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 4298CE77188 for ; Fri, 20 Dec 2024 13:18:30 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id B0BCB10E3B5; Fri, 20 Dec 2024 13:18:29 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=pass (2048-bit key; unprotected) header.d=kernel.org header.i=@kernel.org header.b="QZHwdAFA"; dkim-atps=neutral Received: from nyc.source.kernel.org (nyc.source.kernel.org [147.75.193.91]) by gabe.freedesktop.org (Postfix) with ESMTPS id 04B9110E3B5 for ; Fri, 20 Dec 2024 13:18:28 +0000 (UTC) Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by nyc.source.kernel.org (Postfix) with ESMTP id 752EDA40AA1; Fri, 20 Dec 2024 13:16:36 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 17982C4CECD; Fri, 20 Dec 2024 13:18:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1734700706; bh=lJYxREEplA7hMrw7hlbqIoxsY2oAvj3/FsClJRaiW1Y=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=QZHwdAFABn6Bf5wplvexw48kKzjX7VT/s8VBEJv6exj79gwKvLWID4WKiPYevMWKG /oPc7Cig9XvgWaJkwmicammdH/dqo7szcp+jOypNk78O3p9DotFG0uypxE/Jjzoojt MQQ29enyauowu9R0Gosq5OJLjAZ1n0aZSCDtJEowm7L/1+GgNyfarjwsPNOLLf8mqZ tbPV7ziDdubzLjTlGNgFcACQ9u+V3vnifeP7uG2jOOxSDWEGhSZ6PCfnDFU5/3rRBm knSRnxNlhQxdr9YVOGwmdpIjxRg9gGjiUI0Tx4aOtZ6BmDPpGbWnLfR8vixyv9V5aI JfL59O8qSgocA== Date: Fri, 20 Dec 2024 14:18:20 +0100 From: Danilo Krummrich To: Christian =?iso-8859-1?Q?K=F6nig?= Cc: Philipp Stanner , Luben Tuikov , Matthew Brost , Philipp Stanner , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Sumit Semwal , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org, Tvrtko Ursulin , Andrey Grodzovsky Subject: Re: [PATCH] drm/sched: Document run_job() refcount hazard Message-ID: References: <20241220124515.93169-2-phasta@kernel.org> <5c4c610e-26ec-447c-b4db-ad38e994720b@amd.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <5c4c610e-26ec-447c-b4db-ad38e994720b@amd.com> 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 Fri, Dec 20, 2024 at 01:53:34PM +0100, Christian König wrote: > Am 20.12.24 um 13:45 schrieb Philipp Stanner: > > From: Philipp Stanner > > > > drm_sched_backend_ops.run_job() returns a dma_fence for the scheduler. > > That fence is signalled by the driver once the hardware completed the > > associated job. The scheduler does not increment the reference count on > > that fence, but implicitly expects to inherit this fence from run_job(). > > > > This is relatively subtle and prone to misunderstandings. > > > > This implies that, to keep a reference for itself, a driver needs to > > call dma_fence_get() in addition to dma_fence_init() in that callback. > > > > It's further complicated by the fact that the scheduler even decrements > > the refcount in drm_sched_run_job_work() since it created a new > > reference in drm_sched_fence_scheduled(). It does, however, still use > > its pointer to the fence after calling dma_fence_put() - which is safe > > because of the aforementioned new reference, but actually still violates > > the refcounting rules. > > > > Improve the explanatory comment for that decrement. > > > > Move the call to dma_fence_put() to the position behind the last usage > > of the fence. > > > > Document the necessity to increment the reference count in > > drm_sched_backend_ops.run_job(). > > > > Cc: Christian König > > Cc: Tvrtko Ursulin > > Cc: Andrey Grodzovsky > > Signed-off-by: Philipp Stanner > > --- > > drivers/gpu/drm/scheduler/sched_main.c | 10 +++++++--- > > include/drm/gpu_scheduler.h | 20 ++++++++++++++++---- > > 2 files changed, 23 insertions(+), 7 deletions(-) > > > > diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c > > index 7ce25281c74c..d6f8df39d848 100644 > > --- a/drivers/gpu/drm/scheduler/sched_main.c > > +++ b/drivers/gpu/drm/scheduler/sched_main.c > > + * > > + * @sched_job: the job to run > > + * > > + * Returns: dma_fence the driver must signal once the hardware has > > + * completed the job ("hardware fence"). > > + * > > + * Note that the scheduler expects to 'inherit' its own reference to > > + * this fence from the callback. It does not invoke an extra > > + * dma_fence_get() on it. Consequently, this callback must return a > > + * fence whose refcount is at least 2: One for the scheduler's > > + * reference returned here, another one for the reference kept by the > > + * driver. > > Well the driver actually doesn't need any extra reference. The scheduler > just needs to guarantee that this reference isn't dropped before it is > signaled. I think he means the reference the driver's fence context has to have in order to signal that thing eventually. > > Regards, > Christian. > > > */ > > struct dma_fence *(*run_job)(struct drm_sched_job *sched_job); >