2023-01-16 17:55:50

by Uros Bizjak

Subject: [PATCH] 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 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 <[email protected]>
Cc: "Rafael J. Wysocki" <[email protected]>
Cc: Len Brown <[email protected]>
Cc: Pavel Machek <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
---
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

2023-01-17 12:46:49

by Rafael J. Wysocki

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 <[email protected]> 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 <[email protected]>
> Cc: "Rafael J. Wysocki" <[email protected]>
> Cc: Len Brown <[email protected]>
> Cc: Pavel Machek <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Borislav Petkov <[email protected]>
> Cc: Dave Hansen <[email protected]>
> Cc: "H. Peter Anvin" <[email protected]>

Acked-by: Rafael J. Wysocki <[email protected]>

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
>

2023-01-26 10:49:28

by Ingo Molnar

Subject: Re: [PATCH] x86/ACPI/boot: use try_cmpxchg in __acpi_{acquire,release}_global_lock

* Rafael J. Wysocki <[email protected]> wrote:

> On Mon, Jan 16, 2023 at 5:25 PM Uros Bizjak <[email protected]> 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 <[email protected]>
> > Cc: "Rafael J. Wysocki" <[email protected]>
> > Cc: Len Brown <[email protected]>
> > Cc: Pavel Machek <[email protected]>
> > Cc: Thomas Gleixner <[email protected]>
> > Cc: Ingo Molnar <[email protected]>
> > Cc: Borislav Petkov <[email protected]>
> > Cc: Dave Hansen <[email protected]>
> > Cc: "H. Peter Anvin" <[email protected]>
>
> Acked-by: Rafael J. Wysocki <[email protected]>
>
> 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

2023-01-26 11:38:53

by tip-bot2 for Alexey Makhalov

Subject: [tip: locking/core] x86/ACPI/boot: Use try_cmpxchg() in __acpi_{acquire,release}_global_lock()

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 <[email protected]>
AuthorDate: Mon, 16 Jan 2023 17:25:22 +01:00
Committer: Ingo Molnar <[email protected]>
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 <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Acked-by: Rafael J. Wysocki <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
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;
}

2023-01-26 11:47:05

by Rafael J. Wysocki

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 <[email protected]> wrote:
>
>
> * Rafael J. Wysocki <[email protected]> wrote:
>
> > On Mon, Jan 16, 2023 at 5:25 PM Uros Bizjak <[email protected]> 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 <[email protected]>
> > > Cc: "Rafael J. Wysocki" <[email protected]>
> > > Cc: Len Brown <[email protected]>
> > > Cc: Pavel Machek <[email protected]>
> > > Cc: Thomas Gleixner <[email protected]>
> > > Cc: Ingo Molnar <[email protected]>
> > > Cc: Borislav Petkov <[email protected]>
> > > Cc: Dave Hansen <[email protected]>
> > > Cc: "H. Peter Anvin" <[email protected]>
> >
> > Acked-by: Rafael J. Wysocki <[email protected]>
> >
> > 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!