Commit-ID: 8b19d1dead8413442ba0ff0b4e19b08f69d2f1b7 Gitweb: http://git.kernel.org/tip/8b19d1dead8413442ba0ff0b4e19b08f69d2f1b7 Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Sun, 12 Oct 2014 07:55:47 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Thu, 13 Nov 2014 10:34:53 -0800 documentation: Additional restriction for control dependencies Short-circuit booleans are not defences against compilers breaking your intended control dependencies. Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Reviewed-by: Pranith Kumar <bobby.prani@xxxxxxxxx> --- Documentation/memory-barriers.txt | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 22a969c..1073e01 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -694,6 +694,24 @@ 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: -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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