cgroup: add cgroup_subsys->css_released()

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@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

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);
 

kernel/cgroup.c

@@ -4380,6 +4380,8 @@ static void css_release_work_fn(struct work_struct *work)
 	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);