From patchwork Thu Jan 9 13:37:10 2025 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Philipp Stanner X-Patchwork-Id: 13932558 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 CD532E77199 for ; Thu, 9 Jan 2025 13:37:53 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 55D5810EDD7; Thu, 9 Jan 2025 13:37:53 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=pass (2048-bit key; unprotected) header.d=kernel.org header.i=@kernel.org header.b="eutQ/m08"; 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 600FB10EDD7 for ; Thu, 9 Jan 2025 13:37:52 +0000 (UTC) Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by nyc.source.kernel.org (Postfix) with ESMTP id F403EA41C22; Thu, 9 Jan 2025 13:36:02 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C28D1C4CED2; Thu, 9 Jan 2025 13:37:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1736429871; bh=c9D36Ogvzqnzvebv/xpE/vsoiRoi3LV9aeFt1ahu8h0=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=eutQ/m08AoWt1SE7TjcEnn/nuieJV99R+aj/lc0RdZQHj52YKjLeDw7xdjnAZQPKz W2JiDGQyNhLEovVhxzm3Zk2PEzAkpIYe53rFmhF2dYewyd/TInscE+yRD3ZBHA0Qnp EIkC/BHnJg629yCmn+ZXZmtnzxJfkpfQWcrrj7FzEUyQlr0kLBx5xIPp8FhPTznCGu 3Njc2V9Du6Rw6Oh7Zi3bADioH8y+HU58SCVecPhTvdAPK0jY6CAg4FHbCUfX7IIrid H+44LPOiP1gneym68rjQ25a6WCN/noEHNjHV67z7BZYocyKswTEWDrw6h9hwM1/Qmh W5FOyEtyOU2xQ== From: Philipp Stanner To: Luben Tuikov , Matthew Brost , Danilo Krummrich , Philipp Stanner , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Sumit Semwal , =?utf-8?q?Christian_K=C3=B6nig?= Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org Subject: [PATCH 1/3] drm/sched: Document run_job() refcount hazard Date: Thu, 9 Jan 2025 14:37:10 +0100 Message-ID: <20250109133710.39404-4-phasta@kernel.org> X-Mailer: git-send-email 2.47.1 In-Reply-To: <20250109133710.39404-2-phasta@kernel.org> References: <20250109133710.39404-2-phasta@kernel.org> MIME-Version: 1.0 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" 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(). Signed-off-by: Philipp Stanner Reviewed-by: Danilo Krummrich --- drivers/gpu/drm/scheduler/sched_main.c | 10 +++++++--- include/drm/gpu_scheduler.h | 19 +++++++++++++++---- 2 files changed, 22 insertions(+), 7 deletions(-) diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c index 57da84908752..5f46c01eb01e 100644 --- a/drivers/gpu/drm/scheduler/sched_main.c +++ b/drivers/gpu/drm/scheduler/sched_main.c @@ -1218,15 +1218,19 @@ static void drm_sched_run_job_work(struct work_struct *w) drm_sched_fence_scheduled(s_fence, fence); if (!IS_ERR_OR_NULL(fence)) { - /* Drop for original kref_init of the fence */ - dma_fence_put(fence); - r = dma_fence_add_callback(fence, &sched_job->cb, drm_sched_job_done_cb); if (r == -ENOENT) drm_sched_job_done(sched_job, fence->error); else if (r) DRM_DEV_ERROR(sched->dev, "fence add callback failed (%d)\n", r); + + /* + * s_fence took a new reference to fence in the call to + * drm_sched_fence_scheduled() above. The reference passed by + * run_job() above is now not needed any longer. Drop it. + */ + dma_fence_put(fence); } else { drm_sched_job_done(sched_job, IS_ERR(fence) ? PTR_ERR(fence) : 0); diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h index 95e17504e46a..d5cd2a78f27c 100644 --- a/include/drm/gpu_scheduler.h +++ b/include/drm/gpu_scheduler.h @@ -420,10 +420,21 @@ struct drm_sched_backend_ops { struct drm_sched_entity *s_entity); /** - * @run_job: Called to execute the job once all of the dependencies - * have been resolved. This may be called multiple times, if - * timedout_job() has happened and drm_sched_job_recovery() - * decides to try it again. + * @run_job: Called to execute the job once all of the dependencies + * have been resolved. This may be called multiple times, if + * timedout_job() has happened and drm_sched_job_recovery() decides to + * try it again. + * + * @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 take a + * reference for the scheduler, and additional ones for the driver's + * respective needs. */ struct dma_fence *(*run_job)(struct drm_sched_job *sched_job);