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Date: Wed, 24 Jun 2015 12:27:33 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: KY Srinivasan <kys@microsoft.com>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
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Subject: Re: [PATCH  1/1] cpu-hotplug: export
 cpu_hotplug_enable/cpu_hotplug_disable
Message-ID: <20150624102733.GK3644@twins.programming.kicks-ass.net>
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On Wed, Jun 24, 2015 at 11:43:33AM +0200, Vitaly Kuznetsov wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> > On Tue, Jun 23, 2015 at 04:57:05PM +0000, KY Srinivasan wrote:
> >
> >> > diff --git a/kernel/cpu.c b/kernel/cpu.c
> >> > index 94bbe46..8f35ee6 100644
> >> > --- a/kernel/cpu.c
> >> > +++ b/kernel/cpu.c
> >> > @@ -190,17 +190,19 @@ void cpu_hotplug_done(void)
> >> >  void cpu_hotplug_disable(void)
> >> >  {
> >> >  	cpu_maps_update_begin();
> >> > -	cpu_hotplug_disabled = 1;
> >> > +	cpu_hotplug_disabled++;
> >> >  	cpu_maps_update_done();
> >> >  }
> >> > +EXPORT_SYMBOL_GPL(cpu_hotplug_disable);
> >> > 
> >> >  void cpu_hotplug_enable(void)
> >> >  {
> >> >  	cpu_maps_update_begin();
> >> > -	cpu_hotplug_disabled = 0;
> >> > +	if (cpu_hotplug_disabled)
> >> > +		cpu_hotplug_disabled--;
> >
> > No that just papers over bugs.
> 
> Yes, but these bugs are not in Linux. I don't see any other way for a
> platform to enable/disable cpu hotplug in runtime.

Any code using this had better be open source, if not I really don't
care anyway.

That really ought to be:

	WARN_ON(--cpu_hotplug_disabled < 0);

You do not make underflows go away.