Perhaps I should have pointed out, for Sebastian's benefit, that my second patch uses "timeout" rather than "blocking" since that is the new black in python 3.
Also, I think the "threading" implementation shows clearly the problem of having two independent objects that are only losely bound by some shared common variables:  The threading.py version has to rely on c_types ints for the common counters.
If the two locks were merely two different views of a common RWLock object, this problem could go away, and you could have the threading.RWLock and the multiprocessing.RWLock be different concrete classes derived from a common base class.

I also think it is time to drop the "writer preference" model, since it just adds complexity with doubtful benefits.  Sebastian's model also does that.

I'll provide a new example patch presently.