Analyzing Critical Section Solutions

This handout presents several proposed solutions to the 2 process critical section problem, and analyzes them. In these solutions, one process is numbered 0 and the other is numbered 1. The variable i holds the number corresponding to the process executing the code, and the variable j holds the number corresponding to the other process. All the code shown is shared by both processes, but the variables i and j hold different values.

First Proposed Solution

Here, turn contains the number of the process whose turn it is to execute the critical section.

1	int turn;
				/* entry section */
2	while (turn != i)
3		/* do nothing * / ;
				/* critical section */
	...
				/* exit section */
4	turn = j;

Second Proposed Solution

Here, inCS[0] is true when process 0 is in the critical section, and false otherwise. A similar statement holds for inCS[1].

1	int inCS[2] = { 0, 0 };
				/* entry section */
2	while (inCS[j])
3		/* do nothing * / ;
4	inCS[i] = 1;
				/* critical section */
	...
				/* exit section */
5	inCS[i] = 0;

Third Proposed Solution

Here, interested[0] is true when process 0 wants to enter the critical section, and false otherwise. A similar statement holds for interested[1].

1	int interested[2] = { 0, 0 };
				/* entry section */
2	interested[i] = 1;
3	while (interested[j])
4		/* do nothing * / ;
				/* critical section */
	...
				/* exit section */
5	interested[i] = 0;

Fourth Proposed Solution

This combines the first and third proposed solutions.

1	int interested[2]; = { 0, 0 };
2	int turn;
				/* entry section */
3	interested[i] = 1;
4	turn = j;
5	while (interested[j] && turn == j)
6		/* do nothing * / ;
				/* critical section */
	...
				/* exit section */
7	interested[i] = 0;