Re: [PATCH v1] rust: rcu: Add abstraction for call_rcu()

[Alvin Sun <alvin.sun@linux.dev>]

On 5/20/26 21:17, Philipp Stanner wrote:
> call_rcu() can be expected to be needed by a great variety of users.
> This functionality is almost always used for deallocating resources
> after all accessors are gone. Hence, it appears reasonable to implement
> the abstractions in such a way that the user merely passes data, which
> is later (after a grace period) dropped.
>
> In the rare cases where the user needs special action to take place,
> this could be achieved through implementing a custom drop() method.
>
> Implement a first minimal abstraction for call_rcu().
>
> Signed-off-by: Philipp Stanner <phasta@kernel.org>
> ---
>   rust/helpers/rcu.c      |  1 +
>   rust/kernel/sync.rs     |  1 +
>   rust/kernel/sync/rcu.rs | 89 ++++++++++++++++++++++++++++++++++++++++-
>   3 files changed, 90 insertions(+), 1 deletion(-)
>
> diff --git a/rust/helpers/rcu.c b/rust/helpers/rcu.c
> index 481274c05857..c9cfc99c93d5 100644
> --- a/rust/helpers/rcu.c
> +++ b/rust/helpers/rcu.c
> @@ -1,5 +1,6 @@
>   // SPDX-License-Identifier: GPL-2.0
>   
> +#include <linux/types.h> /* for callback_head */
>   #include <linux/rcupdate.h>
>   
>   __rust_helper void rust_helper_rcu_read_lock(void)
> diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> index 993dbf2caa0e..1ddca3847b19 100644
> --- a/rust/kernel/sync.rs
> +++ b/rust/kernel/sync.rs
> @@ -31,6 +31,7 @@
>   pub use locked_by::LockedBy;
>   pub use refcount::Refcount;
>   pub use set_once::SetOnce;
> +pub use rcu::Callback;
>   
>   /// Represents a lockdep class.
>   ///
> diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs
> index a32bef6e490b..caf71fa46f5e 100644
> --- a/rust/kernel/sync/rcu.rs
> +++ b/rust/kernel/sync/rcu.rs
> @@ -4,7 +4,15 @@
>   //!
>   //! C header: [`include/linux/rcupdate.h`](srctree/include/linux/rcupdate.h)
>   
> -use crate::{bindings, types::NotThreadSafe};
> +use crate::{
> +    bindings,
> +    prelude::*,
> +    types::{
> +        NotThreadSafe,
> +        Opaque,

Suggestion: Append `//` after the last item in each use braces to
prevent rustfmt from condensing lines, since `imports_layout =
"Vertical"` is not yet stable.

Best regards,
Alvin

> +    },
> +    alloc::Flags,
> +};
>   
>   /// Evidence that the RCU read side lock is held on the current thread/CPU.
>   ///
> @@ -50,3 +58,82 @@ fn drop(&mut self) {
>   pub fn read_lock() -> Guard {
>       Guard::new()
>   }
> +
> +
> +/// An RCU callback object. Carries the user's data to drop() it once a grace period ellapsed.
> +///
> +/// This object serves to implement C's `call_rcu()` method. Since it is almost
> +/// always used to free a resource once a grace period ellapsed, the only thing
> +/// this implementation does is drop the user's data. In the rare cases in which
> +/// the user needs more action to take place, said actions need to be implemented
> +/// on the user's data via the [`Drop`] trait.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::sync::rcu::Callback;
> +///
> +/// struct Foo {};
> +///
> +/// impl Drop for Foo {
> +///     fn drop(&mut self) {
> +///         pr_info!("rcu::Foo Dropping.\n");
> +///     }
> +/// }
> +///
> +/// let data = Foo {};
> +///
> +/// let cb = Callback::new(data, GFP_KERNEL)?;
> +/// cb.submit();
> +///
> +/// Ok::<(), Error>(())
> +/// ```
> +#[repr(C)]
> +#[pin_data]
> +pub struct Callback<T: Send + 'static> {
> +    /// The RCU head. Only used (and initialized) by the C backend.
> +    #[pin]
> +    inner: Opaque<bindings::callback_head>,
> +    /// The user's data. This should implement [`Drop`] if the user wants specific
> +    /// actions, besides mere deallocation, to happen.
> +    #[pin]
> +    data: T,
> +}
> +
> +impl<T: Send + 'static> Callback<T> {
> +    /// Create a new callback.
> +    pub fn new(data: impl PinInit<T>, flags: Flags) -> Result<Pin<KBox<Self>>> {
> +        let cb = try_pin_init!(Self {
> +            inner: Opaque::uninit(), // Only needed for the C backend, who will initialize it.
> +            data <- data,
> +        });
> +
> +        KBox::pin_init(cb, flags)
> +    }
> +
> +    extern "C" fn callback(rcu_head: *mut bindings::callback_head) {
> +        let cb_ptr = rcu_head as *mut Self;
> +
> +        // SAFETY: All [`Callback`] objects in this module are always created
> +        // as `Pin<KBox<Self>>`. `Pin` is a transparent container. The action
> +        // below merely serves re-creating the KBox so that it can drop properly.
> +        let _cb = unsafe { KBox::from_raw(cb_ptr) };
> +
> +        // Self::data drops, ensuring the desired cleanup operation.
> +    }
> +
> +    fn as_raw(&self) -> *mut bindings::callback_head {
> +        self.inner.get()
> +    }
> +
> +    /// Arm a [`Callback`]. One grace period after this function was called,
> +    /// the callback object will be dropped.
> +    pub fn submit(self: Pin<KBox<Self>>) {
> +        // SAFETY: The memory is not moved by this code or the C backend.
> +        let cb = unsafe { Pin::into_inner_unchecked(self) };
> +        let ptr = KBox::into_raw(cb);
> +        // SAFETY: `ptr` was just created validly above. `Self::callback` relies
> +        // on the RCU module / code never being unloaded.
> +        unsafe { bindings::call_rcu((*ptr).as_raw(), Some(Self::callback)) };
> +    }
> +}