linux-kernel - Re: [PATCH v3 6/9] dma-buf: Move dma-buf attachment to dynamic locking specification

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Message-ID: <4af793fd-eccc-ad70-65c3-de78dced71f0@collabora.com>
Date:   Wed, 24 Aug 2022 18:03:42 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Christian König <christian.koenig@....com>,
        David Airlie <airlied@...ux.ie>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Daniel Almeida <daniel.almeida@...labora.com>,
        Gert Wollny <gert.wollny@...labora.com>,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Daniel Stone <daniel@...ishbar.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Rob Clark <robdclark@...il.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Alex Deucher <alexander.deucher@....com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Thomas Hellström <thomas_os@...pmail.org>,
        Qiang Yu <yuq825@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Dmitry Osipenko <digetx@...il.com>,
        linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
        amd-gfx@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org,
        linux-rdma@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        lima@...ts.freedesktop.org
Subject: Re: [PATCH v3 6/9] dma-buf: Move dma-buf attachment to dynamic
 locking specification

On 8/24/22 17:08, Christian König wrote:
> Am 24.08.22 um 12:22 schrieb Dmitry Osipenko:
>> Move dma-buf attachment API functions to the dynamic locking
>> specification.
>> The strict locking convention prevents deadlock situations for dma-buf
>> importers and exporters.
>>
>> Previously, the "unlocked" versions of the attachment API functions
>> weren't taking the reservation lock and this patch makes them to take
>> the lock.
> 
> Didn't we concluded that we need to keep the attach and detach callbacks
> without the lock and only move the map/unmap callbacks over?
> 
> Otherwise it won't be possible for drivers to lock multiple buffers if
> they have to shuffle things around for a specific attachment.

We did conclude that. The attach/detach dma-buf ops are unlocked, but
the map_dma_buf/unmap_dma_buf must be invoked under lock and
dma_buf_dynamic_attach_unlocked() maps dma-buf if either importer or
exporter can't handle the dynamic mapping [1].

[1]
https://elixir.bootlin.com/linux/v6.0-rc2/source/drivers/dma-buf/dma-buf.c#L869

Hence I re-arranged the dma_resv_lock() in
dma_buf_dynamic_attach_unlocked() to move both pinning and mapping under
the held lock.

-- 
Best regards,
Dmitry