From patchwork Fri Feb 7 12:36:51 2025 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Philipp Stanner X-Patchwork-Id: 863145 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 B808833EC; Fri, 7 Feb 2025 12:37:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738931831; cv=none; b=B44bmrax3dbfITnbhEYpdd8UcA7vsu5gf+rCN1tqfYraoPmsruDPjdrzY4zU+AR68DXy0eBYIalRLqRg5Ae8IFIZQff1AYNzZ0/THoApNIYxMgzr8gNod5zdxrTZWUskeyuxm4ADvhRpHsSc+DYrjvpdg1zZajjZ7/UZorw09cI= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738931831; c=relaxed/simple; bh=u0tYruvCsRv0U+eALTXlXOhV8iVcop1WlK5I0sT4TPs=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=qlYB/ImweVblT8v6UGg05mVAP2tQz1Mww1UFhoICyUlCWqX8hsStCj8Mu7KmEkyFD+8Nd9rUHwcXqtysa6loxIwbs9l2qxNb4kwZULam3tClBDR2x9oR8H4HfG447xXKaYK8rnzfWW+V4hV6tulQD36lCWscJRa7vgn0UO3WjF0= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=jvWYGipA; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="jvWYGipA" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0A2FAC4CEE6; Fri, 7 Feb 2025 12:37:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1738931831; bh=u0tYruvCsRv0U+eALTXlXOhV8iVcop1WlK5I0sT4TPs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=jvWYGipAe95a+8wY7gJDThP0W3nUXZH4yqglS9bmAkqL6fymQVYLoBg2X0It+0/Q5 8WUSJ1wPiJKChWBBtdf02ELMfVOnFbRXfM8GFDofTsdoQiL4BFk+BUuxxUANWgCgFA 1Y3P2+hSNTf8gDWjX1R0uXnnGl2r/XETKQ1FA1SIaWYv+uPiUUgUAHrEMRsWanBdVY PNjO8w7NqdZz4SCYgovUuv9wR1AI+xGXV26RrUHR4ZLlnroTaIzzF9JWfdtNB6FssQ pl6jvrqS0/jmt8cxP2JSaKEWvBF1vykxZbfscN8pfIAdAXrRxf5RrOGF09s6f8v8Wz 5PhoEabGEorcA== From: Philipp Stanner To: Matthew Brost , Danilo Krummrich , Philipp Stanner , =?utf-8?q?Christian_K=C3=B6nig?= , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Sumit Semwal Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org, Philipp Stanner Subject: [PATCH v3 1/3] drm/sched: Document run_job() refcount hazard Date: Fri, 7 Feb 2025 13:36:51 +0100 Message-ID: <20250207123652.27677-3-phasta@kernel.org> X-Mailer: git-send-email 2.47.1 In-Reply-To: <20250207123652.27677-2-phasta@kernel.org> References: <20250207123652.27677-2-phasta@kernel.org> Precedence: bulk X-Mailing-List: linux-media@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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. 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(). Suggested-by: Danilo Krummrich Signed-off-by: Philipp Stanner Reviewed-by: Danilo Krummrich --- drivers/gpu/drm/scheduler/sched_main.c | 5 ++--- include/drm/gpu_scheduler.h | 19 +++++++++++++++---- 2 files changed, 17 insertions(+), 7 deletions(-) diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c index a48be16ab84f..c80712453bad 100644 --- a/drivers/gpu/drm/scheduler/sched_main.c +++ b/drivers/gpu/drm/scheduler/sched_main.c @@ -1222,15 +1222,14 @@ 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); + + 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 a0ff08123f07..38d2053528c8 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);