On Mon, 30 Oct 2023 02:02:00 +0300 Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote: > SGT isn't refcounted. Once SGT pointer has been obtained, it remains the > same for both locked and unlocked get_pages_sgt(). Return cached SGT > directly without taking a potentially expensive lock. > > Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> > --- > drivers/gpu/drm/drm_gem_shmem_helper.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c > index 560ce565f376..6dd087f19ea3 100644 > --- a/drivers/gpu/drm/drm_gem_shmem_helper.c > +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c > @@ -955,6 +955,9 @@ struct sg_table *drm_gem_shmem_get_pages_sgt(struct drm_gem_shmem_object *shmem) > drm_WARN_ON(obj->dev, drm_gem_shmem_is_evictable(shmem)); > drm_WARN_ON(obj->dev, drm_gem_shmem_is_purgeable(shmem)); > > + if (shmem->sgt) > + return shmem->sgt; > + By doing that, you introduce a race between drm_gem_shmem_put_pages() and drm_gem_shmem_get_pages_sgt(). I know it's only happening if the user doesn't follow the rule listed in drm_gem_shmem_get_pages_sgt() doc (caller must owns a pages ref before calling drm_gem_shmem_get_pages_sgt()), but now you return a possibly invalid pointer instead of returning an error when this happens. If that's what we want, we should at least update the documentation to make it clear. > ret = dma_resv_lock_interruptible(shmem->base.resv, NULL); > if (ret) > return ERR_PTR(ret);
