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;