linux-kernel - [PATCH] documentation: Fix two-CPU control-dependency example

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Date:   Mon, 17 Jul 2017 17:24:42 +0900
From:   Akira Yokosawa <akiyks@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Akira Yokosawa <akiyks@...il.com>
Subject: [PATCH] documentation: Fix two-CPU control-dependency example

>From b798b9b631e237d285aa8699da00bfb8ced33bea Mon Sep 17 00:00:00 2001
From: Akira Yokosawa <akiyks@...il.com>
Date: Mon, 17 Jul 2017 16:25:33 +0900
Subject: [PATCH] documentation: Fix two-CPU control-dependency example

In commit 5646f7acc95f ("memory-barriers: Fix control-ordering
no-transitivity example"), the operator in "if" statement of
the two-CPU example was modified from ">=" to ">".
Now the example misses the point because there is no party
who will modify "x" nor "y". So each CPU performs only the
READ_ONCE().

The point of this example is to use control dependency for ordering,
and the WRITE_ONCE() should always be executed.

So it was correct prior to the above mentioned commit. Partial
revert of the commit (with context adjustments regarding other
changes thereafter) restores the point.

Note that the three-CPU example demonstrating the lack of
transitivity stands regardless of this partial revert.

Signed-off-by: Akira Yokosawa <akiyks@...il.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 c4ddfcd..c1ebe99 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -851,7 +851,7 @@ demonstrated by two related examples, with the initial values of
 	CPU 0                     CPU 1
 	=======================   =======================
 	r1 = READ_ONCE(x);        r2 = READ_ONCE(y);
-	if (r1 > 0)               if (r2 > 0)
+	if (r1 >= 0)              if (r2 >= 0)
 	  WRITE_ONCE(y, 1);         WRITE_ONCE(x, 1);

 	assert(!(r1 == 1 && r2 == 1));
-- 
2.7.4