From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com> Short-circuit booleans are not defences against compilers breaking your intended control dependencies.
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> --- Documentation/memory-barriers.txt | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index d6bc77eb179a..8ebb66128cc8 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -725,6 +725,24 @@ 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: -- 1.8.1.5 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
