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What this mutex-locking C++ concurrency question tests

This is an easy concurrency question that probes your understanding of mutex semantics and lock pairing in C++. It rewards careful reading of how many times a lock is acquired versus released, and whether the code exhibits any undefined behaviour or deadlock conditions.

To approach this, you need to trace through the code path, counting every explicit .lock() call and matching it against the required number of .unlock() calls at the end. You must also watch for patterns that could cause deadlock, race conditions, or violations of mutex contract rules—such as unlocking a mutex that was never locked, or unlocking it more times than it was locked.

• Mutex acquisition and release pairing
• Deadlock detection and prevention
• Undefined behaviour in concurrent code
• Lock counting and state tracking