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Date: Tue, 24 Sep 2013 12:43:41 -0400
From: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()
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To: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>, Srikar Dronamraju <srikar@linux.vnet.ibm.com>, Ingo Molnar <mingo@kernel.org>, Andrea Arcangeli <aarcange@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>, Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, Paul McKenney <paulmck@linux.vnet.ibm.com>, Thomas Gleixner <tglx@linutronix.de>

On Tue, 24 Sep 2013 18:03:59 +0200
Oleg Nesterov <oleg@redhat.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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