Eddie's Blog

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

By Mara Bos

At Topics: Chapter 1. Basics of Rust Concurrency

Extension of interior mutability

Recall

RefCell

  • std::cell::RefCell

  • Unlike Cell, it allows you to borrow its contents (call borrow() or borrow_mut())

    use std::cell::RefCell;

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

  • It has a counter for outstanding (未處理的) borrows

  • It can only be used within a single thread

Notes

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...

References