From: Martijn Coenen <maco@google.com>
Date: Sat, 16 Jan 2016 00:57:49 +0000 (-0800)
Subject: memcg: only free spare array when readers are done
X-Git-Tag: v4.1.12-92~150^2~293
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=97b6ed60e3d771959e059c218a8806c54ca1e0d4;p=users%2Fjedix%2Flinux-maple.git

memcg: only free spare array when readers are done

Orabug: 23330638

[ Upstream commit 6611d8d76132f86faa501de9451a89bf23fb2371 ]

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@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
(cherry picked from commit 2c641f5b0c8e87d43235ce39890bcc4d0c7cd2fb)

Signed-off-by: Dan Duval <dan.duval@oracle.com>
---

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 68dea90334cb9..aac1c98a9bc7c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3824,16 +3824,17 @@ 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);
 }