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

[Paul E. McKenney]

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.
> 
> > * 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.

Plus cmpxchg8b can be quite a bit heavier weight than READ_ONCE(),
in some cases to the point that you would instead use some other
concurrency design.

> It might be interesting to make the Rust side more strict in this regard
> and see where/when we run into trouble.

And maybe have some other name for READ_ONCE() that is permitted to tear.

> > * 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.

On C-language atomics, people who have talked about implementing atomics
for objects too large for tear-free loads and stores have tended to want
ot invent locks.  :-(

							Thanx, Paul