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

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Message-ID: <CAJZ5v0j7i0FG_ivX1xhTPutJOXb8nZEv7kq+TTOCjynts9049w@mail.gmail.com>
Date:   Tue, 17 Jan 2023 13:39:58 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     linux-pm@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.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 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.

> ---
>  arch/x86/kernel/acpi/boot.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> index 907cc98b1938..4177577c173b 100644
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -1840,23 +1840,23 @@ early_param("acpi_sci", setup_acpi_sci);
>
>  int __acpi_acquire_global_lock(unsigned int *lock)
>  {
> -       unsigned int old, new, val;
> +       unsigned int old, new;
> +
> +       old = READ_ONCE(*lock);
>         do {
> -               old = *lock;
>                 new = (((old & ~0x3) + 2) + ((old >> 1) & 0x1));
> -               val = cmpxchg(lock, old, new);
> -       } while (unlikely (val != old));
> +       } while (!try_cmpxchg(lock, &old, new));
>         return ((new & 0x3) < 3) ? -1 : 0;
>  }
>
>  int __acpi_release_global_lock(unsigned int *lock)
>  {
> -       unsigned int old, new, val;
> +       unsigned int old, new;
> +
> +       old = READ_ONCE(*lock);
>         do {
> -               old = *lock;
>                 new = old & ~0x3;
> -               val = cmpxchg(lock, old, new);
> -       } while (unlikely (val != old));
> +       } while (!try_cmpxchg(lock, &old, new));
>         return old & 0x1;
>  }
>
> --
> 2.39.0
>