Re: [PATCH v4] drm/scheduler: Avoid accessing freed bad job.

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

On 2/11/20 10:55 AM, Andrey Grodzovsky wrote:
> On 2/10/20 4:50 PM, Luben Tuikov wrote:
>> Hi Lucas,
>>
>> Thank you for bringing awareness of this issue, publicly.
>>
>> As soon as this patch showed up back in November of 2019,
>> I objected to it, privately.
>
>
> I didn't find this objection in my mail actually
>
>
>>
>> I suggested to instead use a _list_ to store the "state" of
>> all jobs of the same state. Then, at any time, timeout interrupt
>> or whatever, we can atomically (irq spinlock) move the timeout/bad
>> job to the timedout/cleanup/bad job list, and wake someone up
>> to deal with that list asynchronously, and return from the 
>> interrupt/etc.
>> immediately.
>
>
> Sounds a good idea to me, i think enough for us to have 2 lists, 
> timeout list for jobs scheduled to HW and not yet completed 
> (completion fence signaled) and cleanup list for those that did 
> complete. This should give alternative solution to the race condition 
> this patch was addressing without causing the break the Lucas 
> reported. If no one objects I think i can try implement it.
>
> Andrey


Thinking more i realize Luben is right about having also bad job list as 
this is needed for normal job competition (by fence callback from 
amdgpu_fence_process)  and you need to decide if you move it to cleanup 
list from timeout list or not. If it's already in bad job list - meaning 
that it's being processed by GPU recovery code you don't touch it, 
otherwise you move it to cleanup list where it will be freed eventually 
by invocation of drm_sched_get_cleanup_job.

Andrey


>
>
>>
>> Then in due time, if any more interrupts or whatnot take place,
>> the job will either be in the timeout list or not. If it it,
>> then the instigator backs off as someone else (the list handler) will/is
>> awake and handling it (obviously a state variable may be kept as well).
>>
>> This draws somewhat from my days with iSCSI, SCSI and SAS, 15 years ago,
>> where a device can complete a job (task) at anytime regardless
>> of what the SCSI layer "thinks" the task's state is: timed-out, aborted,
>> whatever. It is a very simple and elegant solution which generalizes
>> well.
>>
>> Regards,
>> Luben
>>
>> On 2020-02-10 11:55 a.m., Andrey Grodzovsky wrote:
>>> Lucas - Ping on my question and also I attached this temporary 
>>> solution for etnaviv to clarify my point. If that something 
>>> acceptable for now at least i can do the same for v3d where it 
>>> requires a bit more code changes.
>>>
>>> Andrey
>>>
>>> On 2/6/20 10:49 AM, Andrey Grodzovsky wrote:
>>>>> Well a revert would break our driver.
>>>>>
>>>>> The real solution is that somebody needs to sit down, gather ALL 
>>>>> the requirements and then come up with a solution which is clean 
>>>>> and works for everyone.
>>>>>
>>>>> Christian.
>>>>
>>>> I can to take on this as indeed our general design on this becomes 
>>>> more and more entangled as GPU reset scenarios grow in complexity 
>>>> (at least in AMD driver). Currently I am on a high priority 
>>>> internal task which should take me around a week or 2 to finish and 
>>>> after that I can get to it.
>>>>
>>>> Regarding temporary solution  - I looked into v3d and etnaviv use 
>>>> cases and we in AMD actually face the same scenario where we decide 
>>>> to skip HW reset if the guilty job did finish by the time we are 
>>>> processing the timeout  (see amdgpu_device_gpu_recover and 
>>>> skip_hw_reset goto) - the difference is we always call 
>>>> drm_sched_stop/start irrespectively of whether we are going to 
>>>> actually HW reset or not (same as extend timeout). I wonder if 
>>>> something like this can be done also for ve3 and etnaviv ?
>>>>
>>>> Andrey
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