Patch "sched/fair: Initialize throttle_count for new task-groups lazily" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

sched/fair: Initialize throttle_count for new task-groups lazily

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sched-fair-initialize-throttle_count-for-new-task-groups-lazily.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Konstantin Khlebnikov <[email protected]>
Date: Thu, 16 Jun 2016 15:57:01 +0300
Subject: sched/fair: Initialize throttle_count for new task-groups lazily

From: Konstantin Khlebnikov <[email protected]>

commit 094f469172e00d6ab0a3130b0e01c83b3cf3a98d upstream.

Cgroup created inside throttled group must inherit current throttle_count. Broken throttle_count allows to nominate throttled entries as a next buddy, later this leads to null pointer dereference in pick_next_task_fair().

This patch initialize cfs_rq->throttle_count at first enqueue: laziness allows to skip locking all rq at group creation. Lazy approach also allows to skip full sub-tree scan at throttling hierarchy (not in this patch).

---
 kernel/sched/fair.c  |   20 ++++++++++++++++++++
 kernel/sched/sched.h |    2 +-
 2 files changed, 21 insertions(+), 1 deletion(-)

@@ -3918,6 +3918,26 @@ static void check_enqueue_throttle(struc
 	if (!cfs_bandwidth_used())
 		return;
 
+	/* Synchronize hierarchical throttle counter: */
+	if (unlikely(!cfs_rq->throttle_uptodate)) {
+		struct rq *rq = rq_of(cfs_rq);
+		struct cfs_rq *pcfs_rq;
+		struct task_group *tg;
+
+		cfs_rq->throttle_uptodate = 1;
+
+		/* Get closest up-to-date node, because leaves go first: */
+		for (tg = cfs_rq->tg->parent; tg; tg = tg->parent) {
+			pcfs_rq = tg->cfs_rq[cpu_of(rq)];
+			if (pcfs_rq->throttle_uptodate)
+				break;
+		}
+		if (tg) {
+			cfs_rq->throttle_count = pcfs_rq->throttle_count;
+			cfs_rq->throttled_clock_task = rq_clock_task(rq);
+		}
+	}
+
 	/* an active group must be handled by the update_curr()->put() path */
 	if (!cfs_rq->runtime_enabled || cfs_rq->curr)
 		return;

@@ -417,7 +417,7 @@ struct cfs_rq {
 
 	u64 throttled_clock, throttled_clock_task;
 	u64 throttled_clock_task_time;
-	int throttled, throttle_count;
+	int throttled, throttle_count, throttle_uptodate;
 	struct list_head throttled_list;
 #endif /* CONFIG_CFS_BANDWIDTH */
 #endif /* CONFIG_FAIR_GROUP_SCHED */

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/sched-fair-do-not-announce-throttled-next-buddy-in-dequeue_task_fair.patch queue-4.4/sched-fair-initialize-throttle_count-for-new-task-groups-lazily.patch