[tip:sched/urgent] sched/fair: Do not announce throttled next buddy in dequeue_task_fair()

[tip-bot for Konstantin Khlebnikov <tipbot@zytor.com>]

Commit-ID:  754bd598be9bbc953bc709a9e8ed7f3188bfb9d7
Gitweb:     http://git.kernel.org/tip/754bd598be9bbc953bc709a9e8ed7f3188bfb9d7
Author:     Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
AuthorDate: Thu, 16 Jun 2016 15:57:15 +0300
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 24 Jun 2016 08:26:45 +0200

sched/fair: Do not announce throttled next buddy in dequeue_task_fair()

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

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ben Segall <bsegall@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/146608183552.21905.15924473394414832071.stgit@buzz
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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 8c5d8c0..bdcbeea 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4537,15 +4537,14 @@ 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;