cgroup: remove synchronize_rcu() from cgroup_diput()

Free cgroup via call_rcu(). The actual work is done through
workqueue.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -203,6 +203,7 @@ struct cgroup {
 
 	/* For RCU-protected deletion */
 	struct rcu_head rcu_head;
+	struct work_struct free_work;
 
 	/* List of events which userspace want to receive */
 	struct list_head event_list;

kernel/cgroup.c

@@ -852,20 +852,10 @@ static struct inode *cgroup_new_inode(umode_t mode, struct super_block *sb)
 	return inode;
 }
 
-static void cgroup_diput(struct dentry *dentry, struct inode *inode)
+static void cgroup_free_fn(struct work_struct *work)
 {
-	/* is dentry a directory ? if so, kfree() associated cgroup */
-	if (S_ISDIR(inode->i_mode)) {
-		struct cgroup *cgrp = dentry->d_fsdata;
+	struct cgroup *cgrp = container_of(work, struct cgroup, free_work);
 	struct cgroup_subsys *ss;
-		BUG_ON(!(cgroup_is_removed(cgrp)));
-		/* It's possible for external users to be holding css
-		 * reference counts on a cgroup; css_put() needs to
-		 * be able to access the cgroup after decrementing
-		 * the reference count in order to know if it needs to
-		 * queue the cgroup to be handled by the release
-		 * agent */
-		synchronize_rcu();
 
 	mutex_lock(&cgroup_mutex);
 	/*
@@ -893,6 +883,29 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
 
 	ida_simple_remove(&cgrp->root->cgroup_ida, cgrp->id);
 	kfree(cgrp);
+}
+
+static void cgroup_free_rcu(struct rcu_head *head)
+{
+	struct cgroup *cgrp = container_of(head, struct cgroup, rcu_head);
+
+	schedule_work(&cgrp->free_work);
+}
+
+static void cgroup_diput(struct dentry *dentry, struct inode *inode)
+{
+	/* is dentry a directory ? if so, kfree() associated cgroup */
+	if (S_ISDIR(inode->i_mode)) {
+		struct cgroup *cgrp = dentry->d_fsdata;
+
+		BUG_ON(!(cgroup_is_removed(cgrp)));
+		/* It's possible for external users to be holding css
+		 * reference counts on a cgroup; css_put() needs to
+		 * be able to access the cgroup after decrementing
+		 * the reference count in order to know if it needs to
+		 * queue the cgroup to be handled by the release
+		 * agent */
+		call_rcu(&cgrp->rcu_head, cgroup_free_rcu);
 	} else {
 		struct cfent *cfe = __d_cfe(dentry);
 		struct cgroup *cgrp = dentry->d_parent->d_fsdata;
@@ -1391,6 +1404,7 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
 	INIT_LIST_HEAD(&cgrp->allcg_node);
 	INIT_LIST_HEAD(&cgrp->release_list);
 	INIT_LIST_HEAD(&cgrp->pidlists);
+	INIT_WORK(&cgrp->free_work, cgroup_free_fn);
 	mutex_init(&cgrp->pidlist_mutex);
 	INIT_LIST_HEAD(&cgrp->event_list);
 	spin_lock_init(&cgrp->event_list_lock);