Type Definition parking_lot::FairMutex

pub type FairMutex<T> = Mutex<RawFairMutex, T>;

A mutual exclusive primitive that is always fair, 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.

The regular mutex provided by parking_lot uses eventual fairness (after some time it will default to the fair algorithm), but eventual fairness does not provide the same guarantees an always fair method would. Fair mutexes are generally slower, but sometimes needed.

In a fair mutex the waiters form a queue, and the lock is always granted to the next requester in the queue, in first-in first-out order. This ensures that one thread cannot starve others by quickly re-acquiring the lock after releasing it.

A fair mutex may not be interesting if threads have different priorities (this is known as priority inversion).

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 FairMutex 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 parking_lot::FairMutex;
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(FairMutex::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();