fix: Prevent false-positive deadlocks on try_lock #383

ShoyuVanilla · 2025-06-16T22:34:30Z

Fixes #376

#[test]
fn try_lock_no_block() {
    loom::model(|| {
        let lock = Rc::new(Mutex::new(0));
        let lock2 = lock.clone();

        let th = thread::spawn(move || {
            let _ = lock.try_lock(); // A
        });

        let guard = lock2.try_lock(); // B

        th.join().unwrap();

        drop(guard)
    })
}

With the above code, in some execution paths, the following happens:

Thread 0 tries to acquire B then switched into thread 1 before finishing it

Thread 1 tries to acquire A and blocks thread 0 in the following lines

loom/src/rt/mutex.rs

Lines 115 to 129 in dbf32b0

    
           // Block all **other** threads attempting to acquire the mutex 
        
           for (id, thread) in execution.threads.iter_mut() { 
        
               if id == thread_id { 
        
                   continue; 
        
               } 
        
               if let Some(operation) = thread.operation.as_ref() { 
        
                   if operation.object() == self.state.erase() { 
        
                       let location = operation.location(); 
        
                       trace!(state = ?self.state, thread = ?id, 
        
                           "Mutex::post_acquire"); 
        
                       thread.set_blocked(location); 
        
                   } 
        
               } 
        
           }

Thread 0 is blocked, so it cannot join the thread handle

Similar thing happens with RwLock and the simple short circuit code I added prevents such paths

mox692 · 2025-06-17T11:37:47Z

Hi, I'm happy to review this, but it might take a bit of time since it's been a while since I last looked at the code.

fix: Prevent false-positive deadlocks on try_lock

1dcc6b8

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fix: Prevent false-positive deadlocks on try_lock #383

fix: Prevent false-positive deadlocks on try_lock #383

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	// Block all other threads attempting to acquire the mutex
	for (id, thread) in execution.threads.iter_mut() {
	if id == thread_id {
	continue;
	}

	if let Some(operation) = thread.operation.as_ref() {
	if operation.object() == self.state.erase() {
	let location = operation.location();
	trace!(state = ?self.state, thread = ?id,
	"Mutex::post_acquire");
	thread.set_blocked(location);
	}
	}
	}

fix: Prevent false-positive deadlocks on try_lock #383

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fix: Prevent false-positive deadlocks on try_lock #383

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