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doc: Update control-dependencies section of memory-barriers.txt

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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@gmail.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
pierre Kuo 2017-04-07 14:37:36 +08:00 committed by Paul E. McKenney
parent d3d3a3ccc4
commit b26cfc48e3
1 changed files with 1 additions and 1 deletions
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Documentation/memory-barriers.txt
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@ -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