1. Operating Systems ECE344 Ashvin Goel ECE University of Toronto Classic Synchronization Problems

2. Classic Synchronization Examples  Dining philosophers  Readers and writers 2

3. The Dining Philosophers Problem  Five philosophers sit at a table  A fork lies between every pair of philosophers  Philosophers (1) think, (2) grab one fork, (3) grab another fork, (4) eat, (5) put down one fork, (6) put the other fork 3

4. Dining Philosophers: Try 1  Assume take_fork and put_fork have locks in them to make them atomic o Is this solution correct? 4 #define N 5 Philosopher() { while(TRUE) { Think(); take_fork(i); take_fork((i+1)% N); Eat(); put_fork(i); put_fork((i+1)% N); } Each philosopher is modeled with a thread

5. Dining Philosophers: Try 2 5 #define N 5 Philosopher() { while(TRUE) { Think(); take_fork(i); take_fork((i+1)% N); Eat(); put_fork(i); put_fork((i+1)% N); } take_forks(i) put_forks(i)

6. Take Forks 6 // test whether philosopher i // can take both forks // only called with mutex set! test(int i) { if (state[i] == HUNGRY && state[LEFT] != EATING && state[RIGHT] != EATING){ state[i] = EATING; // Signal Philosopher i V(sem[i]); } int state[N]; lock mutex; sem sem[N] = {0}; take_forks(int i) { lock(mutex); state[i] = HUNGRY; test(i); unlock(mutex); P(sem[i]); }

7. Put Forks 7 // test whether philosopher i // can take both forks // only called with mutex set! test(int i) { if (state[i] == HUNGRY && state[LEFT] != EATING && state[RIGHT] != EATING){ state[i] = EATING; // Signal Philosopher i V(sem[i]); } put_forks(int i) { lock(mutex); state[i] = THINKING; test(LEFT); test(RIGHT); unlock(mutex); } int state[N]; lock mutex; sem sem[N] = {0};

8. The Readers and Writers Problem  Multiple reader and writer threads want to access some shared data  Multiple readers can read concurrently  Writers must synchronize with readers and other writers  Synchronization requirements o Only one writer can write the shared data at a time o When a writer is writing, no readers must access the data  Goals o Maximize concurrency o Prevent starvation 8

9. Readers/Writers - Basics 9 Reader () { rc = rc + 1; // Read shared data rc = rc – 1; // non-critical section } Mutex lock = UNLOCKED; Semaphore data = 1; int rc = 0; Writer () { // non-critical section // Write shared data }

10. Readers/Writers - Mutex 10 Reader () { lock(lock); rc = rc + 1; unlock(lock); // Read shared data lock(lock); rc = rc – 1; unlock(lock); // non-critical section } Mutex lock = UNLOCKED; Semaphore data = 1; int rc = 0; Writer () { // non-critical section // Write shared data }

11. Readers/Writers – Synchronization  Any problems with this solution? 11 Reader () { lock(lock); if (rc == 0) P(data); rc = rc + 1; unlock(lock); // Read shared data lock(lock); rc = rc – 1; if (rc == 0) V(data); unlock(lock); // non-critical section } Mutex lock = UNLOCKED; Semaphore data = 1; int rc = 0; Writer () { // non-critical section P(data); // Write shared data V(data); }