On Thu, Nov 16, 2017 at 10:39:51AM -0800, Paul E. McKenney wrote: > On Thu, Nov 16, 2017 at 10:16:22AM -0800, Nick Desaulniers wrote: > > > On Thu, Nov 16, 2017 at 06:34:17PM +0100, Peter Zijlstra wrote: > > >> So the problem is that its very very hard (and painful) to find these > > >> bugs. Getting the tools people to comment on these specific > > >> optimizations would really help lots. > > > > No doubt; I do not disagree with you. Kernel developers have very > > important use cases for the language. > > > > But the core point I'm trying to make is "do we need to block this > > patch set until issues with the C standards body in regards to the > > kernels memory model are resolved?" I would hope the two are > > orthogonal and that we'd take them and then test them even more > > extensively than the developer has in order to find out. > > Given that I have been working on getting the C and C++ standards to > correctly handle rcu_dereference() for more than ten years, I recommend > -against- waiting on standardization in the strongest possible terms. > And if you think that ten years is bad, the Java standards community has > been struggling with out-of-thin-air (OOTA) values for almost 20 years. > And the C and C++ standards haven't solved OOTA, either. The problem is, if we go ahead with this change, the compiler *will* break some address dependencies and something will eventually go wrong. At that point, what do we do? Turn off some random compiler optimisation? Add a random barrier()? We don't necessarily need standardisation, but we at least need guarantees from the compiler implementation that LTO/PGO will respect source level address dependencies. I don't think we have that today. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
