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-rw-r--r--Documentation/memory-barriers.txt40
1 files changed, 29 insertions, 11 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 4af4cea8cff0..7ee2ae6d5451 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: