This trace follows the actual state transitions behind the companion Synchronization Makes Shared State Predictable. Each step below names its concrete inputs; values illustrate the mechanism and vary across implementations.
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.
Checkpoint: 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. Confirm this state before advancing to Publish state and release.
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 done when this check holds: Two workers each need locks A and B. Compare the outcome against the companion article before making a performance claim.
Hardware, runtime, and observability notes
When tracing Trace: Synchronization Makes Shared State Predictable in a live production environment (subsystem: scheduling, primary layer: kernel), keep the following step-level cost model and diagnostic checklist in mind:
- Step 1 (kernel): Identify the shared invariant — verifies state invariants before advancing the execution pointer.
- Step 2 (kernel): Acquire the coordination primitive — verifies state invariants before advancing the execution pointer.
- Step 3 (kernel): Perform the protected transition — verifies state invariants before advancing the execution pointer.
- Step 4 (kernel): Publish state and release — verifies state invariants before advancing the execution pointer.
- Step 5 (kernel): Wake or unblock dependent work — verifies state invariants before advancing the execution pointer.
Every transition across the kernel boundary carries both a direct CPU/memory cost (cache-line fills, descriptor lookups, lock acquisitions, or context switches) and a tail-latency queueing risk under contention. To verify this trace on your own service, capture latency histograms and hardware/kernel counters across each step boundary, compare cold-cache versus warm-cache execution paths, and confirm that error or timeout branches release all acquired descriptors, locks, and buffers cleanly.