Shared State, Mutex, RwLock, Arc, and Atomics

Advanced⏱90 minutes📖3 min readLesson 3 of 10

Shared State, Mutex, RwLock, Arc, and Atomics

Master shared-state concurrency with Mutex, RwLock, Arc, and atomic types for lock-free programming

Shared State: Mutex, RwLock, Arc, and Atomics

While message passing is great, shared state is sometimes the right tool. Rust's type system ensures shared state is accessed safely.

Mutex<T>: Mutual Exclusion

rust

use std::sync::Mutex;

fn main() {
    let mutex = Mutex::new(42);
    
    // Lock to access data
    {
        let mut guard = mutex.lock().unwrap();
        *guard += 1;
        println!("Inside lock: {guard}");
    } // Lock released here
    
    println!("{:?}", mutex); // Mutex { data: 43, ... }
}

Mutex Internals

Mutex<T> {
    data: T,          // Protected data
    inner: {          // OS-level mutex
        locked: AtomicBool,
        queue: WaitQueue,
    },
}

Poisoning

When a thread panics while holding a lock, the mutex is poisoned:

rust

use std::sync::Mutex;

fn main() {
    let mutex = Mutex::new(10);
    
    // Thread panics while holding lock
    let handle = std::thread::spawn(move || {
        let guard = mutex.lock().unwrap();
        panic!("panic while holding lock");
        drop(guard); // Never reached
    });
    
    assert!(handle.join().is_err());
    // mutex is now poisoned
}

Method	Behavior on Poison
`lock().unwrap()`	Panics if poisoned
`lock().unwrap_or_else(poisoned)`	Can recover with `.into_inner()`
`.into_inner()`	Consumes mutex, returns data

⚠️Warning

A poisoned mutex means the protected data may be in an inconsistent state. Use lock().unwrap_or_else(|e| e.into_inner()) to recover if you're sure it's safe.

MutexGuard: RAII Lock

rust

use std::sync::Mutex;

fn main() {
    let mutex = Mutex::new(vec![1, 2, 3]);
    
    // MutexGuard implements Deref and DerefMut
    let mut guard = mutex.lock().unwrap();
    guard.push(4);
    
    // With map — access a field inside the lock
    let first: std::sync::MutexGuard<'_, i32> = std::sync::MutexGuard::map(guard, |v| &mut v[0]);
    *first = 10;
    // guard is no longer valid, first holds lock
    drop(first); // Lock released
}

RwLock<T>: Read-Write Lock

Multiple readers OR one writer:

rust

use std::sync::RwLock;

fn main() {
    let rwlock = RwLock::new(42);
    
    // Multiple concurrent readers
    {
        let r1 = rwlock.read().unwrap();
        let r2 = rwlock.read().unwrap(); // OK: multiple readers
        println!("{r1} {r2}");
    } // Readers released
    
    // Exclusive writer
    {
        let mut w = rwlock.write().unwrap();
        *w += 1;
    }
}

Lock	Readers	Writers	Use Case
`Mutex<T>`	1 at a time	1 at a time	Simple exclusive access
`RwLock<T>`	Unlimited	1 at a time	Read-heavy workloads

✅Success

Use RwLock when reads vastly outnumber writes. For workloads with frequent writes, Mutex is usually faster due to lower overhead.

RwLock Starvation

rust

use std::sync::RwLock;
use std::thread;

fn main() {
    let lock = RwLock::new(0);
    let lock = std::sync::Arc::new(lock);
    
    // Writer may starve if readers are continuous
    // Use RwLock::try_read() and RwLock::try_write() for non-blocking
    // Use parking_lot::RwLock for fair locking
}

Method	Blocking?	Returns
`read()`	Yes	`RwLockReadGuard<T>`
`try_read()`	No	`Result<RwLockReadGuard<T>, TryLockError>`
`write()`	Yes	`RwLockWriteGuard<T>`
`try_write()`	No	`Result<RwLockWriteGuard<T>, TryLockError>`

Arc: Atomic Reference Counting

Arc<T> enables shared ownership across threads:

rust

use std::sync::Arc;
use std::thread;

fn main() {
    let data = Arc::new(vec![1, 2, 3]);
    let mut handles = vec![];
    
    for _ in 0..5 {
        let data = Arc::clone(&data); // Atomic increment
        handles.push(thread::spawn(move || {
            println!("{:?}", data);
        }));
    }
    
    for handle in handles {
        handle.join().unwrap();
    }
    // All clones dropped, data freed
}

Arc Internals

Arc<T> {
    ptr: NonNull<ArcInner<T>>,
}

ArcInner<T> {
    strong: AtomicUsize,  // Reference count
    weak: AtomicUsize,    // Weak reference count
    data: T,
}

Weak: Breaking Cycles

rust

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

fn main() {
    let shared: Arc<String> = Arc::new("hello".to_string());
    let weak: Weak<String> = Arc::downgrade(&shared);
    
