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

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Message-ID: <20130926161426.GA14372@redhat.com>
Date:	Thu, 26 Sep 2013 18:14:26 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On 09/26, Peter Zijlstra wrote:
>
> On Thu, Sep 26, 2013 at 05:53:21PM +0200, Oleg Nesterov wrote:
> > On 09/26, Peter Zijlstra wrote:
> > >  void cpu_hotplug_done(void)
> > >  {
> > > -	cpu_hotplug.active_writer = NULL;
> > > -	mutex_unlock(&cpu_hotplug.lock);
> > > +	/* Signal the writer is done, no fast path yet. */
> > > +	__cpuhp_state = readers_slow;
> > > +	wake_up_all(&cpuhp_readers);
> > > +
> > > +	/*
> > > +	 * The wait_event()/wake_up_all() prevents the race where the readers
> > > +	 * are delayed between fetching __cpuhp_state and blocking.
> > > +	 */
> > > +
> > > +	/* See percpu_up_write(); readers will no longer attempt to block. */
> > > +	synchronize_sched();
> >
> > Shouldn't you move wake_up_all(&cpuhp_readers) down after
> > synchronize_sched() (or add another one) ? To ensure that a reader can't
> > see state = BLOCK after wakeup().
>
> Well, if they are blocked, the wake_up_all() will do an actual
> try_to_wake_up() which issues a MB as per smp_mb__before_spinlock().

Yes. Everything is fine with the already blocked readers.

I meant the new reader which still can see state = BLOCK after we
do wakeup(), but I didn't notice it should do __wait_event() which
takes the lock unconditionally, it must see the change after that.

> Right?

Yes, I was wrong, thanks.

Oleg.

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