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

[Tzung-Bi Shih <tzungbi@kernel.org>]

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@kernel.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@kernel.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.