[PATCH 1/4] drm/vkms: Add functions to map GEM backing storage

[Chris Wilson]

Quoting Haneen Mohammed (2018-07-09 16:44:26)
> +struct page **get_pages(struct vkms_gem_object *vkms_obj)
> +{
> +       struct drm_gem_object *gem_obj = &vkms_obj->gem;
> +       struct page **pages = vkms_obj->pages;
> +
> +       if (!pages) {
> +               mutex_lock(&vkms_obj->pages_lock);
> +               pages = drm_gem_get_pages(gem_obj);
> +               if (IS_ERR(pages)) {
> +                       mutex_unlock(&vkms_obj->pages_lock);
> +                       return pages;
> +               }
> +
> +               vkms_obj->pages = pages;
> +               mutex_unlock(&vkms_obj->pages_lock);

You have a race here with two callers setting pages. Trivially you fix
it by checking if (!pages) again inside the lock, but the lock is
superfluous in this case:
	if (!vkms_obj->pages) {
		srtuct pages **pages;

		pages = drm_gem_get_pages(gem_obj);
		if (IS_ERR(pages))
			return pages;
		
		if (cmpxchg(&vkms_obj->pages, NULL, pages))
			put_pages(pages);

		/* barrier() is implied */
	}

	return vkms_obj->pages;
-Chris