documentation: Additional restriction for control dependencies
Short-circuit booleans are not defences against compilers breaking
your intended control dependencies.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 22a969c..1073e01 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -694,6 +694,24 @@
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: