[Day9] Read Rust Atomics and Locks — Mutex, RwLock, Atomics and UnsafeCell

by Mara Bos

2 min readMay 25, 2023

At Topics: Chapter 1. Basics of Rust Concurrency
Extension of interior mutability

RefCell

std::cell::RefCell<T>
Unlike Cell, it allows you to borrow its contents (call borrow() or borrow_mut())

use std::cell::RefCell;

fn f(v: &RefCell<Vec<i32>>) {
    // We can modify the `Vec` directly.
    v.borrow_mut().push(1);
}

Mutex and RwLock

An RwLock or reader-writer lock is the concurrent version of a RefCell
An RwLock<T> holds a T and tracks any outstanding borrows
When conflicting borrows happen, it does not panic. Instead, it blocks the current thread (putting it to sleep) and waits for conflicting borrows to disappear.
Borrowing the contents of an `RwLock` is called locking
RwLock keeps tracking shared and exclusive borrows
A Mutex is very similar to RwLock, but it only allows exclusive borrows

Atomics

While an RwLock is the concurrent version of a RefCell, The atomic types represent the concurrent version of a Cell
They cannot be of arbitrary size, so there is no Atomic<T>
Usually use AtomicU32 and AtomicPtr<T> (depends on the processor)

UnsafeCell

An UnsafeCell is the primitive building unit for interior mutability
No restrictions to avoid undefined behavior
Can only be used in unsafe block
Commonly, an UnsafeCell is wrapped in another type that provides safety through a limited interface, such as Cell or Mutex.
In other words, all types with interior mutability are built on top of UnsafeCell, Including: Cell, RefCell, RwLock, Mutex…