cgroup: use an ordered workqueue for cgroup destruction

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 <filbranden@google.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org # 3.10+
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index e2f46ba37f7243c4278de77a8c8536ddc4c0aad5..aa95591c14304e13fece8b9b54e397b1c7e23f04 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4845,12 +4845,16 @@ 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);
 
        /*