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

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

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; put('a'); a count: 0 head: 0 tail: 0 SIZE: 8 count: 1 head: 1 tail: 0 SIZE: 8 CS 140 Lecture Notes: Locks

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; put('b'); put('c'); a a b c count: 0 head: 0 tail: 0 SIZE: 8 count: 3 head: 3 tail: 0 SIZE: 8 CS 140 Lecture Notes: Locks

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; get() => 'a' a b c a b c count: 3 head: 3 tail: 0 SIZE: 8 count: 2 head: 3 tail: 1 SIZE: 8 CS 140 Lecture Notes: Locks

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; get() => 'b' a b c a b c count: 2 head: 3 tail: 1 SIZE: 8 count: 1 head: 3 tail: 2 SIZE: 8 CS 140 Lecture Notes: Locks

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; put('h'); put('i'); a b c d e f g i b c d e f g h count: 5 head: 7 tail: 2 SIZE: 8 count: 7 head: 1 tail: 2 SIZE: 8 CS 140 Lecture Notes: Locks

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

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

CS 140 Lecture Notes: Locks