cgroup: make sure parent won't be destroyed before its children

Suppose we rmdir a cgroup and there're still css refs, this cgroup won't
be freed. Then we rmdir the parent cgroup, and the parent is freed
immediately due to css ref draining to 0. Now it would be a disaster if
the still-alive child cgroup tries to access its parent.

Make sure this won't happen.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Reviewed-by: NMichal Hocko <mhocko@suse.cz>
Acked-by: NKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -875,6 +875,13 @@ static void cgroup_free_fn(struct work_struct *work)
 	cgrp->root->number_of_cgroups--;
 	mutex_unlock(&cgroup_mutex);
 
+	/*
+	 * We get a ref to the parent's dentry, and put the ref when
+	 * this cgroup is being freed, so it's guaranteed that the
+	 * parent won't be destroyed before its children.
+	 */
+	dput(cgrp->parent->dentry);
+
 	/*
 	 * Drop the active superblock reference that we took when we
 	 * created the cgroup
@@ -4164,6 +4171,9 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 	for_each_subsys(root, ss)
 		dget(dentry);
 
+	/* hold a ref to the parent's dentry */
+	dget(parent->dentry);
+
 	/* creation succeeded, notify subsystems */
 	for_each_subsys(root, ss) {
 		err = online_css(ss, cgrp);