Re: [RFC] Outline of USB process integration in the kernel taskqueue system

On Mon, Nov 8, 2010 at 8:46 AM, Matthew Fleming <mdf356_at_gmail.com> wrote:
> On Mon, Nov 8, 2010 at 8:42 AM, John Baldwin <jhb_at_freebsd.org> wrote:
>> On Monday, November 08, 2010 10:34:33 am Matthew Fleming wrote:
>>> On Mon, Nov 8, 2010 at 6:47 AM, John Baldwin <jhb_at_freebsd.org> wrote:
>>> > On Saturday, November 06, 2010 4:33:17 pm Matthew Fleming wrote:
>>> >> On Sat, Nov 6, 2010 at 7:22 AM, Hans Petter Selasky <hselasky_at_c2i.net> wrote:
>>> >> > Hi,
>>> >> >
>>> >> > On Saturday 06 November 2010 14:57:50 Matthew Fleming wrote:
>>> >> >>
>>> >> >> I think you're misunderstanding the existing taskqueue(9) implementation.
>>> >> >>
>>> >> >> As long as TQ_LOCK is held, the state of ta->ta_pending cannot change,
>>> >> >> nor can the set of running tasks.  So the order of checks is
>>> >> >> irrelevant.
>>> >> >
>>> >> > I agree that the order of checks is not important. That is not the problem.
>>> >> >
>>> >> > Cut & paste from suggested taskqueue patch from Fleming:
>>> >> >
>>> >> >  > +int
>>> >> >> > +taskqueue_cancel(struct taskqueue *queue, struct task *task)
>>> >> >> > +{
>>> >> >> > +       int rc;
>>> >> >> > +
>>> >> >> > +       TQ_LOCK(queue);
>>> >> >> > +       if (!task_is_running(queue, task)) {
>>> >> >> > +               if ((rc = task->ta_pending) > 0)
>>> >> >> > +                       STAILQ_REMOVE(&queue->tq_queue, task, task,
>>> >> >> > ta_link); +               task->ta_pending = 0;
>>> >> >> > +       } else {
>>> >> >> > +               rc = -EBUSY;
>>> >> >
>>> >> > What happens in this case if ta_pending > 0. Are you saying this is not
>>> >> > possible? If ta_pending > 0, shouldn't we also do a STAILQ_REMOVE() ?
>>> >>
>>> >> Ah!  I see what you mean.
>>> >>
>>> >> I'm not quite sure what the best thing to do here is; I agree it would
>>> >> be nice if taskqueue_cancel(9) dequeued the task, but I believe it
>>> >> also needs to indicate that the task is currently running.  I guess
>>> >> the best thing would be to return the old pending count by reference
>>> >> parameter, and 0 or EBUSY to also indicate if there is a task
>>> >> currently running.
>>> >>
>>> >> Adding jhb_at_ to this mail since he has good thoughts on interfacing.
>>> >
>>> > I agree we should always dequeue when possible.  I think it should return
>>> > -EBUSY in that case.  That way code that uses 'cancel' followed by a
>>> > conditional 'drain' to implement a blocking 'cancel' will DTRT.
>>>
>>> Do we not also want the old ta_pending to be returned?  In the case
>>> where a task is pending and is also currently running (admittedly a
>>> narrow window), how would we do this?  This is why I suggested
>>> returning the old ta_pending by reference.  This also allows callers
>>> who don't care about the old pending to pass NULL and ignore it.
>>
>> I would be fine with that then.  I wonder if taskqueue_cancel() could then
>> return a simple true/false.  False if the task is running, and true
>> otherwise?
>
> Sure, though since we don't really have a bool type in the kernel I'd
> still prefer to return an int with EBUSY meaning a task was running.

I'll commit this later today unless there are objections.

http://people.freebsd.org/~mdf/0001-Add-a-taskqueue_cancel-9-to-cancel-a-pending-task-wi.patch

Thanks,
matthew