Re: GPIOLIB locking is broken and how to fix it

[Thierry Reding <thierry.reding@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 2820 bytes --]

On Fri, Dec 08, 2023 at 03:47:00PM +0100, Bartosz Golaszewski wrote:
> On Fri, Dec 8, 2023 at 2:56 PM Thierry Reding <thierry.reding@gmail.com> 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@bgdev.pl> 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?

Yeah, I2C CBUS already uses gpiod_set_value() in the same context as
gpiod_direction_output()/gpiod_direction_input(), so it would've already
warned about a mismatch anyway. Doing a test-run with the regular
direction accessors marked as might_sleep() should flush out any other
abusers.

Thierry

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]