gcd.swift

import Foundation

DispatchQueue.global().sync {
    print("hi")
}

print("good")


let serial = DispatchQueue(label: "Serial")
let concurrent = DispatchQueue(label: "Concurrent", attributes: .concurrent)

concurrent.async {
    print("concurrent async in")
    
    // it works!
    concurrent.sync {
        print("concurrent sync of concurrent async")
    }
    
    print("concurrent async out")
}

serial.async {
    print("serial async in")
    
    // X_X
    serial.sync {
        print("serial sync of serial async")
    }
    
    print("serial async out")
}