linux-kernel - Re: [PATCH RESEND v7 4/6] arm64: futex: refactor futex atomic operation

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Message-ID: <aMLs2LcnflSw2uNr@willie-the-truck>
Date: Thu, 11 Sep 2025 16:38:00 +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 RESEND v7 4/6] arm64: futex: refactor futex atomic
 operation

On Sat, Aug 16, 2025 at 04:19:27PM +0100, Yeoreum Yun wrote:
> Refactor futex atomic operations using ll/sc method with
> clearing PSTATE.PAN to prepare to apply FEAT_LSUI on them.
> 
> Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> ---
>  arch/arm64/include/asm/futex.h | 132 +++++++++++++++++++++------------
>  1 file changed, 84 insertions(+), 48 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
> index bc06691d2062..ab7003cb4724 100644
> --- a/arch/arm64/include/asm/futex.h
> +++ b/arch/arm64/include/asm/futex.h
> @@ -7,17 +7,21 @@
> 
>  #include <linux/futex.h>
>  #include <linux/uaccess.h>
> +#include <linux/stringify.h>
> 
>  #include <asm/errno.h>
> 
> -#define FUTEX_MAX_LOOPS	128 /* What's the largest number you can think of? */
> +#define LLSC_MAX_LOOPS	128 /* What's the largest number you can think of? */
> 
> -#define __futex_atomic_op(insn, ret, oldval, uaddr, tmp, oparg)		\
> -do {									\
> -	unsigned int loops = FUTEX_MAX_LOOPS;				\
> +#define LLSC_FUTEX_ATOMIC_OP(op, insn)					\
> +static __always_inline int						\
> +__llsc_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval)	\
> +{									\
> +	unsigned int loops = LLSC_MAX_LOOPS;				\
> +	int ret, oldval, tmp;						\
>  									\
>  	uaccess_enable_privileged();					\
> -	asm volatile(							\
> +	asm volatile("// __llsc_futex_atomic_" #op "\n"			\
>  "	prfm	pstl1strm, %2\n"					\
>  "1:	ldxr	%w1, %2\n"						\
>  	insn "\n"							\
> @@ -35,45 +39,103 @@ do {									\
>  	: "r" (oparg), "Ir" (-EAGAIN)					\
>  	: "memory");							\
>  	uaccess_disable_privileged();					\
> -} while (0)
> +									\
> +	if (!ret)							\
> +		*oval = oldval;						\

Why push the store to '*oval' down into here?

Will