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From: Kent Gibson <warthog618@gmail.com>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH v9 08/20] gpiolib: cdev: support GPIO_V2_GET_LINEINFO_IOCTL and GPIO_V2_GET_LINEINFO_WATCH_IOCTL
Date: Fri, 25 Sep 2020 13:32:25 +0800	[thread overview]
Message-ID: <20200925053225.GA32865@sol> (raw)
In-Reply-To: <CAHp75Vc05P4-X_ZC6k-EWdDCAXOgPgAJhm4RxF3izvk=vW+X+g@mail.gmail.com>

On Wed, Sep 23, 2020 at 06:41:45PM +0300, Andy Shevchenko wrote:
> On Tue, Sep 22, 2020 at 5:35 AM Kent Gibson <warthog618@gmail.com> wrote:
> >
> > Add support for GPIO_V2_GET_LINEINFO_IOCTL and
> > GPIO_V2_GET_LINEINFO_WATCH_IOCTL.
> >
> > +#ifdef CONFIG_GPIO_CDEV_V1
> > +static int lineinfo_ensure_abi_version(struct gpio_chardev_data *cdata,
> > +                                      unsigned int version)
> > +{
> 
> > +       int abiv = atomic_read(&cdata->watch_abi_version);
> > +
> > +       if (abiv == 0) {
> 
> > +               atomic_cmpxchg(&cdata->watch_abi_version, 0, version);
> > +               abiv = atomic_read(&cdata->watch_abi_version);
> 
> atomic_cmpxchng() returns a value.
> Also there are no barriers here...
> 
> > +       }
> > +       if (abiv != version)
> > +               return -EPERM;
> 
> I'm not sure I understand why this is atomic.
> 
> Also this seems to be racy if cdata changed in background.
> 
> Shouldn't be rather
> 
> if (atomic_cmpxchg() == 0) {
>   if (atomic_read() != version)
>     return ...;
> }
> 

Perhaps this is what you had in mind:

static bool lineinfo_is_abi_version(struct gpio_chardev_data *cdata,
				       unsigned int version)
{
	int abiv = atomic_cmpxchg(&cdata->watch_abi_version, 0, version);

	if (abiv == version)
		return true;
	if (abiv == 0)
		/* first (and only) setter sees initial 0 value */
		return true;

	return false;
}

That is functionally equivalent, but slightly shorter.

I was thinking the atomic_cmpxchg() is more expensive than an
atomic_read(), and so initially checked the value with
that, but for the cmp failure case it is probably the same, or at least
not worth the bother - this isn't even vaguely near a hot path.

I've also switched the return value to bool in response to your other
comments.

Cheers,
Kent.

  parent reply	other threads:[~2020-09-25  5:32 UTC|newest]

