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

[Gary Guo <gary@garyguo.net>]

On Thu, 26 Oct 2023 10:13:45 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Oct 25, 2023 at 12:53:39PM -0700, Boqun Feng wrote:
> > In theory, `read_volatile` and `write_volatile` in Rust can have UB in
> > case of the data races [1]. However, kernel uses volatiles to implement
> > READ_ONCE() and WRITE_ONCE(), and expects races on these marked accesses
> > don't cause UB. And they are proven to have a lot of usages in kernel.
> > 
> > To close this gap, `read_once` and `write_once` are introduced, they
> > have the same semantics as `READ_ONCE` and `WRITE_ONCE` especially
> > regarding data races under the assumption that `read_volatile` and
> > `write_volatile` have the same behavior as a volatile pointer in C from
> > a compiler point of view.
> > 
> > Longer term solution is to work with Rust language side for a better way
> > to implement `read_once` and `write_once`. But so far, it should be good
> > enough.  
> 
> So the whole READ_ONCE()/WRITE_ONCE() thing does two things we care
> about (AFAIR):
> 
>  - single-copy-atomicy; this can also be achieved using the C11
>    __atomic_load_n(.memorder=__ATOMIC_RELAXED) /
>    __atomic_store_n(.memorder=__ATOMIC_RELAXED) thingies.
> 
>  - the ONCE thing; that is inhibits re-materialization, and here I'm not
>    sure C11 atomics help, they might since re-reading an atomic is
>    definitely dodgy -- after all it could've changed.
> 
> Now, traditionally we've relied on the whole volatile thing simply
> because there was no C11, or our oldest compiler didn't do C11. But
> these days we actually *could*.
> 
> Now, obviously C11 has issues vs LKMM, but perhaps the load/store
> semantics are near enough to be useful.  (IIRC this also came up in the
> *very* long x86/percpu thread)
> 
> So is there any distinction between the volatile load/store and the C11
> atomic load/store that we care about and could not Rust use the atomic
> load/store to avoid their UB ?

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.
* 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.
* Atomics doesn't work for complex structs. Although I am not quite sure
  of the value of supporting it.