linux-kernel - Re: [PATCH v2 4/5] gpiolib: cdev: reduce locking in gpio_desc_to

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Date:   Thu, 14 Dec 2023 23:34:44 +0800
From:   Kent Gibson <warthog618@...il.com>
To:     Andy Shevchenko <andy@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
        brgl@...ev.pl, linus.walleij@...aro.org
Subject: Re: [PATCH v2 4/5] gpiolib: cdev: reduce locking in
 gpio_desc_to_lineinfo()

On Thu, Dec 14, 2023 at 05:27:29PM +0200, Andy Shevchenko wrote:
> On Thu, Dec 14, 2023 at 11:19:01PM +0800, Kent Gibson wrote:
> > On Thu, Dec 14, 2023 at 05:10:23PM +0200, Andy Shevchenko wrote:
> > > On Thu, Dec 14, 2023 at 05:58:13PM +0800, Kent Gibson wrote:
> > > > Reduce the time holding the gpio_lock by snapshotting the desc flags,
> > > > rather than testing them individually while holding the lock.
> > > >
> > > > Accept that the calculation of the used field is inherently racy, and
> > > > only check the availability of the line from pinctrl if other checks
> > > > pass, so avoiding the check for lines that are otherwise in use.
>
> ...
>
> > > > -	spin_lock_irqsave(&gpio_lock, flags);
> > >
> > > Shouldn't this be covered by patch 1 (I mean conversion to scoped_guard()
> > > instead of spinlock)?
> > >
> >
> > Read the cover letter.
> > Doing that made the change larger, as flags gets removed then restored.
> > I had also thought the flag tests would get indented then unindented, but
> > if we use guard() the indentation should remain unchanged.
>
> I'm fine with that as I pointed out (have you received that mail? I had
> problems with my mail server) the dflags is better semantically, so restoration
> with _different_ name is fine.
>

I have noted that some of your replies have been delayed, and I can't be sure
of what I might not've received. I can't say I've seen one that mentions the
dflags name being preferable.

I prefer the plain flags name, if there is only one flag variable in the
function.

> > Can do it in 1 if you are happy with the flags declaration being
> > removed in patch 1 and restored in 4.
>
> Definitely.
>

Ok will re-arrange in v3.

Cheers,
Kent.