linux-kernel - Re: [PATCH v4 2/5] arm64: barrier: Add smp_cond_load_relaxed

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Message-ID: <aLWHzEAx_-BozA4V@arm.com>
Date: Mon, 1 Sep 2025 12:47:24 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org,
	arnd@...db.de, will@...nel.org, peterz@...radead.org,
	akpm@...ux-foundation.org, mark.rutland@....com,
	harisokn@...zon.com, cl@...two.org, ast@...nel.org,
	memxor@...il.com, zhenglifeng1@...wei.com,
	xueshuai@...ux.alibaba.com, joao.m.martins@...cle.com,
	boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v4 2/5] arm64: barrier: Add
 smp_cond_load_relaxed_timewait()

On Fri, Aug 29, 2025 at 01:07:32AM -0700, Ankur Arora wrote:
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index f5801b0ba9e9..9b29abc212db 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -219,6 +219,28 @@ do {									\
>  	(typeof(*ptr))VAL;						\
>  })
>  
> +extern bool arch_timer_evtstrm_available(void);

In theory, this doesn't work if CONFIG_ARM_ARCH_TIMER is disabled,
though that's not the case for arm64. Maybe add a short comment that's
re-declared here to avoid include dependencies.

> +
> +#define smp_cond_load_relaxed_timewait(ptr, cond_expr, time_check_expr)	\
> +({									\
> +	typeof(ptr) __PTR = (ptr);					\
> +	__unqual_scalar_typeof(*ptr) VAL;				\
> +	bool __wfe = arch_timer_evtstrm_available();			\
> +									\
> +	for (;;) {							\
> +		VAL = READ_ONCE(*__PTR);				\
> +		if (cond_expr)						\
> +			break;						\
> +		if (time_check_expr)					\
> +			break;						\
> +		if (likely(__wfe))					\
> +			__cmpwait_relaxed(__PTR, VAL);			\
> +		else							\
> +			cpu_relax();					\
> +	}								\
> +	(typeof(*ptr)) VAL;						\
> +})
> +
>  #include <asm-generic/barrier.h>
>  
>  #endif	/* __ASSEMBLY__ */

Reviewed-by: Catalin Marinas <catalin.marinas@....com>