mm/memcg: iteration skip memcgs not yet fully initialized

It is surprising that the mem_cgroup iterator can return memcgs which
have not yet been fully initialized.  By accident (or trial and error?)
this appears not to present an actual problem; but it may be better to
prevent such surprises, by skipping memcgs not yet online.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>	[3.12+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 4 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index aa66cc4c9e79..9537e1389ee6 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1119,10 +1119,8 @@ skip_node:
          * protected by css_get and the tree walk is rcu safe.
          */
         if (next_css) {
-                struct mem_cgroup *mem = mem_cgroup_from_css(next_css);
-
-                if (css_tryget(&mem->css))
-                        return mem;
+                if ((next_css->flags & CSS_ONLINE) && css_tryget(next_css))
+                        return mem_cgroup_from_css(next_css);
                 else {
                         prev_css = next_css;
                         goto skip_node;