diff --git a/[refs] b/[refs]
index 65a61f1468f7..26380c414599 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 4b3bde4c983de36c59e6c1a24701f6fe816f9f55
+refs/heads/master: 261fb61a8bf6d3bd964ae6f1e6af49585d30db51
diff --git a/trunk/mm/memcontrol.c b/trunk/mm/memcontrol.c
index b0757660663f..eb9571815f0c 100644
--- a/trunk/mm/memcontrol.c
+++ b/trunk/mm/memcontrol.c
@@ -1134,6 +1134,13 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *mem,
 	}
 
 	pc->mem_cgroup = mem;
+	/*
+	 * We access a page_cgroup asynchronously without lock_page_cgroup().
+	 * Especially when a page_cgroup is taken from a page, pc->mem_cgroup
+	 * is accessed after testing USED bit. To make pc->mem_cgroup visible
+	 * before USED bit, we need memory barrier here.
+	 * See mem_cgroup_add_lru_list(), etc.
+ 	 */
 	smp_wmb();
 	switch (ctype) {
 	case MEM_CGROUP_CHARGE_TYPE_CACHE: