virtio/s390: avoid race on vcdev->config

virtio/s390: avoid race on vcdev->config

BugLink: https://bugs.launchpad.net/bugs/1788432
Currently we have a race on vcdev->config in virtio_ccw_get_config() and
in virtio_ccw_set_config().

This normally does not cause problems, as these are usually infrequent
operations. However, for some devices writing to/reading from the config
space can be triggered through sysfs attributes. For these, userspace can
force the race by increasing the frequency.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Cc: stable@vger.kernel.org
Message-Id: <20180925121309.58524-2-pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 2448a299ec416a80f699940a86f4a6d9a4f643b1)
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Khalid Elmously <khalid.elmously@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>