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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>
This commit is contained in:
KAMEZAWA Hiroyuki 2009-09-23 15:56:33 -07:00 committed by Linus Torvalds
parent 4b3bde4c98
commit 261fb61a8b
1 changed files with 7 additions and 0 deletions
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7
mm/memcontrol.c
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@ -1134,6 +1134,13 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *mem,
} }
pc->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(); smp_wmb();
switch (ctype) { switch (ctype) {
case MEM_CGROUP_CHARGE_TYPE_CACHE: case MEM_CGROUP_CHARGE_TYPE_CACHE: