> I doubt subclassability of RLock matters but who knows, people do code
> things.

I've recently done this to implement potential deadlock detection. I 
keep a record of the sequences of acquired locks, find unique 
sequences, then check for conflicts between each sequence. There's not 
much overhead and it highlighted some potential deadlocks where lock A 
and B were acquired AB in one route through code and BA in another 
route. The algorithm is a simplified version of that used in Linux - 
see http://www.mjmwired.net/kernel/Documentation/lockdep-design.txt

Hugh