[PATCH -mm v2 3/7] cgroup: release css->id after css_free

[Vladimir Davydov]

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@xxxxxxxxxxxxx>
---
 kernel/cgroup.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 6ffd3ae52bf8..7bd3e0f0f341 100644
--- a/kernel/cgroup.c
+++ b/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 {
-- 
1.7.10.4

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