1. Synchronization (other solutions …)

2. Announcements Assignment 2 is graded Project 1 is due today

3. Critical Section Problem Problem: Design a protocol for processes to cooperate, such that only one process is in its critical section –How to make multiple instructions seem like one? Processes progress with non-zero speed, no assumption on clock speed Used extensively in operating systems: Queues, shared variables, interrupt handlers, etc. Process 1 Process 2 CS 1 Time  CS 2

4. Solution Structure Shared vars: Initialization: Process:... Entry Section Critical Section Exit Section Added to solve the CS problem

5. Solution Requirements Mutual Exclusion –Only one process can be in the critical section at any time Progress –Decision on who enters CS cannot be indefinitely postponed No deadlock Bounded Waiting –Bound on #times others can enter CS, while I am waiting No livelock Also efficient (no extra resources), fair, simple, …

6. Solution 1 (Two Processes) Process 0 turn = 0; while(turn != 0); Critical Section turn = 1; Shared: int turn; Initialize: turn = 0; Problem: Does not satisfy progress! Assumes strict alternation Process 1 turn = 1; while(turn != 1); Critical Section turn = 0;

7. Solution 1 (Two Processes) Process 0 flag[0] = true; while(flag[1]); Critical Section flag[0] = false; Shared: bool flag[2]; Initialize: flag[0] = false; flag[1] = false; Problem: Does not satisfy progress! Timing dependent What if you swap statement in the entry section? Process 1 flag[1] = true; while(flag[0]); Critical Section flag[1] = false;

8. Peterson’s Algorithm Process i flag[i] = true; turn = j; while(flag[j] && turn == j); Critical Section flag[i] = false; Shared: bool flag[2]; int turn; Initialize: flag[0] = false; flag[1] = false; turn = 0; Works for atomic load/stores! Special problem for multiprocessors

9. Solution 2: Disable Interrupts Disable interrupts while process is in critical section 2 approaches: –Scheduler checks if program counter is in critical section Do not interrupt if it is Pro: Fast Con: needs compiler support –Process disables interrupts before entering critical section Enables on exit Pro: Easy Con: when infinite loop in CS Cons –Response time –Multiprocessors  Critical section should be small! Process Disable interrupts Critical Section Enable interrupts Entry Section Exit Section

10. Solution 3: Hardware Primitives Modern hardware provides better atomic instructions: –test_and_set –swap –compare_and_swap –load_linked/store_conditional –fetch_and_add Entire function is atomic –Possible by locking bus for both load and store bool test_and_set(bool *target) { bool temp = *target; *target = TRUE; return temp; } void swap(bool *a, bool *b) { bool temp = *a; *a = *b; *b = temp; }

11. Solution 3: Hardware Primitives Process i While(test_and_set(&lock)); Critical Section lock = false; Share: int lock; Initialize: lock = false; Problem: Does not satisfy bounded waiting (see book for correct solution)

12. Hardware Primitives Pros: –Simple primitive –Easy to program critical sections Cons –Busy waiting for entire duration of critical section! Also called spinlocks

13. Implementing Semaphores P() and V() should be executed atomically –Disable interrupts, or –Busy waiting solutions Move busy wait from entry of CS to P() and V() P() and V() code is small

14. Semaphores Atomicity typedef struct semaphore { int lock; int value: ProcessList L; } Semaphore; void P(Semaphore *S) { while(test_and_set(&S->lock) == 1) /* do nothing */; S->value = S->value - 1; if (S.value < 0) { add this process to S.L; atomic_clear_and block(&S->lock); } else atomicclear(&S->lock); } void V(S) { while(test_and_set(&S->lock) == 1) /* do nothing */; S->value = S->value + 1; if (S->value <= 0) { remove a process P from S.L; wakeup P } atomicclear(&S->lock); }

15. A Real Race Condition Customer to sales executive: “This is the second time I have written to you, and I don't blame you for not answering me, because I sounded crazy, but it is a fact that as a tradition in our family we have ice-cream for dessert after dinner each night. …”