Don't attempt to cleanup Mutex when threads are deadlocked

Closes tokio-rs#173

src/rt/mutex.rs

@@ -58,6 +58,11 @@ impl Mutex {
             // Release the lock flag
             state.lock = None;
 
+            // Execution has deadlocked, cleanup does not matter.
+            if !execution.threads.is_active() {
+                return;
+            }
+
             state
                 .synchronize
                 .sync_store(&mut execution.threads, Release);

tests/deadlock.rs

@@ -0,0 +1,38 @@
+// #![deny(warnings, rust_2018_idioms)]
+
+use loom::sync::Mutex;
+use loom::thread;
+
+use std::rc::Rc;
+
+#[test]
+#[should_panic]
+fn two_mutexes_deadlock() {
+    loom::model(|| {
+        let a = Rc::new(Mutex::new(1));
+        let b = Rc::new(Mutex::new(2));
+
+        let th1 = {
+            let a = a.clone();
+            let b = b.clone();
+
+            thread::spawn(move || {
+                let a_lock = a.lock().unwrap();
+                let b_lock = b.lock().unwrap();
+                assert_eq!(*a_lock + *b_lock, 3);
+            })
+        };
+        let th2 = {
+            let a = a.clone();
+            let b = b.clone();
+
+            thread::spawn(move || {
+                let b_lock = b.lock().unwrap();
+                let a_lock = a.lock().unwrap();
+                assert_eq!(*a_lock + *b_lock, 3);
+            })
+        };
+        th1.join().unwrap();
+        th2.join().unwrap();
+    });
+}