linux-kernel - Re: [libgpiod v1.6.x][PATCH] tests: mockup: unbind mockup devices before unloading the module

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Message-ID: <CAMRc=Mep_AZPht6cQFQ-Pz8UJC-q1r4F29SJ=+_WgJX1cz5wtw@mail.gmail.com>
Date:   Tue, 22 Aug 2023 09:08:08 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Kent Gibson <warthog618@...il.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [libgpiod v1.6.x][PATCH] tests: mockup: unbind mockup devices
 before unloading the module

On Tue, Aug 22, 2023 at 2:21 AM Kent Gibson <warthog618@...il.com> wrote:
>
> On Mon, Aug 21, 2023 at 05:33:39PM +0200, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> >
> > gpio-mockup relies on the GPIO devices being registered in module's __init
> > function and them being unregistered in __exit. This works with the GPIO
> > subsystem as it only takes a reference to the underlying owner module when
> > a GPIO descriptor is requested and not when the GPIO device is
> > instantiated.
> >
> > This behavior may change in the future in the kernel so make the behavior
> > of libgpiomockup more correct and have it unbind all mockup devices over
> > sysfs before unloading the module.
> >
>
> Never knew that unbinding was even an option.
> Maybe update gpio-mockup's documentation?
>

Yeah, I might once we agree on that reference counting patch.

> Just clarifying what the potential impact of the existing libgpiomockup
> behaviour and future kernel behaviour is - the kernel may log errors but
> otherwise correctly handle userspace unloading behaving badly?
> So this patch is pre-emptory noise reduction?
>

No, it's a bug-fix-in-advance. gpio-mockup will fail to unload (until
we unbind all devices anyway) if we couple the module's reference with
struct gpio_device. So will every driver that registers devices from
its module_init() function and tears them down in module_exit(). But
these drivers are wrong to do so in the first place and unloading them
sound like a rare thing to do anyway, so I'm willing to give it a try.

Bartosz