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-22  2:31 [PATCH v9 00/20] gpio: cdev: add uAPI v2 Kent Gibson
2020-09-22  2:31 ` [PATCH v9 01/20] gpiolib: cdev: gpio_desc_to_lineinfo() should set info offset Kent Gibson
2020-09-22  2:31 ` [PATCH v9 02/20] gpiolib: cdev: replace strncpy() with strscpy() Kent Gibson
2020-09-22  2:31 ` [PATCH v9 03/20] gpio: uapi: define GPIO_MAX_NAME_SIZE for array sizes Kent Gibson
2020-09-22  2:31 ` [PATCH v9 04/20] gpio: uapi: define uAPI v2 Kent Gibson
2020-09-22  7:41   ` Arnd Bergmann
2020-09-22  9:50     ` Bartosz Golaszewski
2020-09-22 10:07       ` Kent Gibson
2020-09-23 10:04   ` Andy Shevchenko
2020-09-23 10:30     ` Kent Gibson
2020-09-23 11:16       ` Andy Shevchenko
2020-09-23 12:18         ` Arnd Bergmann
2020-09-23 15:15           ` Andy Shevchenko
2020-09-22  2:31 ` [PATCH v9 05/20] gpiolib: make cdev a build option Kent Gibson
2020-09-22  2:31 ` [PATCH v9 06/20] gpiolib: add build option for CDEV v1 ABI Kent Gibson
2020-09-22  2:31 ` [PATCH v9 07/20] gpiolib: cdev: support GPIO_V2_GET_LINE_IOCTL and GPIO_V2_LINE_GET_VALUES_IOCTL Kent Gibson
2020-09-23 11:11   ` Andy Shevchenko
2020-09-24  8:09     ` Kent Gibson
2020-09-25 10:06       ` Andy Shevchenko
2020-09-26  9:16         ` Kent Gibson
2020-09-27  9:00           ` Andy Shevchenko
2020-09-27 12:39             ` Kent Gibson
2020-09-24 14:29     ` Bartosz Golaszewski
2020-09-22  2:31 ` [PATCH v9 08/20] gpiolib: cdev: support GPIO_V2_GET_LINEINFO_IOCTL and GPIO_V2_GET_LINEINFO_WATCH_IOCTL Kent Gibson
2020-09-23 15:41   ` Andy Shevchenko
2020-09-24  2:39     ` Kent Gibson
2020-09-24  8:39       ` Andy Shevchenko
2020-09-24  9:48         ` Kent Gibson
2020-09-25 10:12           ` Andy Shevchenko
2020-09-25 11:56             ` Kent Gibson
2020-09-25 14:43               ` Andy Shevchenko
2020-09-25  5:32     ` Kent Gibson [this message]
2020-09-22  2:31 ` [PATCH v9 09/20] gpiolib: cdev: support edge detection for uAPI v2 Kent Gibson
2020-09-23 15:47   ` Andy Shevchenko
2020-09-24  3:07     ` Kent Gibson
2020-09-25  9:35       ` Andy Shevchenko
2020-09-25 12:26         ` Kent Gibson
2020-09-25 14:48           ` Andy Shevchenko
2020-09-22  2:31 ` [PATCH v9 10/20] gpiolib: cdev: support GPIO_V2_LINE_SET_CONFIG_IOCTL Kent Gibson
2020-09-23 16:14   ` Andy Shevchenko
2020-09-23 16:15     ` Andy Shevchenko
2020-09-24  3:24       ` Kent Gibson
2020-09-24  8:26         ` Andy Shevchenko
2020-09-24  9:26           ` Kent Gibson
2020-09-25  9:49             ` Andy Shevchenko
2020-09-22  2:31 ` [PATCH v9 11/20] gpiolib: cdev: support GPIO_V2_LINE_SET_VALUES_IOCTL Kent Gibson
2020-09-23 16:18   ` Andy Shevchenko
2020-09-24  7:32     ` Kent Gibson
2020-09-24  8:21       ` Andy Shevchenko
2020-09-24  9:08         ` Kent Gibson
2020-09-24 12:46       ` Kent Gibson
2020-09-25  9:57         ` Andy Shevchenko
2020-09-25 12:16           ` Kent Gibson
2020-09-22  2:31 ` [PATCH v9 12/20] gpiolib: cdev: support setting debounce Kent Gibson
2020-09-23 16:27   ` Andy Shevchenko
2020-09-24  7:48     ` Kent Gibson
2020-09-25  9:43       ` Andy Shevchenko
2020-09-22  2:31 ` [PATCH v9 13/20] gpio: uapi: document uAPI v1 as deprecated Kent Gibson
2020-09-22  7:49   ` Arnd Bergmann
2020-09-22  8:11     ` Andy Shevchenko
2020-09-22  9:05     ` Kent Gibson
2020-09-22  2:31 ` [PATCH v9 14/20] tools: gpio: port lsgpio to v2 uAPI Kent Gibson
2020-09-22  2:31 ` [PATCH v9 15/20] tools: gpio: port gpio-watch " Kent Gibson
2020-09-22  2:31 ` [PATCH v9 16/20] tools: gpio: rename nlines to num_lines Kent Gibson
2020-09-22  2:31 ` [PATCH v9 17/20] tools: gpio: port gpio-hammer to v2 uAPI Kent Gibson
2020-09-23 16:30   ` Andy Shevchenko
2020-09-24  7:50     ` Kent Gibson
2020-09-22  2:31 ` [PATCH v9 18/20] tools: gpio: port gpio-event-mon " Kent Gibson
2020-09-22  2:31 ` [PATCH v9 19/20] tools: gpio: add multi-line monitoring to gpio-event-mon Kent Gibson
2020-09-22  2:31 ` [PATCH v9 20/20] tools: gpio: add debounce support " Kent Gibson
2020-09-23 16:35 ` [PATCH v9 00/20] gpio: cdev: add uAPI v2 Andy Shevchenko
2020-09-24  8:00   ` Kent Gibson
2020-09-25  9:44     ` Andy Shevchenko

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