Documentation about memory barriers of linux kernel(https://www.kernel.org/doc/Documentation/memory-barriers.txt) has this example to illustrate the read barrier in the SMP case can be used to causes all effects prior to storage from another cpu be perceptible. why dose read barrier can do that?

+-------+       :      :                :       :
|       |       +------+                +-------+
|       |------>| A=1  |------      --->| A->0  |
|       |       +------+      \         +-------+
| CPU 1 |   wwwwwwwwwwwwwwww   \    --->| B->9  |
|       |       +------+        |       +-------+
|       |------>| B=2  |---     |       :       :
|       |       +------+   \    |       :       :       +-------+
+-------+       :      :    \   |       +-------+       |       |
                             ---------->| B->2  |------>|       |
                                |       +-------+       | CPU 2 |
                                |       :       :       |       |
                                |       :       :       |       |
  At this point the read ---->   \  rrrrrrrrrrrrrrrrr   |       |
  barrier causes all effects      \     +-------+       |       |
  prior to the storage of B        ---->| A->1  |------>|       |
  to be perceptible to CPU 2            +-------+       |       |
                                        :       :       +-------+