From 261fb61a8bf6d3bd964ae6f1e6af49585d30db51 Mon Sep 17 00:00:00 2001
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Date: Wed, 23 Sep 2009 15:56:33 -0700
Subject: [PATCH] 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>
---
mm/memcontrol.c | 7 +++++++
1 file changed, 7 insertions(+)
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:
--
GitLab