cgroup: fix umount vs cgroup_cfts_commit() race

commit 084457f284abf6789d90509ee11dae383842b23b upstream.

cgroup_cfts_commit() uses dget() to keep cgroup alive after cgroup_mutex is dropped, but dget() won't prevent cgroupfs from being umounted. When the race happens, vfs will see some dentries with non-zero refcnt while umount is in process.

Keep running this:
mount -t cgroup -o blkio xxx /cgroup
umount /cgroup

And this:
modprobe cfq-iosched
rmmod cfs-iosched

After a while, the BUG() in shrink_dcache_for_umount_subtree() may be triggered:

  BUG: Dentry xxx{i=0,n=blkio.yyy} still in use (1) [umount of cgroup cgroup]

[ backported to 3.10:
- fixed build error: added '&' to atomic_inc_not_zero parameter;

    this was fixed upstream by commit e8c82d20a9f729cf4b9f73043f7fd4e0872bebfd ]
---
 kernel/cgroup.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

index a7c9e6d..28d150e 100644
@@ -2769,13 +2769,17 @@ 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);

@@ -2798,6 +2802,9 @@ static void cgroup_cfts_commit(struct cgroup_subsys *ss,
 		dput(cgrp->dentry);
 	}
 
+	if (sb)
+		deactivate_super(sb);
+
 	mutex_unlock(&cgroup_cft_mutex);
 }
 

1.8.0.2