When most people talk about async code it is with single threaded (hence the need not to block the event executor) applications (web-browsers, nodejs ect.). How

When most people talk about async code it is with single threaded (hence the need not to block the event executor) applications (web-browsers, nodejs ect.). How-ever on the other end of the delirium when people talk about mutex's (locks) they're talking about multiple threadeds working in parallel which you inherently need to consider if the code you're calling is going to either direct or indirect acquire the same lock or like you pointed out calls a api that blocks. In the single threaded environment the only cognitive load you need to worry about is one or two yield points, how-ever in a parallel system your main concern is mutex's, race-conditions, or dead-locks.