Critical Section

• Critical section or region is the part of a program, where the program accesses the shared (modifiable) resources
• Only one process can be in the critical section at any one time, hence the general concept of :
  entry protocol
  critical section
  exit protocol
• Concept of mutual exclusion : processing mutually agreeing to exclude one another (software) to allow the shared resource to be used efficiently - some solutions are enforced (hardware) however
• Consider the concept of a narrow waterway : Suez Canal, Panama Canal
• Must be carefully coded to prevent one process from sabotaging the machine, and remaining in the critical section forever
• If a process in mutual exclusion terminates - the operating system must release the mutual exclusion held by that process

Requirements and Implementation :

1. Mutual Exclusion : If process is executing in its critical section, then no other process can be executing in that critical section
2. Progress : Only processes outside their critical sections can participate in deciding who will enter the critical section - and selection of a waiting one cannot be postponed indefinitely. However, processes cannot prevent any process from entering their critical section if requirements are valid
3. Bounded Waiting : There must be a bound on the number of times that other processes are allowed to enter their critical section, after a process has made a request to enter is critical section and before that request is granted
4. No assumption may be made or should be necessary about the relative speed of asynchronous concurrent processes

Two common algorithms :

• Dekker's Algorithm
• Petersons Algorithm.