Re: [PATCH v5 15/27] drm/scheduler: Fix hang when sched_entity released

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

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Andrey

On 2021-04-30 12:10 p.m., Andrey Grodzovsky wrote:
> 
> 
> On 2021-04-30 2:47 a.m., Christian König wrote:
>>
>>
>> Am 29.04.21 um 19:06 schrieb Andrey Grodzovsky:
>>>
>>>
>>> On 2021-04-29 3:18 a.m., Christian König wrote:
>>>> I need to take another look at this part when I don't have a massive 
>>>> headache any more.
>>>>
>>>> Maybe split the patch set up into different parts, something like:
>>>> 1. Adding general infrastructure.
>>>> 2. Making sure all memory is unpolated.
>>>> 3. Job and fence handling
>>>
>>> I am not sure you mean this patch here, maybe another one ?
>>> Also note you already RBed it.
>>
>> No what I meant was to send out the patches before this one as #1 and #2.
>>
>> That is the easier stuff which can easily go into the drm-misc-next or 
>> amd-staging-drm-next branch.
>>
>> The scheduler stuff certainly need to go into drm-misc-next.
>>
>> Christian.
> 
> Got you. I am fine with it. What we have here is a working hot-unplug
> code but, one with potential use after free MMIO ranges frpom the zombie
> device. The followup patches after this patch are all about preventing
> this and so the patch-set up until this patch including, is functional
> on it's own. While it's necessary to solve the above issue, it's has
> complications as can be seen from the discussion with Daniel on later
> patch in this series. Still, in my opinion it's better to rollout some
> initial support to hot-unplug without use after free protection then
> having no support for hot-unplug at all. It will also make the merge
> work easier as I need to constantly rebase the patches on top latest
> kernel and solve new regressions.
> 
> Daniel - given the arguments above can you sound your opinion on this
> approach ?
> 
> Andrey
>>
>>>
>>> Andrey
>>>
>>>>
>>>> Christian.
>>>>
>>>> Am 28.04.21 um 17:11 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.
>>>>>
>>>>> v3:
>>>>> Drop drm_sched_rq_remove_entity, only modify entity->stopped
>>>>> and check for it in drm_sched_entity_is_idle
>>>>>
>>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
>>>>> Reviewed-by: Christian König <christian.koenig@amd.com>
>>>>> ---
>>>>>   drivers/gpu/drm/scheduler/sched_entity.c |  3 ++-
>>>>>   drivers/gpu/drm/scheduler/sched_main.c   | 24 
>>>>> ++++++++++++++++++++++++
>>>>>   2 files changed, 26 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c 
>>>>> b/drivers/gpu/drm/scheduler/sched_entity.c
>>>>> index f0790e9471d1..cb58f692dad9 100644
>>>>> --- a/drivers/gpu/drm/scheduler/sched_entity.c
>>>>> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
>>>>> @@ -116,7 +116,8 @@ static bool drm_sched_entity_is_idle(struct 
>>>>> drm_sched_entity *entity)
>>>>>       rmb(); /* for list_empty to work without lock */
>>>>>       if (list_empty(&entity->list) ||
>>>>> -        spsc_queue_count(&entity->job_queue) == 0)
>>>>> +        spsc_queue_count(&entity->job_queue) == 0 ||
>>>>> +        entity->stopped)
>>>>>           return true;
>>>>>       return false;
>>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c 
>>>>> b/drivers/gpu/drm/scheduler/sched_main.c
>>>>> index 908b0b56032d..ba087354d0a8 100644
>>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>>> @@ -897,9 +897,33 @@ EXPORT_SYMBOL(drm_sched_init);
>>>>>    */
>>>>>   void drm_sched_fini(struct drm_gpu_scheduler *sched)
>>>>>   {
>>>>> +    struct drm_sched_entity *s_entity;
>>>>> +    int i;
>>>>> +
>>>>>       if (sched->thread)
>>>>>           kthread_stop(sched->thread);
>>>>> +    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;
>>>>> +
>>>>> +        spin_lock(&rq->lock);
>>>>> +        list_for_each_entry(s_entity, &rq->entities, list)
>>>>> +            /*
>>>>> +             * Prevents reinsertion and marks job_queue as idle,
>>>>> +             * it will removed from rq in drm_sched_entity_fini
>>>>> +             * eventually
>>>>> +             */
>>>>> +            s_entity->stopped = true;
>>>>> +        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);
>>>>
>>