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What this concurrency interview question tests

This is a medium-difficulty C++ concurrency question that probes understanding of memory ordering and inter-thread visibility on x86 architectures. It is representative of questions asked at trading firms and systems companies where correctness under multi-threaded execution is non-negotiable.

To work through this problem, candidates must reason about how writes in one thread become visible to reads in another, how CPU cores interact with cache hierarchies, and what guarantees the C++ memory model provides. The question forces you to distinguish between what the language promises and what the hardware delivers—a critical skill for writing correct lock-free code and debugging subtle race conditions.

• C++ memory ordering semantics (relaxed, acquire, release, sequential consistency)
• x86 memory barriers and cache coherence
• Happens-before relationships between threads
• Visibility and ordering of store and load operations