CS241

Synchronization

Sema-More

This is the idea of a mutex, keep the other person out while you do your thing.

So this is a semaphore.

Lol jk, this is a real semaphore

pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;
Semamore sem;
Stack s; //Thread Safe Stack
void* transaction_listener(void*arg) {
    while(1) {
        semm_wait(&sem);
        stack_push(&s, get_transaction());
    }
}

void* transaction_verifier(void*useless) {
    while(1) {
        semm_post(&sem);
        transaction = stack_pop(&s);
        verify(transaction);
    }
}

int main() {
    pthread_t tid1, tid2;
    pthread_create(&tid1, NULL, transaction_listener, NULL);
    pthread_create(&tid2, NULL, transaction_verifier, NULL);

    pthread_join(tid1,NULL);
    pthread_join(tid2,NULL);
    exit(0);
}

Semamore Outline

Struct

typedef struct {
    int value, max_value;
    pthread_mutext_t m;
    pthread_cond_t cv;
} Semamore;

When max_value is reached
- All thread trying to post should be blocked
- Where/How do you notify these blocked threads when a thread decrease semamore’s value so its not max_value?
When 0 is reached
- All thread trying to wait should be blocked
- Where/How do you notify these blocked thread when a thread increase semamore’s value so it’s not 0?

Remember to not burn CPU

Spurious Wakeup?

if(!condition)
    pthread_cond_wait(&cv, &mutex);

What is wrong with the code above?

Not-So-Broken Barriers

What does a barrier look like? Glad you asked.

void * entry_point(void *arg)
{
    int rank = (int)arg;
    for(int row in thread_range)
        for(int col = 0; col < COLS; ++col)
            DotProduct(row, col, initial_matrix, final_matrix);

    // Make Sure the Threads stop before moving on
    int rc = pthread_barrier_wait(&barr);

    //Check for error
    if(rc != 0 && rc != PTHREAD_BARRIER_SERIAL_THREAD)
    {
        printf("Could not wait on barrier\n");
        exit(-1);
    }

    for(int row in thread_range)
        for(int col = 0; col < COLS; ++col)
            DotProduct(row, col, final_matrix, initial_matrix);
}

Please read the wikibook about this!

Thread Safe Queues

Remember CS125/225! Appending to the head of a linked list and other edge cases etc…

Reminders

Use while loop to check the condition when using cond_wait.
A thread might be awoken even the condition for waking up is not reached.
Google spurious wakeup (https://goo.gl/TEJVOl)
Write a correctly working queue first before making it thread-safe.

PTHREAD_MUTEX_INITIALIZER only works for initialization.
Use pthread_mutex_init(&mtex, NULL) if you are not at the place declare it.
Think of all critical cases to test your queue/semamore
One thread can start working really late
One thread might get modify data structure at any time.
Semamore is not a really term!!

Questions?

Authors: Steve and Bhuvan

Credit Where Credit is Due