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    SubjectRe: [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@kernel.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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