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[165.204.72.6]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49e847ecf52sm51623765e9.2.2026.09.16.05.33.05 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Wed, 16 Sep 2026 05:33:06 -0700 (PDT) Message-ID: <14ad8ea7-c65c-48b0-94c5-bc881d7aaddd@gmail.com> Date: Wed, 16 Sep 2026 14:33:05 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 3/3] drm/sched: Protect entity->last_scheduled with spinlock To: Philipp Stanner , Danilo Krummrich , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Tvrtko Ursulin Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org References: <20260910075942.2000338-2-phasta@kernel.org> <20260910075942.2000338-5-phasta@kernel.org> Content-Language: en-US From: =?UTF-8?Q?Christian_K=C3=B6nig?= In-Reply-To: <20260910075942.2000338-5-phasta@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 9/10/26 09:59, Philipp Stanner wrote: > 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 | 50 ++++++++++-------------- > include/drm/gpu_scheduler.h | 10 ++--- > 2 files changed, 24 insertions(+), 36 deletions(-) > > diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c > index e168f445f2ab..fa4a9388e5c9 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 +318,10 @@ 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); > + spin_lock(&entity->lock); > + prev = entity->last_scheduled; > dma_fence_get(prev); > + spin_unlock(&entity->lock); > while ((job = drm_sched_entity_queue_pop(entity))) { > struct drm_sched_fence *s_fence = job->s_fence; > > @@ -413,8 +413,7 @@ 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 +535,10 @@ 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 +555,15 @@ 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 +587,15 @@ 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..aeeea6efc623 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). > */ > 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.