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Concurrency permutation testing tool for Rust.
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Loom is a testing tool for concurrent Rust code. It runs a test many times, permuting the possible concurrent executions of that test under the C11 memory model. It uses state reduction techniques to avoid combinatorial explosion.

Build Status


Getting started

To use loom, first add this to your Cargo.toml:

loom = "0.2.14"

Next, create a test file and add a test:

use loom::sync::Arc;
use loom::sync::atomic::AtomicUsize;
use loom::sync::atomic::Ordering::{Acquire, Release, Relaxed};
use loom::thread;

fn buggy_concurrent_inc() {
    loom::model(|| {
        let num = Arc::new(AtomicUsize::new(0));

        let ths: Vec<_> = (0..2)
            .map(|_| {
                let num = num.clone();
                thread::spawn(move || {
                    let curr = num.load(Acquire);
           + 1, Release);

        for th in ths {

        assert_eq!(2, num.load(Relaxed));


Loom is an implementation of techniques described in CDSChecker: Checking Concurrent Data Structures Written with C/C++ Atomics.

Thread ordering



In the C++11 memory model, stores to a single atomic cell are totally ordered. This is the modification order. Loom permutes the modification order of each atomic cell within the bounds of the coherence rules.


While already very useful, loom is in its early stages and has a number of limitations.

  • Execution is slow (#5).
  • The full C11 memory model is not implemented (#6).
  • No fence support (#7).


This project is licensed under the MIT license.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in loom by you, shall be licensed as MIT, without any additional terms or conditions.

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