Re: [PATCH v2 0/4] rust: Add i8/i16 atomic xchg helpers

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

On Thu, Dec 25, 2025 at 09:56:55AM +0900, FUJITA Tomonori wrote:
[...]
> >> The architectures supporting Rust, implement atomic xchg families
> >> using architecture-specific instructions. They work for i8/i16 too so
> >> the helpers just call them.
> >> 
> >> Tested on QEMU (86_64, arm64, riscv, loongarch, and armv7).
> >> 
> >> Note the architectures that support Rust handle xchg differently:
> >> 
> >> - arm64 and riscv support xchg with all the orderings.
> >> 
> >> - x86_64 and loongarch support only full-ordering xchg. They calls the
> >>   full-ordering xchg for any orderings.
> > 
> > Maybe it's just that I'm reading this differently, but I think this is a
> > bit confusing, as if there's an optimisation opportunity.
> > 
> > x86 is TSO, so even a relaxed xchg is a full xchg. So in this sense x86
> > has implemented all orderings.
> 
> For x86_64, I agree that the wording is confusing. xchg always implies
> lock so different memory orderings all map to the same full-ordered
> xchg there.
> 

I feel a bit confusing as well about the need of mentioning the exact
ordering of these primitives on each arch. In my opinion, as long as
rust_helper_xchg_X() is mapped to xchg_X() in C, then it's clear that
they have the ordering of the corresponding C APIs. But I keep it as it
is for now, I may remove them from the commit logs later after I
re-think about this.

> 
> > Looking at loongarch ISA manual it's suggested that apart from load/store,
> > all other atomic memory instructions are also always full ordering.
> 
> On loongarch there are two possible implementations: an LL/SC-based
> one, which is effectively always full-ordered (similar to x86), and an
> AMO-based on, where weaker orderings may be possible, as only the AM
> variants with the DBAR function appear to be full-ordered.
> 
> 
> > The change themselves LGTM, so
> > 
> > Reviewed-by: Gary Guo <gary@garyguo.net>
> 
> Thanks a lot!
> 

Thank you all! Applied in rust-sync:

	https://git.kernel.org/pub/scm/linux/kernel/git/boqun/linux.git/ rust-sync

Regards,
Boqun