spdk with rbd

Hi all, Recently I have done some test and read some code about spdk bdev with rbd, and I was really confused about how it works with polling. If I understand correctly, I think in spdk, the thread who get the completed IO have to be the thread who send the IO, every thread can not get the completed IO of other thread.But in ceph rbd, one imange only have one queue, and when calling *rbd_poll_io_events* , it will pop all the IO until it's empty or exceeds the max count. I think there is no way to distinguish which thread an IO belongs to.Why it can works in spdk? Thanks.