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

Hugh Dickins
2014-02-06T23:57:14Z

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@google.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.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);

/*