[PATCH 1/2] memcg: fix endless loop in __mem_cgroup_iter_next

[Hugh Dickins]

Commit 0eef615665ed ("memcg: fix css reference leak and endless loop in
mem_cgroup_iter") got the interaction with the commit a few before it
d8ad30559715 ("mm/memcg: iteration skip memcgs not yet fully initialized")
slightly wrong, and we didn't notice at the time.

It's elusive, and harder to get than the original, but for a couple of
days before rc1, I several times saw a endless loop similar to that
supposedly being fixed.

This time it was a tighter loop in __mem_cgroup_iter_next(): because we
can get here when our root has already been offlined, and the ordering
of conditions was such that we then just cycled around forever.

Fixes: 0eef615665ed ("memcg: fix css reference leak and endless loop in mem_cgroup_iter")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # 3.12+
---
Of course I'd have preferred to send this before that commit went through
to -stable, but priorities kept preempting; I did wonder whether to ask
GregKH to delay it, but decided it's not serious enough to trouble him,
just go with the flow of stable fixing stable.

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

--- 3.14-rc2/mm/memcontrol.c	2014-02-02 18:49:07.897302115 -0800
+++ linux/mm/memcontrol.c	2014-02-12 11:55:02.836035004 -0800
@@ -1127,8 +1127,8 @@ skip_node:
 	 * skipping css reference should be safe.
 	 */
 	if (next_css) {
-		if ((next_css->flags & CSS_ONLINE) &&
-				(next_css == &root->css || css_tryget(next_css)))
+		if ((next_css == &root->css) ||
+		    ((next_css->flags & CSS_ONLINE) && css_tryget(next_css)))
 			return mem_cgroup_from_css(next_css);
 
 		prev_css = next_css;
--
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