Date: Fri, 5 Nov 2010 10:18:28 -0400 From: John Baldwin <jhb@freebsd.org> To: Matthew Fleming <mdf356@gmail.com> Cc: freebsd-usb@freebsd.org, freebsd-current@freebsd.org, freebsd-arch@freebsd.org, Weongyo Jeong <weongyo.jeong@gmail.com> Subject: Re: [RFC] Outline of USB process integration in the kernel taskqueue system Message-ID: <201011051018.28860.jhb@freebsd.org> In-Reply-To: <AANLkTingGuYvcGzmkq4eGwqhcGiZbaXv4fQUq0qG7DX1@mail.gmail.com> References: <201011012054.59551.hselasky@c2i.net> <201011050858.33568.jhb@freebsd.org> <AANLkTingGuYvcGzmkq4eGwqhcGiZbaXv4fQUq0qG7DX1@mail.gmail.com>
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On Friday, November 05, 2010 9:50:10 am Matthew Fleming wrote: > On Fri, Nov 5, 2010 at 5:58 AM, John Baldwin <jhb@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@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
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