Re: [PATCH 0/4] Refine GPU recovery sequence to enhance its stability

["Christian König" <christian.koenig@amd.com>]

Am 08.04.21 um 18:08 schrieb Andrey Grodzovsky:
> On 2021-04-08 4:32 a.m., Christian König wrote:
>> Am 08.04.21 um 10:22 schrieb Christian König:
>>> [SNIP]
>>>>>
>>>>>
>>>>>> Beyond blocking all delayed works and scheduler threads we also 
>>>>>> need to guarantee no  IOCTL can access MMIO post device unplug OR 
>>>>>> in flight IOCTLs are done before we finish pci_remove 
>>>>>> (amdgpu_pci_remove for us).
>>>>>> For this I suggest we do something like what we worked on with 
>>>>>> Takashi Iwai the ALSA maintainer recently when he helped 
>>>>>> implementing PCI BARs move support for snd_hda_intel. Take a look at
>>>>>> https://cgit.freedesktop.org/~agrodzov/linux/commit/?h=yadro/pcie_hotplug/movable_bars_v9.1&id=cbaa324799718e2b828a8c7b5b001dd896748497 
>>>>>> and
>>>>>> https://cgit.freedesktop.org/~agrodzov/linux/commit/?h=yadro/pcie_hotplug/movable_bars_v9.1&id=e36365d9ab5bbc30bdc221ab4b3437de34492440
>>>>>> We also had same issue there, how to prevent MMIO accesses while 
>>>>>> the BARs are migrating. What was done there is a refcount was 
>>>>>> added to count all IOCTLs in flight, for any in flight  IOCTL the 
>>>>>> BAR migration handler would
>>>>>> block for the refcount to drop to 0 before it would proceed, for 
>>>>>> any later IOCTL it stops and wait if device is in migration 
>>>>>> state. We even don't need the wait part, nothing to wait for, we 
>>>>>> just return with -ENODEV for this case.
>>>>>>
>>>>>
>>>>> This is essentially what the DRM SRCU is doing as well.
>>>>>
>>>>> For the hotplug case we could do this in the toplevel since we can 
>>>>> signal the fence and don't need to block memory management.
>>>>
>>>>
>>>> To make SRCU 'wait for' all IOCTLs in flight we would need to wrap 
>>>> every IOCTL ( practically - just drm_ioctl function) with 
>>>> drm_dev_enter/drm_dev_exit - can we do it ?
>>>>
>>
>> Sorry totally missed this question.
>>
>> Yes, exactly that. As discussed for the hotplug case we can do this.
>
>
> Thinking more about it - assuming we are  treating synchronize_srcu as 
> a 'wait for completion' of any in flight {drm_dev_enter, drm_dev_exit} 
> scope, some of those scopes might do dma_fence_wait inside. Since we 
> haven't force signaled the fences yet we will end up a deadlock. We 
> have to signal all the various fences before doing the 'wait for'. But 
> we can't signal the fences before setting 'dev->unplugged = true' to 
> reject further CS and other stuff which might create more fences we 
> were supposed-to force signal and now missed them. Effectively setting 
> 'dev->unplugged = true' and doing synchronize_srcu in one call like 
> drm_dev_unplug does without signalling all the fences in the device in 
> between these two steps looks luck a possible deadlock to me - what do 
> you think ?
>

Indeed, that is a really good argument to handle it the same way as the 
reset lock.

E.g. not taking it at the high level IOCTL, but rather when the frontend 
of the driver has acquired all the necessary locks (BO resv, VM lock 
etc...) before calling into the backend to actually do things with the 
hardware.

Christian.

> Andrey
>
>

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