Concurrency and Thread Safety

When multiple threads touch shared mutable state, interleavings produce race conditions: lost updates, torn reads, corrupted data. Thread safety means correctness under any interleaving. Locks/mutexes enforce mutual exclusion (pessimistic); optimistic concurrency checks for conflicts at commit and retries (compare-and-swap, version columns). Atomic operations skip locks for simple updates. Deadlock shows up when locks are acquired in conflicting orders. AI, ML, and GenAI engineer interviews probe it because inference servers, batching queues, and shared caches are all concurrent, and the classic double-increment bug still shows up in production.