Re: [PATCH v6 14/22] dma-buf: Introduce new locking convention

Thomas Hellström (Intel)
2022-06-28T21:32:41Z

On 5/30/22 15:57, Dmitry Osipenko wrote:
> On 5/30/22 16:41, Christian König wrote:
>> Hi Dmitry,
>>
>> Am 30.05.22 um 15:26 schrieb Dmitry Osipenko:
>>> Hello Christian,
>>>
>>> On 5/30/22 09:50, Christian König wrote:
>>>> Hi Dmitry,
>>>>
>>>> First of all please separate out this patch from the rest of the series,
>>>> since this is a complex separate structural change.
>>> I assume all the patches will go via the DRM tree in the end since the
>>> rest of the DRM patches in this series depend on this dma-buf change.
>>> But I see that separation may ease reviewing of the dma-buf changes, so
>>> let's try it.
>> That sounds like you are underestimating a bit how much trouble this
>> will be.
>>
>>>> I have tried this before and failed because catching all the locks in
>>>> the right code paths are very tricky. So expect some fallout from this
>>>> and make sure the kernel test robot and CI systems are clean.
>>> Sure, I'll fix up all the reported things in the next iteration.
>>>
>>> BTW, have you ever posted yours version of the patch? Will be great if
>>> we could compare the changed code paths.
>> No, I never even finished creating it after realizing how much work it
>> would be.
>>
>>>>> This patch introduces new locking convention for dma-buf users. From
>>>>> now
>>>>> on all dma-buf importers are responsible for holding dma-buf
>>>>> reservation
>>>>> lock around operations performed over dma-bufs.
>>>>>
>>>>> This patch implements the new dma-buf locking convention by:
>>>>>
>>>>>     1. Making dma-buf API functions to take the reservation lock.
>>>>>
>>>>>     2. Adding new locked variants of the dma-buf API functions for
>>>>> drivers
>>>>>        that need to manage imported dma-bufs under the held lock.
>>>> Instead of adding new locked variants please mark all variants which
>>>> expect to be called without a lock with an _unlocked postfix.
>>>>
>>>> This should make it easier to remove those in a follow up patch set and
>>>> then fully move the locking into the importer.
>>> Do we really want to move all the locks to the importers? Seems the
>>> majority of drivers should be happy with the dma-buf helpers handling
>>> the locking for them.
>> Yes, I clearly think so.
>>
>>>>>     3. Converting all drivers to the new locking scheme.
>>>> I have strong doubts that you got all of them. At least radeon and
>>>> nouveau should grab the reservation lock in their ->attach callbacks
>>>> somehow.
>>> Radeon and Nouveau use gem_prime_import_sg_table() and they take resv
>>> lock already, seems they should be okay (?)
>> You are looking at the wrong side. You need to fix the export code path,
>> not the import ones.
>>
>> See for example attach on radeon works like this
>> drm_gem_map_attach->drm_gem_pin->radeon_gem_prime_pin->radeon_bo_reserve->ttm_bo_reserve->dma_resv_lock.
> Yeah, I was looking at the both sides, but missed this one.

Also i915 will run into trouble with attach. In particular since i915
starts a full ww transaction in its attach callback to be able to lock
other objects if migration is needed. I think i915 CI would catch this
in a selftest.

Perhaps it's worthwile to take a step back and figure out, if the
importer is required to lock, which callbacks might need a ww acquire
context?

(And off-topic, Since we do a lot of fancy stuff under dma-resv locks
including waiting for fences and other locks, IMO taking these locks
uninterruptible should ring a warning bell)

/Thomas

>
>> Same for nouveau and probably a few other exporters as well. That will
>> certainly cause a deadlock if you don't fix it.
>>
>> I strongly suggest to do this step by step, first attach/detach and then
>> the rest.
> Thank you very much for the suggestions. I'll implement them in the next
> version.
>