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);

    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];
}