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

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Message-ID: <20130924174717.GH9093@linux.vnet.ibm.com>
Date:	Tue, 24 Sep 2013 10:47:18 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On Tue, Sep 24, 2013 at 07:06:31PM +0200, Oleg Nesterov wrote:
> 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.

If you are saying that there should be a barrier() on all return paths
from get_online_cpus(), I agree.

							Thanx, Paul

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