1 files changed, 29 insertions, 11 deletions

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 22a969cdd476..f7fa63508aba 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -121,22 +121,22 @@ For example, consider the following sequence of events:
 The set of accesses as seen by the memory system in the middle can be arranged
 in 24 different combinations:
 
-        STORE A=3,      STORE B=4,      x=LOAD A->3,    y=LOAD B->4
-        STORE A=3,      STORE B=4,      y=LOAD B->4,    x=LOAD A->3
-        STORE A=3,      x=LOAD A->3,    STORE B=4,      y=LOAD B->4
-        STORE A=3,      x=LOAD A->3,    y=LOAD B->2,    STORE B=4
-        STORE A=3,      y=LOAD B->2,    STORE B=4,      x=LOAD A->3
-        STORE A=3,      y=LOAD B->2,    x=LOAD A->3,    STORE B=4
-        STORE B=4,      STORE A=3,      x=LOAD A->3,    y=LOAD B->4
+        STORE A=3,      STORE B=4,      y=LOAD A->3,    x=LOAD B->4
+        STORE A=3,      STORE B=4,      x=LOAD B->4,    y=LOAD A->3
+        STORE A=3,      y=LOAD A->3,    STORE B=4,      x=LOAD B->4
+        STORE A=3,      y=LOAD A->3,    x=LOAD B->2,    STORE B=4
+        STORE A=3,      x=LOAD B->2,    STORE B=4,      y=LOAD A->3
+        STORE A=3,      x=LOAD B->2,    y=LOAD A->3,    STORE B=4
+        STORE B=4,      STORE A=3,      y=LOAD A->3,    x=LOAD B->4
         STORE B=4, ...
         ...
 
 and can thus result in four different combinations of values:
 
-        x == 1, y == 2
-        x == 1, y == 4
-        x == 3, y == 2
-        x == 3, y == 4
+        x == 2, y == 1
+        x == 2, y == 3
+        x == 4, y == 1
+        x == 4, y == 3
 
 
 Furthermore, the stores committed by a CPU to the memory system may not be
@@ -694,6 +694,24 @@ Please note once again that the stores to 'b' differ.  If they were
 identical, as noted earlier, the compiler could pull this store outside
 of the 'if' statement.
 
+You must also be careful not to rely too much on boolean short-circuit
+evaluation.  Consider this example:
+
+        q = ACCESS_ONCE(a);
+        if (a || 1 > 0)
+                ACCESS_ONCE(b) = 1;
+
+Because the second condition is always true, the compiler can transform
+this example as following, defeating control dependency:
+
+        q = ACCESS_ONCE(a);
+        ACCESS_ONCE(b) = 1;
+
+This example underscores the need to ensure that the compiler cannot
+out-guess your code.  More generally, although ACCESS_ONCE() does force
+the compiler to actually emit code for a given load, it does not force
+the compiler to use the results.
+
 Finally, control dependencies do -not- provide transitivity.  This is
 demonstrated by two related examples, with the initial values of
 x and y both being zero: