Walter is right about transitive readonly - here's the alternative

[Steven Schveighoffer]

"Janice Caron" wrote
> getReadLock()
> {
>    while (wcount != 0)
>    {
>        obtainMutex();
>        releaseMutex();
>    }
>    if (atomicIncrement(rcount) == 1)
>    {
>        obtainMutex();
>    }
> }

I understand that this might not be exactly what you have, but I do see a 
chance for a problem here.

Thread 1 and 2 are attempting to read the value, and nobody is writing it. 
Both threads skip the while loop, but then switch out of context to Thread 3 
which write-locks the data.  Thread 1 does the first increment, tries to 
lock the mutex, gets put to sleep.  Thread 2 switches back into context, 
increments the read counter, then proceeds, however thread 3 can write while 
thread 2 is reading.

Not a very likely situation, but of course those types are the worst when it 
comes to multithreaded programming.  Of course, you may have already solved 
this problem :)  but I can't see how you can do it without making the "read 
write counter, if 0 increment read counter, if that is now 1, lock" atomic.

-Steve