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

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

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@amd.com>
> ---
>   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.

> +
> +			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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