i modified an example , but i dont know why it gets locked!

is it not that , the mutex is not released until the writer function , gets passed the wait(mutex) method ? and after that waits for the mutex ?
and inside reader function , when it is awakened by the notify_all() method by writer function , it waits until writer releases the mutex and then it owns it and there it executes .
so why do i fell into a locked down of some kind .
if i run this example couple of times , some times it works fine and some times it just hangs at the beginning ( getting locked ? ) whats wrong with it . ?