Date: Mon, 8 Nov 2010 08:46:58 -0800 From: Matthew Fleming <mdf356@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-usb@freebsd.org, Weongyo Jeong <weongyo.jeong@gmail.com>, freebsd-current@freebsd.org, Andrew Thompson <thompsa@freebsd.org>, freebsd-arch@freebsd.org, Hans Petter Selasky <hselasky@c2i.net> Subject: Re: [RFC] Outline of USB process integration in the kernel taskqueue system Message-ID: <AANLkTi=SJMF91%2BxtFuc_hG_K5VWjhCvRGcVuVaBfF173@mail.gmail.com> In-Reply-To: <201011081142.46175.jhb@freebsd.org> References: <201011012054.59551.hselasky@c2i.net> <201011080947.00550.jhb@freebsd.org> <AANLkTinfsLB9QqvYZ6gyQL9YyPeVRx10et-oTpR%2B7f3X@mail.gmail.com> <201011081142.46175.jhb@freebsd.org>
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On Mon, Nov 8, 2010 at 8:42 AM, John Baldwin <jhb@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@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@c2i.net=
> wrote:
>> >> > Hi,
>> >> >
>> >> > On Saturday 06 November 2010 14:57:50 Matthew Fleming wrote:
>> >> >>
>> >> >> I think you're misunderstanding the existing taskqueue(9) implemen=
tation.
>> >> >>
>> >> >> As long as TQ_LOCK is held, the state of ta->ta_pending cannot cha=
nge,
>> >> >> nor can the set of running tasks. =A0So 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:
>> >> >
>> >> > =A0> +int
>> >> >> > +taskqueue_cancel(struct taskqueue *queue, struct task *task)
>> >> >> > +{
>> >> >> > + =A0 =A0 =A0 int rc;
>> >> >> > +
>> >> >> > + =A0 =A0 =A0 TQ_LOCK(queue);
>> >> >> > + =A0 =A0 =A0 if (!task_is_running(queue, task)) {
>> >> >> > + =A0 =A0 =A0 =A0 =A0 =A0 =A0 if ((rc =3D task->ta_pending) > 0)
>> >> >> > + =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 STAILQ_REMOVE(&que=
ue->tq_queue, task, task,
>> >> >> > ta_link); + =A0 =A0 =A0 =A0 =A0 =A0 =A0 task->ta_pending =3D 0;
>> >> >> > + =A0 =A0 =A0 } else {
>> >> >> > + =A0 =A0 =A0 =A0 =A0 =A0 =A0 rc =3D -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! =A0I see what you mean.
>> >>
>> >> I'm not quite sure what the best thing to do here is; I agree it woul=
d
>> >> be nice if taskqueue_cancel(9) dequeued the task, but I believe it
>> >> also needs to indicate that the task is currently running. =A0I 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@ to this mail since he has good thoughts on interfacing.
>> >
>> > I agree we should always dequeue when possible. =A0I think it should r=
eturn
>> > -EBUSY in that case. =A0That 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? =A0In the case
>> where a task is pending and is also currently running (admittedly a
>> narrow window), how would we do this? =A0This is why I suggested
>> returning the old ta_pending by reference. =A0This also allows callers
>> who don't care about the old pending to pass NULL and ignore it.
>
> I would be fine with that then. =A0I wonder if taskqueue_cancel() could t=
hen
> return a simple true/false. =A0False 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.
Thanks,
matthew
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