cgroup: fix umount vs cgroup_cfs_commit() race

index 25f1054..482252a 100644
@@ -2788,13 +2788,17 @@ static void cgroup_cfts_commit(struct cgroup_subsys *ss,
 {
 	LIST_HEAD(pending);
 	struct cgroup *cgrp, *n;
+	struct super_block *sb = ss->root->sb;
 
 	/* %NULL @cfts indicates abort and don't bother if @ss isn't attached */
-	if (cfts && ss->root != &rootnode) {
+	if (cfts && ss->root != &rootnode &&
+	    atomic_inc_not_zero(sb->s_active)) {
 		list_for_each_entry(cgrp, &ss->root->allcg_list, allcg_node) {
 			dget(cgrp->dentry);
 			list_add_tail(&cgrp->cft_q_node, &pending);
 		}
+	} else {
+		sb = NULL;
 	}
 
 	mutex_unlock(&cgroup_mutex);

@@ -2817,6 +2821,9 @@ static void cgroup_cfts_commit(struct cgroup_subsys *ss,
 		dput(cgrp->dentry);
 	}
 
+	if (sb)
+		deactivate_super(sb);
+
 	mutex_unlock(&cgroup_cft_mutex);
 }
 

index 482252a..e2dcf08 100644
@@ -3812,6 +3812,23 @@ static int cgroup_write_notify_on_release(struct cgroup *cgrp,
 }
 
 /*
+ * When dput() is called asynchronously, if umount has been done and
+ * then deactivate_super() in cgroup_free_fn() kills the superblock,
+ * there's a small window that vfs will see the root dentry with non-zero
+ * refcnt and trigger BUG().
+ *
+ * That's why we hold a reference before dput() and drop it right after.
+ */
+static void cgroup_dput(struct cgroup *cgrp)
+{
+	struct super_block *sb = cgrp->root->sb;
+
+	atomic_inc(&sb->s_active);
+	dput(cgrp->dentry);
+	deactivate_super(sb);
+}
+
+/*
  * Unregister event and free resources.
  *
  * Gets called from workqueue.

@@ -3831,7 +3848,7 @@ static void cgroup_event_remove(struct work_struct *work)
 
 	eventfd_ctx_put(event->eventfd);
 	kfree(event);
-	dput(cgrp->dentry);
+	cgroup_dput(cgrp);
 }
 
 /*

@@ -4119,12 +4136,8 @@ static void css_dput_fn(struct work_struct *work)
 {
 	struct cgroup_subsys_state *css =
 		container_of(work, struct cgroup_subsys_state, dput_work);
-	struct dentry *dentry = css->cgroup->dentry;
-	struct super_block *sb = dentry->d_sb;
 
-	atomic_inc(&sb->s_active);
-	dput(dentry);
-	deactivate_super(sb);
+	cgroup_dput(css->cgroup);
 }
 
 static void css_release(struct percpu_ref *ref)

index e2dcf08..1d5b1d6 100644
@@ -2879,7 +2879,8 @@ int cgroup_rm_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 
 	list_for_each_entry(set, &ss->cftsets, node) {
 		if (set->cfts == cfts) {
-			list_del_init(&set->node);
+			list_del(&set->node);
+			kfree(set);
 			cgroup_cfts_commit(ss, cfts, false);
 			return 0;
 		}