vhost-scsi: Check for a cleared backend before queueing an event

We currenly hold the vhost_scsi_mutex while clearing the endpoint and
while performing vhost_scsi_do_plug, so tpg->vhost_scsi can't be freed
from uder us, and to make sure anything queued is handled by the
full call in vhost_scsi_clear_endpoint.

This patch removes the need for the vhost_scsi_mutex for the latter
case. In the next patches, we won't hold the vhost_scsi_mutex while
flushing so this patch adds a check for the clearing of the virtqueue
from vhost_scsi_clear_endpoint. We then know that once
vhost_scsi_clear_endpoint has cleared the backend that no new events
will be queued, and the flush after the vhost_vq_set_backend(vq, NULL)
call will see everything that's been queued to that point. So the flush
will then handle all events without the need for the vhost_scsi_mutex.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20230321020624.13323-6-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/scsi.c

@@ -2010,9 +2010,17 @@ vhost_scsi_do_plug(struct vhost_scsi_tpg *tpg,
 
 	vq = &vs->vqs[VHOST_SCSI_VQ_EVT].vq;
 	mutex_lock(&vq->mutex);
+	/*
+	 * We can't queue events if the backend has been cleared, because
+	 * we could end up queueing an event after the flush.
+	 */
+	if (!vhost_vq_get_backend(vq))
+		goto unlock;
+
 	if (vhost_has_feature(vq, VIRTIO_SCSI_F_HOTPLUG))
 		vhost_scsi_send_evt(vs, tpg, lun,
 				   VIRTIO_SCSI_T_TRANSPORT_RESET, reason);
+unlock:
 	mutex_unlock(&vq->mutex);
 }