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From:   Uros Bizjak <ubizjak@gmail.com>
To:     linux-pm@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Uros Bizjak <ubizjak@gmail.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: [PATCH] x86/ACPI/boot: use try_cmpxchg in __acpi_{acquire,release}_global_lock
Date:   Mon, 16 Jan 2023 17:25:22 +0100
Message-Id: <20230116162522.4072-1-ubizjak@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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>
---
 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