1 files changed, 40 insertions, 8 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 3a53c771e503..e9ec642932ee 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3016,7 +3016,7 @@ static int cgroup_event_wake(wait_queue_t *wait, unsigned mode,
        unsigned long flags = (unsigned long)key;
        if (flags & POLLHUP) {
-                remove_wait_queue_locked(event->wqh, &event->wait);
+                __remove_wait_queue(event->wqh, &event->wait);
                spin_lock(&cgrp->event_list_lock);
                list_del(&event->list);
                spin_unlock(&cgrp->event_list_lock);
@@ -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;
@@ -4453,15 +4464,36 @@ unsigned short css_depth(struct cgroup_subsys_state *css)
 }
 EXPORT_SYMBOL_GPL(css_depth);
+/**
+ *  css_is_ancestor - test "root" css is an ancestor of "child"
+ * @child: the css to be tested.
+ * @root: the css supporsed to be an ancestor of the child.
+ *
+ * Returns true if "root" is an ancestor of "child" in its hierarchy. Because
+ * this function reads css->id, this use rcu_dereference() and rcu_read_lock().
+ * But, considering usual usage, the csses should be valid objects after test.
+ * Assuming that the caller will do some action to the child if this returns
+ * returns true, the caller must take "child";s reference count.
+ * If "child" is valid object and this returns true, "root" is valid, too.
+ */
 bool css_is_ancestor(struct cgroup_subsys_state *child,
                    const struct cgroup_subsys_state *root)
 {
-        struct css_id *child_id = rcu_dereference(child->id);
+        struct css_id *child_id;
-        struct css_id *root_id = rcu_dereference(root->id);
+        struct css_id *root_id;
+        bool ret = true;
-        if (!child_id || !root_id || (child_id->depth < root_id->depth))
+        rcu_read_lock();
-                return false;
+        child_id  = rcu_dereference(child->id);
-        return child_id->stack[root_id->depth] == root_id->id;
+        root_id = rcu_dereference(root->id);
+        if (!child_id
+            || !root_id
+            || (child_id->depth < root_id->depth)
+            || (child_id->stack[root_id->depth] != root_id->id))
+                ret = false;
+        rcu_read_unlock();
+        return ret;
 }
 static void __free_css_id_cb(struct rcu_head *head)