1. Lab assignment 1: Synchronization
Operating Systems ECE344
Lab assignment 1: Synchronization
Question: java thread?
Ding Yuan

2. Overview of the lab assignment
Task 1: implementing synchronization primitives
1a: implement lock
1b: implement condition variable
Task 2: use synchronization primitives to solve problems
2a: Mice and cats
2b: traffic lights
Ding Yuan, ECE344 Operating System

3. Ding Yuan, ECE344 Operating System
Lock and cond. var.
P(sem) {
Disable interrupts;
while (sem->count == 0) {
thread_sleep(sem); /* current thread
will sleep on this sem */ }
sem->count--;
Enable interrupts;
V(sem) {
Disable interrupts;
sem->count++;
thread_wakeup (sem); /* this will wake
up all the threads waiting on this
sem. Why wake up all threads? */
Enable interrupts; }
why wake up all threads:
this is to make the threads runnable. Then it’s up to the scheduler to pick one. all these thread will “return” from thread_sleep. only one can pass P.
--- this is also why we need a while (sem->count != 0) instead of “if (sem->count != 0)”.
sleep with interrupts disabled:
thread sleep will eventually call mips_switch to swtich to a new thread. when that thread is waken up, the first thing is to enable interrupt.
Needs atomic region
Atomic region can be done in a similar way to semaphore
If you understand how semaphore is implemented, should be trivial!
Cannot use semaphore to implement lock or cond. var.
Ding Yuan, ECE344 Operating System

4. Synchronization problems
How to start?
First: write operation code
Next: carefully reason about all the possible interleaving and timing scenarios
Add synchronization
Ding Yuan, ECE344 Operating System

5. Ding Yuan, ECE344 Operating System
Mice and cats
Two bowls, multiple cats and mice
Safety criteria:
If a cat is eating at either dishes, no mouse can eat
If a mouse is eating at either dishes, no cat can eat
Ding Yuan, ECE344 Operating System

6. Ding Yuan, ECE344 Operating System
Operation code
void sem_eat(const char *who, int num, int bowl, int iteration) {
kprintf("%s: %d starts eating: bowl %d, iteration %d\n", who, num,
bowl, iteration);
clocksleep(1);
kprintf("%s: %d ends eating: bowl %d, iteration %d\n", who, num, }
void mousesem(void * p, unsigned long mousenumber) {
int bowl, iteration;
for (iteration = 0; iteration < 4; iteration++) {
sem_eat (”mouse", mousenumber, bowl, iteration);
int catmousesem(.. ..) {
for (index = 0; index < NCATS; index++)
thread_fork("catsem Thread”, NULL, index, catsem, NULL);
for (index = 0; index < NMICE; index++)
thread_fork(”mousesem Thread”, NULL, index, mousesem, NULL);
Ding Yuan, ECE344 Operating System

7. Ding Yuan, ECE344 Operating System
About starvation
You do not need to consider priority or starvation
e.g., mice can prevent cat from eating
Since cats/mice will eventually finish eating, won’t starve forever
Ding Yuan, ECE344 Operating System