1. Peculiarity of Peterson’s Solution to Process Synchronization

2. void enter_region(int process) { int other = 1 – process;
turn
Initial State
interested
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

3. void enter_region(int process) { int other = 1 – process;
other 1
Process 0
turn
interested
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

4. void enter_region(int process) { int other = 1 – process;
Process 1
turn
interested
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

5. void enter_region(int process) { int other = 1 – process;
other 1
Process 0
turn
interested
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

6. void enter_region(int process) { int other = 1 – process;
other 1
Process 0
turn
interested 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

7. void enter_region(int process) { int other = 1 – process;
Process 1
turn
interested 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

8. void enter_region(int process) { int other = 1 – process;
Process 1
turn
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

9. void enter_region(int process) { int other = 1 – process;
other 1
Process 0
turn
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

10. void enter_region(int process) { int other = 1 – process;
other 1
Process 0
turn
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

11. void enter_region(int process) { int other = 1 – process;
Process 1
turn
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

12. void enter_region(int process) { int other = 1 – process;
Process 1
turn 1
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

13. void enter_region(int process) { int other = 1 – process;
other 1
Process 0
turn 1
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

14. void enter_region(int process) { int other = 1 – process;
other 1
Process 0
turn 1
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
FALSE
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

15. void enter_region(int process) { int other = 1 – process;
other 1
Process 0
turn 1
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

16. void enter_region(int process) { int other = 1 – process;
Process 1
turn 1
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

17. void enter_region(int process) { int other = 1 – process;
Process 1
turn 1
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
TRUE
TRUE
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

18. void enter_region(int process) { int other = 1 – process;
Process 1
turn 1
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
TRUE
TRUE
So, wait… 
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1

19. void enter_region(int process) { int other = 1 – process;
Process 1
turn 1
But process 1 was supposed to enter its critical region… 
interested 1 1
void enter_region(int process) {
int other = 1 – process;
interested[process] = TRUE;
turn = process;
while (turn==process && interested[other]==TRUE){} } 
void leave_region(int process) {
interested[process] = FALSE }
while (TRUE) {
enter_region(0);
critical_region();
leave_region(0); }
while (TRUE) {
enter_region(1);
critical_region();
leave_region(1); }
Process 0
Process 1