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

[Oleg Nesterov]

On 09/24, Paul E. McKenney wrote:
>
> On Tue, Sep 24, 2013 at 07:06:31PM +0200, Oleg Nesterov wrote:
> >
> > 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.

Paul, Peter, could you provide any (even completely artificial) example
to explain me why do we need this barrier() ? I am puzzled. And
preempt_enable() already has barrier...

	get_online_cpus();
	do_something();

Yes, we need to ensure gcc doesn't reorder this code so that
do_something() comes before get_online_cpus(). But it can't? At least
it should check current->cpuhp_ref != 0 first? And if it is non-zero
we do not really care, we are already in the critical section and
this ->cpuhp_ref has only meaning in put_online_cpus().

Confused...

Oleg.

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