linux-kernel - Re: [PATCH v9 05/15] arm64: barrier: add support for smp_cond_relaxed

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Message-ID: <20241210135052.GB15607@willie-the-truck>
Date: Tue, 10 Dec 2024 13:50:52 +0000
From: Will Deacon <will@...nel.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-pm@...r.kernel.org, kvm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, catalin.marinas@....com,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
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	daniel.lezcano@...aro.org, peterz@...radead.org, arnd@...db.de,
	lenb@...nel.org, mark.rutland@....com, harisokn@...zon.com,
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	zhenglifeng1@...wei.com, joao.m.martins@...cle.com,
	boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v9 05/15] arm64: barrier: add support for
 smp_cond_relaxed_timeout()

On Thu, Nov 07, 2024 at 11:08:08AM -0800, Ankur Arora wrote:
> Support a waited variant of polling on a conditional variable
> via smp_cond_relaxed_timeout().
> 
> This uses the __cmpwait_relaxed() primitive to do the actual
> waiting, when the wait can be guaranteed to not block forever
> (in case there are no stores to the waited for cacheline.)
> For this we depend on the availability of the event-stream.
> 
> For cases when the event-stream is unavailable, we fallback to
> a spin-waited implementation which is identical to the generic
> variant.
> 
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>  arch/arm64/include/asm/barrier.h | 54 ++++++++++++++++++++++++++++++++
>  1 file changed, 54 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index 1ca947d5c939..ab2515ecd6ca 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -216,6 +216,60 @@ do {									\
>  	(typeof(*ptr))VAL;						\
>  })
>  
> +#define __smp_cond_load_timeout_spin(ptr, cond_expr,			\
> +				     time_expr_ns, time_limit_ns)	\
> +({									\
> +	typeof(ptr) __PTR = (ptr);					\
> +	__unqual_scalar_typeof(*ptr) VAL;				\
> +	unsigned int __count = 0;					\
> +	for (;;) {							\
> +		VAL = READ_ONCE(*__PTR);				\
> +		if (cond_expr)						\
> +			break;						\
> +		cpu_relax();						\
> +		if (__count++ < smp_cond_time_check_count)		\
> +			continue;					\
> +		if ((time_expr_ns) >= time_limit_ns)			\
> +			break;						\
> +		__count = 0;						\
> +	}								\
> +	(typeof(*ptr))VAL;						\
> +})

This is a carbon-copy of the asm-generic timeout implementation. Please
can you avoid duplicating that in the arch code?

Will