memcg: use rcu_dereference to access mm->owner

mm->owner should be accessed with rcu_dereference().
Reported-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -75,7 +75,7 @@ int mm_match_cgroup(const struct mm_struct *mm, const struct mem_cgroup *cgroup)
 {
 	struct mem_cgroup *mem;
 	rcu_read_lock();
-	mem = mem_cgroup_from_task((mm)->owner);
+	mem = mem_cgroup_from_task(rcu_dereference((mm)->owner));
 	rcu_read_unlock();
 	return cgroup == mem;
 }