linux-kernel - [PATCH V2 3/3] riscv: xchg: Prefetch the destination word for sc.w

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Date: Sun, 31 Dec 2023 03:29:53 -0500
From: guoren@...nel.org
To: paul.walmsley@...ive.com,
	palmer@...belt.com,
	guoren@...nel.org,
	panqinglin2020@...as.ac.cn,
	bjorn@...osinc.com,
	conor.dooley@...rochip.com,
	leobras@...hat.com,
	peterz@...radead.org,
	keescook@...omium.org,
	wuwei2016@...as.ac.cn,
	xiaoguang.xing@...hgo.com,
	chao.wei@...hgo.com,
	unicorn_wang@...look.com,
	uwu@...nowy.me,
	jszhang@...nel.org,
	wefu@...hat.com,
	atishp@...shpatra.org,
	ajones@...tanamicro.com
Cc: linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Guo Ren <guoren@...ux.alibaba.com>
Subject: [PATCH V2 3/3] riscv: xchg: Prefetch the destination word for sc.w

From: Guo Ren <guoren@...ux.alibaba.com>

The cost of changing a cacheline from shared to exclusive state can be
significant, especially when this is triggered by an exclusive store,
since it may result in having to retry the transaction.

This patch makes use of prefetch.w to prefetch cachelines for write
prior to lr/sc loops when using the xchg_small atomic routine.

This patch is inspired by commit: 0ea366f5e1b6 ("arm64: atomics:
prefetch the destination word for write prior to stxr").

Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
Signed-off-by: Guo Ren <guoren@...nel.org>
---
 arch/riscv/include/asm/cmpxchg.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
index 26cea2395aae..d7b9d7951f08 100644
--- a/arch/riscv/include/asm/cmpxchg.h
+++ b/arch/riscv/include/asm/cmpxchg.h
@@ -10,6 +10,7 @@
 
 #include <asm/barrier.h>
 #include <asm/fence.h>
+#include <asm/processor.h>
 
 #define __arch_xchg_masked(prepend, append, r, p, n)			\
 ({									\
@@ -23,6 +24,7 @@
 									\
 	__asm__ __volatile__ (						\
 	       prepend							\
+	       PREFETCHW_ASM(%5)					\
 	       "0:	lr.w %0, %2\n"					\
 	       "	and  %1, %0, %z4\n"				\
 	       "	or   %1, %1, %z3\n"				\
@@ -30,7 +32,7 @@
 	       "	bnez %1, 0b\n"					\
 	       append							\
 	       : "=&r" (__retx), "=&r" (__rc), "+A" (*(__ptr32b))	\
-	       : "rJ" (__newx), "rJ" (~__mask)				\
+	       : "rJ" (__newx), "rJ" (~__mask), "rJ" (__ptr32b)		\
 	       : "memory");						\
 									\
 	r = (__typeof__(*(p)))((__retx & __mask) >> __s);		\
-- 
2.40.1