1 files changed, 13 insertions, 5 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index da07784dde87..98f80beeac7f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1117,14 +1117,22 @@ skip_node:
          * skipped and we should continue the tree walk.
          * last_visited css is safe to use because it is
          * protected by css_get and the tree walk is rcu safe.
+         *
+         * We do not take a reference on the root of the tree walk
+         * because we might race with the root removal when it would
+         * be the only node in the iterated hierarchy and mem_cgroup_iter
+         * would end up in an endless loop because it expects that at
+         * least one valid node will be returned. Root cannot disappear
+         * because caller of the iterator should hold it already so
+         * skipping css reference should be safe.
          */
         if (next_css) {
-                if ((next_css->flags & CSS_ONLINE) && css_tryget(next_css))
+                if ((next_css->flags & CSS_ONLINE) &&
+                                (next_css == &root->css || css_tryget(next_css)))
                         return mem_cgroup_from_css(next_css);
-                else {
-                        prev_css = next_css;
-                        goto skip_node;
-                }
+
+                prev_css = next_css;
+                goto skip_node;
         }
 
         return NULL;