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

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Message-ID: <20130924180005.GA7148@redhat.com>
Date:	Tue, 24 Sep 2013 20:00:05 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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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