drm/sched: Fix dynamic job-flow control race

Fixes a race condition reported here: #309 (comment)

The whole premise of lockless access to a single-producer-single-
consumer queue is that there is just a single producer and single
consumer.  That means we can't call drm_sched_can_queue() (which is
about queueing more work to the hw, not to the spsc queue) from
anywhere other than the consumer (wq).

This call in the producer is just an optimization to avoid scheduling
the consuming worker if it cannot yet queue more work to the hw.  It
is safe to drop this optimization to avoid the race condition.

Suggested-by: Asahi Lina <lina@asahilina.net>
Fixes: a78422e ("drm/sched: implement dynamic job-flow control")
Closes: #309
Cc: stable@vger.kernel.org
Signed-off-by: Rob Clark <robdclark@chromium.org>

Author: robclark

drivers/gpu/drm/scheduler/sched_entity.c

@@ -380,7 +380,7 @@ static void drm_sched_entity_wakeup(struct dma_fence *f,
 		container_of(cb, struct drm_sched_entity, cb);
 
 	drm_sched_entity_clear_dep(f, cb);
-	drm_sched_wakeup(entity->rq->sched, entity);
+	drm_sched_wakeup(entity->rq->sched);
 }
 
 /**
@@ -612,7 +612,7 @@ void drm_sched_entity_push_job(struct drm_sched_job *sched_job)
 		if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
 			drm_sched_rq_update_fifo(entity, submit_ts);
 
-		drm_sched_wakeup(entity->rq->sched, entity);
+		drm_sched_wakeup(entity->rq->sched);
 	}
 }
 EXPORT_SYMBOL(drm_sched_entity_push_job);

drivers/gpu/drm/scheduler/sched_main.c

@@ -1022,15 +1022,12 @@ EXPORT_SYMBOL(drm_sched_job_cleanup);
 /**
  * drm_sched_wakeup - Wake up the scheduler if it is ready to queue
  * @sched: scheduler instance
- * @entity: the scheduler entity
  *
  * Wake up the scheduler if we can queue jobs.
  */
-void drm_sched_wakeup(struct drm_gpu_scheduler *sched,
-		      struct drm_sched_entity *entity)
+void drm_sched_wakeup(struct drm_gpu_scheduler *sched)
 {
-	if (drm_sched_can_queue(sched, entity))
-		drm_sched_run_job_queue(sched);
+	drm_sched_run_job_queue(sched);
 }
 
 /**

include/drm/gpu_scheduler.h

@@ -574,7 +574,7 @@ void drm_sched_entity_modify_sched(struct drm_sched_entity *entity,
 
 void drm_sched_tdr_queue_imm(struct drm_gpu_scheduler *sched);
 void drm_sched_job_cleanup(struct drm_sched_job *job);
-void drm_sched_wakeup(struct drm_gpu_scheduler *sched, struct drm_sched_entity *entity);
+void drm_sched_wakeup(struct drm_gpu_scheduler *sched);
 bool drm_sched_wqueue_ready(struct drm_gpu_scheduler *sched);
 void drm_sched_wqueue_stop(struct drm_gpu_scheduler *sched);
 void drm_sched_wqueue_start(struct drm_gpu_scheduler *sched);