linux-kernel - Re: [PATCH 22/23] gpiolib: Leverage revocable for other independent lifecycle instances

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Message-ID: <aXT45B6vLf9R3Pbf@hovoldconsulting.com>
Date: Sat, 24 Jan 2026 17:52:52 +0100
From: Johan Hovold <johan@...nel.org>
To: Tzung-Bi Shih <tzungbi@...nel.org>
Cc: Benson Leung <bleung@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>,
	Bartosz Golaszewski <brgl@...ev.pl>,
	Linus Walleij <linusw@...nel.org>, Jonathan Corbet <corbet@....net>,
	Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, chrome-platform@...ts.linux.dev,
	linux-kselftest@...r.kernel.org,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	Simona Vetter <simona.vetter@...ll.ch>,
	Dan Williams <dan.j.williams@...el.com>,
	Jason Gunthorpe <jgg@...dia.com>, linux-gpio@...r.kernel.org
Subject: Re: [PATCH 22/23] gpiolib: Leverage revocable for other independent
 lifecycle instances

On Fri, Jan 16, 2026 at 08:10:35AM +0000, Tzung-Bi Shih wrote:
> There are independent lifecycle instances (e.g., other drivers) can save
> a raw pointer to the struct gpio_device (e.g., via gpio_device_find())
> or struct gpio_desc (e.g., via gpio_to_desc()).  In some operations,
> they have to access the underlying struct gpio_chip.
> 
> Leverage revocable for them so that they don't need to handle the
> synchronization by accessing the SRCU explicitly.
> 
> Signed-off-by: Tzung-Bi Shih <tzungbi@...nel.org>

>  static int gpiod_get_raw_value_commit(const struct gpio_desc *desc)
>  {
> -	struct gpio_device *gdev;
>  	struct gpio_chip *gc;
>  	int value;
> +	DEFINE_REVOCABLE(rev, desc->gdev->chip_rp);

DEFINE_REVOCABLE() is racy and can lead to use-after-free since nothing
prevents chip_rp from being revoked and freed while the
revocable_alloc() hidden in DEFINE_REVOCABLE() is running.

>  
> -	/* FIXME Unable to use gpio_chip_guard due to const desc. */
> -	gdev = desc->gdev;
> -
> -	guard(srcu)(&gdev->srcu);
> -
> -	gc = srcu_dereference(gdev->chip, &gdev->srcu);
> +	REVOCABLE_TRY_ACCESS_WITH(rev, gc);
>  	if (!gc)
>  		return -ENODEV;

Johan