On Thu, May 28, 2026 at 01:23:39PM +0100, Gary Guo wrote:
> On Thu May 28, 2026 at 9:35 AM BST, Onur Özkan wrote:
> > On Thu, 28 May 2026 11:20:10 +0300
> > Onur Özkan <[email protected]> wrote:
> >
> >> On Thu, 28 May 2026 09:27:35 +0300
> >> Onur Özkan <[email protected]> wrote:
> >>
> >> > Add a Rust abstraction for sleepable RCU (SRCU), backed by C srcu_struct.
> >> > Provide FFI helpers and a safe wrapper with a guard-based API for
> >> > read-side
> >> > critical sections.
> >> >
> >> > Cleanup is handled via `PinnedDrop`, which explicitly drains pending
> >> > grace
> >> > periods and callbacks via `synchronize_srcu` and `srcu_barrier` before
> >> > executing `cleanup_srcu_struct` to guarantee memory safety e.g. when
> >> > there
> >> > are leaked guards (via `mem::forget($guard)`).
> >> >
> >> > Signed-off-by: Onur Özkan <[email protected]>
> >> > ---
> >> > rust/kernel/sync.rs | 2 +
> >> > rust/kernel/sync/srcu.rs | 166 +++++++++++++++++++++++++++++++++++++++
> >> > 2 files changed, 168 insertions(+)
> >> > create mode 100644 rust/kernel/sync/srcu.rs
> >> >
> >> > diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> >> > index 993dbf2caa0e..0d6a5f1300c3 100644
> >> > --- a/rust/kernel/sync.rs
> >> > +++ b/rust/kernel/sync.rs
> >> > @@ -21,6 +21,7 @@
> >> > pub mod rcu;
> >> > mod refcount;
> >> > mod set_once;
> >> > +pub mod srcu;
> >> >
> >> > pub use arc::{Arc, ArcBorrow, UniqueArc};
> >> > pub use completion::Completion;
> >> > @@ -31,6 +32,7 @@
> >> > pub use locked_by::LockedBy;
> >> > pub use refcount::Refcount;
> >> > pub use set_once::SetOnce;
> >> > +pub use srcu::Srcu;
> >> >
> >> > /// Represents a lockdep class.
> >> > ///
> >> > diff --git a/rust/kernel/sync/srcu.rs b/rust/kernel/sync/srcu.rs
> >> > new file mode 100644
> >> > index 000000000000..343f00d070c7
> >> > --- /dev/null
> >> > +++ b/rust/kernel/sync/srcu.rs
> >> > @@ -0,0 +1,166 @@
> >> > +// SPDX-License-Identifier: GPL-2.0
> >> > +
> >> > +//! Sleepable read-copy update (SRCU) support.
> >> > +//!
> >> > +//! C header: [`include/linux/srcu.h`](srctree/include/linux/srcu.h)
> >> > +
> >> > +use crate::{
> >> > + bindings,
> >> > + error::to_result,
> >> > + prelude::*,
> >> > + sync::LockClassKey,
> >> > + types::{
> >> > + NotThreadSafe,
> >> > + Opaque, //
> >> > + },
> >> > +};
> >> > +
> >> > +use pin_init::pin_data;
> >> > +
> >> > +/// Creates an [`Srcu`] initialiser with the given name and a
> >> > newly-created lock class.
> >> > +#[doc(hidden)]
> >> > +#[macro_export]
> >> > +macro_rules! new_srcu {
> >> > + ($($name:literal)?) => {
> >> > + $crate::sync::Srcu::new($crate::optional_name!($($name)?),
> >> > $crate::static_lock_class!())
> >> > + };
> >> > +}
> >> > +pub use new_srcu;
> >> > +
> >> > +/// Sleepable read-copy update primitive.
> >> > +///
> >> > +/// SRCU readers may sleep while holding the read-side guard.
> >> > +///
> >> > +/// The destructor waits for active readers and callbacks, so it may
> >> > sleep.
> >> > +/// If a read-side guard has been leaked, dropping an [`Srcu`] may
> >> > never return.
> >> > +///
> >> > +/// # Invariants
> >> > +///
> >> > +/// This represents a valid `struct srcu_struct` initialized by the C
> >> > SRCU API
> >> > +/// and it remains pinned and valid until the pinned destructor runs.
> >> > +#[repr(transparent)]
> >> > +#[pin_data(PinnedDrop)]
> >> > +pub struct Srcu {
> >> > + #[pin]
> >> > + inner: Opaque<bindings::srcu_struct>,
> >> > +}
> >> > +
> >> > +impl Srcu {
> >> > + /// Creates a new SRCU instance.
