AtomicRef's compareAndSet in Kotlin

Question

I am implementing an Elimination back-off stack and I'm confronting a problem trying to invoke compareAndSet on AtomicRef.

I have a function called "write":

private fun write(elementRef: E, index: Int): AtomicRef<Any?>? {
    val positionRef: AtomicRef<Any?> = eliminationArray[index]
    return if (positionRef.compareAndSet(null, elementRef)) {
        positionRef
    } else null
}

This function returns the AtomicRef to the place where the element was set. (eliminationArray is filled with nulls at the beginning:

private val eliminationArray = atomicArrayOfNulls<Any?>(ELIMINATION_ARRAY_SIZE)

). Inside my method "push" I get positionRef via this function "write":

fun push(x: E) {
        val elimIndex = Random.nextInt(ELIMINATION_ARRAY_SIZE)
        val positionRef = write(x, elimIndex)
        if (positionRef != null) {
            for (i in 0 until ELIMINATION_WAIT) {
                if (positionRef.compareAndSet(null, null)) {
                    return
                }
            }

            if (!positionRef.compareAndSet(x, null)) {
                return
            }
        }
        ...
    }

PositionRef is a reference to the place in elimination array where the element was set (or null). If it is not null, I try to invoke compareAndSet, intending to check whether the element was already popped by another thread, changed or stayed the same. If it is not the same, I return, else I execute the code after "if" clause. What I actually get is

Cause: standalone invocation of kotlinx.atomicfu.AtomicRef::compareAndSet that was not traced to previous field load

Obviously it fails on lines with invoking positionRef.compareAndSet(), but why does it happen so and how can I fix it?

P.S. I tried to debug this code and I discovered that during the debugging after the line val positionRef = write(x, elimIndex) I can execute both positionRef.compareAndSet(null, null) (result is false) and positionRef.compareAndSet(x, null) (result is true). So why does it work in debug mode, but not in the runtime?

AtomicRef's compareAndSet in Kotlin

0 Answers0