On Mon, Mar 25, 2024 at 05:14:41PM -0400, Kent Overstreet wrote: > On Mon, Mar 25, 2024 at 12:44:34PM -0700, Linus Torvalds wrote: > > On Mon, 25 Mar 2024 at 11:59, Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote: > > > > > > To be fair, "volatile" dates from an era when we didn't have the haziest > > > understanding of what a working memory model for C would look like or > > > why we'd even want one. > > > > I don't disagree, but I find it very depressing that now that we *do* > > know about memory models etc, the C++ memory model basically doubled > > down on the same "object" model. > > > > > The way the kernel uses volatile in e.g. READ_ONCE() is fully in line > > > with modern thinking, just done with the tools available at the time. A > > > more modern version would be just > > > > > > __atomic_load_n(ptr, __ATOMIC_RELAXED) Note that Rust does have something similiar: https://doc.rust-lang.org/std/ptr/fn.read_volatile.html pub unsafe fn read_volatile<T>(src: *const T) -> T (and also write_volatile()). So they made a good design putting the volatile on the accesses rather than the type. However, per the current Rust memory model these two primitives will be UB when data races happen :-( I mean, sure, if I use read_volatile() on an enum (whose valid values are only 0, 1, 2), and I get a value 3, and the compiler says "you have a logic bug and I refuse to compile the program correctly", I'm OK. But if I use read_volatile() to read something like a u32, and I know it's racy so my program actually handle that, I don't know any sane compiler would miss-compile, so I don't know why that has to be a UB. > > > > Yes. Again, that's the *right* model in many ways, where you mark the > > *access*, not the variable. You make it completely and utterly clear > > that this is a very explicit access to memory. > > > > But that's not what C++ actually did. They went down the same old > > "volatile object" road, and instead of marking the access, they mark > > the object, and the way you do the above is > > > > std::atomic_int value; > > > > and then you just access 'value' and magic happens. > > > > EXACTLY the same way that > > > > volatile int value; > > > > works, in other words. With exactly the same downsides. > > Yeah that's crap. Unfortunate too, because this does need to be a type > system thing and we have all the tools to do it correctly now. > > What we need is for loads and stores to be explict, and that absolutely > can and should be a type system thing. > > In Rust terminology, what we want is > > Volatile<T> > > where T is any type that fits in a machine word, and the only operations > it supports are get(), set(), xchg() and cmpxchG(). > > You DO NOT want it to be possible to transparantly use Volatile<T> in > place of a regular T - in exactly the same way as an atomic_t can't be > used in place of a regular integer. Yes, this is useful. But no it's not that useful, how could you use that to read another CPU's stack during some debug functions in a way you know it's racy? Regards, Boqun
