linux-kernel - Re: fishy code in arch/x86/kernel/tsc.c:time_cpufreq

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Date:	Mon, 01 Jun 2009 16:56:58 -0400
From:	Jon Masters <jonathan@...masters.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Dave Jones <davej@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: fishy code in arch/x86/kernel/tsc.c:time_cpufreq_notifier()

On Mon, 2009-06-01 at 12:39 -0400, Christoph Hellwig wrote:
> On Mon, Jun 01, 2009 at 12:29:55PM -0400, Dave Jones wrote:
> > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> > index d57de05..78c54ea 100644
> > --- a/arch/x86/kernel/tsc.c
> > +++ b/arch/x86/kernel/tsc.c
> > @@ -631,17 +631,15 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
> >  				void *data)
> >  {
> >  	struct cpufreq_freqs *freq = data;
> > -	unsigned long *lpj, dummy;
> > +	unsigned long *lpj;
> >  
> >  	if (cpu_has(&cpu_data(freq->cpu), X86_FEATURE_CONSTANT_TSC))
> >  		return 0;
> >  
> > -	lpj = &dummy;
> > -	if (!(freq->flags & CPUFREQ_CONST_LOOPS))
> > +	lpj = &boot_cpu_data.loops_per_jiffy;
> >  #ifdef CONFIG_SMP
> > +	if (!(freq->flags & CPUFREQ_CONST_LOOPS))
> >  		lpj = &cpu_data(freq->cpu).loops_per_jiffy;
> > -#else
> > -	lpj = &boot_cpu_data.loops_per_jiffy;
> >  #endif
> 
> This makes the code look a lot more sane and should fix the potential
> issue.

Tiny niggle that you wind up setting lpj (loops per jiffy) twice if
you're on SMP and have CPUFREQ_CONST_LOOPS.

Jon.


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