As stated above, CLI/STI will only work on the processor on which they execute. Therefore, only the same processor will be protected from "stepping" on a protected resource. For example, assume the application maintains a linked list of I/O packets for a device. Whenever packets are inserted or removed, the list must be protected as a critical resource. Under the uniprocessor model, a CLI/STI around the manipulation of the list would be sufficient protection. However, in an MP environment, the CLI/STI would only protect the resource on the same processor. Another processor could enter a section of code that attempted to manipulate the linked list. The results would be unpredictable. Possibilities would range from no effect to deadlock. Code that uses CLI/STI is not reliable and should be eliminated.
The solution is to replace CLI/STIs with spinlocks. Each critical resource will have associated with it a spinlock. Before accessing the resource the spinlock must be acquired, and when complete, the spinlock is released.