ABA Problem

ABA means you read a value twice and each time it returns the same value A. Therefore you conclude that nothing changed in between. However, you missed the fact that the value was updated to B somewhere in between.

Let me first use a simple scenario to introduce the problem.

An Analogy

The scenario consists of you sitting in a car and waiting for the traffic light to become green. Green stands in our case for B, and red for A. What’s happening?

1. You look at the traffic light, and it is red (A).
2. Because you are bored, you begin to check the news on your smartphone and forget the time.
3. You look once more at the traffic light. Damn, it is still red (A).

Of course, the traffic light became green (B) between your two checks. Therefore, what seems to be one red phase was a full cycle.

What does this mean for threads (processes)? Now more formally.

1. Thread 1 reads the variable var with value A.
2. Thread 1 is preempted, and thread 2 runs.
3. Thread 2 changes...