memcg: add comments explaining memory barriers

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

[[email protected]: coding-style fixes]
Cc: Daisuke Nishimura <[email protected]>
Cc: Balbir Singh <[email protected]>
Signed-off-by: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b0757660663f47e570208abd18182c44c4e47798..eb9571815f0c0eeb07f60dbeae9abcee35926bf0 100644 (file)
--- 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: