[PATCH] cgroup: use an ordered workqueue for cgroup destruction

[Hugh Dickins <hughd-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>]

Sometimes the cleanup after memcg hierarchy testing gets stuck in
mem_cgroup_reparent_charges(), unable to bring non-kmem usage down to 0.

There may turn out to be several causes, but a major cause is this: the
workitem to offline parent can get run before workitem to offline child;
parent's mem_cgroup_reparent_charges() circles around waiting for the
child's pages to be reparented to its lrus, but it's holding cgroup_mutex
which prevents the child from reaching its mem_cgroup_reparent_charges().

Just use an ordered workqueue for cgroup_destroy_wq.

Fixes: e5fca243abae ("cgroup: use a dedicated workqueue for cgroup destruction")
Suggested-by: Filipe Brandenburger <filbranden-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Signed-off-by: Hugh Dickins <hughd-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org # 3.10+
---

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

--- 3.14-rc1/kernel/cgroup.c	2014-02-02 18:49:07.737302111 -0800
+++ linux/kernel/cgroup.c	2014-02-06 15:20:35.548904965 -0800
@@ -4845,12 +4845,12 @@ static int __init cgroup_wq_init(void)
 	/*
 	 * There isn't much point in executing destruction path in
 	 * parallel.  Good chunk is serialized with cgroup_mutex anyway.
-	 * Use 1 for @max_active.
+	 * Must be ordered to make sure parent is offlined after children.
 	 *
 	 * We would prefer to do this in cgroup_init() above, but that
 	 * is called before init_workqueues(): so leave this until after.
 	 */
-	cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
+	cgroup_destroy_wq = alloc_ordered_workqueue("cgroup_destroy", 0);
 	BUG_ON(!cgroup_destroy_wq);
 
 	/*