Thanks for your response. What I don't understand is why is to so hard to give a more fine-grained locking support? In some sense, "lock" in my example takes an address (in some shared memory space) and puts a semaphore around it. Why should it be so hard to put a semaphore around the address space corresponding to the address space of the value of $h{$key}? Intuitively speaking, lock($h) and lock($h{$key}} both seem so similar in spirit.