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

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Message-ID: <Y9Jagh6eOEkcudL4@gmail.com>
Date:   Thu, 26 Jan 2023 11:48:34 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     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


* 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.

Thanks,

	Ingo