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

diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index 907cc98..4177577 100644 (file)
--- 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;
 }