Re: GPIOLIB locking is broken and how to fix it

[Bartosz Golaszewski <brgl@xxxxxxxx>]

On Fri, Dec 8, 2023 at 2:56 PM Thierry Reding <thierry.reding@xxxxxxxxx> wrote:
>
> On Fri, Dec 08, 2023 at 02:12:45PM +0100, Linus Walleij wrote:
> > On Thu, Dec 7, 2023 at 7:38 PM Bartosz Golaszewski <brgl@xxxxxxxx> wrote:
> >
> > > The reason for that is that I'm stuck on some corner-cases related to
> > > the GPIO <-> pinctrl interaction. Specifically the fact that we have
> > > GPIOLIB API functions that may be called from atomic context which may
> > > end up calling into pinctrl where a mutex will be acquired.
> >
> > OK I see the problem.
> >
> > > An example of that is any of the GPIO chips that don't set the
> > > can_sleep field in struct gpio_chip but still use
> > > gpiochip_generic_config() (e.g. tegra186). We can then encounter the
> > > following situation:
> > >
> > > irq_handler() // in atomic context
> > >   gpiod_direction_output() // line is open-drain
> > >     gpio_set_config()
> > >       gpiochip_generic_config()
> > >         pinctrl_gpio_set_config()
> > >           mutex_lock()
> > >
> > > Currently we don't take any locks nor synchronize in any other way
> > > (which is wrong as concurrent gpiod_direction_output() and
> > > gpiod_direction_input() will get in each other's way).
> >
> > The only thing that really make sense to protect from here is
> > concurrent access to the same register (such as if a single
> > register contains multiple bits to set a number of GPIOs at
> > output or input).
> >
> > The real usecases for gpiod_direction_* I know of are limited to:
> >
> > 1. Once when the GPIO is obtained.
> >
> > 2. In strict sequence switching back and forth as in
> >     drivers/i2c/busses/i2c-cbus-gpio.c
> >     cbus_transfer()
>
> Isn't this a very special case already? cbus_transfer() holds the spin
> lock across the entire function, so it will only work for a very small
> set of GPIO providers anyway, right? Anything that's sleepable just is
> not going to work. I suspect that direction configuration is then also
> not going to sleep, so this should be fine.
>

Maybe we could switch to using gpiod_direction_*_raw() here and then
mark regular gpiod_direction_input/output() as might_sleep() and be
done with it? Treat this one as a special-case and then not accept
anyone new calling these from atomic context?

Bart

> Thierry