[RFC] Outline of USB process integration in the kernel taskqueue system

[Matthew Fleming]

On Thu, Nov 4, 2010 at 12:11 PM, Hans Petter Selasky <hselasky at c2i.net> wrote:
> On Thursday 04 November 2010 20:01:57 Matthew Fleming wrote:
>> On Thu, Nov 4, 2010 at 11:41 AM, Hans Petter Selasky <hselasky at c2i.net>
> wrote:
>> > On Thursday 04 November 2010 15:29:51 John Baldwin wrote:
>> >>  (and there is in Jeff's OFED branch)
>> >
>> > Is there a link to this branch? I would certainly have a look at his work
>> > and re-base my patch.
>>
>> It's on svn.freebsd.org:
>>
>> http://svn.freebsd.org/viewvc/base/projects/ofed/head/sys/kern/subr_taskque
>> ue.c?view=log
>> http://svn.freebsd.org/viewvc/base?view=revision&revision=209422
>>
>> For the purpose of speed, I'm not opposed to breaking the KBI by using
>> a doubly-linked TAILQ, but I don't think the difference will matter
>> all that often (perhaps I'm wrong and some taskqueues have dozens of
>> pending tasks?)
>>
>> Thanks,
>> matthew
>
> At first look I see that I need a non-blocking version of:
>
> taskqueue_cancel(
>
> At the point in the code where these functions are called I cannot block. Is
> this impossible to implement?

It depends on whether the queue uses a MTX_SPIN or MTX_DEF.  It is not
possible to determine whether a task is running without taking the
taskqueue lock.  And it is certainly impossible to dequeue a task
without the lock that was used to enqueue it.

However, a variant that dequeued if the task was still pending, and
returned failure otherwise (rather than sleeping) is definitely
possible.

Thanks,
matthew