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

[Boqun Feng]

On Sat, Jan 17, 2026 at 08:11:17PM +0800, Boqun Feng wrote:
[...]
> > > > 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?

I see, I need to change my reply to "RcuOld" below..

[...]

> > 
> > 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 I don't think so, `RcuOld` represents an unpublished object while `Rcu`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.
> 

I think a more accurate reply should be `RcuOld` is still not designed
for the usage of `RcuBox`. You're right that `RcuBox` is not an `Rcu<_>`
since `RcuBox` don't have the atomic pointer part, instead it relies
other atomic pointer operations to work (for example, the
rcu_dereference() in mtree_load()).

`RcuBox` represents an object pointed (and protected) by RCU.

`Rcu<_>` is an atomic pointer that maintains read and update for RCU, in
your usage, you don't need it because maple tree does that for you.

`RcuOld<_>` works with `Rcu<_>` to provide an API for users to decide
how to handle RCU reclaim. In Rcu + RcuOld design, RcuBox is just a Box
because these two pointer types handle reclaim + accesses.


We will need to use `Rcu` and `RcuOld` where the RCU access code is in
Rust.


I think there are similarities between `RcuOld` and `RcuBox`, but they
are sort of designed with different usages in mind, lemme think more..

Regards,
Boqun
 
> > synchronize_rcu() for cleanup - I need kfree_rcu().
> > 
> 
> That's something we can add later, for example, we can give `Rcu` (we
> can add the similar thing to `RcuOld`) a generic const like:
> 
>     struct Rcu<P, const ASYNC: u64 = 0>(..)
> 
> where Rcu<P, 0> use synchronize_rcu() and Rcu<P, 1> use kfree_rcu() or
> call_rcu() (once we have HasRcuHead support).
> 
> Regards,
> Boqun
> 
> > Alice