Re: [RFC] rust: types: Add read_once and write_once

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Oct 26, 2023 at 07:21:53AM -0700, Boqun Feng wrote:
> On Thu, Oct 26, 2023 at 01:16:25PM +0200, Peter Zijlstra wrote:
> > On Thu, Oct 26, 2023 at 11:36:10AM +0100, Gary Guo wrote:
> > 
> > > There's two reasons that we are using volatile read/write as opposed to
> > > relaxed atomic:
> > > * Rust lacks volatile atomics at the moment. Non-volatile atomics are
> > >   not sufficient because the compiler is allowed (although they
> > >   currently don't) optimise atomics. If you have two adjacent relaxed
> > >   loads, they could be merged into one.
> > 
> > Ah yes, that would be problematic, eg, if lifted out of a loop things
> > could go sideways fast.
> > 
> 
> Maybe we can workaround this limitation by using compiler barriers, i.e.
> 
> 	compiler_fence(SeqCst);
> 	load(Relaxed);
> 	compiler_fence(Acquire);
> 
> this is slightly stronger than a volatile atomic.
> 
> > > * Atomics only works for integer types determined by the platform. On
> > >   some 32-bit platforms you wouldn't be able to use 64-bit atomics at
> > >   all, and on x86 you get less optimal sequence since volatile load is
> > >   permitted to tear while atomic load needs to use LOCK CMPXCHG8B.
> > 
> > We only grudgingly allowed u64 READ_ONCE() on 32bit platforms because
> > the fallout was too numerous to fix. Some of them are probably bugs.
> > 
> > Also, I think cmpxchg8b without lock prefix would be sufficient, but
> > I've got too much of a head-ache to be sure. Worse is that we still
> > support targets without cmpxchg8b.
> > 
> > It might be interesting to make the Rust side more strict in this regard
> > and see where/when we run into trouble.
> > 
> 
> Sounds good to me. If the compiler barriers make sense for now, then
> we can do:
> 
> 	pub unsafe fn read_once_usize(ptr: *const usize) -> usize {
> 		core::sync::atomic::compiler_fence(SeqCst);
> 		let r = unsafe { *ptr.cast::<AtomicUsize>() }.load(Relaxed);
> 		core::sync::atomic::compiler_fence(Acquire);
> 		r
> 	}
> 

I forgot to mention, this can also resolve the comments from Marco, i.e.
switching implemention to Acquire if ARM64 & LTO.

Regards,
Boqun

> and if the other side (i.e. write) is also atomic (e.g. WRITE_ONCE()),
> we don't have data race.
> 
> However, there are still cases where data races are ignored in C code,
> for example inode::i_state: reads out of locks race with writes inside
> locks, since writes are done by plain accesses. Nothing can be done to
> fix that from Rust side only, and fixing the C side is a separate topic.
> 
> Thoughts?
> 
> Regards,
> Boqun
> 
> > > * Atomics doesn't work for complex structs. Although I am not quite sure
> > >   of the value of supporting it.
> > 
> > So on the C side we mandate the size is no larger than machine word,
> > with the exception of the u64 on 32bit thing. We don't mandate strict
> > integer types because things like pte_t are wrapper types.
> > 
> >