[PATCH 3.12 118/127] cgroup: use an ordered workqueue for cgroup destruction

From: Hugh Dickins <hughd@google.com>

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

===============

commit ab3f5faa6255a0eb4f832675507d9e295ca7e9ba upstream.

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.

tj: Committing as the temporary fix until the reverse dependency can
    be removed from memcg.  Comment updated accordingly.

Fixes: e5fca243abae ("cgroup: use a dedicated workqueue for cgroup destruction")
Suggested-by: Filipe Brandenburger <redacted>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jiri Slaby <redacted>
---
 kernel/cgroup.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 5d9d542c0bb5..e89f6cec01c9 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c

@@ -5168,12 +5168,16 @@ 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.
+	 *
+	 * XXX: Must be ordered to make sure parent is offlined after
+	 * children.  The ordering requirement is for memcg where a
+	 * parent's offline may wait for a child's leading to deadlock.  In
+	 * the long term, this should be fixed from memcg side.
 	 *
 	 * 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);
 	return 0;
 }

-- 
2.10.2