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

[Dmitry Osipenko]

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.

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."

> 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?

-- 
Best regards,
Dmitry