xen/gntdevt: Fix race condition in gntdev_release()
xen/gntdevt: Fix race condition in gntdev_release()

While gntdev_release() is called the MMU notifier is still registered
and can traverse priv->maps list even if no pages are mapped (which is
the case -- gntdev_release() is called after all). But
gntdev_release() will clear that list, so make sure that only one of
those things happens at the same time.

Signed-off-by: Marek Marczykowski-Górecki <b37867d4e08020cf5bb17f05a2c036fbc82d5481@invisiblethingslab.com>
Cc: <4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org>
Signed-off-by: David Vrabel <b9d45bd1f671e508cd8daa63dfc3cabb597562df@citrix.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs