drm/virtio: add fence sanity check

Make sure we don't leak half-initialized fences outside the driver.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NChia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190829103301.3539-19-kraxel@redhat.com

drivers/gpu/drm/virtio/virtgpu_fence.c

@@ -41,6 +41,10 @@ bool virtio_fence_signaled(struct dma_fence *f)
 {
 	struct virtio_gpu_fence *fence = to_virtio_fence(f);
 
+	if (WARN_ON_ONCE(fence->f.seqno == 0))
+		/* leaked fence outside driver before completing
+		 * initialization with virtio_gpu_fence_emit */
+		return false;
 	if (atomic64_read(&fence->drv->last_seq) >= fence->f.seqno)
 		return true;
 	return false;