linux-kernel - Re: [PATCH 2/2] gpiolib: cdev: release IRQs when the gpio chip device is removed

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Date: Thu, 22 Feb 2024 13:21:15 +0100
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Herve Codina <herve.codina@...tlin.com>
Cc: Kent Gibson <warthog618@...il.com>, Linus Walleij <linus.walleij@...aro.org>, 
	linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Luca Ceresoli <luca.ceresoli@...tlin.com>, Thomas Petazzoni <thomas.petazzoni@...tlin.com>, 
	Saravana Kannan <saravanak@...gle.com>
Subject: Re: [PATCH 2/2] gpiolib: cdev: release IRQs when the gpio chip device
 is removed

On Thu, Feb 22, 2024 at 12:36 PM Herve Codina <herve.codina@...tlincom> wrote:
>
> Hi Bartosz,
>
> On Thu, 22 Feb 2024 00:31:08 -0800
> Bartosz Golaszewski <brgl@...ev.pl> wrote:
>
> > On Thu, 22 Feb 2024 02:05:30 +0100, Kent Gibson <warthog618@...il.com> said:
> > > On Thu, Feb 22, 2024 at 08:57:44AM +0800, Kent Gibson wrote:
> > >> On Tue, Feb 20, 2024 at 10:29:59PM +0800, Kent Gibson wrote:
> > >> > On Tue, Feb 20, 2024 at 12:10:18PM +0100, Herve Codina wrote:
> > >>
> > >> ...
> > >>
> > >> > >  }
> > >> > >
> > >> > > +static int linereq_unregistered_notify(struct notifier_block *nb,
> > >> > > +                                     unsigned long action, void *data)
> > >> > > +{
> > >> > > +      struct linereq *lr = container_of(nb, struct linereq,
> > >> > > +                                        device_unregistered_nb);
> > >> > > +      int i;
> > >> > > +
> > >> > > +      for (i = 0; i < lr->num_lines; i++) {
> > >> > > +              if (lr->lines[i].desc)
> > >> > > +                      edge_detector_stop(&lr->lines[i]);
> > >> > > +      }
> > >> > > +
> > >> >
> > >> > Firstly, the re-ordering in the previous patch creates a race,
> > >> > as the NULLing of the gdev->chip serves to numb the cdev ioctls, so
> > >> > there is now a window between the notifier being called and that numbing,
> > >> > during which userspace may call linereq_set_config() and re-request
> > >> > the irq.
> > >> >
> > >> > There is also a race here with linereq_set_config().  That can be prevented
> > >> > by holding the lr->config_mutex - assuming the notifier is not being called
> > >> > from atomic context.
> > >> >
> > >>
> > >> It occurs to me that the fixed reordering in patch 1 would place
> > >> the notifier call AFTER the NULLing of the ioctls, so there will no longer
> > >> be any chance of a race with linereq_set_config() - so holding the
> > >> config_mutex semaphore is not necessary.
> > >>
> > >
> > > NULLing -> numbing
> > >
> > > The gdev->chip is NULLed, so the ioctls are numbed.
> > > And I need to let the coffee soak in before sending.
> > >
> > >> In which case this patch is fine - it is only patch 1 that requires
> > >> updating.
> > >>
> > >> Cheers,
> > >> Kent.
> > >
> >
> > The fix for the user-space issue may be more-or-less correct but the problem is
> > deeper and this won't fix it for in-kernel users.
> >
> > Herve: please consider the following DT snippet:
> >
> >       gpio0 {
> >               compatible = "foo";
> >
> >               gpio-controller;
> >               #gpio-cells = <2>;
> >               interrupt-controller;
> >               #interrupt-cells = <1>;
> >               ngpios = <8>;
> >       };
> >
> >       consumer {
> >               compatible = "bar";
> >
> >               interrupts-extended = <&gpio0 0>;
> >       };
> >
> > If you unbind the "gpio0" device after the consumer requested the interrupt,
> > you'll get the same splat. And device links will not help you here (on that
> > note: Saravana: is there anything we could do about it? Have you even
> > considered making the irqchip subsystem use the driver model in any way? Is it
> > even feasible?).
> >
> > I would prefer this to be fixed at a lower lever than the GPIOLIB character
> > device.
>
> I think this use case is covered.
> When the consumer device related to the consumer DT node is added, a
> consumer/supplier relationship is created:
> parse_interrupts() parses the 'interrups-extended' property
>   https://elixir.bootlin.com/linux/v6.8-rc1/source/drivers/of/property.c#L1316
> and so, of_link_to_phandle() creates the consumer/supplier link.
>   https://elixir.bootlin.com/linux/v6.8-rc1/source/drivers/of/property.c#L1316
>
> We that link present, if the supplier is removed, the consumer is removed
> before.
> The consumer should release the interrupt during its remove process (i.e
> explicit in its .remove() or explicit because of a devm_*() call).
>
> At least, it is my understanding.

Well, then it doesn't work, because I literally just tried it before
sending my previous email.

Please try it yourself, you'll see.

Also: an interrupt controller may not even have a device consuming its
DT node (see IRQCHIP_DECLARE()), what happens then?

Bart

>
> Best regards,
> Hervé