On Fri, Nov 5, 2010 at 5:58 AM, John Baldwin <j...@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 <j...@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.

> 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.


> If that works ok (I think it does), I would rather have taskqueue_cancel()
> always be non-blocking.  Even though there is a "race" where the task could
> be rescheduled by another thread in between cancel and drain, the race still
> exists since if the task could be scheduled between the two, it could also
> 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).  The caller still always has to
> provide synchronization for preventing a task's execution outright via their
> own locking.
> --
> John Baldwin
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