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

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Message-ID: <alpine.LFD.2.02.1106231131300.11814@ionos>
Date:	Thu, 23 Jun 2011 11:33:16 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	Suresh Siddha <suresh.b.siddha@...el.com>, mingo@...e.hu,
	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 Thu, 23 Jun 2011, Peter Zijlstra wrote:

> 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?
 
You cannot block on a mutex when you are not online, in fact you
cannot block on it when not active, so the check is wrong anyway.

Thanks,

	tglx


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