An automatic mutual exclusion computer programming system is disclosed which allows a programmer to produce concurrent programming code that is synchronized by default without the need to write any synchronization code. The programmer creates asynchronous methods which are not permitted make changes to shared memory that they cannot reverse, and can execute concurrently with other asynchronous methods. Changes to shared memory are committed if no other thread has accessed shared memory while the asynchronous method executed. Changes are reversed and the asynchronous method is re-executed if another thread has made changes to shared memory. The resulting program executes in a serialized order. A blocking system method is disclosed which causes the asynchronous method to re-execute until the blocking method's predicate results in an appropriate value. A yield system call is disclosed which divides asynchronous methods into atomic fragments. When a yield method call is made, shared memory changes are committed if possible or reversed and the atomic fragment is re-executed.