Question

I have been reading JCIP by Brian Goetz. He explains the implementation of a non-blocking counter using CAS instruction. I could not understand how the increment is happening using CAS instruction. Can anyone help me understand this.

public class CasCounter {
    private SimulatedCAS value;

    public int getValue() {
        return value.get();
    }

    public int increment() {
        int v;
        do {
            v = value.get();
        }
        while (v != value.compareAndSwap(v, v + 1));
        return v + 1;
    }
}

Answer 1

value.compareAndSwap(v, v + 1) is equivalent to the following, except that the entire block is atomic: (see compare-and-swap for details)

int old = value.val;
if (old == v) {
  value.val = v + 1;
}
return old;

Now v = value.get() gets the current value of the counter, and if nobody else is trying to update the counter at the same time, old == v will be true, so the value is set to v+1 (i.e. it is incremented) and old is returned. The loop terminates since v == old.

Suppose someone else incremented the counter just after we did v = value.get(), then old == v would be false, and the method will immediately return old, which is the updated value. Since v != old now, the loop continues.