4.2.1. Synchronization Requirements

• The shared buffer problem (see Data Inconsistency) and most synchronization problems require mutual exclusion: Only one process or thread can be in the critical section at a time.

• Other problems, such as Readers and Writers allow more than one read only thread in the critical section at the same time. (See Classic Synchronization Problems)

• Even to implement a simple lock, requires mutual exclusion:

  shared boolean locked;
  ...
  
  while( locked ) WAIT();
  /* THIS IS A BAD TIME FOR A CONTEXT SWITCH  */
  locked = True;
  /**********************/
     Critical Section
  /**********************/
  locked = False;
  

• Without mutual exclusion, results of multiple execution are not determinate.

• Need an Operating System provided mechanism.