Re: [PATCH v9 08/20] gpiolib: cdev: support GPIO_V2_GET_LINEINFO_IOCTL and GPIO_V2_GET_LINEINFO_WATCH_IOCTL

[Kent Gibson]

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@xxxxxxxxx> 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.