[PATCH] Improve data-dependency memory barrier example in documentation

In the memory barrier document, improve the example of the data dependency
barrier situation by:

 (1) showing the initial values of the variables involved; and

 (2) repeating the instruction sequence description, this time with the data
     dependency barrier actually shown to make it clear what the revised
     sequence actually is.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 528d52f..92f0056 100644 (file)
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -610,6 +610,7 @@ loads.  Consider the following sequence of events:
 
        CPU 1                   CPU 2
        ======================= =======================
+               { B = 7; X = 9; Y = 8; C = &Y }
        STORE A = 1
        STORE B = 2
        <write barrier>
@@ -651,7 +652,20 @@ In the above example, CPU 2 perceives that B is 7, despite the load of *C
 (which would be B) coming after the the LOAD of C.
 
 If, however, a data dependency barrier were to be placed between the load of C
-and the load of *C (ie: B) on CPU 2, then the following will occur:
+and the load of *C (ie: B) on CPU 2:
+
+       CPU 1                   CPU 2
+       ======================= =======================
+               { B = 7; X = 9; Y = 8; C = &Y }
+       STORE A = 1
+       STORE B = 2
+       <write barrier>
+       STORE C = &B            LOAD X
+       STORE D = 4             LOAD C (gets &B)
+                               <data dependency barrier>
+                               LOAD *C (reads B)
+
+then the following will occur:
 
        +-------+       :      :                :       :
        |       |       +------+                +-------+