On Sat, May 30, 2026 at 09:27:29AM +0300, Onur Özkan wrote:
> On Fri, 29 May 2026 08:36:16 -0700
> "Paul E. McKenney" <[email protected]> wrote:
>
> > 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?
>
> Anyone can use them if they want from `bindings::` once it's exposed to Rust.
Is there a Rust-language counterpart to checkpatch.pl that could warn
when it sees patches adding calls to this function?
Thanx, Paul
> Regards,
> Onur
>
> > Thanx, Paul
>
