This trace follows the actual state transitions behind the companion Synchronization Makes Shared State Predictable. It describes a common execution path; implementation details can vary, so keep the contract separate from the mechanism.
Step 1: Identify the shared invariant
Concurrency means operations overlap in time; it does not require multiple CPU cores. When concurrent operations access shared mutable state, their interleaving can violate assumptions such as “read, increment, write happens atomically.” Synchronization establishes which transitions are allowed and who can observe them.
Step 2: Acquire the coordination primitive
A mutex can protect a critical section so only one thread updates an invariant at a time. A condition variable lets a thread sleep until a predicate may have changed; the predicate must be checked in a loop after waking. Semaphores represent a count of available permits or events.
Step 3: Perform the protected transition
A mutex makes a critical update exclusive, while a condition variable waits for a predicate; recheck that predicate after waking because another thread may change it first.
At this point, record the state that changed and check the invariant before advancing. If the operation repeats, make clear which values persist and which are recomputed.
Step 4: Publish state and release
Locks prevent some races but can create deadlocks when threads acquire them in conflicting orders. Holding a lock during slow I/O can stall unrelated work. Atomics are useful for narrow state transitions but do not automatically make a multi-field invariant safe.
Step 5: Wake or unblock dependent work
Two workers each need locks A and B. Describe a global lock-order rule that removes circular wait, and explain why merely adding timeouts changes rather than proves correctness.
The trace is complete when the result satisfies the stated contract. Compare this model with the concrete runtime or system you are studying before making a performance claim.