linux-kernel - Re: [RFC PATCH 1/4] Refcount Based Cpu-Hotplug Implementation

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Message-ID: <20071017053754.GB9940@in.ibm.com>
Date:	Wed, 17 Oct 2007 11:07:54 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>, Ingo Molnar <mingo@...e.hu>,
	Paul E McKenney <paulmck@...ibm.com>
Subject: Re: [RFC PATCH 1/4] Refcount Based Cpu-Hotplug Implementation

On Wed, Oct 17, 2007 at 10:47:41AM +1000, Rusty Russell wrote:
> On Tuesday 16 October 2007 20:34:17 Gautham R Shenoy wrote:
> > This patch implements a Refcount + Waitqueue based model for
> > cpu-hotplug.
> 
> Hi Gautham,

Hi Rusty, 

> 
> 	I can't see where you re-initialize the completion.  

The cpu_hotplug.readers_done is a global variable which has been
initialized in cpu_hotplug_init. 

So I am wondering is the re-initialization required ? 

> 
> > +static void cpu_hotplug_begin(void)
> > +{
> > +	mutex_lock(&cpu_hotplug.lock);
> > +	cpu_hotplug.active_writer = current;
> > +	while (cpu_hotplug.refcount) {
> > +		mutex_unlock(&cpu_hotplug.lock);
> > +		wait_for_completion(&cpu_hotplug.readers_done);
> > +		mutex_lock(&cpu_hotplug.lock);
> > +	}
> 
> AFAICT this will busy-wait on the second CPU hotplug.
> 

Well when the first cpu_hotplug comes out of wait_for_completion, it
would have decremented the ->done count, so it's as good as new 
for the second CPU hotplug, no?

> Cheers,
> Rusty.

Thanks and Regards
gautham.
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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