Re: condvar

Ingo Weinhold <bonefish-CFLBMwTPW48UNGrzBIF7/[email protected]> wrote:
> On 2005-03-17 at 19:14:04 [+0100], Axel Dörfler wrote:
> > to be able to efficiently implement a condvar mechanism (and 
> > similar
> > things), it would be great to atomically release one semaphore 
> > while
> > aquiring another.
> Not having thought much about condition variables yet, I suppose 
> doing this 
> atomically is necessary since the implementation would require to be 
> able 
> to switch the semaphores in both directions, right?

Not really, although our implementation should allow this :)
There is a condvar implementation in the BeOS sample-code (portability/
condvar) where you can easily see how a) condvar works, and b) how 
complicated BeOS makes its implementation.

> > Together with the new B_RELEASE_ALL flag, condvar could then be
> > implemented very easily.
> > 
> > The functionality is very trivial to implement, that's why I've 
> > already
> > done it - the question is on how we make it public.
> > There are basically 3 possibilities:
> > 1) keep it private for R1
> > 2) (the messy version) add another flag to acquire_sem_etc() that
> > reinterpretes the "count" parameter as semaphore to be released - 
> > this
> > has the advantage that no new system call is added;
> System calls are cheap, so this isn't a valid advantage.

I meant "advantage" in the sense that it's closer to R5/Zeta.

> > 3) (the clean version) add a new pair of system calls: switch_sem() 
> > and
> > switch_sem_etc() (other naming suggestions are welcome). 
> > Applications
> > using these functions would not run on R5 or Zeta anymore.
> I don't really see anything that speaks for 2), so I fully agree to 
> go with 
> 3). In principle this doesn't even collide with 1). So, why not 
> introduce 
> the functions, but keep them private (or document them being 
> private).

Okay, good idea. I'll add them and make them semi-public; ie. I defer 
the final decision to a later point :-)

Bye,
   Axel.



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