linux-kernel - [PATCH] LoongArch: Fixup cmpxchg sematic for memory barrier

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Date:   Mon, 31 Jul 2023 21:15:54 -0400
From:   guoren@...nel.org
To:     chenhuacai@...nel.or, kernel@...0n.name, arnd@...db.de,
        andi.shyti@...ux.intel.com, wangrui@...ngson.cn,
        andrzej.hajda@...el.com, peterz@...radead.org, will@...nel.org,
        boqun.feng@...il.com, mark.rutland@....com, guoren@...nel.org
Cc:     loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: [PATCH] LoongArch: Fixup cmpxchg sematic for memory barrier

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

When cmpxchg failed, no memory barrier was needed to take. Only
when cmpxchg success and the new value is written, then the memory
barriers needed.

 - cmpxchg_asm:   Unnecessary __WEAK_LLSC_WB for the fail path would
		  reduce the performance of the cmpxchg loop trying.
 - cmpxchg_small: Miss an necessary __WEAK_LLSC_WB when sc succeeds.
		  It's a bug because there is no memory synchronization
		  when sc succeeds.

Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
Signed-off-by: Guo Ren <guoren@...nel.org>
---
 arch/loongarch/include/asm/cmpxchg.h | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/loongarch/include/asm/cmpxchg.h b/arch/loongarch/include/asm/cmpxchg.h
index 979fde61bba8..6a05b92814b6 100644
--- a/arch/loongarch/include/asm/cmpxchg.h
+++ b/arch/loongarch/include/asm/cmpxchg.h
@@ -102,8 +102,8 @@ __arch_xchg(volatile void *ptr, unsigned long x, int size)
 	"	move	$t0, %z4			\n"		\
 	"	" st "	$t0, %1				\n"		\
 	"	beqz	$t0, 1b				\n"		\
-	"2:						\n"		\
 	__WEAK_LLSC_MB							\
+	"2:						\n"		\
 	: "=&r" (__ret), "=ZB"(*m)					\
 	: "ZB"(*m), "Jr" (old), "Jr" (new)				\
 	: "t0", "memory");						\
@@ -148,10 +148,8 @@ static inline unsigned int __cmpxchg_small(volatile void *ptr, unsigned int old,
 	"	or		%1, %1, %z6	\n"
 	"	sc.w		%1, %2		\n"
 	"	beqz		%1, 1b		\n"
-	"	b		3f		\n"
-	"2:					\n"
 	__WEAK_LLSC_MB
-	"3:					\n"
+	"2:					\n"
 	: "=&r" (old32), "=&r" (temp), "=ZC" (*ptr32)
 	: "ZC" (*ptr32), "Jr" (mask), "Jr" (old), "Jr" (new)
 	: "memory");
-- 
2.36.1