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Re: [PATCH 4/12] arch/mips: remove references to cpu_*_map.

To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Subject: Re: [PATCH 4/12] arch/mips: remove references to cpu_*_map.
From: Rusty Russell <rusty@rustcorp.com.au>
Date: Thu, 16 Feb 2012 10:29:24 +1030
Cc: Ralf Baechle <ralf@linux-mips.org>, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org, Venkatesh Pallipadi <venki@google.com>, "akpm\@linux-foundation.org" <akpm@linux-foundation.org>
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On Wed, 15 Feb 2012 14:50:02 +0530, "Srivatsa S. Bhat" 
<srivatsa.bhat@linux.vnet.ibm.com> wrote:
> > -           cpu_clear(smp_processor_id(), mask);
> > -           for_each_cpu_mask(cpu, mask)
> > -                   if (cpu_context(cpu, mm))
> > +           for_each_online_cpu(cpu) {
> > +                   if (cpu != smp_processor_id() && cpu_context(cpu, mm))
> >                             cpu_context(cpu, mm) = 0;
> > +           }
> 
> 
> Strictly speaking, this one is not a mere cleanup. It causes a subtle change 
> in
> behaviour: earlier, it used to iterate over a local copy of cpu_online_mask, 
> which
> wouldn't change. However, with this patch, it will iterate directly over
> cpu_online_mask, which can change underneath. (The preempt_disable() won't 
> stop
> new CPUs from coming in.. it only prevents CPUs from going offline, that too
> provided that we use stop_machine stuff for CPU offline, which we do 
> currently.)

There's a preempt_disable() around this whole function, so online_mask
can't change.

Same with the others.
> > +           mask = *cpu_online_mask;
> > +   cpumask_clear(&mask, cpu);
> 
> 
> This should be cpumask_clear_cpu(cpu, &mask);

Good catch.  I copied the bitmask ops, and continually regret it.

I've rolled all these together with your fixes, added your ia64 patch,
and am rebasing to -next now, so I can hand this all across to akpm.

Thanks,
Rusty.

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