cgroup: fix umount vs cgroup_cfts_commit() race
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]

Signed-off-by: Li Zefan <a8167b9d27aca9c6624754deb52c4c6420eded04@huawei.com>
Signed-off-by: Tejun Heo <546b05909706652891a87f7bfe385ae147f61f91@kernel.org>
Signed-off-by: Greg Kroah-Hartman <4645f7897fd33786a2ee1264d590b3c400559d85@linuxfoundation.org>


TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas