It would be nice if RUnlock() panicked if called when the mutex is not read locked,
similar to how Unlock() panics when the mutex is not write locked. As it is, bugs
related to mismatched rlocking are harder to find. The documentation states that
"It is a run-time error if rw is not locked for reading on entry to RUnlock."
which I interpret as we intended this to panic.

Go v1.2.1

The program:

http://play.golang.org/p/GmAqwC5WJo

Now prints:

Read locked
Also, write locked

While I would expect it to panic on line 10.