Re: [PATCH v2] drm/scheduler: Fix hang when sched_entity released

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Am 18.02.21 um 16:05 schrieb Andrey Grodzovsky:

On 2/18/21 3:07 AM, Christian König wrote:


Am 17.02.21 um 22:59 schrieb Andrey Grodzovsky:
Problem: If scheduler is already stopped by the time sched_entity
is released and entity's job_queue not empty I encountred
a hang in drm_sched_entity_flush. This is because drm_sched_entity_is_idle
never becomes false.

Fix: In drm_sched_fini detach all sched_entities from the
scheduler's run queues. This will satisfy drm_sched_entity_is_idle.
Also wakeup all those processes stuck in sched_entity flushing
as the scheduler main thread which wakes them up is stopped by now.

v2:
Reverse order of drm_sched_rq_remove_entity and marking
s_entity as stopped to prevent reinserion back to rq due
to race.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@xxxxxxx>
---
  drivers/gpu/drm/scheduler/sched_main.c | 31 +++++++++++++++++++++++++++++++
  1 file changed, 31 insertions(+)

diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
index 908b0b5..c6b7947 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -897,9 +897,40 @@ EXPORT_SYMBOL(drm_sched_init);
   */
  void drm_sched_fini(struct drm_gpu_scheduler *sched)
  {
+    int i;
+    struct drm_sched_entity *s_entity;

BTW: Please order that so that i is declared last.

      if (sched->thread)
          kthread_stop(sched->thread);
  +    /* Detach all sched_entites from this scheduler once it's stopped */ +    for (i = DRM_SCHED_PRIORITY_COUNT - 1; i >= DRM_SCHED_PRIORITY_MIN; i--) {
+        struct drm_sched_rq *rq = &sched->sched_rq[i];
+
+        if (!rq)
+            continue;
+
+        /* Loop this way because rq->lock is taken in drm_sched_rq_remove_entity */
+        spin_lock(&rq->lock);
+        while ((s_entity = list_first_entry_or_null(&rq->entities,
+                                struct drm_sched_entity,
+                                list))) {
+            spin_unlock(&rq->lock);
+
+            /* Prevent reinsertion and remove */
+            spin_lock(&s_entity->rq_lock);
+            s_entity->stopped = true;
+            drm_sched_rq_remove_entity(rq, s_entity);
+            spin_unlock(&s_entity->rq_lock);

Well this spin_unlock/lock dance here doesn't look correct at all now.

Christian.


In what way ? It's in the same same order as in other call sites (see drm_sched_entity_push_job and drm_sched_entity_flush). If i just locked rq->lock and did list_for_each_entry_safe while manually deleting entity->list instead of calling drm_sched_rq_remove_entity this still would not be possible as the order of lock acquisition between  s_entity->rq_lock
and rq->lock would be reverse compared to the call sites mentioned above.

Ah, now I understand. You need this because drm_sched_rq_remove_entity() will grab the rq lock again!

Problem is now what prevents the entity from being destroyed while you remove it?

Christian.


Andrey




+
+            spin_lock(&rq->lock);
+        }
+        spin_unlock(&rq->lock);
+
+    }
+
+    /* Wakeup everyone stuck in drm_sched_entity_flush for this scheduler */
+    wake_up_all(&sched->job_scheduled);
+
      /* Confirm no work left behind accessing device structures */
      cancel_delayed_work_sync(&sched->work_tdr);


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