linux-kernel - Re: [PATCH] hotplug: Optimize {get,put}_online

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Message-ID: <20130924124341.64d57912@gandalf.local.home>
Date:	Tue, 24 Sep 2013 12:43:41 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On Tue, 24 Sep 2013 18:03:59 +0200
Oleg Nesterov <oleg@...hat.com> wrote:

> On 09/24, Peter Zijlstra wrote:
> >
> > +static inline void get_online_cpus(void)
> > +{
> > +	might_sleep();
> > +
> > +	if (current->cpuhp_ref++) {
> > +		barrier();
> > +		return;
> 
> I don't undestand this barrier()... we are going to return if we already
> hold the lock, do we really need it?

I'm confused too. Unless gcc moves this after the release, but the
release uses preempt_disable() which is its own barrier.

If anything, it requires a comment.

-- Steve

> 
> The same for put_online_cpus().
> 
> > +void __get_online_cpus(void)
> >  {
> > -	if (cpu_hotplug.active_writer == current)
> > +	if (cpuhp_writer_task == current)
> >  		return;
> 
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