linux-kernel - Re: [PATCH v3 22/24] gpio: protect the pointer to gpio_chip in gpio

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Date: Tue, 13 Feb 2024 09:10:52 +0100
From: Bartosz Golaszewski <brgl@...ev.pl>
To: "Paul E . McKenney" <paulmck@...nel.org>
Cc: oe-lkp@...ts.linux.dev, lkp@...el.com, 
	Linus Walleij <linus.walleij@...aro.org>, linux-gpio@...r.kernel.org, 
	Kent Gibson <warthog618@...il.com>, Alex Elder <elder@...aro.org>, 
	Geert Uytterhoeven <geert+renesas@...der.be>, 
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Wolfram Sang <wsa@...-dreams.de>, 
	linux-kernel@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>, kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH v3 22/24] gpio: protect the pointer to gpio_chip in
 gpio_device with SRCU

On Mon, Feb 12, 2024 at 10:20 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:
>

[snip]

> >
> >
> > [   76.432519][    T1] gpiochip_find_base_unlocked: found new base at 512
> > [   76.434591][    T1]
> > [   76.435240][    T1] =============================
> > [   76.436545][    T1] WARNING: suspicious RCU usage
> > [   76.437813][    T1] 6.8.0-rc1-00050-gc21131f83abc #1 Tainted: G                 N
> > [   76.439873][    T1] -----------------------------
> > [   76.441158][    T1] drivers/gpio/gpiolib.c:219 suspicious rcu_dereference_check() usage!
> > [   76.443364][    T1]

[snip]

>
> Paul,
>
> Could you help me out here? It seems that lockdep complains (with
> "suspicious RCU usage") whenever an RCU-protected pointer is passed to
> rcu_dereference() but is not actually dereferenced later - in which
> case switching to rcu_access_pointer() helps. But in the case of the
> of_unittests() it also emits the same warning for
> gpiod_direction_input() where gdev->chip is fetched with
> rcu_dereference() using CLASS(gpio_chip_guard) and later actually
> dereferenced by calling guard.gc->...
>
> Any hints as to what I'm doing wrong?
>
> Thanks,
> Bartosz

Seems like these can be silenced with rcu_dereference_protected() so
I'll use it for now.

Bart