linux-kernel - [PATCH tip/core/rcu 3/5] documentation: Additional restriction for control dependencies

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Message-Id: <1414533617-25933-3-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Tue, 28 Oct 2014 15:00:15 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 3/5] documentation: Additional restriction for control dependencies

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

Short-circuit booleans are not defences against compilers breaking
your intended control dependencies.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 Documentation/memory-barriers.txt | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index d6bc77eb179a..8ebb66128cc8 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -725,6 +725,24 @@ 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:
-- 
1.8.1.5

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