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

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Date:	Thu, 23 Jun 2011 11:16:40 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"mingo@...e.hu" <mingo@...e.hu>, "hpa@...or.com" <hpa@...or.com>,
	"trenn@...ell.com" <trenn@...ell.com>,
	"prarit@...hat.com" <prarit@...hat.com>,
	"tj@...nel.org" <tj@...nel.org>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Song, Youquan" <youquan.song@...el.com>,
	"stable@...nel.org" <stable@...nel.org>
Subject: Re: [patch 1/4] x86, mtrr: lock stop machine during MTRR
 rendezvous sequence

On Thu, 2011-06-23 at 02:33 -0700, Thomas Gleixner wrote:
> 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.
> 

Ok. Thanks for educating me on that.

Here we are neither online nor active. So we should be ok. But to be
safe, I changed the online checks to active checks and updated the above
comment also in the new revision.

thanks,
suresh

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