Retain cycle with strong reference to dispatch_queue

In a class, I've declared a thread like:

@property (nonatomic, strong) dispatch_queue_t databaseQueue;

and then I perform an operation this thread like

dispatch_async(self.databaseQueue, ^{
        [self.dao deleteRetries];
    });

Can this potentially create a retain cycle?

AND

the current class holds a strong reference to viewControllerToDismiss and there is a code which looks like:

[viewControllerToDismiss dismissViewControllerAnimated:shouldAnimateDismiss completion:^{

   [self performSomeAction];
}

is this a retain cycle?