Date: Fri, 5 Nov 2010 10:15:01 -0700 From: Matthew Fleming <mdf356@gmail.com> To: John Baldwin <jhb@freebsd.org> 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: <AANLkTimR7MpZ3nyoWqkCR9a=-S6DR_MCNjPA0q5qg3U4@mail.gmail.com> In-Reply-To: <201011051018.28860.jhb@freebsd.org> References: <201011012054.59551.hselasky@c2i.net> <201011050858.33568.jhb@freebsd.org> <AANLkTingGuYvcGzmkq4eGwqhcGiZbaXv4fQUq0qG7DX1@mail.gmail.com> <201011051018.28860.jhb@freebsd.org>
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On Fri, Nov 5, 2010 at 7:18 AM, John Baldwin <jhb@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@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 b= e a drain >> >> >> method for that. I.E. two methods: One to stop and one to cancel/d= rain. 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. =A0Note that this builds but is not yet test= ed. >> >> >> >> >> >> Implement a taskqueue_cancel(9), to cancel a task from a queue. >> >> >> >> Requested by: =A0 =A0 =A0 hps >> >> Original code: =A0 =A0 =A0jeff >> >> MFC after: =A01 week >> >> >> >> >> >> http://people.freebsd.org/~mdf/bsd-taskqueue-cancel.diff >> > >> > For FreeBSD taskqueue_cancel() should return EBUSY, not -EBUSY. =A0How= ever, I >> > would prefer that it follow the semantics of callout_stop() and return= true >> > if it stopped the task and false otherwise. =A0The Linux wrapper for >> > taskqueue_cancel() can convert the return value. >> >> I used -EBUSY since positive return values reflect the old pending >> count. =A0ta_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. =A0I'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. =A0In 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()). =A0It is a bit >> > unfortunate that taskqueue_drain() already exists and has different se= mantics >> > than callout_drain(). =A0It 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 ju= st >> > doing: >> > >> > =A0 =A0 =A0 =A0if (!taskqueue_cancel(queue, task, M_NOWAIT) >> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0taskqueue_drain(queue, task); >> >> This seems reasonable and correct. =A0I will add a note to the manpage a= bout 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'll try to set up something to test it today too. Thanks, matthew
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