Why is this Swift Readers-Writers code causing deadlock?

I seem to have a classic solution for Readers-Writers problem in Swift using concurrent DispatchQueue with a barrier for writes. However, then I run my test code, it deadlocks.

Here's my wanna-be-thread-safe data structure:

class Container<T> {
    private var _value: T
    private let _queue = DispatchQueue(label: "containerQueue", attributes: .concurrent)
    init(_ value: T) {
        self._value = value
    var value: T {
        get {
            _queue.sync {
        set {
            _queue.async(flags: .barrier) {
                self._value = newValue

And here's my test code:

class ContainerTest {
    let testQueue = DispatchQueue(label: "testQueue", attributes: .concurrent)
    var container = Container(0)
    let group = DispatchGroup()
    func runTest() {
        for i in 0..<1000 {
            testQueue.async(group: group) {
                self.container.value = max(i, self.container.value)
        group.notify(queue: .main) {

The piece of code that's run repeatedly is just some random read and write operations. It's not attempting to produce anything sensible, it's just there to stress-test the data structure.

So when I run this, "Finished" is never printed. However, if I change _queue.async(flags: .barrier) to _queue.sync(flags: .barrier), then I see "Finished" printed.

I'm guessing when I'm using the async write version, I'm getting a deadlock, but why? It's the textbook Readers-Writers solution that's typically used. Perhaps it's my test code that is at fault, but again, why?


  • You're almost certainly getting thread-pool exhaustion. This is a pattern Apple engineers repeatedly warn against using because of that risk (I'm aware it's also documented as a common pattern in many places). Each call to .async is spawning a thread, and if there is severe contention (as you're creating), eventually there will be no threads available, and everything will lock up.

    In modern Swift, the correct tool for this is an actor. In pre-concurrency Swift, you'd likely want to use an OSAllocatedUnfairLock, though when you find yourself making an atomic property, thinking you mean thread-safe, you are often on the wrong road.