Situation when several concurrent processes operate on the same variable and the result of the computation depends upon the order in which they execut.
In the preceding example, the value of count could be 4, 5 or 6.
C1,P1,P2,P3,C2,C3 final value 4
C1,C2,C3,P1,P2,P3 final value 5
P1,C1,C2,C3,P2,P3 final value 6
Critical Section Problem
We model the problem using the notion of Critical Sections.
Critical sections are with respect to the shared data.
There are n processes, each sharing some common resource.
Each wants to modify the common resource.
For each process, define the region of code where it accesses a shared piece of data as a critical section.
In a correct system of multiple cooperating processes,
Only one process must be inside its critical section.
Problem Abstraction
Processes execute code similar to the following.
while (1) {
Non critical code Entry code
Critical Section Code Exit Code
Non Critical Code
}
final value 4
