Re: workqueue_drain

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

On Wed, Dec 20, 2017 at 4:57 PM, Paul Goyette <paul%whooppee.com@localhost> wrote:
> 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")

Good idea!

I updated the patch:
  http://www.netbsd.org/~ozaki-r/workqueue_drain.v2.diff

Thanks,
  ozaki-r