1 files changed, 10 insertions, 1 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 5db8138a0482..b0d5f53ae5e1 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -225,6 +225,9 @@ struct cgroup {
         struct list_head pidlists;
         struct mutex pidlist_mutex;
 
+        /* dummy css with NULL ->ss, points back to this cgroup */
+        struct cgroup_subsys_state dummy_css;
+
         /* For css percpu_ref killing and RCU-protected deletion */
         struct rcu_head rcu_head;
         struct work_struct destroy_work;
@@ -668,7 +671,13 @@ struct cgroup_subsys_state *css_parent(struct cgroup_subsys_state *css)
 {
         struct cgroup *parent_cgrp = css->cgroup->parent;
 
-        return parent_cgrp ? parent_cgrp->subsys[css->ss->subsys_id] : NULL;
+        if (!parent_cgrp)
+                return NULL;
+
+        if (css->ss)
+                return parent_cgrp->subsys[css->ss->subsys_id];
+        else
+                return &parent_cgrp->dummy_css;
 }
 
 /**