linux-kernel - [PATCH v4 3/4] LoongArch: Use spinlock to emulate 128-bit cmpxchg

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Message-Id: <20251205-2-v4-3-e5ab932cf219@linux.dev>
Date: Fri, 05 Dec 2025 14:29:06 +0800
From: George Guo <dongtai.guo@...ux.dev>
To: Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>
Cc: loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org, 
 George Guo <dongtai.guo@...ux.dev>, George Guo <guodongtai@...inos.cn>
Subject: [PATCH v4 3/4] LoongArch: Use spinlock to emulate 128-bit cmpxchg

From: George Guo <guodongtai@...inos.cn>

For LoongArch CPUs lacking 128-bit atomic instruction(e.g.,
the SCQ instruction on 3A5000), provide a fallback implementation
of __cmpxchg128 using a spinlock to emulate the atomic operation.

Signed-off-by: George Guo <guodongtai@...inos.cn>
---
 arch/loongarch/include/asm/cmpxchg.h | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/arch/loongarch/include/asm/cmpxchg.h b/arch/loongarch/include/asm/cmpxchg.h
index f7a0a9a032c513196ef186a5493b500787e0e9b6..814097bfc334184018747e47fb90fd2d2fb27ee2 100644
--- a/arch/loongarch/include/asm/cmpxchg.h
+++ b/arch/loongarch/include/asm/cmpxchg.h
@@ -8,6 +8,7 @@
 #include <linux/bits.h>
 #include <linux/build_bug.h>
 #include <asm/barrier.h>
+#include <asm/cpu-features.h>
 
 #define __xchg_asm(amswap_db, m, val)		\
 ({						\
@@ -149,6 +150,23 @@ union __u128_halves {
 	__ret.full;							\
 })
 
+#define __cmpxchg128_locked(ptr, old, new)				\
+({									\
+	u128 __ret;							\
+	static DEFINE_SPINLOCK(lock);					\
+	unsigned long flags;						\
+									\
+	spin_lock_irqsave(&lock, flags);				\
+									\
+	__ret = *(volatile u128 *)(ptr);				\
+	if (__ret == (old))						\
+		*(volatile u128 *)(ptr) = (new);			\
+									\
+	spin_unlock_irqrestore(&lock, flags);				\
+									\
+	__ret;								\
+})
+
 static inline unsigned int __cmpxchg_small(volatile void *ptr, unsigned int old,
 					   unsigned int new, unsigned int size)
 {
@@ -242,7 +260,8 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, unsigned int
 #define arch_cmpxchg128(ptr, o, n)					\
 ({									\
 	BUILD_BUG_ON(sizeof(*(ptr)) != 16);				\
-	__cmpxchg128_asm(ptr, o, n);					\
+	cpu_has_scq ? __cmpxchg128_asm(ptr, o, n) :			\
+			__cmpxchg128_locked(ptr, o, n);			\
 })
 
 #ifdef CONFIG_64BIT

-- 
2.49.0