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

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Date:   Wed, 21 Jun 2023 22:13:24 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Benjamin Gaignard <benjamin.gaignard@...labora.com>,
        Brian Starkey <Brian.Starkey@....com>,
        John Stultz <jstultz@...gle.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Tomi Valkeinen <tomba@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Emil Velikov <emil.l.velikov@...il.com>
Cc:     linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        linux-tegra@...r.kernel.org, kernel@...labora.com
Subject: Re: [PATCH v4 0/6] Move dma-buf mmap() reservation locking down to
 exporters

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