drm_sched with panfrost crash on T820

[Steven Price]

On 04/10/2019 16:03, Neil Armstrong wrote:
> On 04/10/2019 16:53, Grodzovsky, Andrey wrote:
>>
>> On 10/3/19 4:34 AM, Neil Armstrong wrote:
>>> Hi Andrey,
>>>
>>> Le 02/10/2019 à 16:40, Grodzovsky, Andrey a écrit :
>>>> On 9/30/19 10:52 AM, Hillf Danton wrote:
>>>>> On Mon, 30 Sep 2019 11:17:45 +0200 Neil Armstrong wrote:
>>>>>> Did a new run from 5.3:
>>>>>>
>>>>>> [   35.971972] Call trace:
>>>>>> [   35.974391]  drm_sched_increase_karma+0x5c/0xf0
>>>>>> 			ffff000010667f38	FFFF000010667F94
>>>>>> 			drivers/gpu/drm/scheduler/sched_main.c:335
>>>>>>
>>>>>> The crashing line is :
>>>>>>                                   if (bad->s_fence->scheduled.context ==
>>>>>>                                       entity->fence_context) {
>>>>>>
>>>>>> Doesn't seem related to guilty job.
>>>>> Bail out if s_fence is no longer fresh.
>>>>>
>>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>>> @@ -333,6 +333,10 @@ void drm_sched_increase_karma(struct drm
>>>>>    
>>>>>    			spin_lock(&rq->lock);
>>>>>    			list_for_each_entry_safe(entity, tmp, &rq->entities, list) {
>>>>> +				if (!smp_load_acquire(&bad->s_fence)) {
>>>>> +					spin_unlock(&rq->lock);
>>>>> +					return;
>>>>> +				}
>>>>>    				if (bad->s_fence->scheduled.context ==
>>>>>    				    entity->fence_context) {
>>>>>    					if (atomic_read(&bad->karma) >
>>>>> @@ -543,7 +547,7 @@ EXPORT_SYMBOL(drm_sched_job_init);
>>>>>    void drm_sched_job_cleanup(struct drm_sched_job *job)
>>>>>    {
>>>>>    	dma_fence_put(&job->s_fence->finished);
>>>>> -	job->s_fence = NULL;
>>>>> +	smp_store_release(&job->s_fence, 0);
>>>>>    }
>>>>>    EXPORT_SYMBOL(drm_sched_job_cleanup);
>>> This fixed the problem on the 10 CI runs.
>>>
>>> Neil
>>
>>
>> These are good news but I still fail to see how this fixes the problem - 
>> Hillf, do you mind explaining how you came up with this particular fix - 
>> what was the bug you saw ?
> 
> As Steven explained, seems the same job was submitted on both HW slots,
> and then when timeout occurs each thread calls panfrost_job_timedout
> which leads to drm_sched_stop() on the first call and on the
> second call the job was already freed.
> 
> Steven proposed a working fix, and this one does the same but on
> the drm_sched side. This one looks cleaner, but panfrost should
> not call drm_sched_stop() twice for the same job.

I'm not sure that Hillf's fix is sufficient. In particular in
drm_sched_increase_karma() I don't understand how the smp_load_acquire()
call prevents bad->s_fence becoming NULL immediately afterwards (but
admittedly the window is much smaller). But really this is just a
Panfrost bug (calling drm_sched_stop() twice on the same job).

The part of my change that I'd welcome feedback on is changing
cancel_delayed_work() to cancel_delayed_work_sync() in drm_sched_stop()
when called on different scheduler to the bad job. It's not clear to me
exactly what the semantics of the function should be, and I haven't
tested the effect of the change on drivers other than Panfrost.

Steve

> Neil
> 
>>
>> Andrey
>>
>>>
>>>> Does this change help the problem ? Note that drm_sched_job_cleanup is
>>>> called from scheduler thread which is stopped at all times when work_tdr
>>>> thread is running and anyway the 'bad' job is still in the
>>>> ring_mirror_list while it's being accessed from
>>>> drm_sched_increase_karma so I don't think drm_sched_job_cleanup can be
>>>> called for it BEFORE or while drm_sched_increase_karma is executed.
>>>>
>>>> Andrey
>>>>
>>>>
>>>>>    
>>>>> --
>>>>>
> 
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