cgroup: fix umount vs cgroup_cfts_commit() race

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]

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org

1 files changed, 8 insertions, 1 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index c2c64005bbc2..0224f6b3103e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2798,13 +2798,17 @@ 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);
@@ -2827,6 +2831,9 @@ static void cgroup_cfts_commit(struct cgroup_subsys *ss,
                 dput(cgrp->dentry);
         }
 
+        if (sb)
+                deactivate_super(sb);
+
         mutex_unlock(&cgroup_cft_mutex);
 }