}
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: