This fails in iOS 9, but passes for iOS 10/11. What changed around serial/concurrent dispatch queues in iOS 10 so that this now works. Or rather, why does this not work in iOS 9?

DispatchQueueSurprise.m

- (void)testDispatchConcurrentExecutionWithNamedQueuesRaw {
    let expectation = [self expectationWithDescription:@"Concurrent execution must happen"];
    let semaphore = dispatch_semaphore_create(0);

    // global queue, concurrent
    let attributes = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_CONCURRENT, QOS_CLASS_DEFAULT, 0);
    let defaultQueue = dispatch_queue_create("com.pspdfkit.default-concurrent", attributes);

    // serial sub-queues that dispatch to concurrent queue
    let queue1 = dispatch_queue_create("com.pspdfkit.queue1", DISPATCH_QUEUE_SERIAL);
    dispatch_set_target_queue(queue1, defaultQueue);

    let queue2 = dispatch_queue_create("com.pspdfkit.queue2", DISPATCH_QUEUE_SERIAL);
    dispatch_set_target_queue(queue2, defaultQueue);

    // This only passes if the blocks dispatched to the global queue run concurrently.
    // Surprisingly, this works on iOS 10/11 but fails on iOS 9.
    dispatch_async(queue1, ^{
        // dispatch second async operation which needs to start concurrently
         dispatch_async(queue2, ^{
            dispatch_semaphore_signal(semaphore);
        });

        // we cannot lock again, first have to unlock
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        [expectation fulfill];
    });

    [self waitForExpectations:@[expectation] timeout:10];
}

This is the result from a few hours of deadlock-hunting on our CI tests for iOS 9 specifically. (only happened sometimes, timings).

It seems that using let defaultQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); instead of our own custom global queue does make things pass on iOS 9, so I assume the DISPATCH_QUEUE_CONCURRENT behaviour of custom queues changed and was buggy/did not behave as the documentation said on iOS < 10.

Twitter yielded no results, opened a DTS.

Seems this indeed was a bug in iOS 9 and got fixed: https://twitter.com/pedantcoder/status/960912480665968640