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

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

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@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.
>
>
> 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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