linux-kernel - [PATCH tip/core/rcu 9/9] doc: Update control-dependencies section of memory-barriers.txt

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Message-Id: <1492015222-16373-9-git-send-email-paulmck@linux.vnet.ibm.com>
Date:   Wed, 12 Apr 2017 09:40:22 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.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, pierre Kuo <vichy.kuo@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 9/9] doc: Update control-dependencies section of memory-barriers.txt

From: pierre Kuo <vichy.kuo@...il.com>

In the following example, if MAX is defined to be 1, then the compiler
knows (Q % MAX) is equal to zero.  The compiler can therefore throw
away the "then" branch (and the "if"), retaining only the "else" branch.

	q = READ_ONCE(a);
	if (q % MAX) {
		WRITE_ONCE(b, 1);
		do_something();
	} else {
		WRITE_ONCE(b, 2);
		do_something_else();
	}

It is therefore necessary to modify the example like this:

        q = READ_ONCE(a);
-       WRITE_ONCE(b, 1);
+       WRITE_ONCE(b, 2);
        do_something_else();

Signed-off-by: pierre Kuo <vichy.kuo@...il.com>
Acked-by: Will Deacon <will.deacon@....com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 Documentation/memory-barriers.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index d2b0a8d81258..08329cb857ed 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -768,7 +768,7 @@ equal to zero, in which case the compiler is within its rights to
 transform the above code into the following:
 
 	q = READ_ONCE(a);
-	WRITE_ONCE(b, 1);
+	WRITE_ONCE(b, 2);
 	do_something_else();
 
 Given this transformation, the CPU is not required to respect the ordering
-- 
2.5.2