Date: Fri, 5 Nov 2010 06:50:10 -0700 From: Matthew Fleming <mdf356@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-usb@freebsd.org, Hans Petter Selasky <hselasky@c2i.net>, freebsd-current@freebsd.org, Andrew Thompson <thompsa@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: <AANLkTingGuYvcGzmkq4eGwqhcGiZbaXv4fQUq0qG7DX1@mail.gmail.com> In-Reply-To: <201011050858.33568.jhb@freebsd.org> References: <201011012054.59551.hselasky@c2i.net> <201011041722.46673.jhb@freebsd.org> <AANLkTim6YH7TzcEFuimVhuF9k-n5%2B%2BO5wAbKrmScRFc4@mail.gmail.com> <201011050858.33568.jhb@freebsd.org>
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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/drai= n. 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 tested. >> >> >> 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. =A0Howeve= r, I > would prefer that it follow the semantics of callout_stop() and return tr= ue > 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. 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. > I'm not sure I like reusing the memory allocation flags (M_NOWAIT / M_WAI= TOK) > for this blocking flag. =A0In the case of callout(9) we just have two fun= ctions > 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 seman= tics > than callout_drain(). =A0It would have been nice to have the two APIs mir= ror each > other instead. > > Hmm, I wonder if the blocking behavior cannot safely be provided by just > 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. I will add a note to the manpage about = this. Thanks, matthew > > If that works ok (I think it does), I would rather have taskqueue_cancel(= ) > always be non-blocking. =A0Even though there is a "race" where the task c= ould > be rescheduled by another thread in between cancel and drain, the race st= ill > exists since if the task could be scheduled between the two, it could als= o > be scheduled just before the call to taskqueue_cancel() (in which case a > taskqueue_cancel(queue, task, M_WAITOK) would have blocked to wait for it > matching the taskqueue_drain() above). =A0The caller still always has to > provide synchronization for preventing a task's execution outright via th= eir > own locking. > > -- > John Baldwin >
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