Re: [PATCH RFC 0/2] rcu box container for Rust + maple tree load_rcu

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Jan 21, 2026 at 01:21:46PM +0000, Alice Ryhl wrote:
> On Wed, Jan 21, 2026 at 09:14:05PM +0800, Boqun Feng wrote:
> > On Wed, Jan 21, 2026 at 12:10:54PM +0000, Alice Ryhl wrote:
> > > On Sat, Jan 17, 2026 at 10:00:19PM +0800, Boqun Feng wrote:
> > > > On Sat, Jan 17, 2026 at 01:12:08PM +0000, Alice Ryhl wrote:
> > > > [...]
> > > > > > > 1) "relaxed atomic" does not sound like something that provides an
> > > > > > >    address dependency to me.
> > > > > > 
> > > > > > If you look at rcu_dereference(), it's a READ_ONCE(), which is the same
> > > > > > as a relaxed atomic load, and yes in LKMM, relaxed atomic load provides
> > > > > > address dependency (Please see the DEPENDENCY part in
> > > > > > tools/memory-model/Documentation/explanation.txt).
> > > > > 
> > > > > You argued that we should rename READ_ONCE() to atomic load on that
> > > > > other patch series because "atomic load" naming is better than what LKMM
> > > > > normally uses. Fine, but relaxed atomic load is a much worse name than
> > > > 
> > > > To be clear, in that series, my argument was not about naming, it's
> > > > about READ_ONCE() being more powerful than atomic load (no, not because
> > > > of address dependency, they are the same on that, it's because of the
> > > > behaviors of them regarding a current access on the same memory
> > > > location), and we want user to specify the intention more clearly.
> > > 
> > > Expressing intent more clearly is fine with me. I still think it's weird
> > > for us to not have READ_ONCE() when it's a primitive operation of our
> > > memory model, though.
> > > 
> > 
> > But in our memory model, it's exact the same as atomic_read() (see
> > tools/memory-model/linux-kernel.def and search for "atomic_read"), so
> > why do we want to have both? ;-)
> 
> I've been saying Rust should have both because I've been repeatedly told
> that they are different. If READ_ONCE() and atomic_load() are the same,
> then I retract my concern.

I confess some bemusement on the allergy to READ_ONCE() and WRITE_ONCE().
However, we have a similar thing within the Linux kernel.  READ_ONCE()
is the same as atomic_read() from am memory-ordering perspective,
and WRITE_ONCE() is the same as atomic_set() from a memory-ordering
perspective.  The difference is typing.  READ_ONCE() and WRITE_ONCE()
will eat anything, but atomic_read() and atomic_set() insist on pointers
to atomic_t.  There is also atomic_long_t and atomic64_t, just in case
you want more API members.  ;-)

							Thanx, Paul

> > > And I also think we should consider using an implementation along the
> > > lines of what I shared for our atomic_load() or READ_ONCE() or whatever
> > > you wish to call it. The perf impact of helpers makes me sad.
> > > 
> > 
> > I'm not totally against that, it'll actually help Atomic as well, I also
> > hope that we can use `asm!()` to implement the cases where
> > `{read,write}_volatile()` cannot cover. However currently I would rely
> > on helper inlining to resolve this to avoid duplicate implementations.
> 
> I'm in favor of using helpers to begin with. I think it's probably worth
> to do atomic_load() before we do the other ops, since it's so much
> simpler to implement that particular operation than the ones using asm.
> 
> Alice