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

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.

Thanks,
matthew

>
>> > +       }
>> > +       TQ_UNLOCK(queue);
>> > +       return (rc);
>> > +}
>>
>
>>
>> As John said, the taskqueue(9) implementation cannot protect consumers
>> of it from re-queueing a task; that kind of serialization needs to
>> happen at a higher level.
>
> Agreed, but that is not what I'm pointing at. I'm pointing at what happens if
> you re-queue a task and then cancel while it is actually running. Will the
> task still be queued for execution after taskqueue_cancel()?
>
>> taskqueue(9) is not quite like callout(9); the taskqueue(9)
>> implementation drops all locks before calling the task's callback
>> function.  So once the task is running, taskqueue(9) can do nothing
>> about it until the task stops running.
>
> This is not the problem.
>
>>
>> This is why Jeff's
>> implementation of taskqueue_cancel(9) slept if the task was running,
>> and why mine returns an error code.  The only way to know for sure
>> that a task is "about" to run is to ask taskqueue(9), because there's
>> a window where the TQ_LOCK is dropped before the callback is entered.
>
> And if you re-queue and cancel in this window, shouldn't this also be handled
> like in the other cases?
>
> Cut & paste from kern/subr_taskqueue.c:
>
>                task->ta_pending = 0;
>                tb.tb_running = task;
>                TQ_UNLOCK(queue);
>
> If a concurrent thread at exactly this point in time calls taskqueue_enqueue()
> on this task, then we re-add the task to the "queue->tq_queue". So far we
> agree. Imagine now that for some reason a following call to taskqueue_cancel()
> on this task at same point in time. Now, shouldn't taskqueue_cancel() also
> remove the task from the "queue->tq_queue" in this case aswell? Because in
> your implementation you only remove the task if we are not running, and that
> is not true when we are at exactly this point in time.
>
>                task->ta_func(task->ta_context, pending);
>
>                TQ_LOCK(queue);
>                tb.tb_running = NULL;
>                wakeup(task);
>
>
> Another issue I noticed is that the ta_pending counter should have a wrap
> protector.
>
> --HPS
>