Rust's Mutex, Atomics and UnsafeCell – Spooky Action at a Distance?
A defining feature of Rust is its concept of aliasing ⊕
mutability. This rule governs that at any time a value may either
have multiple immutable shared references, or a single mutable
unique reference, and never both. While this greatly helps in
producing fast, efficient and correct code, it can be limiting. To
this end, Rust also features types that bend these rules, like
Mutex, RwLock,
Cell, RefCell, and
the ominous UnsafeCell types. In this post we
explore how these types interact with Rust's type system and concepts
of references and aliasing ⊕ mutability. We do so by looking at
how the AtomicUsize and Mutex types are implemented, how violating Rust's assumptions
can lead to incorrect optimizations by the compiler, and the
surprising global impact of synchronization primitives.