memcg: add comments explaining memory barriers

Add comments for the reason of smp_wmb() in mem_cgroup_commit_charge().

[akpm@linux-foundation.org: coding-style fixes]
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b0757660663f..eb9571815f0c 100644
--- a/mm/memcontrol.c
+++ b/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: