Re: [PATCH v4 0/6] Move dma-buf mmap() reservation locking down to exporters

Dmitry Osipenko
2023-06-21T19:13:34Z

On 5/30/23 01:39, Dmitry Osipenko wrote:
> This patchset makes dma-buf exporters responisble for taking care of
> the reservation lock. I also included patch that moves drm-shmem to use
> reservation lock, to let CI test the whole set. I'm going to take all
> the patches via the drm-misc tree, please give an ack.
>
> Previous policy stated that dma-buf core takes the lock around mmap()
> callback. Which meant that both importers and exporters shouldn't touch
> the reservation lock in the mmap() code path. This worked well until
> Intel-CI found a deadlock problem in a case of self-imported dma-buf [1].
>
> The problem happens when userpace mmaps a self-imported dma-buf, i.e.
> mmaps the dma-buf FD. DRM core treats self-imported dma-bufs as own GEMs
> [2]. There is no way to differentiate a prime GEM from a normal GEM for
> drm-shmem in drm_gem_shmem_mmap(), which resulted in a deadlock problem
> for drm-shmem mmap() code path once it's switched to use reservation lock.
>
> It was difficult to fix the drm-shmem problem without adjusting dma-buf
> locking policy. In parctice not much changed from importers perspective
> because previosly dma-buf was taking the lock in between of importers
> and exporters. Now this lock is shifted down to exporters.
>
> [1] https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_114671v2/shard-snb5/igt@prime_vgem@sync@rcs0.html
> [2] https://elixir.bootlin.com/linux/v6.3-rc4/source/drivers/gpu/drm/drm_prime.c#L924

Applied to misc-next

--
Best regards,
Dmitry