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

[Andrey Grodzovsky <andrey.grodzovsky@amd.com>]

Ping

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 ?
>
> 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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