ABA Problem

ABA Problem — A lock-free concurrency hazard in which a value changes from A to B and back to A, causing compare-and-swap logic to miss the intervening state transition.

Concurrency Context

In pointer-based lock-free structures, a node can be removed and later reused while the pointer value returns to the same address. The structure has changed, but a simple value comparison can no longer reveal that transition. Version tags, hazard pointers, or epoch-based reclamation are common ways to control the risk.

Correctness Boundary

ABA is not merely an equality problem. It matters when the intermediate state change has algorithmic meaning and a successful compare-and-swap would otherwise validate a stale assumption.

Related Concurrency Concepts

• Compare-and-Swap
• Lock-Free
• Hazard Pointer
• Memory Reclamation