Re: [PATCH v4 2/6] dma-buf/heaps: Don't assert held reservation lock for dma-buf mmapping

From: "T.J. Mercier" <tjmercier@google.com>
To: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc: "Sumit Semwal" <sumit.semwal@linaro.org>,
	"Christian König" <christian.koenig@amd.com>,
	"Benjamin Gaignard" <benjamin.gaignard@collabora.com>,
	"Brian Starkey" <Brian.Starkey@arm.com>,
	"John Stultz" <jstultz@google.com>,
	"Gerd Hoffmann" <kraxel@redhat.com>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Jani Nikula" <jani.nikula@linux.intel.com>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Thomas Zimmermann" <tzimmermann@suse.de>,
	"Tomi Valkeinen" <tomba@kernel.org>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Tomasz Figa" <tfiga@chromium.org>,
	"Marek Szyprowski" <m.szyprowski@samsung.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	"Emil Velikov" <emil.l.velikov@gmail.com>,
	intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	dri-devel@lists.freedesktop.org, linux-tegra@vger.kernel.org,
	kernel@collabora.com, linux-media@vger.kernel.org
Subject: Re: [PATCH v4 2/6] dma-buf/heaps: Don't assert held reservation lock for dma-buf mmapping
Date: Wed, 21 Jun 2023 12:24:15 -0700	[thread overview]
Message-ID: <CABdmKX0SJvW30PEriUhW9kmKQuhrEHHPiVbyUYPbgYCyh40hFg@mail.gmail.com> (raw)
In-Reply-To: <106bc72b-9358-de06-6f3f-06fdfa3859cd@collabora.com>

On Wed, Jun 21, 2023 at 11:16 AM Dmitry Osipenko
<dmitry.osipenko@collabora.com> wrote:
>
> Hi,
>
> On 6/21/23 20:21, T.J. Mercier wrote:
> > On Mon, May 29, 2023 at 3:46 PM Dmitry Osipenko
> > <dmitry.osipenko@collabora.com> wrote:
> >>
> >> Don't assert held dma-buf reservation lock on memory mapping of exported
> >> buffer.
> >>
> >> We're going to change dma-buf mmap() locking policy such that exporters
> >> will have to handle the lock. The previous locking policy caused deadlock
> >> problem for DRM drivers in a case of self-imported dma-bufs once these
> >> drivers are moved to use reservation lock universally. The problem
> >> solved by moving the lock down to exporters. This patch prepares dma-buf
> >> heaps for the locking policy update.
> >>
> > Hi Dmitry,
> >
> > I see that in patch 6 of this series calls to
> > dma_resv_lock/dma_resv_unlock have been added to the
> > drm_gem_shmem_helper functions and some exporters. But I'm curious why
> > no dma_resv_lock/dma_resv_unlock calls were added to these two dma-buf
> > heap exporters for mmap?
>
> DMA-buf heaps are exporters, drm_gem_shmem_helper is importer. Locking
> rules are different for importers and exporters.
>
> DMA-heaps use own locking, they can be moved to resv lock in the future.
>
> DMA-heaps don't protect internal data in theirs mmap() implementations,
> nothing to protect there.
>
Thanks.