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

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Message-ID: <20130924170631.GB5059@redhat.com>
Date:	Tue, 24 Sep 2013 19:06:31 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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 09/24, Steven Rostedt wrote:
>
> 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.

And I am still confused even after emails from Paul and Peter...

If gcc can actually do something wrong, then I suspect this barrier()
should be unconditional.

Oleg.

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