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

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Message-Id: <200906011649.38694.lkml@morethan.org>
Date:	Mon, 1 Jun 2009 16:49:36 -0500
From:	"Michael S. Zick" <lkml@...ethan.org>
To:	Jon Masters <jonathan@...masters.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	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 June 1 2009, Jon Masters wrote:
> 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.
> 

At least it is consistent. ;)
You set lpj twice if your not on SMP and have CPUFREG_CONST_LOOPS.

Mike
> Jon.
> 
> 
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