Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Tue, 17 Jan 2023 13:39:58 +0100 | Subject | Re: [PATCH] x86/ACPI/boot: use try_cmpxchg in __acpi_{acquire,release}_global_lock |
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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 >
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