Commit b26cfc48 authored Apr 07, 2017 by pierre Kuo Committed by Paul E. McKenney Apr 12, 2017

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



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>

parent d3d3a3cc

Documentation/memory-barriers.txt

+1 −1

Original line number	Original line	Diff line number	Diff line
	@@ -768,7 +768,7 @@ equal to zero, in which case the compiler is within its rights to
	transform the above code into the following:		transform the above code into the following:

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

	Given this transformation, the CPU is not required to respect the ordering		Given this transformation, the CPU is not required to respect the ordering