> >> > + #[inline]
> >> > + pub fn new(name: &'static CStr, key: Pin<&'static LockClassKey>) ->
> >> > impl PinInit<Self, Error> {
> >> > + try_pin_init!(Self {
> >> > + // INVARIANT: On success, the C initializer creates a valid
> >> > `srcu_struct` and
> >> > + // it remains pinned until `PinnedDrop` runs.
> >> > + 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_with_key(ptr,
> >> > name.as_char_ptr(), key.as_ptr())
> >> > + })
> >> > + }),
> >> > + })
> >> > + }
> >> > +
> >> > + /// Enters an SRCU read-side critical section.
> >> > + ///
> >> > + /// Leaking the returned [`Guard`] leaves the SRCU read-side
> >> > critical
> >> > + /// section active and makes `drop` sleep forever.
> >> > + #[inline]
> >> > + pub fn read_lock(&self) -> Guard<'_> {
> >> > + // SAFETY: By the type invariants, `self` contains a valid
> >> > `struct srcu_struct`.
> >> > + let idx = unsafe { bindings::srcu_read_lock(self.inner.get()) };
> >> > +
> >> > + // INVARIANT: `idx` was returned by `srcu_read_lock()` for this
> >> > `Srcu`.
> >> > + Guard {
> >> > + srcu: self,
> >> > + idx,
> >> > + _not_send: NotThreadSafe,
> >> > + }
> >> > + }
> >> > +
> >> > + /// Waits until all pre-existing SRCU readers have completed.
> >> > + #[inline]
> >> > + pub fn synchronize(&self) {
> >> > + // SAFETY: By the type invariants, `self` contains a valid
> >> > `struct 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.
> >> > + #[inline]
> >> > + pub fn synchronize_expedited(&self) {
> >> > + // SAFETY: By the type invariants, `self` contains a valid
> >> > `struct srcu_struct`.
> >> > + unsafe { bindings::synchronize_srcu_expedited(self.inner.get())
> >> > };
> >> > + }
> >> > +}
> >> > +
> >> > +#[pinned_drop]
> >> > +impl PinnedDrop for Srcu {
> >> > + fn drop(self: Pin<&mut Self>) {
> >> > + let ptr = self.inner.get();
> >> > +
> >> > + // SAFETY: By the type invariants, `self` contains a valid and
> >> > pinned `struct srcu_struct`
> >> > + // and `srcu_readers_active()` only checks the active reader
> >> > count.
> >> > + if unsafe { bindings::srcu_readers_active(ptr) } {
> >> > + crate::pr_warn!(
> >> > + "Leaked `Guard` detected while dropping SRCU; drop will
> >> > block forever.\n"
> >> > + );
>
> I think this could be a `warn_on` similar to how cleanup_srcu_struct handle
> the
> condition.
>
> >> > + }
> >> > +
> >> > + // `cleanup_srcu_struct()` may return early if readers are
> >> > still active. Because `Srcu`
> >> > + // owns the embedded `srcu_struct`, returning from `drop` in
> >> > that state could free memory
> >> > + // that is still referenced by the C side.
> >> > + //
> >> > + // Wait for all readers to complete first. If any `Guard` was
> >> > leaked, `synchronize_srcu()`
> >> > + // will sleep forever.
> >> > + //
> >> > + // SAFETY: By the type invariants, `self` contains a valid and
> >> > pinned `struct srcu_struct`.
> >> > + unsafe { bindings::synchronize_srcu(ptr) };
> >>
> >> Sashiko got a good point here which is calling synchronize_srcu() only if
> >> there
> >> are active readers. That's a nice low-effort improvement we can have in
> >> the next
> >> version.
> >>
> >> Onur
> >
> > Actually, now I am now thinking about whether we can come up with a better
> > approach when we detect leaked guards. Initially I came up with the
> > synchronize_srcu() solution because it would handle leaked guards
> > automatically
> > without requiring any additional checks. But now that we can actually detect
> > whether guards are leaked the question becomes:
> >
> > "Is there a better option than effectively sleeping forever when leaked
> > guards are detected?"
> >
> > I have no plans for tomorrow other than finalizing this series including the
> > question above.
>
> The best solution is to proceed cleanups anyway, given Rust rules ensure that
> these are actual leaks and not just srcu read-side critical section that
> failed
> to synchronize with the destruction of SRCU.
>
> This obviously require changes to the SRCU code though.
Right now, the C-language SRCU code would splat and leak the srcu_struct
structure. Of course, we *could* provide something that zeroed the
reader counts to avoid the splat and leak, but yikes! Or is there a
way to make such a function available *only* to the Rust code associated
with cleanup_srcu_struct(), and not to other C/Rust code?
Thanx, Paul
