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

From: "Christian König" <ckoenig.leichtzumerken@gmail.com>
To: Dmitry Osipenko <dmitry.osipenko@collabora.com>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	Daniel Stone <daniel@fooishbar.org>,
	David Airlie <airlied@linux.ie>,
	Gerd Hoffmann <kraxel@redhat.com>,
	Gurchetan Singh <gurchetansingh@chromium.org>,
	Chia-I Wu <olvaffe@gmail.com>,
	Daniel Almeida <daniel.almeida@collabora.com>,
	Gert Wollny <gert.wollny@collabora.com>,
	Gustavo Padovan <gustavo.padovan@collabora.com>,
	Tomeu Vizoso <tomeu.vizoso@collabora.com>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Maxime Ripard <mripard@kernel.org>, Rob Herring <robh@kernel.org>,
	Steven Price <steven.price@arm.com>,
	Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>,
	Rob Clark <robdclark@gmail.com>,
	Emil Velikov <emil.l.velikov@gmail.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Dmitry Osipenko <digetx@gmail.com>,
	linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
	virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v4 10/15] drm/shmem-helper: Take reservation lock instead of drm_gem_shmem locks
Date: Thu, 12 May 2022 09:29:35 +0200	[thread overview]
Message-ID: <f6e42cfb-0252-1273-2ba3-76af818e0799@gmail.com> (raw)
In-Reply-To: <YnwI5UX/zvmnAHvg@phenom.ffwll.local>

Am 11.05.22 um 21:05 schrieb Daniel Vetter:
> [SNIP]
>>>> 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?
> Yeah I think long-term it's where we want to be, just trying to find
> clever ways to get there.
>
> And I think Christian agrees with that?

Yes, completely.

A design where most DMA-buf functions are supposed to be called with the 
reservation lock held is exactly what I have in mind for the long term.

>>>>> 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
> So throwing it at intel-gfx-ci can't hurt I think, but that only covers
> i915 so doesn't really help with the bigger issue of catching all the
> drivers.

BTW: We have now somebody working on converting the existing 
libdrm_amdgpu unit tests over to igt.

Regards,
Christian.

>
> Cheers, Daniel

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization