Question

Is there any way to implement a type of reference whose value can be exchanged with another atomically?

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In Java we have AtomicReference which can be swapped with a local variable but not with another AtomicReference.

You can do:

AtomicReference r1 = new AtomicReference("hello");
AtomicReference r2 = new AtomicReference("world");

and swap them with a combination of two operations:

r1.set(r2.getAndSet(r1.get()));

But this leaves them in an inconsistent state in between, where both contain "hello". Also even if you could swap them atomically, you still could not read them (as a pair) atomically.

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What I would like to be able to do is:

PairableAtomicReference r1 = new PairableAtomicReference("hello");
PairableAtomicReference r2 = new PairableAtomicReference("world");
AtomicRefPair rp = new AtomicRefPair(r1, r2);

then

Object[] oldVal, newVal;
do {
    oldVal = rp.get();
    newVal = new Object[] {oldVal[1], oldVal[0]};
} while (! rp.compareAndSet(oldVal, newVal));

to swap the values, and in another thread:

AtomicRefPair otherRP = new AtomicRefPair(r1, r2);
System.out.println(Arrays.toString(otherRP.get()));

and be certain that the output will be either [hello, world] or [world, hello].

Notes:

• r1 and r2 are paired for this operation, but it’s possible that another thread will independently pair, say r1 and another r3 (unfortunately that means I cannot use this solution.)
• There will be hundreds of thousands of these references, so a global ReentrantLock would be a major bottleneck.
• rp and otherRP are not necessarily shared between threads, so simply locking them will not work. They could be interned, but the intern pool would need its own synchronization which would be another bottleneck.
• I have only made groups of 2 references here, but the ability to group 3 or more would be a bonus.

Is it possible to implement a lock-free version of AtomicRefPair? I have a hunch that it isn’t, but if not then maybe there is an article somewhere that explains why?

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Related: How do I atomically swap 2 ints in C#?

Answer 1

I don’t know if there’s a nice solution, but the following ugly one could work:

public final class MyReference<T> extends ReentrantLock implements Comparable<MyReference<T>> {
    public MyReference() {
        id = counter.incrementAndGet();
    }

    public void swap(MyReference<T> other) {
        if (id < other.id) {
            lock();
            other.lock();
        } else {
            other.lock();
            lock();
        }
        final T tmp = value;
        value = other.value;
        other.value = tmp;
        unlock();
        other.unlock();
    }

    public static <T> List<T> consistentGet(List<MyReference<T>> references) {
        final ArrayList<MyReference<T>> sortedReferences = Lists.newArrayList(references);
        Collections.sort(sortedReferences);
        for (val r : sortedReferences) r.lock();
        final List<T> result = Lists.newArrayListWithExpectedSize(sortedReferences.size());
        for (val r : references) result.add(r.value);
        for (val r : sortedReferences) r.unlock();
        return result;
    }

    @Override
    public int compareTo(MyReference<T> o) {
        return id < o.id ? -1 : id > o.id ? 1 : 0;
    }

    private final static AtomicInteger counter = new AtomicInteger();

    private T value;
    private final int id;
}

• Use MyReference instead of AtomicReference.
• It uses a lot of locks, but none of them is global.
• It acquires locks in a fixed order, so it’s deadlock-free.
• It compiles using lombok and guava (take it as pseudocode without them).