1. Dining Philosophers (1) Philosophers eat/think Eating needs 2 forks Pick one fork at a time How to prevent deadlock

2. Dining Philosophers A nonsolution to the dining philosophers problem

3. semaphore Forks[N] = {1,1,1,1,1} ; void take_fork(int i) { wait (Forks[i]) ; } void put_fork(int i) { signal(Forks[i]) ; } Problems?

4. Approach 2: Semaphore Forks[N] = {1,1,1,1,1} ; Semaphore mutex = 1 ; while (1) { think(); wait(mutex) ; take_fork(i) ; take_fork((i+1) % N) ; eat() ; put_fork(i) ; put_fork((i+1) % N) ; signal(mutex) ; } Will this work?

5. Dining Philosophers Solution to dining philosophers problem (part 1)

6. Dining Philosophers Solution to dining philosophers problem (part 2)

7. NULL Mutex.queue T T T T T 0 0 0 0 0 State Array S Array Mutex = 1 P0: take_forks(0) down(mutex) ; test(0) ;

8. NULL Mutex.queue E T T T T 2 1 1 1 1 State Array S Array Mutex = 0 P0: take_forks(0) down(mutex) ; test(0) ; 1 0 0 0 0

9. NULL Mutex.queue E T T T T 1 0 0 0 0 State Array S Array Mutex = 0 P0: take_forks(0) down(mutex) ; test(0) ; up(mutex) ; down(S[0]) ;

10. NULL Mutex.queue E T T T T 0 0 0 0 0 State Array S Array Mutex = 1 P0: take_forks(0) down(mutex) ; test(0) ; up(mutex) ; down(S[0]) ; // falls through to //critical section Preempted

11. NULL Mutex.queue E T T T T 0 0 0 0 0 State Array S Array Mutex = 1 P0: take_forks(0) down(mutex) ; test(0) ; up(mutex) ; down(S[0]) ; // falls through to //critical section Preempted P2: take_forks(2) ; down(mutex) ; test(2) ;

12. NULL Mutex.queue E T E T T 1 1 2 1 1 State Array S Array Mutex = 0 P0: take_forks(0) down(mutex) ; test(0) ; down(S[0]) ; Preempted 0 0 1 0 0 P2: take_forks(2) ; down(mutex) ; test(2) ;

13. NULL Mutex.queue E T E T T 1 1 2 1 1 State Array S Array Mutex = 1 P0: take_forks(0) down(mutex) ; test(0) ; down(S[0]) ; Preempted 0 0 0 0 0 P2: take_forks(2) ; down(mutex) ; test(2) ; up(mutex) ; down(S[2]) ; //Falls through to critical section

14. NULL Mutex.queue E T E T T 1 1 1 1 1 State Array S Array Mutex = 1 P0: take_forks(0) down(mutex) ; test(0) ; down(S[0]) ; Preempted P3: take_forks(3) ; down(mutex) ; test(3) ; up(mutex) ; down(S[3]) ; 0 0 0 0 0 P2: take_forks(2) ; down(mutex) ; test(2) ; up(mutex) ; down(S[2]) ; eat() !!!

15. NULL Mutex.queue E T E H T 1 1 1 0 1 State Array S Array Mutex = 1 P0: take_forks(0) down(mutex) ; test(0) ; down(S[0]) ; P3: take_forks(3) ; down(mutex) ; test(3) ; up(mutex) ; down(S[3]) ; P3 S[3].queue 0 0 0 -1 0 P2: take_forks(2) ; down(mutex) ; test(2) ; up(mutex) ; down(S[2]) ; eat() !!!

16. NULL Mutex.queue E T E H T 1 1 1 0 1 State Array S Array Mutex = 1 P0: take_forks(0) down(mutex) ; test(0) ; down(S[0]) ; P3: take_forks(3) ; down(mutex) ; test(3) ; up(mutex) ; down(S[3]) ; P3 S[3].queue 0 0 0 -1 0 P2: take_forks(2) ; down(mutex) ; test(2) ; up(mutex) ; down(S[2]) ; eat() !!! P2: put_forks(2) ; down(mutex) ; State[2] = Thinking ; test(1) ;

17. NULL Mutex.queue E T T H T 1 1 1 0 1 State Array S Array Mutex = 0 P0: take_forks(0) down(mutex) ; test(0) ; down(S[0]) ; P3: take_forks(3) ; down(mutex) ; test(3) ; up(mutex) ; down(S[3]) ; P3 S[3].queue 0 0 0 -1 0 P2: take_forks(2) ; down(mutex) ; test(2) ; up(mutex) ; down(S[2]) ; eat() !!! P2: put_forks(2) ; down(mutex) ; State[2] = Thinking ; test(1) ;

18. NULL Mutex.queue E T T H T 1 1 1 0 1 State Array S Array Mutex = 0 P0: take_forks(0) down(mutex) ; test(0) ; down(S[0]) ; P3: take_forks(3) ; down(mutex) ; test(3) ; up(mutex) ; down(S[3]) ; P3 S[3].queue 0 0 0 -1 0 P2: take_forks(2) ; down(mutex) ; test(2) ; up(mutex) ; down(S[2]) ; eat() !!! P2: put_forks(2) ; down(mutex) ; State[2] = Thinking ; test(1) ; test(3) ;

19. NULL Mutex.queue E T T E T 1 1 1 0 1 State Array S Array Mutex = 0 P0: take_forks(0) down(mutex) ; test(0) ; down(S[0]) ; P3: take_forks(3) ; down(mutex) ; test(3) ; up(mutex) ; down(S[3]) ; 0 0 0 0 0 P2: take_forks(2) ; down(mutex) ; test(2) ; up(mutex) ; down(S[2]) ; eat() !!! P2: put_forks(2) ; down(mutex) ; State[2] = Thinking ; test(1) ; test(3) ; //performs a signal on S[3] P3 Ready Queue

20. NULL Mutex.queue E T T E T 1 1 1 0 1 State Array S Array Mutex = 1 P0: take_forks(0) down(mutex) ; test(0) ; down(S[0]) ; P3: take_forks(3) ; down(mutex) ; test(3) ; up(mutex) ; down(S[3]) ; 0 0 0 0 0 P2: take_forks(2) ; down(mutex) ; test(2) ; up(mutex) ; down(S[2]) ; eat() !!! P2: put_forks(2) ; down(mutex) ; State[2] = Thinking ; test(1) ; test(3) ; //performs a signal on S[3] up(mutex) ; P3 Ready Queue