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 08:11:17PM +0800, Boqun Feng wrote:
> On Sat, Jan 17, 2026 at 11:55:06AM +0000, Alice Ryhl wrote:
> > On Sat, Jan 17, 2026 at 08:06:40AM +0800, Boqun Feng wrote:
> > > On Fri, Jan 16, 2026 at 03:46:35PM +0000, Alice Ryhl wrote:
> > > > I'm sending this RFC to share an experiment I'm looking at. This may let
> > > > us replace the range allocator in Rust Binder with a maple tree.
> > > > 
> > > 
> > > Thank you, Alice.
> > > 
> > > > An RcuBox is like a Box except that it lets you obtain a &T that
> > > > outlives the box by a grace period. It does not allow mutable access to
> > > 
> > > I think the `RcuBox` can be folded into the more generic RCU pointer api
> > > [1], e.g. Rcu<Box<RcuBoxInner<T>>> where RcuBoxInner<T>: HasRcuHead. The
> > > benefits are at least 1) we use relaxed atomic read for RCU readers
> > > which guarantees address dependency that RCU needs under LKMM (while in
> > > the RcuBox here, we just use plain reads), 2) we also support mutable
> > > access as well.
> > 
> > 1) But mtree_load() does use rcu_dereference() to obtain the pointer?
> > 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
READ_ONCE() if what you want to convey is "has address dependency".
That's not what "relaxed" means!

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).

> > > As for the progress of that effort, the Rcu atomic pointer is almost
> > > ready [2], I will likely send it early next week. For the `HasRcuHead`
> > > part, as you may be aware, I'm working on a generic `HasField` approach
> > > to avoid duplication of `Has*` trait and macros [3], that requires some
> > > syn adjustments from Gary and Benno, but they should be available next
> > > cycle. I will probably send the patches for reviews before that. Once we
> > > have that `HasRcuHead` should be easily to add.
> > > 
> > > Given the WIP code I have, I *think* we are not that far from providing
> > > what you need for binder.
> > 
> > Hmm, so I looked over [2], and I think my RcuBox is an RcuOld<_> rather
> > than an Rcu<_> under this model. Though I can't afford to pay
> 
> I don't think so, `RcuOld` represents an unpublished object while `Rcu`
> represents a published object, you can update an `Rcu` pointer to
> another object, which is normally how you update with RCU. But maybe
> it's easy to discuss this with updater side code in picture.

When the RcuBox<_> is an owned value in Rust code, it's unpublished.
It's only published while it's foreign-owned by the maple tree.

Alice