Open Radar

 
Summary:
Thread Sanitizer stops on a false positive when deferring mutex unlock.

Steps to Reproduce:
(run sample project with thread sanitizer enabled)
- init a mutex
- lock the mutex on a non-main thread
- unlock the mutex on a non-main thread
- lock the mutex on the main thread 
- defer unlocking mutex on main thread

Expected Results:
- Thread sanitizer does not pause on valid mutex usage

Actual Results:
- WARNING: ThreadSanitizer: Swift access race

Version/Build:
Xcode 10.2


Configuration:

```
class ViewController: UIViewController {
    var mutex = pthread_mutex_t()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        pthread_mutex_init(&mutex, nil)
        DispatchQueue(label: "NotMainQueue", attributes: []).async {
            pthread_mutex_lock(&self.mutex)
            pthread_mutex_unlock(&self.mutex)
        }
        pthread_mutex_lock(&self.mutex)
        defer { pthread_mutex_unlock(&self.mutex) }
    }
}
```