linux-kernel - Re: [PATCH v7 RESEND 5/6] arm64: futex: small optimisation for __llsc_futex_atomic

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Message-ID: <aMLqsez5y9R6FIdJ@willie-the-truck>
Date: Thu, 11 Sep 2025 16:28:49 +0100
From: Will Deacon <will@...nel.org>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: catalin.marinas@....com, broonie@...nel.org, maz@...nel.org,
	oliver.upton@...ux.dev, joey.gouly@....com, james.morse@....com,
	ardb@...nel.org, scott@...amperecomputing.com,
	suzuki.poulose@....com, yuzenghui@...wei.com, mark.rutland@....com,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 RESEND 5/6] arm64: futex: small optimisation for
 __llsc_futex_atomic_set()

On Sat, Aug 16, 2025 at 04:19:28PM +0100, Yeoreum Yun wrote:
> __llsc_futex_atomic_set() is implmented using
> LLSC_FUTEX_ATOMIC_OP() macro with "mov  %w3, %w5".
> But this instruction isn't required to implement fux_atomic_set()
> so make a small optimisation by implementing __llsc_futex_atomic_set()
> as separate function.
> 
> This will make usage of LLSC_FUTEX_ATOMIC_OP() macro more simple.
> 
> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> ---
>  arch/arm64/include/asm/futex.h | 43 ++++++++++++++++++++++++++++------
>  1 file changed, 36 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
> index ab7003cb4724..22a6301a9f3d 100644
> --- a/arch/arm64/include/asm/futex.h
> +++ b/arch/arm64/include/asm/futex.h
> @@ -13,7 +13,7 @@
> 
>  #define LLSC_MAX_LOOPS	128 /* What's the largest number you can think of? */
> 
> -#define LLSC_FUTEX_ATOMIC_OP(op, insn)					\
> +#define LLSC_FUTEX_ATOMIC_OP(op, asm_op)				\
>  static __always_inline int						\
>  __llsc_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval)	\
>  {									\
> @@ -24,7 +24,7 @@ __llsc_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval)	\
>  	asm volatile("// __llsc_futex_atomic_" #op "\n"			\
>  "	prfm	pstl1strm, %2\n"					\
>  "1:	ldxr	%w1, %2\n"						\
> -	insn "\n"							\
> +"	" #asm_op "	%w3, %w1, %w5\n"				\
>  "2:	stlxr	%w0, %w3, %2\n"						\
>  "	cbz	%w0, 3f\n"						\
>  "	sub	%w4, %w4, %w0\n"					\
> @@ -46,11 +46,40 @@ __llsc_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval)	\
>  	return ret;							\
>  }
> 
> -LLSC_FUTEX_ATOMIC_OP(add, "add	%w3, %w1, %w5")
> -LLSC_FUTEX_ATOMIC_OP(or, "orr	%w3, %w1, %w5")
> -LLSC_FUTEX_ATOMIC_OP(and, "and	%w3, %w1, %w5")
> -LLSC_FUTEX_ATOMIC_OP(eor, "eor	%w3, %w1, %w5")
> -LLSC_FUTEX_ATOMIC_OP(set, "mov	%w3, %w5")
> +LLSC_FUTEX_ATOMIC_OP(add, add)
> +LLSC_FUTEX_ATOMIC_OP(or, orr)
> +LLSC_FUTEX_ATOMIC_OP(and, and)
> +LLSC_FUTEX_ATOMIC_OP(eor, eor)
> +
> +static __always_inline int
> +__llsc_futex_atomic_set(int oparg, u32 __user *uaddr, int *oval)
> +{
> +	unsigned int loops = LLSC_MAX_LOOPS;
> +	int ret, oldval;
> +
> +	uaccess_enable_privileged();
> +	asm volatile("//__llsc_futex_xchg\n"
> +"	prfm	pstl1strm, %2\n"
> +"1:	ldxr	%w1, %2\n"
> +"2:	stlxr	%w0, %w4, %2\n"
> +"	cbz	%w3, 3f\n"
> +"	sub	%w3, %w3, %w0\n"
> +"	cbnz	%w3, 1b\n"
> +"	mov	%w0, %w5\n"
> +"3:\n"
> +"	dmb	ish\n"
> +	_ASM_EXTABLE_UACCESS_ERR(1b, 3b, %w0)
> +	_ASM_EXTABLE_UACCESS_ERR(2b, 3b, %w0)
> +	: "=&r" (ret), "=&r" (oldval), "+Q" (*uaddr), "+r" (loops)
> +	: "r" (oparg), "Ir" (-EAGAIN)
> +	: "memory");
> +	uaccess_disable_privileged();
> +
> +	if (!ret)
> +		*oval = oldval;

Hmm, I'm really not sure this is worthwhile. I doubt the "optimisation"
actually does anything and adding a whole new block of asm just for the
SET case isn't much of an improvement on the maintainability side, either.

Will