How Race Conditions Happen — Two threads, one variable, and a bug that only shows up sometimes.

Two threads, one variable, and a bug that only shows up sometimes.

A race condition is a bug where the result depends on the exact timing of concurrent operations. It hides in the gap between reading a value and writing it back.

The classic lost update

1. Both read. Thread A and Thread B both read count = 5.
2. Both compute. Each independently decides the new value is 6.
3. Both write. They both store 6 — but two increments happened, so it should be 7.
4. Non-determinism. Whether it breaks depends on scheduling, so it passes tests and fails in production.

How to prevent them

Locks. A mutex makes read-modify-write atomic, one thread at a time.

Atomics. Hardware atomic operations do increment as a single indivisible step.

Avoid shared state. Message passing and immutability sidestep the problem entirely.

The one-line mental model

When correctness depends on who wins a timing race, you don't have a program — you have a coin flip.

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