linux-kernel - Re: [PATCH 01/51] CPU hotplug: Provide lockless versions of callback registration functions

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Message-ID: <1392140115.5612.135.camel@misato.fc.hp.com>
Date:	Tue, 11 Feb 2014 10:35:15 -0700
From:	Toshi Kani <toshi.kani@...com>
To:	ego@...ux.vnet.ibm.com
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"tj@...nel.org" <tj@...nel.org>,
	"walken@...gle.com" <walken@...gle.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH 01/51] CPU hotplug: Provide lockless versions of
 callback registration functions

On Tue, 2014-02-11 at 22:48 +0530, Gautham R Shenoy wrote:
> On Tue, Feb 11, 2014 at 09:33:56AM -0700, Toshi Kani wrote:
> > 
> > I agree that introducing a reader-writer semaphore allows concurrent
> > executions.  Adding yet another hotplug lock is a bit unfortunate,
> > though.
> > 
> 
> I agree with this last part. We already have enough locks for
> cpu-hotplug. Another one sounds one too many!!
> 
> 
> > This may be a dumb question, but can't we simply do this way?
> > 
> >         get_online_cpus();
> > 
> >         for_each_online_cpu(cpu)
> >                 init_cpu(cpu);
> > 
> >         put_online_cpus();
> >
>       -------- Someone chooses to hotplug a cpu here ------
>       -------- But this subsystem might miss out on knowing
>                about it since it hasn't registered its
>                notifier yet! 
> 
> >         register_cpu_notifier(&foobar_cpu_notifier);


How about this?  foo_cpu_notifier returns NOP when foo_notifier_ready is
false.

        register_cpu_notifier(&foobar_cpu_notifier);

        get_online_cpus();

        for_each_online_cpu(cpu)
                init_cpu(cpu);

	foo_notifier_ready = true;

        put_online_cpus();

Thanks,
-Toshi

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