cgroup: release css->id after css_free

Currently, we release css->id in css_release_work_fn, right before calling
css_free callback, so that when css_free is called, the id may have
already been reused for a new cgroup.

I am going to use css->id to create unique names for per memcg kmem
caches.  Since kmem caches are destroyed only on css_free, I need css->id
to be freed after css_free was called to avoid name clashes.  This patch
therefore moves css->id removal to css_free_work_fn.  To prevent
css_from_id from returning a pointer to a stale css, it makes
css_release_work_fn replace the css ptr at css_idr:css->id with NULL.
Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cgroup.c

@@ -4373,16 +4373,20 @@ static void css_free_work_fn(struct work_struct *work)
 {
 	struct cgroup_subsys_state *css =
 		container_of(work, struct cgroup_subsys_state, destroy_work);
+	struct cgroup_subsys *ss = css->ss;
 	struct cgroup *cgrp = css->cgroup;
 
 	percpu_ref_exit(&css->refcnt);
 
-	if (css->ss) {
+	if (ss) {
 		/* css free path */
+		int id = css->id;
+
 		if (css->parent)
 			css_put(css->parent);
 
-		css->ss->css_free(css);
+		ss->css_free(css);
+		cgroup_idr_remove(&ss->css_idr, id);
 		cgroup_put(cgrp);
 	} else {
 		/* cgroup free path */
@@ -4434,7 +4438,7 @@ static void css_release_work_fn(struct work_struct *work)
 
 	if (ss) {
 		/* css release path */
-		cgroup_idr_remove(&ss->css_idr, css->id);
+		cgroup_idr_replace(&ss->css_idr, NULL, css->id);
 		if (ss->css_released)
 			ss->css_released(css);
 	} else {