linux-kernel - Re: [PATCH 00/23] gpiolib: Adopt revocable mechanism for UAF prevention

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Message-ID: <CAMRc=MejWmGb02akR6TZVPzM4rnZ041TRkJG94=JDYdA5z=xVg@mail.gmail.com>
Date: Wed, 21 Jan 2026 11:42:00 +0100
From: Bartosz Golaszewski <brgl@...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>, 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 00/23] gpiolib: Adopt revocable mechanism for UAF prevention

On Wed, Jan 21, 2026 at 5:17 AM Tzung-Bi Shih <tzungbi@...nel.org> wrote:
>
> On Mon, Jan 19, 2026 at 09:33:21AM +0100, Bartosz Golaszewski wrote:
> > On Sat, Jan 17, 2026 at 1:48 PM Tzung-Bi Shih <tzungbi@...nel.org> wrote:
> > >
> > > On Fri, Jan 16, 2026 at 11:35:00AM +0100, Bartosz Golaszewski wrote:
> > > > On Fri, Jan 16, 2026 at 9:11 AM Tzung-Bi Shih <tzungbi@...nel.org> wrote:
> > > > >
> > > > > This series transitions the UAF prevention logic within the GPIO core
> > > > > (gpiolib) to use the 'revocable' mechanism.
> > > > >
> > > > > The existing code aims to prevent UAF issues when the underlying GPIO
> > > > > chip is removed.  This series replaces that custom logic with the
> > > > > generic 'revocable' API, which is designed to handle such lifecycle
> > > > > dependencies.  There should be no change in behavior.
> > > > >
> > > > > This series depends on the 'revocable' API, introduced in [1].  Some
> > > > > build bots may report errors due to undefined symbols related to
> > > > > 'revocable' until the dependency is merged.
> > > > >
> > > >
> > > > Hi Tzung-Bi!
> > > >
> > > > Thank you for doing this and considering my suggestions from LPC. I
> > > > haven't looked at the code yet but I quickly tested the series with my
> > > > regular test-suites. The good news is: nothing is broken, every test
> > > > works fine. The bad news is: there seems to be a significant impact on
> > > > performance. With the user-space test-suite from libgpiod (for core C
> > > > library - gpiod-test) I'm seeing a consistent 40% impact on
> > > > performance. That's not really acceptable. :( I will try to bisect the
> > > > series later and see which part exactly breaks it.
> > > >
> > > > I can also help you with user-space testing with libgpiod, if you need
> > > > it? Some documentation is available here:
> > > > https://libgpiod.readthedocs.io/en/latest/testing.html
> > >
> > > How to get the performance data?
> > >
> > > I tried on libgpiod-2.2.2.tar.xz:
> > > - ./configure --enable-tools --enable-tests
> > > - make
> > > - ./tests/gpiod-test
> > >
> > > There is only TAP output.  Also I don't see the difference between:
> > > `./tests/gpiod-test` vs. `./tests/gpiod-test -m perf`.
> >
> > Yeah, no, there's no dedicated performance measurement in GLib tests,
> > I just timed the test-suite and it runs 40% slower with this series.
>
> I think this is mostly introduced by a redundant synchronize_srcu() call in
> revocable_provider_alloc().  Proposed a fix in
> https://lore.kernel.org/all/20260121040204.2699886-1-tzungbi@kernel.org/.
>
> The replacement still brings a few overhead (e.g., for allocating some in
> the .open() file operations).  Especially the test approach can accumulate
> them.

People do care about open()/close() performance, please see the
following discussion:
https://lore.kernel.org/all/20250311110034.53959031@erd003.prtnl/

It required us to use raw notifiers instead of atomic ones which call
rcu_synchronize() to fix the performance regression.

Bartosz