rust.arc_mutex_contention

Performance Medium

Detects Arc<Mutex<T>> or Arc<RwLock<T>> patterns that may cause lock contention in async code.

Why It Matters

Lock contention:

• Blocks async runtime — Mutex guards block the executor thread
• Serializes operations — Concurrent tasks wait for lock
• Defeats concurrency — Parallelism lost to synchronization

Example

// ❌ Before (holding lock across await)
let data = Arc::new(Mutex::new(vec![]));

async fn process(data: Arc<Mutex<Vec<i32>>>) {
    let mut guard = data.lock().unwrap();
    some_async_op().await; // Lock held across await!
    guard.push(1);
}

// ✅ After (minimize lock scope)
async fn process(data: Arc<Mutex<Vec<i32>>>) {
    some_async_op().await;
    {
        let mut guard = data.lock().unwrap();
        guard.push(1);
    } // Lock released immediately
}

What Unfault Detects

• Lock guards held across .await points
• Long-held mutex guards
• Mutex where RwLock would be better

Related Rules

• rust.blocking_in_async