linux-kernel - Re: [PATCH v2 15/17] x86: Add rdtsc_ordered() and use it in trivial call sites

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Date:	Sat, 13 Jun 2015 11:02:11 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
	John Stultz <john.stultz@...aro.org>,
	linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
	Huang Rui <ray.huang@....com>,
	Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH v2 15/17] x86: Add rdtsc_ordered() and use it in trivial
 call sites

On Fri, Jun 12, 2015 at 04:44:55PM -0700, Andy Lutomirski wrote:
> barrier_before_rdtsc(); rdtsc_unordered() is an unnecessary mouthful and
> requires more thought than should be necessary.  Add an rdtsc_ordered()
> helper and replace the trivial call sites with it.
> 
> This should not change generated code.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---

...

> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index a47fb11af5f5..22d69d2d1f0d 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -148,6 +148,20 @@ static __always_inline void barrier_before_rdtsc(void)
>  			  "lfence", X86_FEATURE_LFENCE_RDTSC);
>  }
>  
> +/**
> + * rdtsc_ordered() - read the current TSC in program order
> + *
> + * rdtsc_ordered() returns the result of RDTSC as a 64-bit integer.
> + * It is ordered like a load to a global in-memory counter.  It should
> + * be impossible to observe non-monotonic rdtsc_unordered() behavior
> + * across multiple CPUs as long as the TSC is synced.
> + */
> +static __always_inline unsigned long long rdtsc_ordered(void)
> +{
> +	barrier_before_rdtsc();
> +	return rdtsc_unordered();
> +}
> +

I don't see the final tree state with all those applied (too lazy to
apply them) but why not simply kill barrier_before_rdtsc() and inline
the alternative into rdtsc_ordered()?

I mean, I don't see usage for it somewhere else...

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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