Re: [PATCH v3] CPU hotplug: active_writer not woken up in some cases - deadlock

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Dec 10, 2014 at 01:26:19AM +0100, Oleg Nesterov wrote:
> On 12/09, Paul E. McKenney wrote:
> >
> > Would wait_event()/wake_up() work for the wakeup-writer case?
> 
> Yes, and in this case we could probably kill this puts_pending logic
> and avoid cpu_hotplug.lock in put_online_cpus() altogether? Can't we
> just make cpu_hotplug.refcount atomic_t?

Seems like that should be possible.  That would certainly simplify the
wakeup logic from put_online_cpus().  It might even be possible to
avoid acquiring cpu_hotplug.lock in put_online_cpus(), though that
would of course require more luck than anyone deserves.

> Anyway, this makes me think again that this code should use percpu_rwsem.
> Perhaps I'll try to make a patch next week...
> 
> (we need down_write_recursive_readers(), and probably rcusync patches).

Careful!  You might end up re-introducing the deadlock that I used
puts_pending to get rid of.  ;-)

							Thanx, Paul