Re: ✗ Fi.CI.BAT: failure for series starting with [1/6] dma-buf: add dynamic DMA-buf handling v13

["Koenig, Christian" <Christian.Koenig@amd.com>]

Am 08.08.19 um 09:29 schrieb Daniel Vetter:
> On Thu, Aug 8, 2019 at 9:09 AM Koenig, Christian
> <Christian.Koenig@amd.com> wrote:
>> Am 07.08.19 um 23:19 schrieb Daniel Vetter:
>>> On Wed, Jul 31, 2019 at 10:55:02AM +0200, Daniel Vetter wrote:
>>>> On Thu, Jun 27, 2019 at 09:28:11AM +0200, Christian König wrote:
>>>>> Hi Daniel,
>>>>>
>>>>> those fails look like something random to me and not related to my patch
>>>>> set. Correct?
>>>> First one I looked at has the reservation_obj all over:
>>>>
>>>> https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_13438/fi-cml-u/igt@gem_exec_fence@basic-busy-default.html
>>>>
>>>> So 5 second guees is ... probably real?
>>>>
>>>> Note that with the entire lmem stuff going on right now there's massive
>>>> discussions about how we're doing resv_obj vs obj->mm.lock the wrong way
>>>> round in i915, so I'm not surprised at all that you managed to trip this.
>>>>
>>>> The way I see it right now is that obj->mm.lock needs to be limited to
>>>> dealing with the i915 shrinker interactions only, and only for i915 native
>>>> objects. And for dma-bufs we need to make sure it's not anywhere in the
>>>> callchain. Unfortunately that's a massive refactor I guess ...
>>> Thought about this some more, aside from just breaking i915 or waiting
>>> until it's refactored (Both not awesome) I think the only option is get
>>> back to the original caching. And figure out whether we really need to
>>> take the direction into account for that, or whether upgrading to
>>> bidirectional unconditionally won't be ok. I think there's only really two
>>> cases where this matters:
>>>
>>> - display drivers using the cma/dma_alloc helpers. Everything is allocated
>>>     fully coherent, cpu side wc, no flushing.
>>>
>>> - Everyone else (on platforms where there's actually some flushing going
>>>     on) is for rendering gpus, and those always map bidirectional and want
>>>     the mapping cached for as long as possible.
>>>
>>> With that we could go back to creating the cached mapping at attach time
>>> and avoid inflicting the reservation object lock to places that would keel
>>> over.
>>>
>>> Thoughts?
>> Actually we had a not so nice internal mail thread with our hardware
>> guys and it looks like we have tons of hardware bugs/exceptions that
>> sometimes PCIe BARs are only readable or only writable. So it turned out
>> that always caching with bidirectional won't work for us either.
>>
>> Additional to that I'm not sure how i915 actually triggered the issue,
>> cause with the current code that shouldn't be possible.
>>
>> But independent of that I came to the conclusion that we first need to
>> get to a common view of what the fences in the reservation mean or
>> otherwise the whole stuff here isn't going to work smooth either.
>>
>> So working on that for now and when that's finished I will come back to
>> this problem here again.
> Yeah makes sense. I think we also need to clarify a bit the existing
> rules around reservatrion_object, dma_fence signaling, and how that
> nests with everything else (like memory allocation/fs_reclaim critical
> sections, or mmap_sem).
>
> Ignore the drivers which just pin everything system memory (mostly
> just socs) I think we have a bunch of groups, and they're all somewhat
> incompatible with each another. Examples:
>
> - old ttm drivers (anything except amdgpu) nest the mmap_sem within
> the reservation_object. That allows you to do copy_*_user while
> holding reservations, simplifying command submission since you don't
> need fallback paths when you take a fault. But means you have this
> awkward trylock in the mmap path with no forward progress guarantee at
> all.
>
> amdgpu fixed that (but left ttm alone), i915 also works like that with
> mmap_sem being the outer lock.

By the way that is incorrect. Both amdgpu as well as readeon don't use 
copy_to/from_user while holding the reservation lock.

The last time I checked the only driver still doing that was nouveau.

Maybe time to add a might_lock() so that we will be informed about 
misuse by lockdep?

Christian.

>
> - other is reservation_object vs memory allocations. Currently all
> drivers assume you can allocate memory while holding a reservation,
> but i915 gem folks seem to have some plans to change that for i915.
> Which isn't going to work I think, so we need to clarify that before
> things get more inconsistent.
>
> Above two can at least be ensured by adding somme lockdep annotations
> and dependency priming, see i915_gem_shrinker_taints_mutex for what I
> have in mind for reservation_obj.
>
> The real pain/scary thing is dma_fence. All the
> shrinkers/mmu_notifiers/hmm_mirrors we have assume that you can wait
> for a fence from allocation contexts/direct reclaim. Which means
> nothing you do between publishing a fence somewhere (dma-buf, syncobj,
> syncpt fd) and signalling that fence is allowed to allocate memory or
> pull in any dependencies which might need memory allocations. I think
> we're mostly ok with this, but there's some i915 patches that break
> this.
>
> Much worse is that lockdep can't help us check this: dma_fence is
> essentially struct completion on steroids, and the cross-release
> lockdep support for struct completion looks like it's never going to
> get merged. So no debugging aids to make sure we get this right, all
> we have is review and testing and machines deadlocking in really
> complicated ways if we get it wrong.
>
> Cheers, Daniel

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