linux-kernel - Re: [PATCH 1/1] cpu-hotplug: export cpu_hotplug_enable/cpu_hotplug

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Date:	Wed, 24 Jun 2015 13:14:02 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	KY Srinivasan <kys@...rosoft.com>,
	"gregkh\@linuxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel\@linuxdriverproject.org" <devel@...uxdriverproject.org>,
	"mingo\@kernel.org" <mingo@...nel.org>,
	"rafael.j.wysocki\@intel.com" <rafael.j.wysocki@...el.com>,
	"tglx\@linutronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH  1/1] cpu-hotplug: export cpu_hotplug_enable/cpu_hotplug_disable

Peter Zijlstra <peterz@...radead.org> writes:

> On Wed, Jun 24, 2015 at 11:43:33AM +0200, Vitaly Kuznetsov wrote:
>> Peter Zijlstra <peterz@...radead.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.

Sorry, I was under an impression you were referring to the patch as a
whole :-) Yes, I agree, let's add this WARN_ON().

-- 
  Vitaly
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