From 261fb61a8bf6d3bd964ae6f1e6af49585d30db51 Mon Sep 17 00:00:00 2001 From: KAMEZAWA Hiroyuki 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 Cc: Balbir Singh Signed-off-by: KAMEZAWA Hiroyuki Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index b075766..eb95718 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: -- 1.8.2.3