How are reads and writes kept in order in raid5.c?

I've gotten through most of handle_stripe, but I'm still confused on one big thing:  The reads and the writes are separated into two separate lists as soon as they come into make_request.  What guarantees they are done in order?  I can't see any mechanism that maintains any specific ordering of the bion.  Just that reads are done first, when possible, then writes.  What am I missing?