[PATCH 1/3 cgroup/for-3.19] cgroup: add cgroup_subsys->css_released()

[Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

Add a new cgroup subsys callback css_released().  This is called when
the reference count of the css (cgroup_subsys_state) reaches zero
before RCU scheduling free.

Signed-off-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
---
 include/linux/cgroup.h |    1 +
 kernel/cgroup.c        |    2 ++
 2 files changed, 3 insertions(+)

--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -612,6 +612,7 @@ struct cgroup_subsys {
 	struct cgroup_subsys_state *(*css_alloc)(struct cgroup_subsys_state *parent_css);
 	int (*css_online)(struct cgroup_subsys_state *css);
 	void (*css_offline)(struct cgroup_subsys_state *css);
+	void (*css_released)(struct cgroup_subsys_state *css);
 	void (*css_free)(struct cgroup_subsys_state *css);
 	void (*css_reset)(struct cgroup_subsys_state *css);
 
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4380,6 +4380,8 @@ static void css_release_work_fn(struct w
 	if (ss) {
 		/* css release path */
 		cgroup_idr_remove(&ss->css_idr, css->id);
+		if (ss->css_released)
+			ss->css_released(css);
 	} else {
 		/* cgroup release path */
 		cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);