Rust Atomics and Locks

Chapter 2. Atomics

The word atomic comes from the Greek word ἄτομος, meaning indivisible, something that cannot be cut into smaller pieces. In computer science, it is used to describe an operation that is indivisible: it is either fully completed, or it didn’t happen yet.

As mentioned in “Borrowing and Data Races”, multiple threads concurrently reading and modifying the same variable normally results in undefined behavior. However, atomic operations do allow for different threads to safely read and modify the same variable. Since such an operation is indivisible, it either happens completely before or completely after another operation, avoiding undefined behavior. Later, in Chapter 7, we’ll see how this works at the hardware level.

Atomic operations are the main building block for anything involving multiple threads. All the other concurrency primitives, such as mutexes and condition variables, are implemented using atomic operations.

In Rust, atomic operations are available as methods on the standard atomic types that live in std::sync::atomic. They all have names starting with Atomic, such as AtomicI32 or AtomicUsize. Which ones are available depends on the hardware architecture and sometimes operating system, but almost all platforms provide at least all atomic types up to the size of a pointer.

Unlike most types, they allow modification through a shared reference (e.g., &AtomicU8). This is possible thanks to interior mutability, as discussed in “Interior ...