Thinking about kqueue's and pthread_cond_wait

[Daniel Eischen]

On Thu, 11 Feb 2010, John Baldwin wrote:

> On Wednesday 10 February 2010 7:10:53 pm Daniel Eischen wrote:
>> On Feb 10, 2010, at 3:06 PM, Alfred Perlstein <alfred at freebsd.org>
>> wrote:
>>
>>> * Daniel Eischen <deischen at freebsd.org> [100210 12:01] wrote:
>>>>
>>>>
>>>> I strongly disagree.  Using mutexes and condition variables in the
>>>> proper way is not as easy as it sounds, let alone trying to mix
>>>> them as userland thingies into kqueue.
>>>>
>>>> I will strongly oppose this...
>>>
>>> Well then you "win".  I have no desire to engage in such discussion.
>>>
>>> I do hope that when you see this facility leveraged elsewhere for
>>> an application that you reflect on this conversation and think back
>>> on it the next time an opportunity presents itself to lead in
>>> functionality.
>>
>> Don't misunderstand me, I just don't think running around the tree and
>> adapting all the userland leaves to kqueue-isize them is the right
>> approach.  IMHO, it's better to extend the kqueue/kevent mechanism to
>> allow a generic object to be added to the event list and the kqueue to
>> be signaled from userland.  All the pthread and semaphore functions
>> are userland operations that also rely on userland structures anyway.
>
> kqueue/kevent already support that via EVFILT_USER, and Apple's GCD depends on
> this extensively.  However, my point from my earlier post still stands and I
> think it is the right way to implement something like NT's
> WaitForMultipleObjects().

I guess that didn't exist in my 9.0-current that I looked at, so
I replied privately with something very similar ;-)  With this
one could wrap pthread objects, semaphores, etc.  The wrapper
functions would have to additionally call kevent() to trigger
the event if the object was being waited on in a kqueue, as in:

 	typedef struct {
#define MY_MTX_IN_KQUEUE	0x0001
 		int flags;
 		pthread_mutex_t mutex;
 	} my_pthread_mutex_t;

 	my_pthread_mutex_unlock(my_pthread_mutex_t *m)
 	{
 		if (m->flags & MY_MTX_IN_KQUEUE) != 0) {
 			/* Trigger the event. */
 			kevent(...);
 		}
 		ret = pthread_mutex_unlock(m->mutex);
 	}

-- 
DE