linux-kernel - Re: [PATCH v2 07/11] gpio: Add revocable provider handle for struct gpio

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Message-ID: <CAMRc=MeyfAMVOMVtC3zobv5XXGtthJxYGsTogNzRK3uhKo1TvQ@mail.gmail.com>
Date: Thu, 5 Feb 2026 17:57:15 +0100
From: Bartosz Golaszewski <brgl@...nel.org>
To: Tzung-Bi Shih <tzungbi@...nel.org>
Cc: Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>, 
	Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
	Wolfram Sang <wsa+renesas@...g-engineering.com>, Jason Gunthorpe <jgg@...dia.com>, 
	Johan Hovold <johan@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, chrome-platform@...ts.linux.dev, 
	Dan Williams <dan.j.williams@...el.com>, linux-gpio@...r.kernel.org, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Danilo Krummrich <dakr@...nel.org>, Linus Walleij <linusw@...nel.org>
Subject: Re: [PATCH v2 07/11] gpio: Add revocable provider handle for struct gpio_chip

On Thu, Feb 5, 2026 at 9:52 AM Tzung-Bi Shih <tzungbi@...nel.org> wrote:
>
> On Wed, Feb 04, 2026 at 07:58:44AM -0500, Bartosz Golaszewski wrote:
> > On Tue, 3 Feb 2026 07:10:54 +0100, Tzung-Bi Shih <tzungbi@...nel.org> said:
> > > diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
> > > index 3abb90385829..cd136d5b52e9 100644
> > > --- a/drivers/gpio/gpiolib.h
> > > +++ b/drivers/gpio/gpiolib.h
> > > @@ -52,6 +52,7 @@
> > >   * @device_notifier: used to notify character device wait queues about the GPIO
> > >   *                   device being unregistered
> > >   * @srcu: protects the pointer to the underlying GPIO chip
> > > + * @chip_rp: revocable provider handle for the corresponding struct gpio_chip.
> > >   * @pin_ranges: range of pins served by the GPIO driver
> > >   *
> > >   * This state container holds most of the runtime variable data
> > > @@ -79,6 +80,7 @@ struct gpio_device {
> > >     struct workqueue_struct *line_state_wq;
> > >     struct blocking_notifier_head device_notifier;
> > >     struct srcu_struct      srcu;
> > > +   struct revocable_provider __rcu *chip_rp;
> > >
> >
> > Why __rcu? This doesn't live in a different address space, only the internal
> > resource it protects does. If anything - this could be __attribute__((noderef))
> > but even that is questionable as this is an opaque structure.
>
> For fixing a race on the pointer itself.  See also [1].
>
> [1] https://lore.kernel.org/all/20260129143733.45618-2-tzungbi@kernel.org

So we're just using a double RCU here? One to protect the resource and
another to protect the protector of the resource? I can't say I'm a
fan of this. I really want to like this interface but is there really
no way to hide the implementation details from the caller? Isn't this
the whole point? As it is: the user still has to care about an
RCU-protected pointer.

Bartosz