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

[Christian König]

Am 17.02.21 um 22:36 schrieb Andrey Grodzovsky:
>
> On 2/17/21 4:32 PM, Christian König wrote:
>> Am 16.02.21 um 18:07 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.
>>>
>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky at 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..11abf5d 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;
>>>       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);
>>> +            drm_sched_rq_remove_entity(rq, s_entity);
>>> +
>>> +            /* Mark as stopped to reject adding to any new rq */
>>> +            spin_lock(&s_entity->rq_lock);
>>> +            s_entity->stopped = true;
>>
>> Why not marking it as stopped and then removing it?
>>
>> Regards,
>> Christian.
>
>
> You mean just reverse the order of operations here to prevent a race 
> where someone adding it again to rq before marking it as stopped ?

Exactly that, yeah.

Christian.

>
> Andrey
>
>
>>
>>> + spin_unlock(&s_entity->rq_lock);
>>> +
>>> +            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);
>>