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

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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
This commit is contained in:
Uros Bizjak
2023-01-16 17:25:22 +01:00
committed by Ingo Molnar
parent 50fd4d5e69
commit 890a0794b3
1 changed files with 8 additions and 8 deletions
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16
arch/x86/kernel/acpi/boot.c
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@@ -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;
}