Building on the previous exercise, suppose that stack nodes are dynamically allocated. If we read a pointer and then are delayed (e.g., due to preemption), the node to which the pointer refers may be reclaimed and then reallocated for a different purpose. A subsequent compare-and-swap may then succeed when logically it should not. This issue is known as the ABA problem.

Give a concrete example—an interleaving of operations in two or more threads—where the ABA problem may result in incorrect behavior for your stack. Explain why this behavior cannot occur in systems with automatic garbage collection. Suggest what might be done to avoid it in systems with manual storage management.