Type Definition parking_lot::Mutex

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 also be statically initialized or created via a 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

A typical unfair lock can often end up in a situation where a single thread quickly acquires and releases the same mutex in succession, which can starve other threads waiting to acquire the mutex. While this improves performance because it doesn't force a context switch when a thread tries to re-acquire a mutex it has just released, this can starve other threads.

This mutex uses eventual fairness to ensure that the lock will be fair on average without sacrificing performance. This is done by forcing a fair unlock on average every 0.5ms, which will force the lock to go to the next thread waiting for the mutex.

Additionally, any critical section longer than 1ms will always use a fair unlock, which has a negligible performance impact compared to the length of the critical section.

You can also force a fair unlock by calling MutexGuard::unlock_fair when unlocking a mutex instead of simply dropping the MutexGuard.

Differences from the standard library Mutex

• No poisoning, the lock is released normally on panic.
• Only requires 1 byte of space, whereas the standard library boxes the Mutex due to platform limitations.
• Can be statically constructed (requires the const_fn nightly feature).
• 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.
• Supports eventual fairness so that the mutex is fair on average.
• Optionally allows making the mutex fair by calling MutexGuard::unlock_fair.

Examples

use std::sync::Arc;
use parking_lot::Mutex;
use std::thread;
use std::sync::mpsc::channel;

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) = (data.clone(), 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();