khlebni...@yandex-team.ru>
Cc: Stable <sta...@vger.kernel.org> # v3.2+
---
 kernel/sched/fair.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fe809fe169d2..3c6b038cb734 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4519,15 +4519,14 @@ static void dequeue_task_fair(struct rq *rq, struct 
task_struct *p, int flags)
 
                /* Don't dequeue parent if it has other entities besides us */
                if (cfs_rq->load.weight) {
+                       /* avoid re-evaluating load for this entity */
+                       se = parent_entity(se);
                        /*
                         * Bias pick_next to pick a task from this cfs_rq, as
                         * p is sleeping when it is within its sched_slice.
                         */
-                       if (task_sleep && parent_entity(se))
-                               set_next_buddy(parent_entity(se));
-
-                       /* avoid re-evaluating load for this entity */
-                       se = parent_entity(se);
+                       if (task_sleep && se && !throttled_hierarchy(cfs_rq))
+                               set_next_buddy(se);
                        break;
                }
                flags |= DEQUEUE_SLEEP;