Received-SPF: pass (google.com: domain of linux-kernel+bounces-46678-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Precedence: bulk
MIME-Version: 1.0
References: <20240130124828.14678-1-brgl@bgdev.pl> <20240130124828.14678-2-brgl@bgdev.pl>
In-Reply-To: <20240130124828.14678-2-brgl@bgdev.pl>
From: Linus Walleij <linus.walleij@linaro.org>
Date: Wed, 31 Jan 2024 16:01:40 +0100
Message-ID: <CACRpkdbJOqmPH8T2_MAbNHWVboudDNXpCPc8OpLzB8z20axxfQ@mail.gmail.com>
Subject: Re: [PATCH 01/22] gpio: protect the list of GPIO devices with SRCU
To: Bartosz Golaszewski <brgl@bgdev.pl>
Cc: Kent Gibson <warthog618@gmail.com>, Alex Elder <elder@linaro.org>, 
	Geert Uytterhoeven <geert+renesas@glider.be>, "Paul E . McKenney" <paulmck@kernel.org>, 
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>, Wolfram Sang <wsa@the-dreams.de>, 
	linux-gpio@vger.kernel.org, linux-kernel@vger.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Tue, Jan 30, 2024 at 1:48=E2=80=AFPM Bartosz Golaszewski <brgl@bgdev.pl>=
 wrote:

> From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
>
> We're working towards removing the "multi-function" GPIO spinlock that's
> implemented terribly wrong. We tried using an RW-semaphore to protect
> the list of GPIO devices but it turned out that we still have old code
> using legacy GPIO calls that need to translate the global GPIO number to
> the address of the associated descriptor and - to that end - traverse
> the list while holding the lock. If we change the spinlock to a sleeping
> lock then we'll end up with "scheduling while atomic" bugs.
>
> Let's allow lockless traversal of the list using SRCU and only use the
> mutex when modyfing the list.
>
> While at it: let's protect the period between when we start the lookup
> and when we finally request the descriptor (increasing the reference
> count of the GPIO device) with the SRCU read lock.
>
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

This looks to be doing the right thing to my RCU-untrained eye, so:
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

Yours,
Linus Walleij