Re: [PATCH 5/5] gpiolib: notify user-space about in-kernel line state changes

[Kent Gibson <warthog618@xxxxxxxxx>]

On Sat, Oct 05, 2024 at 11:42:34AM +0200, Bartosz Golaszewski wrote:
> On Sat, Oct 5, 2024 at 9:46 AM Kent Gibson <warthog618@xxxxxxxxx> wrote:
> >
> > On Fri, Oct 04, 2024 at 04:43:26PM +0200, Bartosz Golaszewski wrote:
> > > From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> > >
> > > There is a problem with gpiod_direction_output/input(), namely the fact
> > > that they can be called both from sleeping as well as atomic context. We
> > > cannot call the blocking notifier from atomic and we cannot switch to
> > > atomic notifier because the pinctrl functions we call higher up the stack
> > > take a mutex. Let's instead use a workqueue and schedule a task to emit
> > > the event from process context on the unbound system queue for minimal
> > > latencies.
> > >
> >
> > So now there is a race between the state of the desc changing and the
> > notified reading it?
> >
>
> Theoretically? Well, yes... In practice I don't think this would
> matter. But I understand the concern and won't insist if it's a
> deal-breaker for you.
>

I don't like that correctness depends on timing, so this is a deal
breaker for me as it stands.  I would like to see the relevant state passed
via the notifier chain, rather than assuming it can be pulled from the desc
when the notifier is eventually called.

Cheers,
Kent.

> Ideally we'd switch to an atomic notifier but I don't have a good idea
> on how to handle pinctrl_gpio_can_use_line(). It digs deep into the
> pinctrl code and it's all synchronized with a mutex. Unlike GPIO, it
> doesn't make any sense to spend days converting pinctrl to SRCU for a
> single corner-case.
>
> I wanted to use in_atomic() to determine whether we can emit the event
> immediately or (if we're in interrupt or with a spinlock taken) we
> should use a workqueue as a fallback but checkpatch.pl is very adamant
> about it being an error (in capital reds).
>
> Bart