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

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

Am 25.02.21 um 22:27 schrieb Andrey Grodzovsky:
>
> On 2021-02-25 1:42 p.m., Christian König wrote:
>>
>>
>> Am 25.02.21 um 17:03 schrieb Andrey Grodzovsky:
>>>
>>> On 2021-02-25 2:53 a.m., Christian König wrote:
>>>> Am 24.02.21 um 16:13 schrieb Andrey Grodzovsky:
>>>>> Ping
>>>>
>>>> Sorry, I've been on vacation this week.
>>>>
>>>>>
>>>>> Andrey
>>>>>
>>>>> On 2021-02-20 7:12 a.m., Andrey Grodzovsky wrote:
>>>>>>
>>>>>> On 2/20/21 3:38 AM, Christian König wrote:
>>>>>>>
>>>>>>>
>>>>>>> Am 18.02.21 um 17:41 schrieb Andrey Grodzovsky:
>>>>>>>>
>>>>>>>> On 2/18/21 10:15 AM, Christian König wrote:
>>>>>>>>> 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.
>>>>>>>>
>>>>>>>> Right, well, since (unfortunately) sched_entity is part of 
>>>>>>>> amdgpu_ctx_entity and amdgpu_ctx_entity is refcounted
>>>>>>>> there is a problem here that we don't increment 
>>>>>>>> amdgpu_ctx.refcount when assigning  sched_entity
>>>>>>>> to new rq (e.g. before drm_sched_rq_add_entity) and not 
>>>>>>>> decrement before removing. We do it for
>>>>>>>> amdgpu_cs_parser.entity for example (in amdgpu_cs_parser_init 
>>>>>>>> and amdgpu_cs_parser_fini by
>>>>>>>> calling amdgpu_ctx_get and amdgpu_ctx_put). But this seems a 
>>>>>>>> bit tricky due to all the drm_sched_entity_select_rq
>>>>>>>> logic.
>>>>>>>>
>>>>>>>> Another, kind of a band aid fix, would probably be just locking 
>>>>>>>> amdgpu_ctx_mgr.lock around drm_sched_fini
>>>>>>>> when finalizing the fence driver and around idr iteration in 
>>>>>>>> amdgpu_ctx_mgr_fini (which should be lock protected
>>>>>>>> anyway as I see from other idr usages in the code) ... This 
>>>>>>>> should prevent this use after free.
>>>>>>>
>>>>>>> Puh, that's rather complicated as well. Ok let's look at it from 
>>>>>>> the other side for a moment.
>>>>>>>
>>>>>>> Why do we have to remove the entities from the rq in the first 
>>>>>>> place?
>>>>>>>
>>>>>>> Wouldn't it be sufficient to just set all of them to stopped?
>>>>>>>
>>>>>>> Christian.
>>>>>>
>>>>>>
>>>>>> And adding it as another  condition in drm_sched_entity_is_idle ?
>>>>
>>>> Yes, I think that should work.
>>>
>>>
>>> In this case reverse locking order is created, In 
>>> drm_sched_entity_push_job and drm_sched_entity_flush lock 
>>> entity->rq_lock locked first and rq->lock locked second. In 
>>> drm_sched_fini on the other hand, I need to lock rq->lock first to 
>>> iterate rq->entities and then lock s_entity->rq_lock for each entity 
>>> to modify s_entity->stopped. I guess we could change 
>>> s_entity->stopped to atomic ?
>>
>> Good point. But I think the memory barrier inserted by 
>> wake_up_all(&sched->job_scheduled); should be sufficient.
>>
>> If I see this correctly we have the entity->rq_lock mainly to protect 
>> concurrent changes of entity->rq.
>>
>> But when two CPUs both set entity->stopped to true at the same time 
>> we don't really care about it as long drm_sched_entity_is_idle() sees 
>> it.
>>
>> Regards,
>> Christian.
>
>
> I was more thinking about integrity of reading/writing entity->stopped 
> from different threads. I guess since it's bool it's guaranteed to be 
> atomic from HW perspective anyway ?

More or less yes. The key point is that we only change it from false -> 
true and never the other way around. All that you need then for other 
CPUs to see the value is a barrier.

Christian.

> Will send V3 soon.
>
> Andrey
>
>
>>
>>>
>>> Andrey
>>>
>>>
>>>>
>>>> Christian.
>>>>
>>>>
>>>>>>
>>>>>> Andrey
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> Andrey
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> 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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