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Subject: Re: [patch 1/4] x86, mtrr: lock stop machine during MTRR
 rendezvous sequence
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Suresh Siddha <suresh.b.siddha@intel.com>
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Date: Thu, 23 Jun 2011 11:05:05 +0200
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On Wed, 2011-06-22 at 15:20 -0700, Suresh Siddha wrote:
> +#ifdef CONFIG_SMP
> +       /*
> +        * If we are not yet online, then there can be no stop_machine() in
> +        * parallel. Stop machine ensures this by using get_online_cpus().
> +        *
> +        * If we are online, then we need to prevent a stop_machine() happening
> +        * in parallel by taking the stop cpus mutex.
> +        */
> +       if (cpu_online(raw_smp_processor_id()))
> +               mutex_lock(&stop_cpus_mutex);
> +#endif 

This reads like an optimization, is it really worth-while to not take
the mutex in the rare offline case?