Synchronous and Asynchronous Synchronization Primitives

Word-sized low-level synchronization primitives providing both asynchronous and synchronous interfaces.

Features

• No heap allocation.
• No hidden global variables.
• Provides both asynchronous and synchronous interfaces.
• Small memory footprint.
• Loom support: features = ["loom"].

Lock

saa::Lock is a low-level shared-exclusive lock providing both asynchronous and synchronous interfaces. Synchronous locking methods such as lock_sync and share_sync can be used alongside their asynchronous counterparts lock_async and share_async simultaneously. saa::Lock implements an allocation-free fair wait queue shared between both synchronous and asynchronous methods.

Examples

use saa::Lock;

let lock = Lock::default();

lock.lock_sync();

assert!(!lock.try_lock());
assert!(!lock.try_share());

assert!(!lock.release_share());
assert!(lock.release_lock());

async {
    lock.share_async();
    assert!(lock.release_share());
};

Barrier

saa::Barrier is a synchronization primitive to enable a number of tasks to start execution at the same time.

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

use saa::Barrier;

let barrier = Arc::new(Barrier::with_count(8));

let mut threads = Vec::new();

for _ in 0..8 {
    let barrier = barrier.clone();
    threads.push(thread::spawn(move || {
        for _ in 0..4 {
            barrier.wait_sync();
        }
    }));
}

for thread in threads {
    thread.join().unwrap();
}

Semaphore

saa::Semaphore is a synchronization primitive that allows a fixed number of threads to access a resource concurrently.

Examples

use saa::Semaphore;

let semaphore = Semaphore::default();

semaphore.acquire_many_sync(Semaphore::MAX_PERMITS - 1);

assert!(semaphore.try_acquire());
assert!(!semaphore.try_acquire());

assert!(semaphore.release());
assert!(!semaphore.release_many(Semaphore::MAX_PERMITS));
assert!(semaphore.release_many(Semaphore::MAX_PERMITS - 1));

async {
    semaphore.acquire_async().await;
    assert!(semaphore.release());
};

Gate

saa::Gate is an unbounded barrier that can be opened or sealed manually as needed.

Examples

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

use saa::Gate;
use saa::gate::State;

let gate = Arc::new(Gate::default());

let mut threads = Vec::new();

for _ in 0..4 {
    let gate = gate.clone();
    threads.push(thread::spawn(move || {
        assert_eq!(gate.enter_sync(), Ok(State::Controlled));
    }));
}

let mut count = 0;
while count != 4 {
    if let Ok(n) = gate.permit() {
        count += n;
    }
}

for thread in threads {
    thread.join().unwrap();
}

Pager

saa::Pager enables remotely waiting for a resource to become available.

Examples

use std::pin::pin;

use saa::{Gate, Pager};
use saa::gate::State;

let gate = Gate::default();

let mut pinned_pager = pin!(Pager::default());

assert!(gate.register_pager(&mut pinned_pager, true));
assert_eq!(gate.open().1, 1);

assert_eq!(pinned_pager.poll_sync(), Ok(State::Open));

Notes

Using synchronous methods in an asynchronous context may lead to deadlocks. Consider a scenario where an asynchronous runtime uses two threads to execute three tasks.

• ThreadId(0): task-0: share-waiting / pending || task-1: "synchronous"-lock-waiting.
• ThreadId(1): task-2: release-lock / ready: wake-up task-0 -> task-2: lock-waiting / pending.

In this example, task-0 has logically acquired a shared lock transferred from task-2; however, it may remain in the task queue indefinitely depending on the asynchronous runtime's scheduling policy.

Changelog