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 {
        /**
         * Since "data race" need to satisfy 4 conditions
         *  - target the same location
         *  - are performed concurrently by two threads
         *  - are not reads
         *  - are not synchronization operations
         * Therefore, we can break any one of above to avoid "data race".
         * The most easy implemented one I think should be "synchronization" in swift.
         * The code here presents how to avoid "data race" utilizing "synchronization"
         */
        objc_sync_enter(counter) // Start sync
        dispatch_group_enter(grp)
        dispatch_async(queue) {
            counter += 1
            dispatch_group_leave(grp)
        }
        objc_sync_exit(counter) // Exit sync
    }
    
    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)