linux-kernel - Re: [PATCH 3/4] arm64: barrier: Add smp_cond_load_relaxed

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Message-ID: <Z8dUg5zzclvDpPtZ@arm.com>
Date: Tue, 4 Mar 2025 19:29:07 +0000
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, arnd@...db.de,
	will@...nel.org, peterz@...radead.org, mark.rutland@....com,
	harisokn@...zon.com, cl@...two.org, memxor@...il.com,
	zhenglifeng1@...wei.com, joao.m.martins@...cle.com,
	boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH 3/4] arm64: barrier: Add smp_cond_load_relaxed_timewait()

On Mon, Feb 03, 2025 at 01:49:10PM -0800, Ankur Arora wrote:
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index 1ca947d5c939..25721275a5a2 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -216,6 +216,44 @@ do {									\
>  	(typeof(*ptr))VAL;						\
>  })
>  
> +#define __smp_cond_load_relaxed_timewait(ptr, cond_expr,		\
> +					 time_expr_ns, time_limit_ns)	\
> +({									\
> +	typeof(ptr) __PTR = (ptr);					\
> +	__unqual_scalar_typeof(*ptr) VAL;				\
> +	for (;;) {							\
> +		VAL = READ_ONCE(*__PTR);				\
> +		if (cond_expr)						\
> +			break;						\
> +		__cmpwait_relaxed(__PTR, VAL);				\
> +		if ((time_expr_ns) >= (time_limit_ns))			\
> +			break;						\
> +	}								\
> +	(typeof(*ptr))VAL;						\
> +})

Rename this to something like *_evstrm as this doesn't really work
unless we have the event stream. Another one would be *_wfet.

> +
> +/*
> + * For the unlikely case that the event-stream is unavailable,
> + * ward off the possibility of waiting forever by falling back
> + * to the generic spin-wait.
> + */
> +#define smp_cond_load_relaxed_timewait(ptr, cond_expr,			\
> +				       time_expr_ns, time_limit_ns)	\
> +({									\
> +	__unqual_scalar_typeof(*ptr) _val;				\
> +	int __wfe = arch_timer_evtstrm_available();			\

This should be a bool.

> +									\
> +	if (likely(__wfe))						\
> +		_val = __smp_cond_load_relaxed_timewait(ptr, cond_expr,	\
> +							time_expr_ns,	\
> +							time_limit_ns);	\
> +	else								\
> +		_val = __smp_cond_load_relaxed_spinwait(ptr, cond_expr,	\
> +							time_expr_ns,	\
> +							time_limit_ns);	\
> +	(typeof(*ptr))_val;						\
> +})

Not sure there's much to say here, this depends on the actual interface
introduced by patch 1. If we make some statements about granularity of
some time_cond_expr check, we'll have to take that into account.

-- 
Catalin