On Fri, Mar 22, 2024 at 07:26:28PM -0700, Boqun Feng wrote: > On Fri, Mar 22, 2024 at 10:07:31PM -0400, Kent Overstreet wrote: > [...] > > > Boqun already mentioned the "mixing access sizes", which is actually > > > quite fundamental in the kernel, where we play lots of games with that > > > (typically around locking, where you find patterns line unlock writing > > > a zero to a single byte, even though the whole lock data structure is > > > a word). And sometimes the access size games are very explicit (eg > > > lib/lockref.c). > > > > I don't think mixing access sizes should be a real barrier. On the read > > Well, it actually is, since mixing access sizes is, guess what, > an undefined behavior: > > (example in https://doc.rust-lang.org/std/sync/atomic/#memory-model-for-atomic-accesses) > > thread::scope(|s| { > // This is UB: using different-sized atomic accesses to the same data > s.spawn(|| atomic.store(1, Ordering::Relaxed)); > s.spawn(|| unsafe { > let differently_sized = transmute::<&AtomicU16, &AtomicU8>(&atomic); > differently_sized.store(2, Ordering::Relaxed); > }); > }); > > Of course, you can say "I will just ignore the UB", but if you have to > ignore "compiler rules" to make your code work, why bother use compiler > builtin in the first place? Being UB means they are NOT guaranteed to > work. That's not what I'm proposing - you'd need additional compiler support. but the new intrinsic would be no different, semantics wise for the compiler to model, than a "lock orb".
