Re: [RFC PATCH 0/5] Refcount based Cpu Hotplug. V2

[Gautham R Shenoy <ego@in.ibm.com>]

On Wed, Oct 24, 2007 at 10:17:54PM +0400, Oleg Nesterov wrote:
> On 10/24, Gautham R Shenoy wrote:
> >
> > On Wed, Oct 24, 2007 at 10:04:41AM -0700, Christoph Lameter wrote:
> > > On Wed, 24 Oct 2007, Gautham R Shenoy wrote:
> > > 
> > > > This is the version 2 of the refcount based cpu-hotplug "locking" 
> > > > implementation. 
> > > 
> > > Uggh. This introduces a global lock that has to be taken always when 
> > > scanning over cpus? 
> > 
> > Well, no! we take the global lock only while bumping up the refcount. 
> > We don't hold the lock while scanning over the cpus. And this is
> > definitely an improvement over the lock_cpu_hotplug() global mutex
> > we have now.
> 
> Just to be sure I didn't miss something... preempt_disable() still works,
> yes?

Yes it does. But it doesn't prevent onlining of new cpus. 
I am checking if __cpu_up() can also be called safely using
stop_machine_run() so that we can use preempt_disable() for safe atomic
access of the cpu_online_map.

> 
> Oleg.
> 

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!"