Type Definition usync::Mutex

pub type Mutex<T> = Mutex<RawMutex, T>;

A mutual exclusion primitive useful for protecting shared data

This mutex will block threads waiting for the lock to become available. The mutex can be statically initialized or created by the new constructor. Each mutex has a type parameter which represents the data that it is protecting. The data can only be accessed through the RAII guards returned from lock and try_lock, which guarantees that the data is only ever accessed when the mutex is locked.

Fairness

This lock is a wrapper for RwLock underneath and has similar fairness guarantees. To reiterate, the mutex is unfair by default. This means that a thread which unlocks the mutex is allowed to re-acquire it again even when other threads are waiting for the lock.

This greatly improves throughput (read “performance”) but could potentially starve an unlucky thread when there’s constant lock contention. The mutex tries to at least wake up threads in the order that they we’re queued as an attempt to avoid starvation, but it is entirely up to the OS scheduler.

Differences from the standard library Mutex

• No poisoning, the lock is released normally on panic.
• Only requires 1 word (usize) of space, whereas the standard library boxes the Mutex due to platform limitations.
• Can be statically constructed.
• Does not require any drop glue when dropped.
• Inline fast path for the uncontended case.
• Efficient handling of micro-contention using adaptive spinning.
• Allows raw locking & unlocking without a guard.

Examples

use usync::Mutex;
use std::sync::{Arc, mpsc::channel};
use std::thread;

const N: usize = 10;

// Spawn a few threads to increment a shared variable (non-atomically), and
// let the main thread know once all increments are done.
//
// Here we're using an Arc to share memory among threads, and the data inside
// the Arc is protected with a mutex.
let data = Arc::new(Mutex::new(0));

let (tx, rx) = channel();
for _ in 0..10 {
    let (data, tx) = (Arc::clone(&data), tx.clone());
    thread::spawn(move || {
        // The shared state can only be accessed once the lock is held.
        // Our non-atomic increment is safe because we're the only thread
        // which can access the shared state when the lock is held.
        let mut data = data.lock();
        *data += 1;
        if *data == N {
            tx.send(()).unwrap();
        }
        // the lock is unlocked here when `data` goes out of scope.
    });
}

rx.recv().unwrap();