Why Double-Checked Locking is used at all?

Question

I keep on running across code that uses double-checked locking, and I'm still confused as to why it's used at all.

I initially didn't know that double-checked locking is broken, and when I learned it, it magnified this question for me: why do people use it in the first place? Isn't compare-and-swap better?

if (field == null)
    Interlocked.CompareExchange(ref field, newValue, null);
return field;

(My question applies to both C# and Java, although the code above is for C#.)

Does double-checked locking have some sort of inherent advantage compared to atomic operations?

Answer 1

Does double-checked locking have some sort of inherent advantage compared to atomic operations?

(This answer only covers C#; I have no idea what Java's memory model is like.)

The principle difference is the potential race. If you have:

if (f == null)
    CompareExchange(ref f, FetchNewValue(), null)

then FetchNewValue() can be called arbitrarily many times on different threads. One of those threads wins the race. If FetchNewValue() is extremely expensive and you want to ensure that it is called only once, then:

if (f == null)
    lock(whatever)
        if (f == null)
            f = FetchNewValue();

Guarantees that FetchNewValue is only called once.

If I personally want to do a low-lock lazy initialization then I do what you suggest: I use an interlocked operation and live with the rare race condition where two threads both run the initializer and only one wins. If that's not acceptable then I use locks.