drm/nouveau: add reservation to nouveau_gem_ioctl_cpu_prep

Apart from some code inside ttm itself and nouveau_bo_vma_del,
this is the only place where ttm_bo_wait is used without a reservation.
Fix this so we can remove the fence_lock later on.

After the switch to rcu the reservation lock will be
removed again.
Signed-off-by: NMaarten Lankhorst <maarten.lankhorst@canonical.com>
Acked-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_gem.c

@@ -884,17 +884,31 @@ nouveau_gem_ioctl_cpu_prep(struct drm_device *dev, void *data,
 	struct drm_gem_object *gem;
 	struct nouveau_bo *nvbo;
 	bool no_wait = !!(req->flags & NOUVEAU_GEM_CPU_PREP_NOWAIT);
-	int ret = -EINVAL;
+	int ret;
+	struct nouveau_fence *fence = NULL;
 
 	gem = drm_gem_object_lookup(dev, file_priv, req->handle);
 	if (!gem)
 		return -ENOENT;
 	nvbo = nouveau_gem_object(gem);
 
+	ret = ttm_bo_reserve(&nvbo->bo, true, false, false, NULL);
+	if (!ret) {
 		spin_lock(&nvbo->bo.bdev->fence_lock);
-	ret = ttm_bo_wait(&nvbo->bo, true, true, no_wait);
+		ret = ttm_bo_wait(&nvbo->bo, true, true, true);
+		if (!no_wait && ret)
+			fence = nouveau_fence_ref(nvbo->bo.sync_obj);
 		spin_unlock(&nvbo->bo.bdev->fence_lock);
+
+		ttm_bo_unreserve(&nvbo->bo);
+	}
 	drm_gem_object_unreference_unlocked(gem);
+
+	if (fence) {
+		ret = nouveau_fence_wait(fence, true, no_wait);
+		nouveau_fence_unref(&fence);
+	}
+
 	return ret;
 }