linux-kernel - [PATCH] x86/asm: Implement local_xchg using CMPXCHG without lock prefix

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Date: Wed, 24 Jan 2024 11:58:16 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86/asm: Implement local_xchg using CMPXCHG without lock prefix

Implement local_xchg using CMPXCHG instruction without lock prefix.
XCHG is expensive due to the implied lock prefix.  The processor
cannot prefetch cachelines if XCHG is used.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
---
 arch/x86/include/asm/local.h | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/local.h b/arch/x86/include/asm/local.h
index 73dba8b94443..f9af6908aa2f 100644
--- a/arch/x86/include/asm/local.h
+++ b/arch/x86/include/asm/local.h
@@ -131,8 +131,20 @@ static inline bool local_try_cmpxchg(local_t *l, long *old, long new)
 				 (typeof(l->a.counter) *) old, new);
 }
 
-/* Always has a lock prefix */
-#define local_xchg(l, n) (xchg(&((l)->a.counter), (n)))
+/*
+ * Implement local_xchg using CMPXCHG instruction without lock prefix.
+ * XCHG is expensive due to the implied lock prefix.  The processor
+ * cannot prefetch cachelines if XCHG is used.
+ */
+static __always_inline long
+local_xchg(local_t *l, long n)
+{
+	long c = local_read(l);
+
+	do { } while (!local_try_cmpxchg(l, &c, n));
+
+	return c;
+}
 
 /**
  * local_add_unless - add unless the number is already a given value
-- 
2.31.1