Commit 890a0794 authored by Uros Bizjak's avatar Uros Bizjak Committed by Ingo Molnar
Browse files

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: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Acked-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20230116162522.4072-1-ubizjak@gmail.com
parent 50fd4d5e

arch/x86/kernel/acpi/boot.c

+8 −8
Original line number Original line Diff line number Diff line
@@ -1840,23 +1840,23 @@ early_param("acpi_sci", setup_acpi_sci); 




int __acpi_acquire_global_lock(unsigned int *lock)

int __acpi_acquire_global_lock(unsigned int *lock)

{

{

	unsigned int old, new, val;

	unsigned int old, new;



	old = READ_ONCE(*lock);

	do {

	do {

		old = *lock;

		new = (((old & ~0x3) + 2) + ((old >> 1) & 0x1));

		new = (((old & ~0x3) + 2) + ((old >> 1) & 0x1));

		val = cmpxchg(lock, old, new);

	} while (!try_cmpxchg(lock, &old, new));

	} while (unlikely (val != old));

	return ((new & 0x3) < 3) ? -1 : 0;

	return ((new & 0x3) < 3) ? -1 : 0;

}

}





int __acpi_release_global_lock(unsigned int *lock)

int __acpi_release_global_lock(unsigned int *lock)

{

{

	unsigned int old, new, val;

	unsigned int old, new;



	old = READ_ONCE(*lock);

	do {

	do {

		old = *lock;

		new = old & ~0x3;

		new = old & ~0x3;

		val = cmpxchg(lock, old, new);

	} while (!try_cmpxchg(lock, &old, new));

	} while (unlikely (val != old));

	return old & 0x1;

	return old & 0x1;

}

}