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

[Alice Ryhl <aliceryhl@google.com>]

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.

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.

> > READ_ONCE() if what you want to convey is "has address dependency".
> > That's not what "relaxed" means!
> > 
> 
> Also note that my previous reply was explaining why we don't need to
> call rcu_dereference() from Rust, because implementation-wise the
> LKMM relaxed atomic load provides the address dependency. Depending on
> what we want to do, we can limit this address dependency only to
> rcu_dereference() and make it a special case, this means we disallow the
> address dependency provided by the "relaxed" in normal cases. Or we can
> add a Consume ordering (a type alias to Relaxed) that makes user to
> explicitly use it when they rely on the address dependency. I think
> either would resolve your concern about the name of "relaxed".

Yes, I suppose that would resolve my concern about the name.

Well, I do have one partial concern there, which is that the wrong name
will show up in stack traces as long as helpers are used.

> > I suppose you can argue that the word "relaxed" means different things
> > in LKMM than it does elsewhere, but I looked over the doc you mentioned,
> > and there the LKMM calls said operation READ_ONCE(). The word "relaxed"
> > does not appear even once. If we're going to change terminology / use
> > new terminology, let's at least pick terminology that's not
> > contradictory with the rest of the world.
> > 
> > > > 2) How do you intend to provide mutable access? By waiting a grace
> > > >    period?
> > > 
> > > Please see the {read_}copy_update() in the RCU patches that I linked.
> > > In short, you don't wait a grace for mutable access, since in RCU,
> > > readers don't block updaters, but instead updater will copy the object,
> > > atomically update the pointer and then get an `RcuOld`,
> > > which you can either synchronize_rcu() or {call,kfree}_rcu().
> > 
> > Hm, ok. I don't really need that. What I want rcu for is the internal
> > maple tree data structure, so mtree_load() doesn't need to block on the
> > maple tree internal spinlock. The contents of the box would be protected
> > by a separate lock (probably via LockedBy).
> > 
> 
> You mean after `load_rcu()`, we could access mutably by a lock? You need
> to hold that lock and the rcu_read_lock() while mutably accessing the
> return of `load_rcu()`, right? That is basically using RCU as a proof
> for existence.

Yeah, that's right.

Alice