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

On Fri, Nov 5, 2010 at 10:36 AM, Hans Petter Selasky <hselasky_at_c2i.net> wrote:
> On Friday 05 November 2010 18:15:01 Matthew Fleming wrote:
>> On Fri, Nov 5, 2010 at 7:18 AM, John Baldwin <jhb_at_freebsd.org> wrote:
>> > On Friday, November 05, 2010 9:50:10 am Matthew Fleming wrote:
>> >> On Fri, Nov 5, 2010 at 5:58 AM, John Baldwin <jhb_at_freebsd.org> wrote:
>> >> > On Thursday, November 04, 2010 5:49:22 pm Matthew Fleming wrote:
>> >> >> On Thu, Nov 4, 2010 at 2:22 PM, John Baldwin <jhb_at_freebsd.org> wrote:
>> >> >> > On Thursday, November 04, 2010 4:15:16 pm Hans Petter Selasky wrote:
>> >> >> >> I think that if a task is currently executing, then there should
>> >> >> >> be a drain method for that. I.E. two methods: One to stop and one
>> >> >> >> to cancel/drain. Can you implement this?
>> >> >> >
>> >> >> > I agree, this would also be consistent with the callout_*() API if
>> >> >> > you had both "stop()" and "drain()" methods.
>> >> >>
>> >> >> Here's my proposed code.  Note that this builds but is not yet
>> >> >> tested.
>> >> >>
>> >> >>
>> >> >> Implement a taskqueue_cancel(9), to cancel a task from a queue.
>> >> >>
>> >> >> Requested by:       hps
>> >> >> Original code:      jeff
>> >> >> MFC after:  1 week
>> >> >>
>> >> >>
>> >> >> http://people.freebsd.org/~mdf/bsd-taskqueue-cancel.diff
>> >> >
>> >> > For FreeBSD taskqueue_cancel() should return EBUSY, not -EBUSY.
>> >> >  However, I would prefer that it follow the semantics of
>> >> > callout_stop() and return true if it stopped the task and false
>> >> > otherwise.  The Linux wrapper for taskqueue_cancel() can convert the
>> >> > return value.
>> >>
>> >> I used -EBUSY since positive return values reflect the old pending
>> >> count.  ta_pending was zero'd, and I think needs to be to keep the
>> >> task sane, because all of taskqueue(9) assumes a non-zero ta_pending
>> >> means the task is queued.
>> >>
>> >> I don't know that the caller often needs to know the old value of
>> >> ta_pending, but it seems simpler to return that as the return value
>> >> and use -EBUSY than to use an optional pointer to a place to store the
>> >> old ta_pending just so we can keep the error return positive.
>> >>
>> >> Note that phk (IIRC) suggested using -error in the returns for
>> >> sbuf_drain to indicate the difference between success (> 0 bytes
>> >> drained) and an error, so FreeBSD now has precedent.  I'm not entirely
>> >> sure that's a good thing, since I am not generally fond of Linux's use
>> >> of -error, but for some cases it is convenient.
>> >>
>> >> But, I'll do this one either way, just let me know if the above hasn't
>> >> convinced you.
>> >
>> > Hmm, I hadn't considered if callers would want to know the pending count
>> > of the cancelled task.
>> >
>> >> > I'm not sure I like reusing the memory allocation flags (M_NOWAIT /
>> >> > M_WAITOK) for this blocking flag.  In the case of callout(9) we just
>> >> > have two functions that pass an internal boolean to the real routine
>> >> > (callout_stop() and callout_drain() are wrappers for
>> >> > _callout_stop_safe()).  It is a bit unfortunate that
>> >> > taskqueue_drain() already exists and has different semantics than
>> >> > callout_drain().  It would have been nice to have the two APIs mirror
>> >> > each other instead.
>> >> >
>> >> > Hmm, I wonder if the blocking behavior cannot safely be provided by
>> >> > just doing:
>> >> >
>> >> >        if (!taskqueue_cancel(queue, task, M_NOWAIT)
>> >> >                taskqueue_drain(queue, task);
>> >>
>> >> This seems reasonable and correct.  I will add a note to the manpage
>> >> about this.
>> >
>> > In that case, would you be fine with dropping the blocking functionality
>> > from taskqueue_cancel() completely and requiring code that wants the
>> > blocking semantics to use a cancel followed by a drain?
>>
>> New patch is at
>> http://people.freebsd.org/~mdf/0001-Implement-taskqueue_cancel-9-to-cancel-
>> a-task-from-a.patch
>
> I think the:
>
> +       if (!task_is_running(queue, task)) {
>
> check needs to be omitted. Else you block the possibility of enqueue and
> cancel a task while it is actually executing/running ??

Huh?  If the task is currently running, there's nothing to do except
return failure.  Task running means it can't be canceled, because...
it's running.

Thanks,
matthew