memcg: css_put after remove_list

mem_cgroup_uncharge_page does css_put on the mem_cgroup before uncharging from
it, and before removing page_cgroup from one of its lru lists: isn't there a
danger that struct mem_cgroup memory could be freed and reused before
completing that, so corrupting something?  Never seen it, and for all I know
there may be other constraints which make it impossible; but let's be
defensive and reverse the ordering there.

mem_cgroup_force_empty_list is safe because there's an extra css_get around
all its works; but even so, change its ordering the same way round, to help
get in the habit of doing it like this.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Hirokazu Takahashi <taka@valinux.co.jp>
Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -665,15 +665,15 @@ void mem_cgroup_uncharge_page(struct page *page)
 		page_assign_page_cgroup(page, NULL);
 		unlock_page_cgroup(page);
 
-		mem = pc->mem_cgroup;
-		css_put(&mem->css);
-		res_counter_uncharge(&mem->res, PAGE_SIZE);
-
 		mz = page_cgroup_zoneinfo(pc);
 		spin_lock_irqsave(&mz->lru_lock, flags);
 		__mem_cgroup_remove_list(pc);
 		spin_unlock_irqrestore(&mz->lru_lock, flags);
 
+		mem = pc->mem_cgroup;
+		res_counter_uncharge(&mem->res, PAGE_SIZE);
+		css_put(&mem->css);
+
 		kfree(pc);
 		return;
 	}
@@ -774,9 +774,9 @@ retry:
 		if (page_get_page_cgroup(page) == pc) {
 			page_assign_page_cgroup(page, NULL);
 			unlock_page_cgroup(page);
-			css_put(&mem->css);
-			res_counter_uncharge(&mem->res, PAGE_SIZE);
 			__mem_cgroup_remove_list(pc);
+			res_counter_uncharge(&mem->res, PAGE_SIZE);
+			css_put(&mem->css);
 			kfree(pc);
 		} else {
 			/* racing uncharge: let page go then retry */