linux-kernel - Re: [PATCH RFC 0/2] rcu box container for Rust + maple tree load

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Message-ID: <aWutQuJAaBXG2JY1@tardis-2.local>
Date: Sat, 17 Jan 2026 23:39:46 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Gary Guo <gary@...yguo.net>, Miguel Ojeda <ojeda@...nel.org>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Josh Triplett <josh@...htriplett.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang@...ux.dev>,
	Andrew Ballance <andrewjballance@...il.com>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	rcu@...r.kernel.org, maple-tree@...ts.infradead.org,
	linux-mm@...ck.org
Subject: Re: [PATCH RFC 0/2] rcu box container for Rust + maple tree load_rcu

On Sat, Jan 17, 2026 at 10:05:18PM +0800, Boqun Feng wrote:
> On Sat, Jan 17, 2026 at 01:29:25PM +0000, Alice Ryhl wrote:
> > On Sat, Jan 17, 2026 at 09:11:49PM +0800, Boqun Feng wrote:
> > > In Rcu + RcuOld design, RcuBox is just a Box
> > > because these two pointer types handle reclaim + accesses.
> > 
> > How would that work? Dropping my RcuBox<_> must use kfree_rcu() or
> > synchronize_rcu() or it is unsound. So it can't just be a Box.
> > 
> 
> RcuOld<P: ForeignOwnable> will call synchronize_rcu() before dropping
> `P`. And I think we can have an asynchronous drop pointer type like
> `RcuOld<P, Async>` that requires `P` is a type like `Box<HasRcuHead>`,

One blocker on that is Drop impls cannot be specialized. We probably
need a different pointer type like RcuOldAsync...

Regards,
Boqun

> and the drop of `RcuOld<Box<HasRcuHead>, Async>` would be `call_rcu()`
> or `kfree_rcu()`.
> 
> Regards,
> Boqun
> 
> > Alice