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

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

On 2/11/20 7:53 PM, Luben Tuikov wrote:
> On 2020-02-11 4:27 p.m., Andrey Grodzovsky wrote:
>> 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
> Yes, I didn't send it to you.
>
>>>> 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.
> Yep...
>
> Perhaps fewer lists, than "timeout", "bad" and "cleanup" could be had.
> I'd also name the "bad" list as "recovery" list, as that is what would
> be done to commands on that list.
>
> "Timeout" is a status "timed-out", so perhaps just set the timeout
> flag and move it to a "done" list. (Note that the command can still
> complete asynchronously while on that list and while it has status
> "timed-out'.)
>
> The idea is that,
> 1) it avoid contention and races when more than one context
>     can update the job at the same time, and
> 2) easy to process all jobs of a certain state and/or
>     move them around, etc.
>
> Let's discuss it and come up with a plan. :-)
>
> Regards,
> Luben


Sure, let me maybe come up with a draft patch so we have more concrete 
stuff to discuss and review.

Andrey



>
>
>
>
>> 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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