Swift access race with os_unfair_lock_lock

Question

I made a custom property wrapper which provides a method to access data in a mutually exclusive context using an os_unfair_lock. After testing my wrapper with TSAN enabled, an access race error was reported at the point of lock acquisition using os_unfair_lock_lock (shown in the image below)

Somehow a locking structure which is supposedly thread-safe is reported by TSAN as not being so. What is going on here?

Answer 1

According to the WWDC 2016 talk "Concurrent Programming with GCD in Swift 3" at around 18:07 the speaker states that

Traditionally you would use a lock. And in Swift, since you have the entire Darwin module at your disposition, you will actually see the struct based traditional C locks. However [emphasis added], Swift assumes that anything that's a struct can be moved, and that doesn't work with a mutex or with a lock.

The solution is to bridge to Objective-C and create a class which wraps the os_unfair_lock as an ivar:

And if you want something that's smaller and that looks like the locks that you have in C, then you have to call into Objective-C and introduce a base class in Objective-C that has your lock as an ivar

In this case, something like

UnfairLock.h

#ifndef UnfairLock_h
#define UnfairLock_h

@import Foundation;
@import os;

@interface UnfairLock : NSObject

-(void)unfairlyAcquire;
-(void)unlock;

@end


#endif /* UnfairLock_h */

UnfairLock.m

#import <Foundation/Foundation.h>
#import "UnfairLock.h"

@implementation UnfairLock {
    os_unfair_lock _lock;
}

-(instancetype)init {
    self = [super init];
    
    if (self) {
        _lock = OS_UNFAIR_LOCK_INIT;
    }
    
    return self;
}


-(void)unfairlyAcquire {
    os_unfair_lock_lock(&_lock);
}

-(void)unlock {
    os_unfair_lock_unlock(&_lock);
}

@end