linux-kernel - Re: [patch 1/4] x86, mtrr: lock stop machine during MTRR rendezvous sequence

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Date:	Thu, 23 Jun 2011 11:05:05 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com, trenn@...ell.com,
	prarit@...hat.com, tj@...nel.org, rusty@...tcorp.com.au,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, youquan.song@...el.com,
	stable@...nel.org
Subject: Re: [patch 1/4] x86, mtrr: lock stop machine during MTRR
 rendezvous sequence

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?
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