MB [1] ensures that the writes done to save the state of the current process happen before the ownership is passed.
MB [2] ensures that the reads done to load the state of the new process happen after the ownership is picked up and hence are reliably the values written by the processor saving the old state. Leaving this MB out makes the code fail if an old value of the context remains in the second processor’s cache and invalidates from the writes done on the first processor are not delivered soon enough.
The TB on the second processor must be made coherent with any write to the page tables that may have occurred on the first processor just before the save of the process state. This must be done with a series of TB invalidate instructions to remove any nonglobal page mapping for this process, or by assigning an ASN that is unused on the second processor to the process. One of these actions must occur sometime before starting execution of the code for the new process that accesses memory (instruction or data) that is not common to all processes. A common method is to assign a new ASN after gaining ownership of the new process and before loading its context, which includes its ASN.
The
The
Combining all these considerations gives the following, where, on a single processor, there is no need for the barriers:
System Architecture and Programming Implications
