Again I think here we have abstraction vs. implementation, where the key is how atomic is implemented, how it is implemented differently compared with lock, and why its specific implementation allows the atomicity, and the key lies in transactional memory.

Kunle mentions in lecture that one problem/limitation with transactions is that "its not possible to do something within a transaction which you cannot undo." A transaction is like a big undo button from the point of view of the memory system. By definition, you can't do I/O within a transaction b/c a transaction must satisfy the isolation property. For example, if part of a withdrawal involves giving somebody cash at an ATM, undoing that will be difficult :)