[Natty][SRU][PATCH] Kernel Oops : Dentry still in use (1) [unmount of nfs4 0:1d]

[Chris J Arges]

BugLink:  http://bugs.launchpad.net/bugs/769927

SRU Justification:

Impact:
If two processes attempt to cause automounting on the same mountpoint at
the same time, the vfsmount holding the mountpoint will be left with one
too few references on it, causing a BUG when the kernel tries to clean up.

Fix:
Fix has been cherry picked from 8aef18845266f5c05904c610088f2d1ed58f6be3t.
It has been accepted in 3.0 and is present in ubuntu-oneiric.
It applies cleanly to ubuntu-natty.

Testcase:
The procedure for replicating this bug is outlined in the patch
description. Essentially mounting an nfs mount with another mount in it.
Running a test program, and unmount the submount and original mount.

Patch is attached.
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