JLS Compliance and Program Termination:

Your understanding is correct. According to the Java Language Specification (JLS), the program is not guaranteed to halt. The JMM allows executions where the write to the volatile flag is ordered after an infinite sequence of reads in the synchronization order. This means the main thread could theoretically observe false indefinitely, even after the write occurs, due to the JMM's lack of strict liveness guarantees. While volatile ensures visibility and happens-before ordering, it does not enforce real-time scheduling or bounded latency. Thus, the JLS permits behaviors where the program hangs, even if such scenarios are improbable in practice.

Real JVM Behavior:

In practice, no real JVM will hang for this program when flag is volatile. Real JVMs (like OpenJDK) implement volatile with strong visibility semantics, ensuring that writes are promptly propagated. The assembly you observed confirms that the loop checks the volatile variable directly (not cached), and the second thread’s write will eventually be observed. While the JLS theoretically allows non-termination, actual JVMs prioritize practical correctness over such edge cases. Non-termination would require a JVM that violates the intent of volatile, which no mainstream implementation does.

Final Answer :
Yes, the program is allowed to not halt under the JMM, but it will likely halt in practice on modern JVMs. ​