[PATCH 1/2] drm/sched: add WARN_ON and BUG_ON to drm_sched_fini

[Christian König]

Tearing down the scheduler with jobs still on the pending list can
lead to use after free issues. Add a warning if drivers try to
destroy a scheduler which still has work pushed to the HW.

When there are still entities with jobs the situation is even worse
since the dma_fences for those jobs can never signal we can just
choose between potentially locking up core memory management and
random memory corruption. When drivers really mess it up that well
let them run into a BUG_ON().

Signed-off-by: Christian König <christian.koenig at amd.com>
---
 drivers/gpu/drm/scheduler/sched_main.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
index f093616fe53c..8a46fab5cdc8 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -1333,17 +1333,34 @@ void drm_sched_fini(struct drm_gpu_scheduler *sched)
 
 	drm_sched_wqueue_stop(sched);
 
+	/*
+	 * Tearing down the scheduler wile there are still unprocessed jobs can
+	 * lead to use after free issues in the scheduler fence.
+	 */
+	WARN_ON(!list_empty(&sched->pending_list));
+
 	for (i = DRM_SCHED_PRIORITY_KERNEL; i < sched->num_rqs; i++) {
 		struct drm_sched_rq *rq = sched->sched_rq[i];
 
 		spin_lock(&rq->lock);
-		list_for_each_entry(s_entity, &rq->entities, list)
+		list_for_each_entry(s_entity, &rq->entities, list) {
+			/*
+			 * The justification for this BUG_ON() is that tearing
+			 * down the scheduler while jobs are pending leaves
+			 * dma_fences unsignaled. Since we have dependencies
+			 * from the core memory management to eventually signal
+			 * dma_fences this can trivially lead to a system wide
+			 * stop because of a locked up memory management.
+			 */
+			BUG_ON(spsc_queue_count(&s_entity->job_queue));
+
 			/*
 			 * Prevents reinsertion and marks job_queue as idle,
 			 * it will removed from rq in drm_sched_entity_fini
 			 * eventually
 			 */
 			s_entity->stopped = true;
+		}
 		spin_unlock(&rq->lock);
 		kfree(sched->sched_rq[i]);
 	}
-- 
2.34.1