[4.2.y-ckt stable] Patch "memcg: only free spare array when readers are done" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    memcg: only free spare array when readers are done

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt3.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 65f46bf6343ab128a811c4931f230f607521c0d1 Mon Sep 17 00:00:00 2001
From: Martijn Coenen <maco at google.com>
Date: Fri, 15 Jan 2016 16:57:49 -0800
Subject: memcg: only free spare array when readers are done

commit 6611d8d76132f86faa501de9451a89bf23fb2371 upstream.

A spare array holding mem cgroup threshold events is kept around to make
sure we can always safely deregister an event and have an array to store
the new set of events in.

In the scenario where we're going from 1 to 0 registered events, the
pointer to the primary array containing 1 event is copied to the spare
slot, and then the spare slot is freed because no events are left.
However, it is freed before calling synchronize_rcu(), which means
readers may still be accessing threshold->primary after it is freed.

Fixed by only freeing after synchronize_rcu().

Signed-off-by: Martijn Coenen <maco at google.com>
Cc: Johannes Weiner <hannes at cmpxchg.org>
Acked-by: Michal Hocko <mhocko at suse.com>
Cc: Vladimir Davydov <vdavydov at virtuozzo.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 mm/memcontrol.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 364f972..48540e5 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3833,16 +3833,17 @@ static void __mem_cgroup_usage_unregister_event(struct mem_cgroup *memcg,
 swap_buffers:
 	/* Swap primary and spare array */
 	thresholds->spare = thresholds->primary;
-	/* If all events are unregistered, free the spare array */
-	if (!new) {
-		kfree(thresholds->spare);
-		thresholds->spare = NULL;
-	}

 	rcu_assign_pointer(thresholds->primary, new);

 	/* To be sure that nobody uses thresholds */
 	synchronize_rcu();
+
+	/* If all events are unregistered, free the spare array */
+	if (!new) {
+		kfree(thresholds->spare);
+		thresholds->spare = NULL;
+	}
 unlock:
 	mutex_unlock(&memcg->thresholds_lock);
 }
--
1.9.1