1. Too Much Milk With Locks
Both threads:
struct lock l;
...
lock_acquire(&l);
if (milk == 0) {
buy_milk(); }
lock_release(&l);
CS 140 Lecture Notes: Concurrency

2. CS 140 Lecture Notes: Locks
Producer/Consumer, v1
char buffer[SIZE];
int count = 0;
int head = 0, tail = 0;
struct lock l;
lock_init(&l);
void put(char c) {
lock_acquire(&l);
count++;
buffer[head] = c;
head++;
if (head == SIZE) {
head = 0; }
lock_release(&l);
char get() {
char c;
lock_acquire(&l);
count--;
c = buffer[tail];
tail++;
if (tail == SIZE) {
tail = 0; }
lock_release(&l);
return c;
CS 140 Lecture Notes: Locks

3. CS 140 Lecture Notes: Locks
Producer/Consumer v2
char buffer[SIZE];
int count = 0;
int head = 0, tail = 0;
struct lock l;
lock_init(&l);
void put(char c) {
lock_acquire(&l);
while (count == SIZE) {
lock_release(&l); }
count++;
buffer[head] = c;
head++;
if (head == SIZE) {
head = 0;
char get() {
char c;
lock_acquire(&l);
while (count == 0) {
lock_release(&l); }
count--;
c = buffer[tail];
tail++;
if (tail == SIZE) {
tail = 0;
return c;
CS 140 Lecture Notes: Locks

4. CS 140 Lecture Notes: Locks
Producer/Consumer v3
char buffer[SIZE];
int count = 0;
int head = 0, tail = 0;
struct lock l;
struct condition dataAvailable;
struct condition spaceAvailable;
lock_init(&l);
cond_init(&dataAvailable);
cond_init(&spaceAvailable);
void put(char c) {
lock_acquire(&l);
while (count == SIZE) {
cond_wait(&spaceAvailable, &l); }
count++;
buffer[head] = c;
head++;
if (head == SIZE) {
head = 0;
cond_signal(&dataAvailable, &l);
lock_release(&l);
char get() {
char c;
lock_acquire(&l);
while (count == 0) {
cond_wait(&dataAvailable, &l); }
count--;
c = buffer[tail];
tail++;
if (tail == SIZE) {
tail = 0;
cond_signal(&spaceAvailable, &l);
lock_release(&l);
return c; T1 T2 T3
CS 140 Lecture Notes: Locks

5. CS 140 Lecture Notes: Locks