var semaphore = DispatchSemaphore(value: 1)

DispatchQueue.global().async {
    print("T1 Start")
    semaphore.wait()
    print("T1 Enter")
    sleep(2)
    print("T1 Something complete")
    semaphore.signal()
}
DispatchQueue.global().async {
    sleep(1)
    print("T2 Start")
    semaphore.wait()
    print("T2 Enter")
    print("T2 Something complete")
    semaphore.signal()
}