Dead Lock/Process lock


A process-lock is a destruct which provides a mechanism for 
interlocking process execution. A process-lock is either 
free or it is seized by exactly one process. When a process 
is seized, a non-nil value is stored in the lock (in the 
slot named locker). Usually this is the process which 
seized the lock, but can be any Lisp object other than nil. 
Any process which tries to seize the lock before it is 
released will block. This includes the process which has 
seized the lock; the with-process-lock macro protects 
against such recursion.

Dead Lock
An error with the computer that occurs when two programs 
are trying to wait for a signal from the other program at 
the same time.