workqueue_drain

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

Hi,

workqueue_destroy requires that its queue doesn't have
any works (and no new work will be enqueued anymore)
before being called. Currently users of workqueue have
to ensure the contract by theirselves. It's possible
but it may need to add extra synchronization mechanisms
such as mutex and condvar only for that purpose.

workqueue itself can provide an API to help the routine.
It, say workqueue_drain, waits for all pending works to
finish. With it the caller needs to only ensure that no
new work will be enqueued.

Here is a patch:
  http://www.netbsd.org/~ozaki-r/workqueue_drain.diff
(I will update workqueue.4 once the proposal is accepted.)

And this is a patch that applies workqueue_drain to bridge(4):
  http://www.netbsd.org/~ozaki-r/workqueue_drain-example.diff

If we don't have workqueue_drain we had to add an extra
condvar to wait the workqueue worker of bridge to finish.

Any comments or suggestions?

Thanks,
  ozaki-r