cgroup: Use rcu_work instead of explicit rcu and work item

Workqueue now has rcu_work.  Use it instead of open-coding rcu -> work
item bouncing.
Signed-off-by: NTejun Heo <tj@kernel.org>

include/linux/cgroup-defs.h

@@ -151,8 +151,8 @@ struct cgroup_subsys_state {
 	atomic_t online_cnt;
 
 	/* percpu_ref killing and RCU release */
-	struct rcu_head rcu_head;
 	struct work_struct destroy_work;
+	struct rcu_work destroy_rwork;
 
 	/*
 	 * PI: the parent css.	Placed here for cache proximity to following

kernel/cgroup/cgroup.c

@@ -4514,10 +4514,10 @@ static struct cftype cgroup_base_files[] = {
  * and thus involve punting to css->destroy_work adding two additional
  * steps to the already complex sequence.
  */
-static void css_free_work_fn(struct work_struct *work)
+static void css_free_rwork_fn(struct work_struct *work)
 {
-	struct cgroup_subsys_state *css =
-		container_of(work, struct cgroup_subsys_state, destroy_work);
+	struct cgroup_subsys_state *css = container_of(to_rcu_work(work),
+				struct cgroup_subsys_state, destroy_rwork);
 	struct cgroup_subsys *ss = css->ss;
 	struct cgroup *cgrp = css->cgroup;
 
@@ -4563,15 +4563,6 @@ static void css_free_work_fn(struct work_struct *work)
 	}
 }
 
-static void css_free_rcu_fn(struct rcu_head *rcu_head)
-{
-	struct cgroup_subsys_state *css =
-		container_of(rcu_head, struct cgroup_subsys_state, rcu_head);
-
-	INIT_WORK(&css->destroy_work, css_free_work_fn);
-	queue_work(cgroup_destroy_wq, &css->destroy_work);
-}
-
 static void css_release_work_fn(struct work_struct *work)
 {
 	struct cgroup_subsys_state *css =
@@ -4621,7 +4612,8 @@ static void css_release_work_fn(struct work_struct *work)
 
 	mutex_unlock(&cgroup_mutex);
 
-	call_rcu(&css->rcu_head, css_free_rcu_fn);
+	INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
+	queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
 }
 
 static void css_release(struct percpu_ref *ref)
@@ -4755,7 +4747,8 @@ static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
 err_list_del:
 	list_del_rcu(&css->sibling);
 err_free_css:
-	call_rcu(&css->rcu_head, css_free_rcu_fn);
+	INIT_RCU_WORK(&css->destroy_rwork, css_free_rwork_fn);
+	queue_rcu_work(cgroup_destroy_wq, &css->destroy_rwork);
 	return ERR_PTR(err);
 }