linux-kernel - [tip: locking/core] x86/ACPI/boot: Use try_cmpxchg() in __acpi_{acquire,release}_global

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Date:   Thu, 26 Jan 2023 11:38:42 -0000
From:   "tip-bot2 for Uros Bizjak" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Ingo Molnar <mingo@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
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 <ubizjak@...il.com>
AuthorDate:    Mon, 16 Jan 2023 17:25:22 +01:00
Committer:     Ingo Molnar <mingo@...nel.org>
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 <ubizjak@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Link: https://lore.kernel.org/r/20230116162522.4072-1-ubizjak@gmail.com
---
 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;
 }