io_submit() blocks until a previous operation will be completed

I'm using the Linux kernel AIO through libaio, and I have to submit the next reading operation before the previous one was completed. The problem is that io_submit() blocks for some time and, as I can deduce from the interval, it waits the previous operation to be completed.

I know that I can enqueue a several operations with a single io_submit(), but it is not an option for me, because I don't know how exactly the next read operation would like when it's already a time to submit the first one.

Is it working like that only for me, or for everyone? In the second case, may I ask if I'm looking for something feasible, or I have to fallback to a threaded model?