Re: [PATCH v2 8/8] drm/amdgpu: Prevent any job recoveries after device is unplugged.

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 18.11.20 um 17:20 schrieb Andrey Grodzovsky:
>
> On 11/18/20 7:01 AM, Christian König wrote:
>> Am 18.11.20 um 08:39 schrieb Daniel Vetter:
>>> On Tue, Nov 17, 2020 at 9:07 PM Andrey Grodzovsky
>>> <Andrey.Grodzovsky@amd.com> wrote:
>>>>
>>>> On 11/17/20 2:49 PM, Daniel Vetter wrote:
>>>>> On Tue, Nov 17, 2020 at 02:18:49PM -0500, Andrey Grodzovsky wrote:
>>>>>> On 11/17/20 1:52 PM, Daniel Vetter wrote:
>>>>>>> On Tue, Nov 17, 2020 at 01:38:14PM -0500, Andrey Grodzovsky wrote:
>>>>>>>> On 6/22/20 5:53 AM, Daniel Vetter wrote:
>>>>>>>>> On Sun, Jun 21, 2020 at 02:03:08AM -0400, Andrey Grodzovsky 
>>>>>>>>> wrote:
>>>>>>>>>> No point to try recovery if device is gone, just messes up 
>>>>>>>>>> things.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
>>>>>>>>>> ---
>>>>>>>>>>      drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c | 16 
>>>>>>>>>> ++++++++++++++++
>>>>>>>>>>      drivers/gpu/drm/amd/amdgpu/amdgpu_job.c | 8 ++++++++
>>>>>>>>>>      2 files changed, 24 insertions(+)
>>>>>>>>>>
>>>>>>>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c 
>>>>>>>>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>>>>>>>>>> index 6932d75..5d6d3d9 100644
>>>>>>>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>>>>>>>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
>>>>>>>>>> @@ -1129,12 +1129,28 @@ static int amdgpu_pci_probe(struct 
>>>>>>>>>> pci_dev *pdev,
>>>>>>>>>>           return ret;
>>>>>>>>>>      }
>>>>>>>>>> +static void amdgpu_cancel_all_tdr(struct amdgpu_device *adev)
>>>>>>>>>> +{
>>>>>>>>>> +        int i;
>>>>>>>>>> +
>>>>>>>>>> +        for (i = 0; i < AMDGPU_MAX_RINGS; ++i) {
>>>>>>>>>> +                struct amdgpu_ring *ring = adev->rings[i];
>>>>>>>>>> +
>>>>>>>>>> +                if (!ring || !ring->sched.thread)
>>>>>>>>>> +                        continue;
>>>>>>>>>> +
>>>>>>>>>> + cancel_delayed_work_sync(&ring->sched.work_tdr);
>>>>>>>>>> +        }
>>>>>>>>>> +}
>>>>>>>>> I think this is a function that's supposed to be in 
>>>>>>>>> drm/scheduler, not
>>>>>>>>> here. Might also just be your cleanup code being ordered 
>>>>>>>>> wrongly, or your
>>>>>>>>> split in one of the earlier patches not done quite right.
>>>>>>>>> -Daniel
>>>>>>>> This function iterates across all the schedulers per amdgpu 
>>>>>>>> device and accesses
>>>>>>>> amdgpu specific structures , drm/scheduler deals with single 
>>>>>>>> scheduler at most
>>>>>>>> so looks to me like this is the right place for this function
>>>>>>> I guess we could keep track of all schedulers somewhere in a 
>>>>>>> list in
>>>>>>> struct drm_device and wrap this up. That was kinda the idea.
>>>>>>>
>>>>>>> Minimally I think a tiny wrapper with docs for the
>>>>>>> cancel_delayed_work_sync(&sched->work_tdr); which explains what 
>>>>>>> you must
>>>>>>> observe to make sure there's no race.
>>>>>> Will do
>>>>>>
>>>>>>
>>>>>>> I'm not exactly sure there's no
>>>>>>> guarantee here we won't get a new tdr work launched right 
>>>>>>> afterwards at
>>>>>>> least, so this looks a bit like a hack.
>>>>>> Note that for any TDR work happening post amdgpu_cancel_all_tdr
>>>>>> amdgpu_job_timedout->drm_dev_is_unplugged
>>>>>> will return true and so it will return early. To make it water 
>>>>>> proof tight
>>>>>> against race
>>>>>> i can switch from drm_dev_is_unplugged to drm_dev_enter/exit
>>>>> Hm that's confusing. You do a work_cancel_sync, so that at least 
>>>>> looks
>>>>> like "tdr work must not run after this point"
>>>>>
>>>>> If you only rely on drm_dev_enter/exit check with the tdr work, then
>>>>> there's no need to cancel anything.
>>>>
>>>> Agree, synchronize_srcu from drm_dev_unplug should play the role
>>>> of 'flushing' any earlier (in progress) tdr work which is
>>>> using drm_dev_enter/exit pair. Any later arising tdr will terminate 
>>>> early when
>>>> drm_dev_enter
>>>> returns false.
>>> Nope, anything you put into the work itself cannot close this race.
>>> It's the schedule_work that matters here. Or I'm missing something ...
>>> I thought that the tdr work you're cancelling here is launched by
>>> drm/scheduler code, not by the amd callback?
>
>
> My bad, you are right, I am supposed to put drm_dev_enter.exit pair 
> into drm_sched_job_timedout
>
>
>>
>> Yes that is correct. Canceling the work item is not the right 
>> approach at all, nor is adding dev_enter/exit pair in the recovery 
>> handler.
>
>
> Without adding the dev_enter/exit guarding pair in the recovery 
> handler you are ending up with GPU reset starting while
> the device is already unplugged, this leads to multiple errors and 
> general mess.
>
>
>>
>> What we need to do here is to stop the scheduler thread and then wait 
>> for any timeout handling to have finished.
>>
>> Otherwise it can scheduler a new timeout just after we have canceled 
>> this one.
>>
>> Regards,
>> Christian.
>
>
> Schedulers are stopped from amdgpu_driver_unload_kms which indeed 
> happens after drm_dev_unplug
> so yes, there is still a chance for new work being scheduler and 
> timeout armed after but, once i fix the code
> to place drm_dev_enter/exit pair into drm_sched_job_timeout I don't 
> see why that not a good solution ?

Yeah that should work as well, but then you also don't need to cancel 
the work item from the driver.

> Any tdr work started after drm_dev_unplug finished will simply abort 
> on entry to drm_sched_job_timedout
> because drm_dev_enter will be false and the function will return 
> without rearming the timeout timer and
> so will have no impact.
>
> The only issue i see here now is of possible use after free if some 
> late tdr work will try to execute after
> drm device already gone, for this we probably should add 
> cancel_delayed_work_sync(sched.work_tdr)
> to drm_sched_fini after sched->thread is stopped there.

Good point, that is indeed missing as far as I can see.

Christian.

>
> Andrey

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