On Thu, Nov 16, 2017 at 06:45:08PM +0000, Will Deacon wrote: > 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. Ah, if "this patch set" meant "adding LTO", I stand corrected and I apologize for my confusion. I agree that we need LTO/PGO to be housebroken from an LKMM viewpoint before it is enabled. Thanx, Paul -- 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
