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@gmail.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Len Brown <len.brown@intel.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
 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

On Mon, Jan 16, 2023 at 5:25 PM Uros Bizjak <ubizjak@gmail.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@gmail.com>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: Len Brown <len.brown@intel.com>
> Cc: Pavel Machek <pavel@ucw.cz>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.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
>

* Rafael J. Wysocki <rafael@kernel.org> wrote:

> On Mon, Jan 16, 2023 at 5:25 PM Uros Bizjak <ubizjak@gmail.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@gmail.com>
> > Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> > Cc: Len Brown <len.brown@intel.com>
> > Cc: Pavel Machek <pavel@ucw.cz>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> 
> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.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

On Thu, Jan 26, 2023 at 11:48 AM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> > On Mon, Jan 16, 2023 at 5:25 PM Uros Bizjak <ubizjak@gmail.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@gmail.com>
> > > Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> > > Cc: Len Brown <len.brown@intel.com>
> > > Cc: Pavel Machek <pavel@ucw.cz>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Ingo Molnar <mingo@redhat.com>
> > > Cc: Borislav Petkov <bp@alien8.de>
> > > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > > Cc: "H. Peter Anvin" <hpa@zytor.com>
> >
> > Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.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!

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     890a0794b34f89fcd90e94ec970ad2bc18b70e73
Gitweb:        https://git.kernel.org/tip/890a0794b34f89fcd90e94ec970ad2bc18b70e73
Author:        Uros Bizjak <ubizjak@gmail.com>
AuthorDate:    Mon, 16 Jan 2023 17:25:22 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Thu, 26 Jan 2023 11:49:40 +01:00

x86/ACPI/boot: Use try_cmpxchg() in __acpi_{acquire,release}_global_lock()

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 MOV 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@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20230116162522.4072-1-ubizjak@gmail.com
---
 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 907cc98..4177577 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;
 }