Introduction of Process Synchronization

On the basis of synchronization, processes are categorized as one of the following two types:

  • Independent Process : Execution of one process does not affects the execution of other processes.
  • Cooperative Process : Execution of one process affects the execution of other processes.

Process synchronization problem arises in the case of Cooperative process also because resources are shared in Cooperative processes.
 
Race Condition
When more than one processes are executing the same code or accessing the same memory or any shared variable in that condition there is a possibility that the output or the value of the shared variable is wrong so for that all the processes doing the race to say that my output is correct this condition known as a race condition. Several processes access and process the manipulations over the same data concurrently, then the outcome depends on the particular order in which the access takes place.
A race condition is a situation that may occur inside a critical section. This happens when the result of multiple thread execution in the critical section differs according to the order in which the threads execute.

Race conditions in critical sections can be avoided if the critical section is treated as an atomic instruction. Also, proper thread synchronization using locks or atomic variables can prevent race conditions.
 
Critical Section Problem

Critical section is a code segment that can be accessed by only one process at a time. Critical section contains shared variables which need to be synchronized to maintain consistency of data variables.
critical section problem

In the entry section, the process requests for entry in the Critical Section.





Any solution to the critical section problem must satisfy three requirements:

  • Mutual Exclusion : If a process is executing in its critical section, then no other process is allowed to execute in the critical section.
  • Progress : If no process is executing in the critical section and other processes are waiting outside the critical section, then only those processes that are not executing in their remainder section can participate in deciding which will enter in the critical section next, and the selection can not be postponed indefinitely.
  • Bounded Waiting : A bound must exist 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.

 
Peterson’s Solution
Peterson’s Solution is a classical software based solution to the critical section problem.

In Peterson’s solution, we have two shared variables:

  • boolean flag[i] :Initialized to FALSE, initially no one is interested in entering the critical section
  • int turn : The process whose turn is to enter the critical section.

peterson
 
Peterson’s Solution preserves all three conditions :