Merge branch 'rcu/next' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu into core/rcu

Pull RCU updates from Paul E. McKenney:

 - Streamline RCU's use of per-CPU variables, shifting from "cpu"
   arguments to functions to "this_"-style per-CPU variable accessors.

 - Signal-handling RCU updates.

 - Real-time updates.

 - Torture-test updates.

 - Miscellaneous fixes.

 - Documentation updates.

Signed-off-by: Ingo Molnar <mingo@kernel.org>

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: