From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp1.osuosl.org (smtp1.osuosl.org [140.211.166.138]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6690114AAA for ; Mon, 13 Nov 2023 09:50:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.b="PZPUaFlP" Received: from localhost (localhost [127.0.0.1]) by smtp1.osuosl.org (Postfix) with ESMTP id B317981763 for ; Mon, 13 Nov 2023 09:50:04 +0000 (UTC) DKIM-Filter: OpenDKIM Filter v2.11.0 smtp1.osuosl.org B317981763 Authentication-Results: smtp1.osuosl.org; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.a=rsa-sha256 header.s=mail header.b=PZPUaFlP X-Virus-Scanned: amavisd-new at osuosl.org X-Spam-Flag: NO X-Spam-Score: -2.1 X-Spam-Level: Received: from smtp1.osuosl.org ([127.0.0.1]) by localhost (smtp1.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 8zeD50MgVM38 for ; Mon, 13 Nov 2023 09:50:04 +0000 (UTC) Received: from madras.collabora.co.uk (madras.collabora.co.uk [46.235.227.172]) by smtp1.osuosl.org (Postfix) with ESMTPS id E2244814B8 for ; Mon, 13 Nov 2023 09:50:03 +0000 (UTC) DKIM-Filter: OpenDKIM Filter v2.11.0 smtp1.osuosl.org E2244814B8 Received: from localhost (cola.collaboradmins.com [195.201.22.229]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: bbrezillon) by madras.collabora.co.uk (Postfix) with ESMTPSA id 599A966071C9; Mon, 13 Nov 2023 09:50:00 +0000 (GMT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1699869001; bh=wCfkXVzFh9+4/eq4jvC4jcqKh2QCYCyEjmq0UU8BsDw=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=PZPUaFlPSKPp3o6zruu/QciXsc7IUJUUWsGdEFqHyBk79f3gt7ldiZCkGCTcQByFL URjfigZn911fDQha95PLuUss6T6/Nq6Xa9150A0aij5ByTPs4+LWV81CMmWU0pKdDN ub2bKS2aO6rbcVIlOedm1FilPSCI6wWT5q5AUiGSD9k6OOhl+C3HuLMsZKiKt/No7C 2D3u/BqrowOvvKcxvDzyDx4nAIVz/YaEAHhlXosdQ2IYxlCsNpl+I0CBYFmnZ+nTiT Zf/gVe1drWYIyqbxIQXJBAaS/LrJL6Qiu4njP416DMYHN6h9C5laL8f+xy9uHeZDKs XC5cvNPCqPXpg== Date: Mon, 13 Nov 2023 10:49:56 +0100 From: Boris Brezillon To: Dmitry Osipenko Cc: David Airlie , Gerd Hoffmann , Gurchetan Singh , Chia-I Wu , Daniel Vetter , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Christian =?UTF-8?B?S8O2bmln?= , Qiang Yu , Steven Price , Emma Anholt , Melissa Wen , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, kernel@collabora.com, virtualization@lists.linux-foundation.org Subject: Re: [PATCH v18 21/26] drm/shmem-helper: Optimize unlocked get_pages_sgt() Message-ID: <20231113104956.2255269a@collabora.com> In-Reply-To: <20231029230205.93277-22-dmitry.osipenko@collabora.com> References: <20231029230205.93277-1-dmitry.osipenko@collabora.com> <20231029230205.93277-22-dmitry.osipenko@collabora.com> Organization: Collabora X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; x86_64-redhat-linux-gnu) Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 30 Oct 2023 02:02:00 +0300 Dmitry Osipenko 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 > --- > 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);