Document the necessary steps which needs to be done before calling
drm_sched_fini().
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() to prevent further damage.
Signed-off-by: Christian König <christian.koenig@xxxxxxx>
---
drivers/gpu/drm/scheduler/sched_main.c | 33 ++++++++++++++++++++++++--
1 file changed, 31 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
index b5e9f657d829..2082cacada69 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -1324,7 +1324,16 @@ EXPORT_SYMBOL(drm_sched_init);
*
* @sched: scheduler instance
*
- * Tears down and cleans up the scheduler.
+ * Before calling this function all entities which potentially used this
+ * scheduler instance should be forced idle using drm_sched_entity_flush() and
+ * detached from their scheduler using drm_entity_fini().
+ *
+ * Special care must be taken if drivers allocate scheduler instances
+ * dynamically. Since the dma_fence signaling doesn't guarantee any processing
+ * order of callbacks it is possible that the scheduler is still cleaning up
+ * when fences have already signaled. The easiest way to avoid that is to keep a
+ * reference from the job to the scheduler and tear down the scheduler from a
+ * work item after the last job was cleaned up.
*/
void drm_sched_fini(struct drm_gpu_scheduler *sched)
{
@@ -1333,17 +1342,37 @@ 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() 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 lead to a system wide stop
+ * because of a locked up memory management.
+ */
+ if (spsc_queue_count(&s_entity->job_queue))
+ BUG();
+ else
+ WARN();
+
/*
* Prevents reinsertion and marks job_queue as idle,
* it will be removed from the rq in drm_sched_entity_fini()
* eventually
*/
s_entity->stopped = true;
+ }
spin_unlock(&rq->lock);
kfree(sched->sched_rq[i]);
}
--
2.34.1
