It depends on the application. If you only need to read one word (32 bits) then you don't need a mutex. If you need to read a whole block made of several words, and need that block to be consistent, then you need a mutex. Without the mutex, if CPU1 is writing the block while CPU2 is reading it, then the CPU2 will get a mix of the old and the new block, which may have some undesirable consequences.
