On Tue, Jul 10, 2018 at 09:12:36AM +0100, Chris Wilson wrote:
> 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
Thank you for the feedback!
- Haneen
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