The Critical Section Problem

critical section problem

The critical section problem

Consider a system consisting of n processes {Po, P1, …, Pn-1). Each process has a segment of code, called a critical section problem, in which the process may be changing common variables, updating a table, writing a file, and so on. The important feature of the system is that, when one process is executing in its critical section, no other process is to be allowed to execute in its critical section. Thus, the execution of critical sections by the processes is mutually exclusive in time. The critical-section problem is to design a protocol that the processes can use to cooperate. Each process must request permission to enter its critical section. The section of code implementing this request is the entry section. The critical section may be followed by an exit section. The remaining code is the remainder section.




            Entry section

                        Critical section

            Exit section

                        Remainder section


A solution to the critical-section problem must satisfy the following three requirements:


  1. Mutual Exclusion: If process Pi is executing in its critical section, then no other processes can be executing in their critical sections.
  2. Progress: If no process is executing in its critical section and some processes wish to enter their critical sections, then only those processes that are not executing in their remainder section can participate in the decision on which will enter its critical section next, and this selection cannot be postponed indefinitely.
  3. Bounded Waiting: There exists a bound on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted.

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