On Tue, Jul 07, 2020 at 11:29:15AM +0100, Dave Martin wrote: > On Mon, Jul 06, 2020 at 10:36:28AM -0700, Paul E. McKenney wrote: > > On Mon, Jul 06, 2020 at 06:05:57PM +0100, Dave Martin wrote: [ . . . ] > > > The underlying problem here seems to be that the necessary ordering > > > rule is not part of what passes for the C memory model prior to C11. > > > If we want to control the data flow, don't we have to wrap the entire > > > dereference in a macro? > > > > Yes, exactly. Because we are relying on things that are not guaranteed > > by the C memory model, we need to pay attention to the implementations. > > As I have said elsewhere, the price of control dependencies is eternal > > vigilance. > > > > And this also applies, to a lesser extent, to address and data > > dependencies, which are also not well supported by the C standard. > > > > There is one important case in which the C memory model -does- support > > control dependencies, and that is when the dependent write is a normal > > C-language write that is not involved in a data race. In that case, > > if the compiler broke the control dependency, it might have introduced > > a data race, which it is forbidden to do. However, this rule can also > > be broken when the compiler knows too much, as it might be able to prove > > that breaking the dependency won't introduce a data race. In that case, > > according to the standard, it is free to break the dependency. > > Which only matters because the C abstract machine may not match reality. > > LTO has no bearing on the abstract machine though. > > If specific compiler options etc. can be added to inhibit the > problematic optimisations, that would be ideal. I guess that can't > happen overnight though. Sadly, I must agree. > > > > > > We likely won't realise if/when this goes wrong, other than impossible to > > > > > > debug, subtle breakage that crops up seemingly randomly. Ideally, we'd be > > > > > > able to detect this sort of thing happening at build time, and perhaps > > > > > > even prevent it with compiler options or annotations, but none of that is > > > > > > close to being available and I'm keen to progress the LTO patches in the > > > > > > meantime because they are a requirement for CFI. > > > > > > > > > > My concern was not so much why LTO makes things dangerous, as why !LTO > > > > > makes things safe... > > > > > > > > Because ignorant compilers are safe compilers! ;-) > > > > > > AFAICT ignorance is no gurantee of ordering in general -- the compiler > > > is free to speculatively invent knowledge any place that the language > > > spec allows it to. !LTO doesn't stop this happening. > > > > Agreed, according to the standard, the compiler has great freedom. > > > > We have two choices: (1) Restrict ourselves to live within the confines of > > the standard or (2) Pay continued close attention to the implementation. > > We have made different choices at different times, but for many ordering > > situations we have gone with door #2. > > > > Me, I have been working to get the standard to better support our > > use case. This is at best slow going. But don't take my word for it, > > ask Will. > > I can believe it. They want to enable optimisations rather than prevent > them... Right in one! ;-) > > > Hopefully some of the knowledge I invented in my reply is valid... > > > > It is. It is just that there are multiple valid strategies, and the > > Linux kernel is currently taking a mixed-strategy approach. > > Ack. The hope that there is a correct way to fix everything dies > hard ;) Either that, or one slowly degrades ones definition of "correct". :-/ > Life was cosier before I started trying to reason about language specs. Same here! Thanx, Paul
