linux-kernel - Re: [PATCH] x86/ACPI/boot: use try_cmpxchg in __acpi_{acquire,release}_global

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Date:   Thu, 26 Jan 2023 12:46:46 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Uros Bizjak <ubizjak@...il.com>, linux-pm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH] x86/ACPI/boot: use try_cmpxchg in __acpi_{acquire,release}_global_lock

On Thu, Jan 26, 2023 at 11:48 AM Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> > On Mon, Jan 16, 2023 at 5:25 PM Uros Bizjak <ubizjak@...il.com> wrote:
> > >
> > > Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
> > > __acpi_{acquire,release}_global_lock.  x86 CMPXCHG instruction returns
> > > success in ZF flag, so this change saves a compare after cmpxchg
> > > (and related move instruction in front of cmpxchg).
> > >
> > > Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
> > > fails. There is no need to re-read the value in the loop.
> > >
> > > Note that the value from *ptr should be read using READ_ONCE to prevent
> > > the compiler from merging, refetching or reordering the read.
> > >
> > > No functional change intended.
> > >
> > > Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> > > Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> > > Cc: Len Brown <len.brown@...el.com>
> > > Cc: Pavel Machek <pavel@....cz>
> > > Cc: Thomas Gleixner <tglx@...utronix.de>
> > > Cc: Ingo Molnar <mingo@...hat.com>
> > > Cc: Borislav Petkov <bp@...en8.de>
> > > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > > Cc: "H. Peter Anvin" <hpa@...or.com>
> >
> > Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > but please CC linux-acpi on all ACPI-related changes in the future.
>
> Thanks Rafael - I'll take this patch through the locking tree, if that's
> fine with you.

It is, thanks!