From: Li Zefan <lizefan@huawei.com>
Date: Tue, 18 Jun 2013 10:40:19 +0000 (+0800)
Subject: cgroup: fix umount vs cgroup_cfts_commit() race
X-Git-Tag: omap-for-v3.11/fixes-for-merge-window~44^2~22
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=084457f284abf6789d90509ee11dae383842b23b;p=pandora-kernel.git

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
---

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