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>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index aa66cc4c9e79b1654db117f63256544bf9500683..9537e1389ee6a3551064bcfe58938cddc5a2d908 100644 (file)
--- 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;