m-locking
m-waiting
t-Locking
t-Waiting
t-Setting
t-Signalling  # look ma, nobody received my signal!
t-Locking
t-Waiting     # and now we're both blocked, waiting for the other to call

or (I reconstructed this second scenario from memory)

m-processing
t-Locking
t-Waiting
t-Signalling  # if a tree signals in a forest, and noone's listening
t-Locking
t-Waiting
m-locking
m-waiting     # and again, we both wait...


##</code><code>##

If there are no threads blocked in a "cond_wait" on the variable,
the signal is discarded. By always locking before signaling, you can
(with care), avoid signaling before another thread has entered
cond_wait().