Why is cpuset_cpus_allowed_fallback() necessary?

Hello, guys.

I'm wondering why cpuset_cpus_allowed_fallback() is necessary.  This
is called from, e.g., try_to_wake_up()->select_task_rq() when none of
the cpus in ->cpus_allowed is useable.  The cpuset callback invokes
do_set_cpus_allowed() w/ the cpuset's cpus_allowed.  This was added by
the following commit,

  commit 9084bb8246ea935b98320554229e2f371f7f52fa
  Author: Oleg Nesterov [off-list ref]
  Date:   Mon Mar 15 10:10:27 2010 +0100


  sched: Make select_fallback_rq() cpuset friendly

  Introduce cpuset_cpus_allowed_fallback() helper to fix the cpuset problems
  with select_fallback_rq(). It can be called from any context and can't use
  any cpuset locks including task_lock(). It is called when the task doesn't
  have online cpus in ->cpus_allowed but ttwu/etc must be able to find a
  suitable cpu.
  ....			  

The problem is, nothing's explaining what "the cpuset problems with
select_fallback_rq()" are.  Oleg, do you remember?  Why do we need
this?

Thanks.

-- 
tejun