DataRaceDemo.swift

import Foundation


func test(completion: (Int) -> ()) {
    let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
    //let queue = dispatch_queue_create("serial queue", DISPATCH_QUEUE_SERIAL)
    
    var counter = 0
    let grp = dispatch_group_create()
    
    for _ in 0..<10000 {
        dispatch_group_enter(grp)
        dispatch_async(queue) {
            counter += 1
            dispatch_group_leave(grp)
        }
    }
        
    dispatch_group_notify(grp, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
        completion(counter)
    }
}


let sem = dispatch_semaphore_create(0)
test() { counter in
    print("counter: \(counter)")
    // The counter should be 10000 - unless a  data race occurred.
    // With a concurrent queue, the variable `counter` is not synchronized and a data race will occure.
    dispatch_semaphore_signal(sem)
}

dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER)

In my example, simply use the other serial queue, which has been commented out, e.g. use:

//let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
let queue = dispatch_queue_create("serial queue", DISPATCH_QUEUE_SERIAL)