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

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?

-- 
John Baldwin