linux-kernel - [PATCH v3 7/9] dma-buf: Document dynamic locking convention

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Message-Id: <20220824102248.91964-8-dmitry.osipenko@collabora.com>
Date:   Wed, 24 Aug 2022 13:22:46 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     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>,
        Christian König <christian.koenig@....com>,
        "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>,
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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,
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        lima@...ts.freedesktop.org
Subject: [PATCH v3 7/9] dma-buf: Document dynamic locking convention

Add documentation for the dynamic locking convention. The documentation
tells dma-buf API users when they should take the reservation lock and
when not.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
---
 Documentation/driver-api/dma-buf.rst |  6 +++
 drivers/dma-buf/dma-buf.c            | 63 ++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+)

diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index 36a76cbe9095..622b8156d212 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -119,6 +119,12 @@ DMA Buffer ioctls
 
 .. kernel-doc:: include/uapi/linux/dma-buf.h
 
+DMA-BUF locking convention
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+   :doc: locking convention
+
 Kernel Functions and Structures Reference
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index f2a5a122da4a..696d132b02f4 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -794,6 +794,69 @@ static struct sg_table * __map_dma_buf(struct dma_buf_attachment *attach,
 	return sg_table;
 }
 
+/**
+ * DOC: locking convention
+ *
+ * In order to avoid deadlock situations between dma-buf exports and importers,
+ * all dma-buf API users must follow the common dma-buf locking convention.
+ *
+ * Convention for importers
+ *
+ * 1. Importers must hold the dma-buf reservation lock when calling these
+ *    functions:
+ *
+ *     - dma_buf_pin()
+ *     - dma_buf_unpin()
+ *     - dma_buf_map_attachment()
+ *     - dma_buf_unmap_attachment()
+ *     - dma_buf_vmap()
+ *     - dma_buf_vunmap()
+ *
+ * 2. Importers must not hold the dma-buf reservation lock when calling these
+ *    functions:
+ *
+ *     - dma_buf_attach_unlocked()
+ *     - dma_buf_dynamic_attach_unlocked()
+ *     - dma_buf_detach_unlocked()
+ *     - dma_buf_export(
+ *     - dma_buf_fd()
+ *     - dma_buf_get()
+ *     - dma_buf_put()
+ *     - dma_buf_begin_cpu_access()
+ *     - dma_buf_end_cpu_access()
+ *     - dma_buf_map_attachment_unlocked()
+ *     - dma_buf_unmap_attachment_unlocked()
+ *     - dma_buf_vmap_unlocked()
+ *     - dma_buf_vunmap_unlocked()
+ *
+ * Convention for exporters
+ *
+ * 1. These &dma_buf_ops callbacks are invoked with unlocked dma-buf
+ *    reservation and exporter can take the lock:
+ *
+ *     - &dma_buf_ops.attach()
+ *     - &dma_buf_ops.detach()
+ *     - &dma_buf_ops.release()
+ *     - &dma_buf_ops.begin_cpu_access()
+ *     - &dma_buf_ops.end_cpu_access()
+ *
+ * 2. These &dma_buf_ops callbacks are invoked with locked dma-buf
+ *    reservation and exporter can't take the lock:
+ *
+ *     - &dma_buf_ops.pin()
+ *     - &dma_buf_ops.unpin()
+ *     - &dma_buf_ops.map_dma_buf()
+ *     - &dma_buf_ops.unmap_dma_buf()
+ *     - &dma_buf_ops.mmap()
+ *     - &dma_buf_ops.vmap()
+ *     - &dma_buf_ops.vunmap()
+ *
+ * 3. Exporters must hold the dma-buf reservation lock when calling these
+ *    functions:
+ *
+ *     - dma_buf_move_notify()
+ */
+
 /**
  * dma_buf_dynamic_attach_unlocked - Add the device to dma_buf's attachments list
  * @dmabuf:		[in]	buffer to attach device to.
-- 
2.37.2