Replaying what Oleg wrote:

Hi,

Sorry I don't understand the context and I can't find this thread
anywhere, so I am not sure I understand...

Replaying what Tejun wrote:
So, we basically need percpu_rwlock.  We already have percpu_rwsem.

Yes, and with -mm patches it becomes reader-friendly. In particular
see http://marc.info/?l=linux-mm-commits&m=135240650828875

Oleg, it seems
CPU hotplug needs big-reader rwlock, ideas on how to proceed?

I am going to convert get_online_cpus() to use percpu_down_read(),
this looks simple.

We already discussed this with Paul, see

	http://marc.info/?l=linux-kernel&m=135248463226031

and the whole thread.

In short, all we need is percpu_down_write_recursive_readers() and
afaics the only complication is lockdep, we need down_read_no_lockdep()
which (like __up_read) doesn't do rwsem_acquire_read().

Oleg.