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

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Message-ID: <20251120111435.1382b52e@pumpkin>
Date: Thu, 20 Nov 2025 11:14:35 +0000
From: david laight <david.laight@...box.com>
To: George Guo <dongtai.guo@...ux.dev>
Cc: Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>,
 loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org, George Guo
 <guodongtai@...inos.cn>
Subject: Re: [PATCH 1/2] LoongArch: Add 128-bit atomic cmpxchg support

On Thu, 20 Nov 2025 15:45:44 +0800
George Guo <dongtai.guo@...ux.dev> wrote:

> 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");					\

I'd add symbolic names for the asm registers to it easier to read.
eg: [ret_low] "=%r" (__ret.low) and replace %0 with %[rel_row]

	David

> +									\
> +	__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)					\
>    ({									\
>