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Date:   Wed, 24 Aug 2022 18:03:42 +0300
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Subject: Re: [PATCH v3 6/9] dma-buf: Move dma-buf attachment to dynamic
 locking specification
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To:     =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
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        Gerd Hoffmann <kraxel@redhat.com>,
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        Gert Wollny <gert.wollny@collabora.com>,
        Gustavo Padovan <gustavo.padovan@collabora.com>,
        Daniel Stone <daniel@fooishbar.org>,
        Tomeu Vizoso <tomeu.vizoso@collabora.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Rob Clark <robdclark@gmail.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        "Pan, Xinhui" <Xinhui.Pan@amd.com>,
        Thierry Reding <thierry.reding@gmail.com>,
        Tomasz Figa <tfiga@chromium.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Alex Deucher <alexander.deucher@amd.com>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
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        Qiang Yu <yuq825@gmail.com>
Cc:     dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        Dmitry Osipenko <digetx@gmail.com>,
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References: <20220824102248.91964-1-dmitry.osipenko@collabora.com>
 <20220824102248.91964-7-dmitry.osipenko@collabora.com>
 <17181951-1b40-cd39-48df-58b43cad117d@amd.com>
From:   Dmitry Osipenko <dmitry.osipenko@collabora.com>
In-Reply-To: <17181951-1b40-cd39-48df-58b43cad117d@amd.com>
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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