Replaying what Tejun wrote:

On 12/06/2012 12:13 AM, Srivatsa S. Bhat wrote:

Some of the atomic hotplug readers cannot tolerate CPUs going offline while
they are in their critical section. That is, they can't get away with just
synchronizing with the updates to the cpu_online_mask; they really need to
synchronize with the entire CPU tear-down sequence, because they are very
much involved in the hotplug related code paths.

Such "full" atomic hotplug readers need a way to *actually* and *truly*
prevent CPUs from going offline while they are active.

I don't think this is a good idea.  You really should just need
get/put_online_cpus() and get/put_online_cpus_atomic().  The former
the same as they are.  The latter replacing what
preempt_disable/enable() was protecting.  Let's please not go
overboard unless we know they're necessary.  I strongly suspect that
breaking up reader side from preempt_disable and making writer side a
bit lighter should be enough.  Conceptually, it really should be a
simple conversion - convert preempt_disable/enable() pairs protecting
CPU on/offlining w/ get/put_cpu_online_atomic() and wrap the
stop_machine() section with the matching write lock.

Thanks.

-- tejun