From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7E32DF417F1 for ; Mon, 9 Mar 2026 14:56:26 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id CCF8E10E18E; Mon, 9 Mar 2026 14:56:25 +0000 (UTC) Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by gabe.freedesktop.org (Postfix) with ESMTP id 0626E10E18E for ; Mon, 9 Mar 2026 14:56:23 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4DE5D1570 for ; Mon, 9 Mar 2026 07:56:17 -0700 (PDT) Received: from [192.168.0.1] (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 55FD73F7BD for ; Mon, 9 Mar 2026 07:56:23 -0700 (PDT) Date: Mon, 9 Mar 2026 14:54:21 +0000 From: Liviu Dudau To: Boris Brezillon Cc: Steven Price , =?utf-8?Q?Adri=C3=A1n?= Larumbe , dri-devel@lists.freedesktop.org, kernel@collabora.com, Nicolas Frattaroli , Tvrtko Ursulin , Philipp Stanner , Christian =?utf-8?B?S8O2bmln?= Subject: Re: [PATCH] drm/panthor: Fix the "done_fence is initialized" detection logic Message-ID: References: <20260309103053.211415-1-boris.brezillon@collabora.com> <20260309141549.3b254c46@fedora> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260309141549.3b254c46@fedora> X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Mon, Mar 09, 2026 at 02:15:49PM +0100, Boris Brezillon wrote: > On Mon, 9 Mar 2026 11:05:06 +0000 > Liviu Dudau wrote: > > > > After commit 541c8f2468b9 ("dma-buf: detach fence ops on signal v3"), > > > dma_fence::ops == NULL can't be used to check if the fence is initialized > > > or not. We could turn this into an "is_signaled() || ops == NULL" test, > > > but that's fragile, since it's still subject to dma_fence internal > > > changes. So let's have the "is_initialized" state encoded directly in > > > the pointer through the lowest bit which is guaranteed to be unused > > > because of the dma_fence alignment constraint. > > > > I'm confused! There is only one place where we end up being interested if the > > fence has been initialized or not, and that is in job_release(). I don't > > see why checking for "ops != NULL" before calling dma_fence_put() should not > > be enough, > > Because after 541c8f2468b9 ("dma-buf: detach fence ops on signal v3"), > dma_fence->ops is set back to NULL at signal time[1]. Yes, I gathered that. What I meant to say was that I don't understand why we need all this infrastructure just for one check. Meanwhile Christian pointed out that a simpler solution already exists. > > > or even better, why don't we call dma_fence_put() regardless, > > as the core code should take care of an uninitialized dma_fence AFAICT. > > When the job is created, we pre-allocate the done_fence, but we leave it > uninitialized until ::run_job() is called. If we call > dma_fence_release() (through dma_fence_put()) on a dma_fence that was > not dma_fence_init()-ialized, we have a NULL deref on the cb_list, and > probably other issues too. I don't see the benefit of not initializing the done_fence until we ::run_job() but I might have missed something obvious. If we want to keep that, maybe we should not be droping the reference in job_release() but when we signal the fence. But that would leak the memory of the uninitialized done_fence. Best regards, Liviu > > [1]https://gitlab.freedesktop.org/drm/misc/kernel/-/blob/drm-misc-next/drivers/dma-buf/dma-fence.c?ref_type=heads#L373 -- ==================== | I would like to | | fix the world, | | but they're not | | giving me the | \ source code! / --------------- ¯\_(ツ)_/¯