drm/gem-vram: Fix deadlock in drm_gem_vram_vmap()

Recently DRM framebuffer core and all drivers were moved to unlocked
vmapping functions that take the reservation lock. The drm_gem_vram_vmap()
was missed out by accident and now deadlocks drivers that use drm_gem_vram
helpers when framebuffer is updated, like Bochs driver. Remove the locking
from drm_gem_vram_v[un]map() functions to fix the deadlock.
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: 79e2cf2e ("drm/gem: Take reservation lock for vmap/vunmap operations")
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221113233850.427704-1-dmitry.osipenko@collabora.com

drivers/gpu/drm/drm_gem_vram_helper.c

@@ -433,25 +433,19 @@ int drm_gem_vram_vmap(struct drm_gem_vram_object *gbo, struct iosys_map *map)
 {
 	int ret;
 
-	ret = ttm_bo_reserve(&gbo->bo, true, false, NULL);
-	if (ret)
-		return ret;
+	dma_resv_assert_held(gbo->bo.base.resv);
 
 	ret = drm_gem_vram_pin_locked(gbo, 0);
 	if (ret)
-		goto err_ttm_bo_unreserve;
+		return ret;
 	ret = drm_gem_vram_kmap_locked(gbo, map);
 	if (ret)
 		goto err_drm_gem_vram_unpin_locked;
 
-	ttm_bo_unreserve(&gbo->bo);
-
 	return 0;
 
 err_drm_gem_vram_unpin_locked:
 	drm_gem_vram_unpin_locked(gbo);
-err_ttm_bo_unreserve:
-	ttm_bo_unreserve(&gbo->bo);
 	return ret;
 }
 EXPORT_SYMBOL(drm_gem_vram_vmap);
@@ -467,16 +461,10 @@ EXPORT_SYMBOL(drm_gem_vram_vmap);
 void drm_gem_vram_vunmap(struct drm_gem_vram_object *gbo,
 			 struct iosys_map *map)
 {
-	int ret;
-
-	ret = ttm_bo_reserve(&gbo->bo, false, false, NULL);
-	if (WARN_ONCE(ret, "ttm_bo_reserve_failed(): ret=%d\n", ret))
-		return;
+	dma_resv_assert_held(gbo->bo.base.resv);
 
 	drm_gem_vram_kunmap_locked(gbo, map);
 	drm_gem_vram_unpin_locked(gbo);
-
-	ttm_bo_unreserve(&gbo->bo);
 }
 EXPORT_SYMBOL(drm_gem_vram_vunmap);