A weakly-ordered processing system implements an execution synchronization bus transaction, or “memory barrier” bus transaction, to enforce strongly-ordered data transfer bus transactions. A slave device that ensures global observability may “opt out” of the memory barrier protocol. In various embodiments, the opt-out decision may be made dynamically by each slave device asserting a signal, may be set system-wide during a Power-On Self Test (POST) by polling the slave devices and setting corresponding bits in a global observability register, or it may be hardwired by system designers so that only slave devices capable of performing out-of-order data transfer operations participate in the memory barrier protocol.