linux-kernel - Re: [PATCH 01/22] gpio: protect the list of GPIO devices with SRCU

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Date: Wed, 31 Jan 2024 16:01:40 +0100
From: Linus Walleij <linus.walleij@...aro.org>
To: Bartosz Golaszewski <brgl@...ev.pl>
Cc: Kent Gibson <warthog618@...il.com>, Alex Elder <elder@...aro.org>, 
	Geert Uytterhoeven <geert+renesas@...der.be>, "Paul E . McKenney" <paulmck@...nel.org>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Wolfram Sang <wsa@...-dreams.de>, 
	linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH 01/22] gpio: protect the list of GPIO devices with SRCU

On Tue, Jan 30, 2024 at 1:48 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:

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

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

Yours,
Linus Walleij