When are mutexs required for producer/consumer problems?

Question

if there is 1 producer, 1 consumer and a buffer of >1 size is insert mutex required? is remove mutex required?

if there is 1 producer, 1 consumer and a buffer of 1 size is insert mutex required? is remove mutex required?

if there is >1 producer, >1 consumer and a buffer of 1 size is insert mutex required? is remove mutex required?

Can someone explain how you get to answer these questions. I know that two threads should never read from a the buffer while its being written into, but doesn't that mean that all of the scenarios require both mutexs?

Answers from professor: first case is yes, second two are no because when buffer is nonempty that is equivalent to a full buffer. When the buffer is empty the consumer is blocked. When the buffer contains an item the producer is blocked. So mutual exclusion is guaranteed with out using mutex. Which didn't help with understanding why that's the case. Good thing more experience has been had since this post was made.

This sounds like homework. It seems like you need to work out the answers yourself, that's how you gain understanding. — Barmar, Feb 25 '14 at 00:42
@Barmar Yes, its a homework question. I just don't think my answer that mutex is required all the time is correct. I just wanted a little guidance. — Gunslinger, Feb 25 '14 at 06:54
If you had posted your answer, explained why you thought it was right, and then asked why you were wrong, that would have been more appropriate. Although maybe cs.stackexchange.com would be a more appropriate place for the discussion. — Barmar, Feb 25 '14 at 15:54
read the last line of the question where I write that I know two threads should never read from the buffer while its being written into, making me think all cases require mutexs. I felt like I was missing something and going over notes did not help. — Gunslinger, Feb 26 '14 at 00:07

score 1 · Accepted Answer · answered Feb 25 '14 at 00:49

Consider the following linked list queue pop method:

Object pop() {
    if(this.head != null) {
        Node n = this.head;
        this.head = n.next;
        return n.data;
    }
    return null;
}

This method is not thread safe. Consider what would happen if the thread paused after executing line 3, and another thread calls pop; both threads would get the same object.

Mutexes ensure that two threads cannot access the same resource at the same time, protecting against this 'race condition'. By ensuring that only one thread can pop an element at a time, the consistency of the queue is maintained.

It is possible to implement a queue without using mutexes (ex. Java's ConcurrentLinkedList), but it is much more difficult.

When are mutexs required for producer/consumer problems?

1 Answers1