Refcount based Cpu Hotplug. V2

Hello everyone,

This is the version 2 of the refcount based cpu-hotplug "locking" 
implementation. 

It incorporates the review comments from the first posting which
can be found here --> http://lkml.org/lkml/2007/10/16/118.

Changes since v1:
- !CONFIG_HOTPLUG_CPU part is now handled correctly, thanks
  to the patch from Paul Jackson.

- The cpu_hotplug_begin() uses a waitqueue instead of a completion struct
  where a writer can wait while there are active readers in the system.

- Provided a new API's cpu_maps_update_begin(), cpu_maps_update_done()
  for serializing the updates to cpu_present_map and cpu_online_map.
  Thus threads which update the cpu_present_map should now call
  cpu_maps_update_begin instead of lock_cpu_hotplug(), since they play the
  role of writers.

- pseries_processor_add() , pseries_processor_remove() now use
  cpu_maps_update_begin()/cpu_maps_update_done() in place of
  lock_cpu_hotplug()/unlock_cpu_hotplug().

- Replaced the workqueue_mutex with workqueue_lock, which is a spinlock
  and guards the workqueues list.

- Updated Documentation/cpu-hotplug.txt to reflect get_online_cpus(), 
  put_online_cpus() in place of the old lock_cpu_hotplug(), 
  unlock_cpu_hotplug().

I'm Cc'ing the different subsystem maintainers who might be affected
by the changes in the patchstack. Especially if they rely on
lock_cpu_hotplug() to provide them protection for their local data 
structures as well.

The patchstack which is based against 2.6.23-mm1 has behaved well
when it was stress tested with kernbench running while continuously
performing cpu-hotplug operations on i386, x86_64 and ppc64.

Awaiting your feedback.

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!"