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(). Further testing showed that an ordered workqueue for cgroup_destroy_wq is not always good enough: percpu_ref_kill_and_confirm's call_rcu_sched stage on the way can mess up the order before reaching the workqueue. Instead bring back v3.11's css kill_cnt, repurposing it to make sure that offline_css() is not called for parent before it has been called for all children. Fixes: e5fca243abae ("cgroup: use a dedicated workqueue for cgroup destruction") Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Reviewed-by: Filipe Brandenburger <filbranden@xxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx # v3.10+ (but will need extra care) --- This is an alternative to Filipe's 1/2: there's no need for both, but each has its merits. I prefer Filipe's, which is much easier to understand: this one made more sense in v3.11, when it was just a matter of extending the use of css_kill_cnt; but might be preferred if offlining children before parent is thought to be a good idea generally. include/linux/cgroup.h | 3 +++ kernel/cgroup.c | 21 +++++++++++++++++++++ 2 files changed, 24 insertions(+) --- 3.14-rc2/include/linux/cgroup.h 2014-02-02 18:49:07.033302094 -0800 +++ linux/include/linux/cgroup.h 2014-02-11 15:59:22.720393186 -0800 @@ -79,6 +79,9 @@ struct cgroup_subsys_state { unsigned long flags; + /* ensure children are offlined before parent */ + atomic_t kill_cnt; + /* percpu_ref killing and RCU release */ struct rcu_head rcu_head; struct work_struct destroy_work; --- 3.14-rc2/kernel/cgroup.c 2014-02-02 18:49:07.737302111 -0800 +++ linux/kernel/cgroup.c 2014-02-11 15:57:56.000391125 -0800 @@ -175,6 +175,7 @@ static int need_forkexit_callback __read static struct cftype cgroup_base_files[]; +static void css_killed_ref_fn(struct percpu_ref *ref); static void cgroup_destroy_css_killed(struct cgroup *cgrp); static int cgroup_destroy_locked(struct cgroup *cgrp); static int cgroup_addrm_files(struct cgroup *cgrp, struct cftype cfts[], @@ -4043,6 +4044,7 @@ static void init_css(struct cgroup_subsy css->cgroup = cgrp; css->ss = ss; css->flags = 0; + atomic_set(&css->kill_cnt, 1); if (cgrp->parent) css->parent = cgroup_css(cgrp->parent, ss); @@ -4292,6 +4294,7 @@ static void css_killed_work_fn(struct wo { struct cgroup_subsys_state *css = container_of(work, struct cgroup_subsys_state, destroy_work); + struct cgroup_subsys_state *parent = css->parent; struct cgroup *cgrp = css->cgroup; mutex_lock(&cgroup_mutex); @@ -4320,6 +4323,12 @@ static void css_killed_work_fn(struct wo * destruction happens only after all css's are released. */ css_put(css); + + /* + * Put the parent's kill_cnt reference from kill_css(), and + * schedule its ->css_offline() if all children are now offline. + */ + css_killed_ref_fn(&parent->refcnt); } /* css kill confirmation processing requires process context, bounce */ @@ -4328,6 +4337,9 @@ static void css_killed_ref_fn(struct per struct cgroup_subsys_state *css = container_of(ref, struct cgroup_subsys_state, refcnt); + if (!atomic_dec_and_test(&css->kill_cnt)) + return; + INIT_WORK(&css->destroy_work, css_killed_work_fn); queue_work(cgroup_destroy_wq, &css->destroy_work); } @@ -4362,6 +4374,15 @@ static void kill_css(struct cgroup_subsy * css is confirmed to be seen as killed on all CPUs. */ percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn); + + /* + * Make sure that ->css_offline() will not be called for parent + * before it has been called for all children: this ordering + * requirement is important for memcg, where parent's offline + * might wait for a child's, leading to deadlock. + */ + atomic_inc(&css->parent->kill_cnt); + css_killed_ref_fn(&css->refcnt); } /** -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>