Re: condvar

> Hi there,
> 
> to be able to efficiently implement a condvar mechanism (and similar 
> things), it would be great to atomically release one semaphore while 
> aquiring another.
> 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; it's by no means 
> backwards compatible, though (the app would run incorrectly on 
> systems).

That would change the semantics of the "count" parameter and its 
name...
Btw, after checking it seems acquire_sem_etc just doesn't care about 
undefined flag values (>16) in R5. Would it return EINVAL or somethign 
we could use it, and have a slower fallback maybe...

> 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.

Shouldn't those condvar and friends be wrapped into objects in 
libraries anyway ?
Btw, there is a big issue with the current pthread implementation as 
well as yours: it doesn't handle the STATIC initializers...
They both declare the basic pthread _t types as pointers to structs, 
for extensibility... and:
#define PTHREAD_MUTEX_INITIALIZER	NULL
#define PTHREAD_COND_INITIALIZER	NULL
#define PTHREAD_RWLOCK_INITIALIZER	NULL

But that prevents apps doing:

pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;
and just using it, since the funcs will get a NULL, and can't use that.
(the R5 lib actually tries to acquire a pseudo-spinlock from that 
struct to lazily initialize it... except it's still NULL, so it 
crashes.

François.



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