I’ll readily agree I need to strengthen my concurrency skills without hesitation 
but really think at least an UnfairLock class should be available somewhere given the subtleties of the implementation pointed out above and DispatchQueue.concurrentPerform seems to offer a very limited vision. Leaving people to fend for themselves seems regrettable when they can be offered better more Swifty abstractions. They can still use other types of lock when they understand they need too.