Re: [PATCH v16 05/20] drm/v3d: Replace open-coded drm_gem_shmem_free() with drm_gem_object_put()

[Boris Brezillon <boris.brezillon@collabora.com>]

On Sun,  3 Sep 2023 20:07:21 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> The drm_gem_shmem_free() doesn't put GEM's kref to zero, which becomes
> important with addition of the shrinker support to drm-shmem that will
> use kref=0 in order to prevent taking lock during special GEM-freeing
> time in order to avoid spurious lockdep warning about locking ordering
> vs fs_reclaim code paths.

As for patch 4, I don't mind the change (I actually think it's better
to have a single error path with a free function handling partially
initialized GEM objects, which is exactly what your patch is doing), but
I disagree with the explanation, especially after you dropped
"drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper"
and went for a drm_gem_shmem_do_release_pages_locked() helper called in
drm_gem_shmem_free().

> 
> Replace open-coded drm_gem_shmem_free() with drm_gem_object_put() that
> drops kref to zero before freeing GEM.
> 
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> ---
>  drivers/gpu/drm/v3d/v3d_bo.c | 22 ++++++++++++----------
>  1 file changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/v3d/v3d_bo.c b/drivers/gpu/drm/v3d/v3d_bo.c
> index 8b3229a37c6d..70c1095d6eec 100644
> --- a/drivers/gpu/drm/v3d/v3d_bo.c
> +++ b/drivers/gpu/drm/v3d/v3d_bo.c
> @@ -33,16 +33,18 @@ void v3d_free_object(struct drm_gem_object *obj)
>  	struct v3d_dev *v3d = to_v3d_dev(obj->dev);
>  	struct v3d_bo *bo = to_v3d_bo(obj);
>  
> -	v3d_mmu_remove_ptes(bo);
> +	if (drm_mm_node_allocated(&bo->node)) {
> +		v3d_mmu_remove_ptes(bo);
>  
> -	mutex_lock(&v3d->bo_lock);
> -	v3d->bo_stats.num_allocated--;
> -	v3d->bo_stats.pages_allocated -= obj->size >> PAGE_SHIFT;
> -	mutex_unlock(&v3d->bo_lock);
> +		mutex_lock(&v3d->bo_lock);
> +		v3d->bo_stats.num_allocated--;
> +		v3d->bo_stats.pages_allocated -= obj->size >> PAGE_SHIFT;
> +		mutex_unlock(&v3d->bo_lock);
>  
> -	spin_lock(&v3d->mm_lock);
> -	drm_mm_remove_node(&bo->node);
> -	spin_unlock(&v3d->mm_lock);
> +		spin_lock(&v3d->mm_lock);
> +		drm_mm_remove_node(&bo->node);
> +		spin_unlock(&v3d->mm_lock);
> +	}
>  
>  	/* GPU execution may have dirtied any pages in the BO. */
>  	bo->base.pages_mark_dirty_on_put = true;
> @@ -142,7 +144,7 @@ struct v3d_bo *v3d_bo_create(struct drm_device *dev, struct drm_file *file_priv,
>  	return bo;
>  
>  free_obj:
> -	drm_gem_shmem_free(shmem_obj);
> +	drm_gem_object_put(&shmem_obj->base);
>  	return ERR_PTR(ret);
>  }
>  
> @@ -160,7 +162,7 @@ v3d_prime_import_sg_table(struct drm_device *dev,
>  
>  	ret = v3d_bo_create_finish(obj);
>  	if (ret) {
> -		drm_gem_shmem_free(&to_v3d_bo(obj)->base);
> +		drm_gem_object_put(obj);
>  		return ERR_PTR(ret);
>  	}
>