rust: add SRCU abstraction

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/helpers/srcu.c      |  10 +++
 rust/kernel/sync.rs      |   2 +
 rust/kernel/sync/srcu.rs | 158 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 170 insertions(+)
 create mode 100644 rust/kernel/sync/srcu.rs

rust/helpers/srcu.c +10 -0

index e9f723d7f8c9..79dd24a104ef 100644
@@ -22,3 +22,13 @@ __rust_helper void rust_helper_srcu_read_unlock(struct srcu_struct *ssp, int idx
 {
 	srcu_read_unlock(ssp, idx);
 }
+
+__rust_helper void rust_helper_srcu_barrier(struct srcu_struct *ssp)
+{
+	srcu_barrier(ssp);
+}
+
+__rust_helper void rust_helper_synchronize_srcu_expedited(struct srcu_struct *ssp)
+{
+	synchronize_srcu_expedited(ssp);
+}

rust/kernel/sync.rs +2 -0

index 993dbf2caa0e..0d6a5f1300c3 100644
@@ -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.
 ///

rust/kernel/sync/srcu.rs new file +104 -0

new file mode 100644
index 000000000000..655ecddd1320
@@ -0,0 +1,158 @@
+// 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();

rust/kernel/sync/srcu.rs new file +44 -0

+
+        // Ensure all SRCU callbacks have been finished before freeing.
+        // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`.
+        unsafe { bindings::srcu_barrier(ptr) };
+
+        // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`.
+        unsafe { bindings::cleanup_srcu_struct(ptr) };
+    }
+}
+
+// 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`].
+///
+/// Leaking this guard with [`core::mem::forget`] leaves the SRCU read-side
+/// critical section active and makes dropping the associated [`Srcu`] sleep forever.
+///
+/// # Invariants
+///
+/// `idx` is the index returned by `srcu_read_lock()` for `srcu`.
+#[must_use = "if unused, the lock will be immediately unlocked"]
+pub struct Guard<'a> {
+    srcu: &'a Srcu,
+    idx: i32,
+    _not_send: NotThreadSafe,
+}
+
+impl Guard<'_> {
+    /// Explicitly releases the SRCU read-side critical section.
+    #[inline]
+    pub fn unlock(self) {}
+}
+
+impl Drop for Guard<'_> {
+    #[inline]
+    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) };
+    }
+}

Signature · 1 line

2.51.2

Quoted 173 lines · Onur Özkan

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/helpers/srcu.c      |  10 +++
 rust/kernel/sync.rs      |   2 +
 rust/kernel/sync/srcu.rs | 158 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 170 insertions(+)
 create mode 100644 rust/kernel/sync/srcu.rs

rust/helpers/srcu.c +10 -0

index e9f723d7f8c9..79dd24a104ef 100644
@@ -22,3 +22,13 @@ __rust_helper void rust_helper_srcu_read_unlock(struct srcu_struct *ssp, int idx
 {
 	srcu_read_unlock(ssp, idx);
 }
+
+__rust_helper void rust_helper_srcu_barrier(struct srcu_struct *ssp)
+{
+	srcu_barrier(ssp);
+}
+
+__rust_helper void rust_helper_synchronize_srcu_expedited(struct srcu_struct *ssp)
+{
+	synchronize_srcu_expedited(ssp);
+}

rust/kernel/sync.rs +2 -0

index 993dbf2caa0e..0d6a5f1300c3 100644
@@ -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.
 ///

rust/kernel/sync/srcu.rs new file +112 -0

new file mode 100644
index 000000000000..655ecddd1320
@@ -0,0 +1,158 @@
+// 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();
+
+        // `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.
+        //

rust/kernel/sync/srcu.rs line 114

Quoted 2 lines

rust/kernel/sync/srcu.rs new file +2 -0

+        // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`.
+        unsafe { bindings::synchronize_srcu(ptr) };

Sorry for being slow on this. But I think your approach is the right one here. However, even though this makes Srcu safe, it's still undesired if an Srcu::drop() blocks forever *silently*. I think we should call srcu_active_readers() here and throw a warning if a leaked Guard is detected.

The rest of the patch set looks good to me.

Regards,
Boqun

Quoted 47 lines

rust/kernel/sync/srcu.rs new file +44 -0

+
+        // Ensure all SRCU callbacks have been finished before freeing.
+        // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`.
+        unsafe { bindings::srcu_barrier(ptr) };
+
+        // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`.
+        unsafe { bindings::cleanup_srcu_struct(ptr) };
+    }
+}
+
+// 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`].
+///
+/// Leaking this guard with [`core::mem::forget`] leaves the SRCU read-side
+/// critical section active and makes dropping the associated [`Srcu`] sleep forever.
+///
+/// # Invariants
+///
+/// `idx` is the index returned by `srcu_read_lock()` for `srcu`.
+#[must_use = "if unused, the lock will be immediately unlocked"]
+pub struct Guard<'a> {
+    srcu: &'a Srcu,
+    idx: i32,
+    _not_send: NotThreadSafe,
+}
+
+impl Guard<'_> {
+    /// Explicitly releases the SRCU read-side critical section.
+    #[inline]
+    pub fn unlock(self) {}
+}
+
+impl Drop for Guard<'_> {
+    #[inline]
+    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) };
+    }
+}

Signature · 1 line

2.51.2