linux-kernel - [PATCH v2 0/3] gpiolib: use a read-write semaphore to protect the GPIO device list

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Message-Id: <20240102155949.73434-1-brgl@bgdev.pl>
Date: Tue,  2 Jan 2024 16:59:46 +0100
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Linus Walleij <linus.walleij@...aro.org>
Cc: linux-gpio@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: [PATCH v2 0/3] gpiolib: use a read-write semaphore to protect the GPIO device list

From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>

I'm still figuring out how to keep GPIO descriptors coherent while
(mostly) lockless. In the meantime, I found a potential race-condition
during GPIO descriptor lookup and also figured that the correct way to
protect the GPIO device list is actually a read-write semaphore as we're
not modifying the list very often and readers should be able to iterate
over it concurrently.

The first patch in this series is new in v2. I realized that we must not
wait until .release() to remove the GPIO device from the list as this is
why pinning down the GPIO device list during lookup would never work -
we always could end up re-taking a reference to an object that was being
released if it got looked up between when the last reference is dropped
and the object is finally removed from the device list.

v1 -> v2:
- add patch 1/3 to fix a release timing issue

Bartosz Golaszewski (3):
  gpiolib: remove the GPIO device from the list when it's unregistered
  gpiolib: replace the GPIO device mutex with a read-write semaphore
  gpiolib: pin GPIO devices in place during descriptor lookup

 drivers/gpio/gpiolib-sysfs.c |  2 +-
 drivers/gpio/gpiolib.c       | 62 ++++++++++++++++++++----------------
 drivers/gpio/gpiolib.h       |  2 +-
 3 files changed, 36 insertions(+), 30 deletions(-)

-- 
2.40.1