linux-kernel - [PATCH 1/2] LoongArch: Add 128-bit atomic cmpxchg support

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Message-Id: <20251120-2-v1-1-705bdc440550@linux.dev>
Date: Thu, 20 Nov 2025 15:45:44 +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 1/2] LoongArch: Add 128-bit atomic cmpxchg support

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

Implement 128-bit atomic compare-and-exchange using LoongArch's
LL.D/SC.Q instructions.

At the same time, fix BPF scheduler test failures (scx_central scx_qmap)
caused by kmalloc_nolock_noprof returning NULL due to missing
128-bit atomics. The NULL returns led to -ENOMEM errors during
scheduler initialization, causing test cases to fail.

Verified by testing with the scx_qmap scheduler (located in
tools/sched_ext/). Building with `make` and running
./tools/sched_ext/build/bin/scx_qmap.

Signed-off-by: George Guo <guodongtai@...inos.cn>
---
 arch/loongarch/include/asm/cmpxchg.h | 46 ++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/arch/loongarch/include/asm/cmpxchg.h b/arch/loongarch/include/asm/cmpxchg.h
index 979fde61bba8a42cb4f019f13ded2a3119d4aaf4..5f8d418595cf62ec3153dd3825d80ac1fb31e883 100644
--- a/arch/loongarch/include/asm/cmpxchg.h
+++ b/arch/loongarch/include/asm/cmpxchg.h
@@ -111,6 +111,43 @@ __arch_xchg(volatile void *ptr, unsigned long x, int size)
 	__ret;								\
 })
 
+union __u128_halves {
+	u128 full;
+	struct {
+		u64 low;
+		u64 high;
+	};
+};
+
+#define __cmpxchg128_asm(ld, st, ptr, old, new)				\
+({									\
+	union __u128_halves __old, __new, __ret;			\
+	volatile u64 *__ptr = (volatile u64 *)(ptr);			\
+									\
+	__old.full = (old);                                             \
+	__new.full = (new);						\
+									\
+	__asm__ __volatile__(						\
+	"1:   " ld "  %0, %4          # 128-bit cmpxchg low  \n"	\
+	"     " ld "  %1, %5          # 128-bit cmpxchg high \n"	\
+	"     bne     %0, %z6, 2f			     \n"	\
+	"     bne     %1, %z7, 2f                            \n"	\
+	"     move    $t0, %z8				     \n"	\
+	"     move    $t1, %z9				     \n"	\
+	"     " st "  $t0, $t1, %2                           \n"	\
+	"     beqz    $t0, 1b                                \n"	\
+	"2:                                                  \n"	\
+	__WEAK_LLSC_MB							\
+	: "=&r" (__ret.low), "=&r" (__ret.high),			\
+	  "=ZB" (__ptr[0]), "=ZB" (__ptr[1])				\
+	: "ZB" (__ptr[0]), "ZB" (__ptr[1]),				\
+	  "Jr" (__old.low), "Jr" (__old.high),				\
+	  "Jr" (__new.low), "Jr" (__new.high)				\
+	: "t0", "t1", "memory");					\
+									\
+	__ret.full;							\
+})
+
 static inline unsigned int __cmpxchg_small(volatile void *ptr, unsigned int old,
 					   unsigned int new, unsigned int size)
 {
@@ -198,6 +235,15 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, unsigned int
 	__res;								\
 })
 
+/* cmpxchg128 */
+#define system_has_cmpxchg128()		1
+
+#define arch_cmpxchg128(ptr, o, n)					\
+({									\
+	BUILD_BUG_ON(sizeof(*(ptr)) != 16);				\
+	__cmpxchg128_asm("ll.d", "sc.d", ptr, o, n);			\
+})
+
 #ifdef CONFIG_64BIT
 #define arch_cmpxchg64_local(ptr, o, n)					\
   ({									\

-- 
2.48.1