[PATCH 3.13 013/120] mm/memcg: iteration skip memcgs not yet fully initialized

[Greg Kroah-Hartman]

3.13-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Hugh Dickins <hughd@xxxxxxxxxx>

commit d8ad30559715ce97afb7d1a93a12fd90e8fff312 upstream.

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@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 mm/memcontrol.c |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1100,10 +1100,8 @@ 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;


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