1 files changed, 16 insertions, 1 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 73d1c730c3c4..ce1c1f34c30c 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -340,8 +340,23 @@ struct cgroup_subsys {
 #define MAX_CGROUP_TYPE_NAMELEN 32
         const char *name;
 
-        struct cgroupfs_root *root;
+        /*
+         * Protects sibling/children links of cgroups in this
+         * hierarchy, plus protects which hierarchy (or none) the
+         * subsystem is a part of (i.e. root/sibling).  To avoid
+         * potential deadlocks, the following operations should not be
+         * undertaken while holding any hierarchy_mutex:
+         *
+         * - allocating memory
+         * - initiating hotplug events
+         */
+        struct mutex hierarchy_mutex;
 
+        /*
+         * Link to parent, and list entry in parent's children.
+         * Protected by this->hierarchy_mutex and cgroup_lock()
+         */
+        struct cgroupfs_root *root;
         struct list_head sibling;
 };