Re: [PATCH v2 RESEND 1/4] rust: add SRCU abstraction

Gary Guo
2026-04-21T16:15:01Z

On Thu Apr 16, 2026 at 6:43 PM BST, Onur Özkan wrote:
> Add a Rust abstraction for sleepable RCU (SRCU), backed by
> srcu_struct. Provide FFI helpers and a safe wrapper with a
> guard-based API for read-side critical sections.
>
> Signed-off-by: Onur Özkan <work@onurozkan.dev>
> ---
> rust/helpers/helpers.c | 1 +
> rust/helpers/srcu.c | 18 +++++++
> rust/kernel/sync.rs | 2 +
> rust/kernel/sync/srcu.rs | 109 +++++++++++++++++++++++++++++++++++++++
> 4 files changed, 130 insertions(+)
> create mode 100644 rust/helpers/srcu.c
> create mode 100644 rust/kernel/sync/srcu.rs
>
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index 875a9788ad40..052fef89d5f0 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -60,6 +60,7 @@
> #include "signal.c"
> #include "slab.c"
> #include "spinlock.c"
> +#include "srcu.c"
> #include "sync.c"
> #include "task.c"
> #include "time.c"
> diff --git a/rust/helpers/srcu.c b/rust/helpers/srcu.c
> new file mode 100644
> index 000000000000..b372b733eb89
> --- /dev/null
> +++ b/rust/helpers/srcu.c
> @@ -0,0 +1,18 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/srcu.h>
> +
> +__rust_helper int rust_helper_init_srcu_struct(struct srcu_struct *ssp)
> +{
> + return init_srcu_struct(ssp);
> +}
> +
> +__rust_helper int rust_helper_srcu_read_lock(struct srcu_struct *ssp)
> +{
> + return srcu_read_lock(ssp);
> +}
> +
> +__rust_helper void rust_helper_srcu_read_unlock(struct srcu_struct *ssp, int idx)
> +{
> + srcu_read_unlock(ssp, idx);
> +}
> \ No newline at end of file

^

> diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> index 993dbf2caa0e..0d6a5f1300c3 100644
> --- a/rust/kernel/sync.rs
> +++ b/rust/kernel/sync.rs
> @@ -21,6 +21,7 @@
> pub mod rcu;
> mod refcount;
> mod set_once;
> +pub mod srcu;
>
> pub use arc::{Arc, ArcBorrow, UniqueArc};
> pub use completion::Completion;
> @@ -31,6 +32,7 @@
> pub use locked_by::LockedBy;
> pub use refcount::Refcount;
> pub use set_once::SetOnce;
> +pub use srcu::Srcu;
>
> /// Represents a lockdep class.
> ///
> diff --git a/rust/kernel/sync/srcu.rs b/rust/kernel/sync/srcu.rs
> new file mode 100644
> index 000000000000..cf0c16248ea3
> --- /dev/null
> +++ b/rust/kernel/sync/srcu.rs
> @@ -0,0 +1,109 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Sleepable read-copy update (SRCU) abstraction.
> +//!
> +//! C header: [`include/linux/srcu.h`](srctree/include/linux/srcu.h)
> +
> +use crate::{
> + bindings,
> + error::to_result,
> + prelude::*,
> + types::{
> + NotThreadSafe,
> + Opaque, //
> + },
> +};
> +
> +use pin_init::pin_data;
> +
> +/// Creates an [`Srcu`] initialiser.
> +#[macro_export]
> +macro_rules! new_srcu {
> + () => {
> + $crate::sync::Srcu::new()
> + };

There's no need to create macro just for the sake of creating a macro. Although,
as Boqun already pointed out, macro will be needed for lockdep.

> +}
> +
> +/// Sleepable read-copy update primitive.
> +///
> +/// SRCU readers may sleep while holding the read-side guard.
> +#[repr(transparent)]
> +#[pin_data(PinnedDrop)]
> +pub struct Srcu {
> + #[pin]
> + inner: Opaque<bindings::srcu_struct>,
> +}
> +
> +impl Srcu {
> + /// Creates a new SRCU instance.
> + pub fn new() -> impl PinInit<Self, Error> {

Most of the methods here can be `#[inline]`

> + try_pin_init!(Self {
> + inner <- Opaque::try_ffi_init(|ptr: *mut bindings::srcu_struct| {
> + // SAFETY: `ptr` points to valid uninitialised memory for a `srcu_struct`.
> + to_result(unsafe { bindings::init_srcu_struct(ptr) })
> + }),
> + })
> + }
> +
> + /// Enters an SRCU read-side critical section.
> + pub fn read_lock(&self) -> Guard<'_> {
> + // SAFETY: By the type invariants, `self.inner.get()` is a valid initialized `srcu_struct`.
> + let idx = unsafe { bindings::srcu_read_lock(self.inner.get()) };
> +
> + Guard {
> + srcu: self,
> + idx,
> + _nts: NotThreadSafe,
> + }
> + }
> +
> + /// Waits until all pre-existing SRCU readers have completed.
> + pub fn synchronize(&self) {
> + // SAFETY: By the type invariants, `self.inner.get()` is a valid initialized `srcu_struct`.
> + unsafe { bindings::synchronize_srcu(self.inner.get()) };
> + }
> +
> + /// Waits until all pre-existing SRCU readers have completed, expedited.
> + ///
> + /// This requests a lower-latency grace period than [`Srcu::synchronize`] typically
> + /// at the cost of higher system-wide overhead. Prefer [`Srcu::synchronize`] by default
> + /// and use this variant only when reducing reset or teardown latency is more important
> + /// than the extra cost.
> + pub fn synchronize_expedited(&self) {
> + // SAFETY: By the type invariants, `self.inner.get()` is a valid initialized `srcu_struct`.
> + unsafe { bindings::synchronize_srcu_expedited(self.inner.get()) };
> + }
> +}
> +
> +#[pinned_drop]
> +impl PinnedDrop for Srcu {
> + fn drop(self: Pin<&mut Self>) {
> + // SAFETY: `self` is pinned and `inner` contains a valid initialized `srcu_struct`.
> + unsafe { bindings::cleanup_srcu_struct(self.as_ref().get_ref().inner.get()) };
> + }
> +}
> +
> +// SAFETY: `srcu_struct` may be shared and used across threads.
> +unsafe impl Send for Srcu {}
> +// SAFETY: `srcu_struct` may be shared and used concurrently.
> +unsafe impl Sync for Srcu {}
> +
> +/// Guard for an active SRCU read-side critical section on a particular [`Srcu`].
> +pub struct Guard<'a> {
> + srcu: &'a Srcu,
> + idx: core::ffi::c_int,
> + _nts: NotThreadSafe,

I'd prefer this to be _not_send so it's self-documenting on why you need this.

Best,
Gary

> +}
> +
> +impl Guard<'_> {
> + /// Explicitly exits the SRCU read-side critical section.
> + pub fn unlock(self) {}
> +}
> +
> +impl Drop for Guard<'_> {
> + fn drop(&mut self) {
> + // SAFETY: `Guard` is only constructible through `Srcu::read_lock()`,
> + // which returns a valid index for the SRCU instance.
> + unsafe { bindings::srcu_read_unlock(self.srcu.inner.get(), self.idx) };
> + }
> +}