blkcg: clean up blkg_tryget_closest()

index f025fd1e22e6..76c61318fda5 100644
@@ -499,22 +499,33 @@ static inline void blkg_get(struct blkcg_gq *blkg)
  */
 static inline bool blkg_tryget(struct blkcg_gq *blkg)
 {
-	return percpu_ref_tryget(&blkg->refcnt);
+	return blkg && percpu_ref_tryget(&blkg->refcnt);
 }
 
 /**
  * blkg_tryget_closest - try and get a blkg ref on the closet blkg
  * @blkg: blkg to get
  *
- * This walks up the blkg tree to find the closest non-dying blkg and returns
- * the blkg that it did association with as it may not be the passed in blkg.
+ * This needs to be called rcu protected.  As the failure mode here is to walk
+ * up the blkg tree, this ensure that the blkg->parent pointers are always
+ * valid.  This returns the blkg that it ended up taking a reference on or %NULL
+ * if no reference was taken.
  */
 static inline struct blkcg_gq *blkg_tryget_closest(struct blkcg_gq *blkg)
 {
-	while (blkg && !percpu_ref_tryget(&blkg->refcnt))
+	struct blkcg_gq *ret_blkg = NULL;
+
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	while (blkg) {
+		if (blkg_tryget(blkg)) {
+			ret_blkg = blkg;
+			break;
+		}
 		blkg = blkg->parent;
+	}
 
-	return blkg;
+	return ret_blkg;
 }
 
 /**