linux-kernel - Re: [PATCH RFC 1/2] rust: rcu: add RcuBox type

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Message-ID: <ef39d1a7-7f13-436d-adcf-cefd7cd854b4@nvidia.com>
Date: Fri, 16 Jan 2026 11:23:25 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: Alice Ryhl <aliceryhl@...gle.com>, "Paul E. McKenney"
 <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: 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>,
 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 1/2] rust: rcu: add RcuBox type

On 1/16/2026 10:46 AM, Alice Ryhl wrote:
> +impl<T: Send> Drop for RcuBox<T> {
> +    fn drop(&mut self) {
> +        // SAFETY: The `rcu_head` field is in-bounds of a valid allocation.
> +        let rcu_head = unsafe { &raw mut (*self.0.as_ptr()).rcu_head };
> +        if core::mem::needs_drop::<T>() {
> +            // SAFETY: `rcu_head` is the `rcu_head` field of `RcuBoxInner<T>`. All users will be
> +            // gone in an rcu grace period. This is the destructor, so we may pass ownership of the
> +            // allocation.
> +            unsafe { bindings::call_rcu(rcu_head, Some(drop_rcu_box::<T>)) };
> +        } else {
> +            // SAFETY: All users will be gone in an rcu grace period.
> +            unsafe { bindings::kvfree_call_rcu(rcu_head, self.0.as_ptr().cast()) };
> +        }
> +    }

We should probably add support for asynchronous callbacks for Rust instead of
directly calling the bindings in Rcubox. I know Boqun and me talked about that a
while back, and there was a usecase missing. Perhaps, RcuBox is a good use case
for the same?

 - Joel