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

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Date:	Tue, 24 Sep 2013 19:02:22 +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 09/24, Peter Zijlstra wrote:
>
> On Tue, Sep 24, 2013 at 09:49:00AM -0700, Paul E. McKenney wrote:
> > > >  void cpu_hotplug_done(void)
> > > >  {
> > > > +	/* Signal the writer is done */
> > > > +	cpuhp_writer = 0;
> > > > +	wake_up_all(&cpuhp_wq);
> > > > +
> > > > +	/* Wait for any pending readers to be running */
> > > > +	cpuhp_writer_wait(!atomic_read(&cpuhp_waitcount));
> > > > +	cpuhp_writer_task = NULL;
> > >
> > > We also need to ensure that the next reader should see all changes
> > > done by the writer, iow this lacks "realease" semantics.
> >
> > Good point -- I was expecting wake_up_all() to provide the release
> > semantics, but code could be reordered into __wake_up()'s critical
> > section, especially in the case where there was nothing to wake
> > up, but where there were new readers starting concurrently with
> > cpu_hotplug_done().
>
> Doh, indeed. I missed this in Oleg's email, but yes I made that same
> assumption about wake_up_all().

Well, I think this is even worse... No matter what the writer does,
the new reader needs mb() after it checks !__cpuhp_writer. Or we
need another synchronize_sched() in cpu_hotplug_done(). This is
what percpu_rw_semaphore() does (to remind, this can be turned into
call_rcu).

Oleg.

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