From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 32781388E6F; Tue, 22 Sep 2026 14:55:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790088922; cv=none; b=Olp/DHhvk5Uh5R6gzZnPMelLokfhPJn8TSWTKn8evyqeVyPlYLVcsz8hFRKfXMV1QdJSYikO1TvO29n7hmwSWA9phDo2o4fLxWfyOfyr6yKIdmd6qQUbQxycVB+BNGlbCVIOhz+f1FnpD2vLOkLBMzB22hbNpYNjjf6+1tPfYz0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790088922; c=relaxed/simple; bh=fWh68O1VVT5hX4kN1vo8CuAlKyBjytQkL5UJjhZ0vk4=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version:Content-Type; b=mBAO/sRRMORzUvlmjJXRBXwhcUKAf8mrFCeuUGiF/PgZGcqDEeGwKrLCg59+c2BujnoeledHAZTr4xDKpezn4WbAYJXl6mZpN+MqJ9mk/+WfqEdJjSuC+U7VteVtt/vpt1MRagX3VmW1JjDNPsJ3RBX3T+JM2Lt7wInM2MRr0IY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=jc58LaCq; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="jc58LaCq" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3BA201F000FF; Tue, 22 Sep 2026 14:55:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790088920; bh=btsAOjmvEG3UfY/0+Dti8pyBnrPe0Fbr1sdzHKjTxq4=; h=From:To:Cc:Subject:Date; b=jc58LaCq7r7ygQXeWHpde/AFMkBFU2GtjMxgvNwS8dv9KoeI0J2lA4lHHadEkl7wu GVtXLfyxtANTFZ42lIzXEe9YktYWrP2MjYtBewLpq5uwy2dhcTddxStRcq0TsFwq7y OQgoBhqrDSvpNaOP1CkS4Jis96nfh8fS3dje8qSco1vp8rq+n8zxlObIcpEs7Hk0qh EDr5wvhKSGVAFEcE36TZXWJpAhqGWUM8/unhcMr2rr0n59ZmcJlXdF4cxCW1Y8bCIp +pzO9r4PJJZIvOJhMU2ukFBN+0JLKDVk19JK3SACYcWvfzTajlqq+CFT49InPSX84Q fOq2y4mU1xDeQ== From: Philipp Stanner To: Matthew Brost , Danilo Krummrich , Philipp Stanner , =?UTF-8?q?Christian=20K=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, =?UTF-8?q?Christian=20K=C3=B6nig?= Subject: [PATCH v4] drm/sched: Protect entity->last_scheduled with spinlock Date: Tue, 22 Sep 2026 16:54:42 +0200 Message-ID: <20260922145441.619097-2-phasta@kernel.org> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The entity->last_scheduled field has always been set and read with special RCU functions in addition to memory barriers. This was added in commit 70102d77ff22 ("drm/scheduler: add drm_sched_entity_error and use rcu for last_scheduled") however, no proper justification for that mechanism was provided. There seems to be no obvious reason, since the entity lock is available and taken at all places that evaluate the last_scheduled field. The only exception is drm_sched_entity_error(), which is not performance critical in any way. Improve robustness, readability and maintainability by replacing RCU and barriers with the lock. Signed-off-by: Philipp Stanner Acked-by: Christian König --- drivers/gpu/drm/scheduler/sched_entity.c | 51 ++++++++++-------------- include/drm/gpu_scheduler.h | 10 ++--- 2 files changed, 24 insertions(+), 37 deletions(-) diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c index e4069fcb0272..7be0387bac3c 100644 --- a/drivers/gpu/drm/scheduler/sched_entity.c +++ b/drivers/gpu/drm/scheduler/sched_entity.c @@ -136,7 +136,6 @@ int drm_sched_entity_init(struct drm_sched_entity *entity, DRM_SCHED_PRIORITY_KERNEL : priority; entity->num_sched_list = num_sched_list; entity->sched_list = num_sched_list > 1 ? sched_list : NULL; - RCU_INIT_POINTER(entity->last_scheduled, NULL); RB_CLEAR_NODE(&entity->rb_tree_node); if (!sched_list[0]->sched_rq) { @@ -233,10 +232,10 @@ int drm_sched_entity_error(struct drm_sched_entity *entity) struct dma_fence *fence; int r; - rcu_read_lock(); - fence = rcu_dereference(entity->last_scheduled); + spin_lock(&entity->lock); + fence = entity->last_scheduled; r = fence ? fence->error : 0; - rcu_read_unlock(); + spin_unlock(&entity->lock); return r; } @@ -319,9 +318,9 @@ void drm_sched_entity_kill(struct drm_sched_entity *entity) /* Make sure this entity is not used by the scheduler at the moment */ wait_for_completion(&entity->entity_idle); - /* The entity is guaranteed to not be used by the scheduler */ - prev = rcu_dereference_check(entity->last_scheduled, true); - dma_fence_get(prev); + spin_lock(&entity->lock); + prev = dma_fence_get(entity->last_scheduled); + spin_unlock(&entity->lock); while ((job = drm_sched_entity_queue_pop(entity))) { struct drm_sched_fence *s_fence = job->s_fence; @@ -413,8 +412,7 @@ void drm_sched_entity_fini(struct drm_sched_entity *entity) entity->dependency = NULL; } - dma_fence_put(rcu_dereference_check(entity->last_scheduled, true)); - RCU_INIT_POINTER(entity->last_scheduled, NULL); + dma_fence_put(entity->last_scheduled); drm_sched_entity_stats_put(entity->stats); } EXPORT_SYMBOL(drm_sched_entity_fini); @@ -536,6 +534,10 @@ drm_sched_job_dependency(struct drm_sched_job *job, struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity) { + /* Helper to avoid dropping the reference while the entity lock is held, + * just to have some more robustness. + */ + struct dma_fence *prev_last_scheduled; struct drm_sched_job *sched_job; sched_job = drm_sched_entity_queue_peek(entity); @@ -552,22 +554,15 @@ struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity) if (entity->guilty && atomic_read(entity->guilty)) dma_fence_set_error(&sched_job->s_fence->finished, -ECANCELED); - dma_fence_put(rcu_dereference_check(entity->last_scheduled, true)); - rcu_assign_pointer(entity->last_scheduled, - dma_fence_get(&sched_job->s_fence->finished)); - - /* - * If the queue is empty we allow drm_sched_entity_select_rq() to - * locklessly access ->last_scheduled. This only works if we set the - * pointer before we dequeue and if we a write barrier here. - */ - smp_wmb(); - spin_lock(&entity->lock); + prev_last_scheduled = entity->last_scheduled; + entity->last_scheduled = dma_fence_get(&sched_job->s_fence->finished); spsc_queue_pop(&entity->job_queue); drm_sched_rq_pop_entity(entity); spin_unlock(&entity->lock); + dma_fence_put(prev_last_scheduled); + /* Jobs and entities might have different lifecycles. Since we're * removing the job from the entities queue, set the jobs entity pointer * to NULL to prevent any future access of the entity through this job. @@ -591,21 +586,15 @@ void drm_sched_entity_select_rq(struct drm_sched_entity *entity) if (spsc_queue_count(&entity->job_queue)) return; - /* - * Only when the queue is empty are we guaranteed that - * drm_sched_run_job_work() cannot change entity->last_scheduled. To - * enforce ordering we need a read barrier here. See - * drm_sched_entity_pop_job() for the other side. - */ - smp_rmb(); - - fence = rcu_dereference_check(entity->last_scheduled, true); + spin_lock(&entity->lock); + fence = entity->last_scheduled; /* stay on the same engine if the previous job hasn't finished */ - if (fence && !dma_fence_is_signaled(fence)) + if (fence && !dma_fence_is_signaled(fence)) { + spin_unlock(&entity->lock); return; + } - spin_lock(&entity->lock); sched = drm_sched_pick_best(entity->sched_list, entity->num_sched_list); rq = sched ? sched->sched_rq[entity->rq_priority] : NULL; if (rq != entity->rq) { diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h index 7a64cc11de08..563d7fb88a6d 100644 --- a/include/drm/gpu_scheduler.h +++ b/include/drm/gpu_scheduler.h @@ -100,8 +100,8 @@ struct drm_sched_entity { * @lock: * * Lock protecting the run-queue (@rq) to which this entity belongs, - * @priority, the list of schedulers (@sched_list, @num_sched_list) and - * the @rr_ts field. + * @priority, @last_scheduled and the list of schedulers (@sched_list, + * @num_sched_list) and @rr_ts. */ spinlock_t lock; @@ -215,11 +215,9 @@ struct drm_sched_entity { /** * @last_scheduled: * - * Points to the finished fence of the last scheduled job. Only written - * by drm_sched_entity_pop_job(). Can be accessed locklessly from - * drm_sched_job_arm() if the queue is empty. + * Points to the finished fence of the last scheduled job. */ - struct dma_fence __rcu *last_scheduled; + struct dma_fence *last_scheduled; /** * @last_user: last group leader pushing a job into the entity. base-commit: bc3f07516a99280d5c5a34b358ee92a30a653cca -- 2.55.0