Yeah but it could be IO parallelism. There could be two instances of callback chains of sequence C1->C2->C3 started such that the the second starts before the first one finished. As in C1->C2 ran then C1 gets called again. If in those callbacks you update a data structure (a database record?), you now accessed that data in parallel. So you have to protect against that with some kind of a lock/mutex. Yeah context switching doesn't potentially happen at every assembly instruction, the granularity is much higher, but it is still there.