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 08:06:40AM +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.
2) How do you intend to provide mutable access? By waiting a grace
   period?

> 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
synchronize_rcu() for cleanup - I need kfree_rcu().

Alice