Re: [PATCH v4 10/15] drm/shmem-helper: Take reservation lock instead of drm_gem_shmem locks

[Dmitry Osipenko]

On 5/11/22 18:29, Daniel Vetter wrote:
> On Wed, May 11, 2022 at 06:14:00PM +0300, Dmitry Osipenko wrote:
>> On 5/11/22 17:24, Christian König wrote:
>>> Am 11.05.22 um 15:00 schrieb Daniel Vetter:
>>>> On Tue, May 10, 2022 at 04:39:53PM +0300, Dmitry Osipenko wrote:
>>>>> [SNIP]
>>>>> Since vmapping implies implicit pinning, we can't use a separate lock in
>>>>> drm_gem_shmem_vmap() because we need to protect the
>>>>> drm_gem_shmem_get_pages(), which is invoked by drm_gem_shmem_vmap() to
>>>>> pin the pages and requires the dma_resv_lock to be locked.
>>>>>
>>>>> Hence the problem is:
>>>>>
>>>>> 1. If dma-buf importer holds the dma_resv_lock and invokes
>>>>> dma_buf_vmap() -> drm_gem_shmem_vmap(), then drm_gem_shmem_vmap() shall
>>>>> not take the dma_resv_lock.
>>>>>
>>>>> 2. Since dma-buf locking convention isn't specified, we can't assume
>>>>> that dma-buf importer holds the dma_resv_lock around dma_buf_vmap().
>>>>>
>>>>> The possible solutions are:
>>>>>
>>>>> 1. Specify the dma_resv_lock convention for dma-bufs and make all
>>>>> drivers to follow it.
>>>>>
>>>>> 2. Make only DRM drivers to hold dma_resv_lock around dma_buf_vmap().
>>>>> Other non-DRM drivers will get the lockdep warning.
>>>>>
>>>>> 3. Make drm_gem_shmem_vmap() to take the dma_resv_lock and get deadlock
>>>>> if dma-buf importer holds the lock.
>>>>>
>>>>> ...
>>>> Yeah this is all very annoying.
>>> Ah, yes that topic again :)
>>>
>>> I think we could relatively easily fix that by just defining and
>>> enforcing that the dma_resv_lock must have be taken by the caller when
>>> dma_buf_vmap() is called.
>>>
>>> A two step approach should work:
>>> 1. Move the call to dma_resv_lock() into the dma_buf_vmap() function and
>>> remove all lock taking from the vmap callback implementations.
>>> 2. Move the call to dma_resv_lock() into the callers of dma_buf_vmap()
>>> and enforce that the function is called with the lock held.
>> I've doubts about the need to move out the dma_resv_lock() into the
>> callers of dma_buf_vmap()..
>>
>> I looked through all the dma_buf_vmap() users and neither of them
>> interacts with dma_resv_lock() at all, i.e. nobody takes the lock
>> in/outside of dma_buf_vmap(). Hence it's easy and more practical to make
>> dma_buf_mmap/vmap() to take the dma_resv_lock by themselves.
> i915_gem_dmabuf_vmap -> i915_gem_object_pin_map_unlocked ->
>   i915_gem_object_lock -> dma_resv_lock
> 
> And all the ttm drivers should work similarly. So there's definitely
> drivers which grab dma_resv_lock from their vmap callback.

Grr.. I'll take another look.

>> It's unclear to me which driver may ever want to do the mapping under
>> the dma_resv_lock. But if we will ever have such a driver that will need
>> to map imported buffer under dma_resv_lock, then we could always add the
>> dma_buf_vmap_locked() variant of the function. In this case the locking
>> rule will sound like this:
>>
>> "All dma-buf importers are responsible for holding the dma-reservation
>> lock around the dmabuf->ops->mmap/vmap() calls."

Are you okay with this rule?

>>> It shouldn't be that hard to clean up. The last time I looked into it my
>>> main problem was that we didn't had any easy unit test for it.
>> Do we have any tests for dma-bufs at all? It's unclear to me what you
>> are going to test in regards to the reservation locks, could you please
>> clarify?
> Unfortunately not really :-/ Only way really is to grab a driver which
> needs vmap (those are mostly display drivers) on an imported buffer, and
> see what happens.
> 
> 2nd best is liberally sprinkling lockdep annotations all over the place
> and throwing it at intel ci (not sure amd ci is accessible to the public)
> and then hoping that's good enough. Stuff like might_lock and
> dma_resv_assert_held.

Alright

-- 
Best regards,
Dmitry