From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Thu, 13 Feb 2014 11:45:11 +0200

vhost_zerocopy_callback accesses VQ right after it drops a ubuf
reference.  In theory, this could race with device removal which waits
on the ubuf kref, and crash on use after free.

Do all accesses within rcu read side critical section, and synchronize
on release.

Since callbacks are always invoked from bh, synchronize_rcu_bh seems
enough and will help release complete a bit faster.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

This is was previously posted as part of patch
series, but it's an independent fix really.
Theoretical race so not needed for stable I think.

Ok, no -stable, applied.