Re: [PATCH v18 21/26] drm/shmem-helper: Optimize unlocked get_pages_sgt()
From: Boris Brezillon <boris.brezillon@collabora.com>
Date: 2023-11-13 09:50:09
Also in:
dri-devel, lkml
On Mon, 30 Oct 2023 02:02:00 +0300 Dmitry Osipenko [off-list ref] wrote:
quoted hunk
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@collabora.com> --- 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);