Rust Concurrency & Atomics

Comprehensive guide based on Rust Atomics and Locks by Mara Bos and The Rust Programming Language Ch. 16.

When to Use This Skill

• Implementing lock-free or wait-free data structures
• Understanding Ordering (Relaxed, Acquire, Release, AcqRel, SeqCst)
• Building custom mutexes, channels, or condition variables
• Debugging data races or incorrect synchronization
• Understanding Send, Sync, and thread safety markers
• Using Arc<Mutex<T>> and Arc<RwLock<T>> patterns
• Working with std::sync::atomic types

Thread Safety Markers

Send

A type is Send if it can be transferred to another thread. Most types are Send. Notable exceptions: Rc<T>, raw pointers, MutexGuard (on some platforms).

Sync

A type is Sync if &T is Send (safe to share references across threads). Examples: Mutex<T> is Sync (even though T isn't necessarily), Cell<T> is NOT Sync.

// Auto-derived by compiler. Manually implement (unsafe) only when wrapping raw pointers:
unsafe impl Send for MyWrapper {}
unsafe impl Sync for MyWrapper {}

Atomic Operations

Types

AtomicBool, AtomicI8..AtomicI64, AtomicU8..AtomicU64, AtomicUsize, AtomicPtr<T>

Core Operations

use std::sync::atomic::{AtomicU64, Ordering};

let counter = AtomicU64::new(0);

counter.store(42, Ordering::Release);           // write
let val = counter.load(Ordering::Acquire);      // read
let old = counter.swap(100, Ordering::AcqRel);  // exchange

// Fetch-and-modify (returns OLD value)
counter.fetch_add(1, Ordering::Relaxed);
counter.fetch_sub(1, Ordering::Relaxed);
counter.fetch_or(mask, Ordering::Relaxed);
counter.fetch_and(!mask, Ordering::Relaxed);

// Compare-and-swap
let result = counter.compare_exchange(
    expected,        // current expected value
    new_value,       // value to store if current == expected
    Ordering::AcqRel,  // ordering on success
    Ordering::Acquire,  // ordering on failure
);
// Returns Ok(old) on success, Err(actual) on failure

Memory Ordering

Acquire-Release Pattern

// Thread A (producer)
DATA.store(value, Ordering::Relaxed);     // write data
READY.store(true, Ordering::Release);      // publish flag

// Thread B (consumer)
while !READY.load(Ordering::Acquire) {}    // wait for flag
let val = DATA.load(Ordering::Relaxed);    // guaranteed to see value

When to Use What

• Relaxed: Simple counters, statistics, progress indicators
• Acquire/Release: Publish data between threads (producer-consumer)
• SeqCst: When you need total ordering (rare; usually overkill)

Building a SpinLock

use std::sync::atomic::{AtomicBool, Ordering};
use std::cell::UnsafeCell;

pub struct SpinLock<T> {
    locked: AtomicBool,
    value: UnsafeCell<T>,
}

unsafe impl<T: Send> Sync for SpinLock<T> {}

impl<T> SpinLock<T> {
    pub fn lock(&self) -> &mut T {
        while self.locked.swap(true, Ordering::Acquire) {
            std::hint::spin_loop();  // CPU hint: we're spinning
        }
        unsafe { &mut *self.value.get() }
    }

    pub fn unlock(&self) {
        self.locked.store(false, Ordering::Release);
    }
}

Standard Library Primitives

Mutex<T>

use std::sync::{Arc, Mutex};

let data = Arc::new(Mutex::new(vec![]));
let data_clone = Arc::clone(&data);

std::thread::spawn(move || {
    let mut lock = data_clone.lock().unwrap();
    lock.push(42);
});  // MutexGuard dropped → lock released

RwLock<T>

use std::sync::RwLock;

let lock = RwLock::new(HashMap::new());
// Multiple readers simultaneously
let r = lock.read().unwrap();
// Exclusive writer
let mut w = lock.write().unwrap();

Channels (mpsc)

use std::sync::mpsc;

let (tx, rx) = mpsc::channel();
let tx2 = tx.clone();  // multiple producers

std::thread::spawn(move || tx.send("hello").unwrap());
std::thread::spawn(move || tx2.send("world").unwrap());

for msg in rx { println!("{msg}"); }

Reference Map

• references/atomics-ordering.md — atomic types, operations, memory ordering in depth
• references/building-primitives.md — implementing Mutex, Channel, Condvar from atomics
• references/thread-patterns.md — Arc+Mutex, scoped threads, thread pools, parking

Key References

• Rust Atomics and Locks by Mara Bos
• The Rust Programming Language, Ch. 16
• std::sync::atomic