Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

[Paul E. McKenney]

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@xxxxxxxxxx> 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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