linux-kernel - [PATCH 1/2] x86/percpu: Define this_percpu_xchg

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Date: Wed, 20 Mar 2024 09:30:40 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH 1/2] x86/percpu: Define this_percpu_xchg_op()

Rewrite percpu_xchg_op() using generic percpu primitives instead
of using asm. The new implementation is similar to local_xchg() and
allows the compiler to perform various optimizations (e.g. the
compiler is able to create fast path through the loop, according
to likely/unlikely annotations in percpu_try_cmpxchg_op).

No functional changes intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
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/percpu.h | 32 +++++++++++---------------------
 1 file changed, 11 insertions(+), 21 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 44958ebaf626..de991e6d050a 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -230,25 +230,15 @@ do {									\
 })
 
 /*
- * xchg is implemented using cmpxchg without a lock prefix. xchg is
- * expensive due to the implied lock prefix.  The processor cannot prefetch
- * cachelines if xchg is used.
+ * this_cpu_xchg() is implemented using cmpxchg without a lock prefix.
+ * xchg is expensive due to the implied lock prefix. The processor
+ * cannot prefetch cachelines if xchg is used.
  */
-#define percpu_xchg_op(size, qual, _var, _nval)				\
+#define this_percpu_xchg_op(_var, _nval)				\
 ({									\
-	__pcpu_type_##size pxo_old__;					\
-	__pcpu_type_##size pxo_new__ = __pcpu_cast_##size(_nval);	\
-	asm qual (__pcpu_op2_##size("mov", __percpu_arg([var]),		\
-				    "%[oval]")				\
-		  "\n1:\t"						\
-		  __pcpu_op2_##size("cmpxchg", "%[nval]",		\
-				    __percpu_arg([var]))		\
-		  "\n\tjnz 1b"						\
-		  : [oval] "=&a" (pxo_old__),				\
-		    [var] "+m" (__my_cpu_var(_var))			\
-		  : [nval] __pcpu_reg_##size(, pxo_new__)		\
-		  : "memory");						\
-	(typeof(_var))(unsigned long) pxo_old__;			\
+	typeof(_var) pxo_old__ = this_cpu_read(_var);			\
+	do { } while (!this_cpu_try_cmpxchg(_var, &pxo_old__, _nval));	\
+	pxo_old__;							\
 })
 
 /*
@@ -534,9 +524,9 @@ do {									\
 #define this_cpu_or_1(pcp, val)		percpu_to_op(1, volatile, "or", (pcp), val)
 #define this_cpu_or_2(pcp, val)		percpu_to_op(2, volatile, "or", (pcp), val)
 #define this_cpu_or_4(pcp, val)		percpu_to_op(4, volatile, "or", (pcp), val)
-#define this_cpu_xchg_1(pcp, nval)	percpu_xchg_op(1, volatile, pcp, nval)
-#define this_cpu_xchg_2(pcp, nval)	percpu_xchg_op(2, volatile, pcp, nval)
-#define this_cpu_xchg_4(pcp, nval)	percpu_xchg_op(4, volatile, pcp, nval)
+#define this_cpu_xchg_1(pcp, nval)	this_percpu_xchg_op(pcp, nval)
+#define this_cpu_xchg_2(pcp, nval)	this_percpu_xchg_op(pcp, nval)
+#define this_cpu_xchg_4(pcp, nval)	this_percpu_xchg_op(pcp, nval)
 
 #define raw_cpu_add_return_1(pcp, val)		percpu_add_return_op(1, , pcp, val)
 #define raw_cpu_add_return_2(pcp, val)		percpu_add_return_op(2, , pcp, val)
@@ -575,7 +565,7 @@ do {									\
 #define this_cpu_and_8(pcp, val)		percpu_to_op(8, volatile, "and", (pcp), val)
 #define this_cpu_or_8(pcp, val)			percpu_to_op(8, volatile, "or", (pcp), val)
 #define this_cpu_add_return_8(pcp, val)		percpu_add_return_op(8, volatile, pcp, val)
-#define this_cpu_xchg_8(pcp, nval)		percpu_xchg_op(8, volatile, pcp, nval)
+#define this_cpu_xchg_8(pcp, nval)		this_percpu_xchg_op(pcp, nval)
 #define this_cpu_cmpxchg_8(pcp, oval, nval)	percpu_cmpxchg_op(8, volatile, pcp, oval, nval)
 #define this_cpu_try_cmpxchg_8(pcp, ovalp, nval)	percpu_try_cmpxchg_op(8, volatile, pcp, ovalp, nval)
 #endif
-- 
2.44.0