From ab3f5faa6255a0eb4f832675507d9e295ca7e9ba Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Thu, 6 Feb 2014 15:56:01 -0800 Subject: [PATCH] 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 Signed-off-by: Hugh Dickins Cc: stable@vger.kernel.org # 3.10+ Signed-off-by: Tejun Heo --- kernel/cgroup.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/kernel/cgroup.c b/kernel/cgroup.c index e2f46ba37f72..aa95591c1430 100644 --- 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); /*