memcg: fix css_id() RCU locking for real

Commit ad4ba375373937817404fd92239ef4cadbded23b ("memcg: css_id() must be
called under rcu_read_lock()") modifies memcontol.c for fixing RCU check
message.  But Andrew Morton pointed out that the fix doesn't seems sane
and it was just for hidining lockdep messages.

This is a patch for do proper things.  Checking again, all places,
accessing without rcu_read_lock, that commit fixies was intentional....
all callers of css_id() has reference count on it.  So, it's not necessary
to be under rcu_read_lock().

Considering again, we can use rcu_dereference_check for css_id().  We know
css->id is valid if css->refcnt > 0.  (css->id never changes and freed
after css->refcnt going to be 0.)

This patch makes use of rcu_dereference_check() in css_id/depth and remove
unnecessary rcu-read-lock added by the commit.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: 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>

1 files changed, 13 insertions, 2 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 3a53c771e503..6db8b7f297a1 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4435,7 +4435,15 @@ __setup("cgroup_disable=", cgroup_disable);
  */
 unsigned short css_id(struct cgroup_subsys_state *css)
 {
-	struct css_id *cssid = rcu_dereference(css->id);
+	struct css_id *cssid;
+
+	/*
+	 * This css_id() can return correct value when somone has refcnt
+	 * on this or this is under rcu_read_lock(). Once css->id is allocated,
+	 * it's unchanged until freed.
+	 */
+	cssid = rcu_dereference_check(css->id,
+			rcu_read_lock_held() || atomic_read(&css->refcnt));
 
 	if (cssid)
 		return cssid->id;
@@ -4445,7 +4453,10 @@ EXPORT_SYMBOL_GPL(css_id);
 
 unsigned short css_depth(struct cgroup_subsys_state *css)
 {
-	struct css_id *cssid = rcu_dereference(css->id);
+	struct css_id *cssid;
+
+	cssid = rcu_dereference_check(css->id,
+			rcu_read_lock_held() || atomic_read(&css->refcnt));
 
 	if (cssid)
 		return cssid->depth;