    // Upgrade to check if still alive
    match weak.upgrade() {
        Some(data) => println!("Still alive: {data}"),
        None => println!("Data was dropped"),
    }
    
    drop(shared);
    assert!(weak.upgrade().is_none());
}

Atomics: Lock-Free Programming

Atomic types provide lock-free, thread-safe operations:

rust

use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use std::thread;

fn main() {
    let flag = AtomicBool::new(false);
    let counter = AtomicUsize::new(0);
    
    let flag = std::sync::Arc::new(flag);
    let counter = std::sync::Arc::new(counter);
    
    let mut handles = vec![];
    for _ in 0..10 {
        let flag = Arc::clone(&flag);
        let counter = Arc::clone(&counter);
        handles.push(thread::spawn(move || {
            counter.fetch_add(1, Ordering::SeqCst);
            flag.store(true, Ordering::SeqCst);
        }));
    }
    
    for handle in handles {
        handle.join().unwrap();
    }
    
    println!("counter: {}", counter.load(Ordering::SeqCst));
    println!("flag: {}", flag.load(Ordering::SeqCst));
}

Atomic Types

Type	Size	Use Case
`AtomicBool`	1 byte	Flags, control signals
`AtomicI32` / `AtomicU32`	4 bytes	Counters, state
`AtomicI64` / `AtomicU64`	8 bytes	Larger counters
`AtomicUsize`	arch bytes	Pointer-sized counters
`AtomicPtr<T>`	arch bytes	Lock-free data structures

Ordering

Memory ordering controls visibility of atomic operations:

rust

use std::sync::atomic::Ordering;

// Relaxed — no ordering guarantees (fastest)
x.store(5, Ordering::Relaxed);
y.load(Ordering::Relaxed);

// Acquire — subsequent reads see the write
// Release — previous writes are visible to acquire
x.store(5, Ordering::Release);
y.load(Ordering::Acquire); // Sees x = 5

// SeqCst — strongest guarantee
// All threads see operations in the same order
x.store(5, Ordering::SeqCst);
y.load(Ordering::SeqCst);

⚠️Warning

Ordering	Guarantee	Cost
`Relaxed`	No ordering	Zero
`Acquire`/`Release`	happens-before	Moderate
`AcqRel`	Acquire + Release	Moderate
`SeqCst`	Sequential consistency	Highest (but usually fine)

Atomic Operations

rust

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

fn main() {
    let counter = AtomicUsize::new(0);
    
    // Fetch-and-modify
    counter.fetch_add(5, Ordering::SeqCst);
    counter.fetch_sub(3, Ordering::SeqCst);
    
    // Compare-and-swap (CAS)
    let mut current = counter.load(Ordering::SeqCst);
    loop {
        let new = current + 1;
        match counter.compare_exchange(
            current,
            new,
            Ordering::SeqCst,
            Ordering::SeqCst,
        ) {
            Ok(_) => break,
            Err(actual) => current = actual, // Retry with updated value
        }
    }
    
    // Swap
    let old = counter.swap(100, Ordering::SeqCst);
    
    println!("Final: {}", counter.load(Ordering::SeqCst)); // 100
}

Real-World: Concurrent Counter

rust

use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::Arc;
use std::thread;

struct ConcurrentCounter {
    value: AtomicUsize,
}

impl ConcurrentCounter {
    fn new() -> Self {
        ConcurrentCounter { value: AtomicUsize::new(0) }
    }
    
    fn increment(&self) -> usize {
        self.value.fetch_add(1, Ordering::SeqCst)
    }
    
    fn get(&self) -> usize {
        self.value.load(Ordering::SeqCst)
    }
}

fn main() {
    let counter = Arc::new(ConcurrentCounter::new());
    let mut handles = vec![];
    
    for _ in 0..100 {
        let counter = Arc::clone(&counter);
        handles.push(thread::spawn(move || {
            for _ in 0..1000 {
                counter.increment();
            }
        }));
    }
    
    for handle in handles {
        handle.join().unwrap();
    }
    
    println!("Final count: {}", counter.get()); // 100000
}

Practice Questions

What's the difference between Mutex and RwLock?
What does it mean when a mutex is poisoned?
How do you recover from a poisoned mutex?
What's the difference between Arc and Rc?
When should you use Weak instead of Arc?
What are atomic types and when are they useful?
What do the different memory orderings mean?
How does compare-and-swap (CAS) work?
What's the advantage of RwLock over Mutex for read-heavy workloads?
How do you atomically increment a shared counter?

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Message Passing, Channels and Mutex Basics

Async/Await and Tokio

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Course Content

1. Threading, spawn, join, and Send + Sync 2. Message Passing, Channels and Mutex Basics 3. Shared State, Mutex, RwLock, Arc, and Atomics 4. Async/Await and Tokio 5. Unsafe Rust 6. FFI and C Interop 7. Declarative Macros (macro_rules!)8. Procedural Macros 9. Smart Pointers, Box, Rc, RefCell, and Interior Mutability 10. Advanced Capstone, Concurrent Key-Value Store