1. Sarah Diesburg Operating Systems COP 4610
Homework 3-4
Sarah Diesburg
Operating Systems
COP 4610

2. 1. All Possible Execution Orders
Thread A Thread B
x = 3; y = 2;
x = y - 1; y = x + 1;
(x = ?, y = ?)
x = 3
y = 2
(x = ?, y = 2)
(x = 3, y = ?)
x = y - 1
y = 2
x = 3
y = x + 1
(x = 3, y = 2)
(x = ?, y = ?)
(x = ?, y = ?)
y = 2
y = x + 1
x = y - 1
y = x + 1
x = 3
x = y - 1
(x = ?, y = 2)
(x = 1, y = 2)
(x = 3, y = 4)
(x = 3, y = ?)
(x = ?, y = ?)
(x = 1, y = 2)
(x = 3, y = 4)
(x = ?, y = ?)

3. 2. Why does disabling interrupts not work on multi-processors?
Interrupts are disabled on a per-CPU basis
A thread running on another CPU could enter the critical area

4. 3. Too Much Milk w/ Busy Waits
//define the busy-wait locks value = 0; Lock::Acquire() { // while the previous value is BUSY, loop while (test_and_set(value) == 1); } Lock::Release() {

5. 3. (cont.)
//Dumb Lock::Acquire(value) if(no milk) { //go get milk Lock::Release(value)
//Dumber Lock::Acquire(value) if(no milk) { //go get milk Lock::Release(value)

6. 4. Does this work?
// consumer waits on 0 semaphore nLoadedBuffers = 0; // producer waits on 0 semaphore nFreeBuffers = N; // N >= 2 // one thread waits when another thread is // modifying the buffer semaphore mutex = 1;
Producer() { 1. P(nFreeBuffers); 2. P(mutex); 3. // put 1 item in the buffer 4. V(nLoadedBuffers); 5. V(mutex); } Consumer() { 6. P(nLoadedBuffers); 7. P(mutex); 8. // take 1 item from the buffer 9. V(mutex); 10. V(nFreeBuffers);

7. 4. Does this work?
// consumer waits on 0 semaphore nLoadedBuffers = 0; // producer waits on 0 semaphore nFreeBuffers = N; // N >= 2 // one thread waits when another thread is // modifying the buffer semaphore mutex = 1;
Producer() { 1. P(nFreeBuffers); 2. P(mutex); 3. // put 1 item in the buffer 4. V(nLoadedBuffers); 5. V(mutex); } Consumer() { 6. P(nLoadedBuffers); 7. P(mutex); 8. // take 1 item from the buffer 9. V(mutex); 10. V(nFreeBuffers);

8. 4. Does this work? Actually, yes.
Even if producer thread hits #4 and wakes up a consumer sleeping on #6, the consumer will not be able to enter the critical section until the producer comes back and hits #5