kernel: cgroup: push rcu read locking from css_is_ancestor() to callsite

Library functions should not grab locks when the callsites can do it,
even if the lock nests like the rcu read-side lock does.

Push the rcu_read_lock() from css_is_ancestor() to its single user,
mem_cgroup_same_or_subtree() in preparation for another user that may
already hold the rcu read-side lock.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 5 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 92675fe8a2ef..faad98e6d17d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1152,12 +1152,16 @@ struct lruvec *mem_cgroup_lru_move_lists(struct zone *zone,
 static bool mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
 		struct mem_cgroup *memcg)
 {
-	if (root_memcg != memcg) {
-		return (root_memcg->use_hierarchy &&
-			css_is_ancestor(&memcg->css, &root_memcg->css));
-	}
+	bool ret;
 
-	return true;
+	if (root_memcg == memcg)
+		return true;
+	if (!root_memcg->use_hierarchy)
+		return false;
+	rcu_read_lock();
+	ret = css_is_ancestor(&memcg->css, &root_memcg->css);
+	rcu_read_unlock();
+	return ret;
 }
 
 int task_in_mem_cgroup(struct task_struct *task, const struct mem_cgroup *memcg)