Re: workqueue_drain

[Paul Goyette <paul%whooppee.com@localhost>]

On Wed, 20 Dec 2017, Ryota Ozaki wrote:

> 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?

You could add a new flag bit for WQ_DRAINING when you start to
drain.  Then workqueue_enque() could enforce the "no new work"
with

	KASSERT_MSG((wq->wg_flags & WQ_DRAINING) != 0,
	    "adding work to a draining workqueue")


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