From: Alice Ryhl aliceryhl@google.com
This adds an RcuBox container, which is like KBox except that the value is freed with kfree_rcu.
To allow containers to rely on the rcu properties of RcuBox, an extension of ForeignOwnable is added.
Signed-off-by: Alice Ryhl aliceryhl@google.com --- rust/bindings/bindings_helper.h | 1 + rust/kernel/sync/rcu.rs | 31 ++++++- rust/kernel/sync/rcu/rcu_box.rs | 145 ++++++++++++++++++++++++++++++++ 3 files changed, 176 insertions(+), 1 deletion(-) create mode 100644 rust/kernel/sync/rcu/rcu_box.rs
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h index 446dbeaf0866..2011645c7cfb 100644 --- a/rust/bindings/bindings_helper.h +++ b/rust/bindings/bindings_helper.h @@ -80,6 +80,7 @@ #include <linux/property.h> #include <linux/pwm.h> #include <linux/random.h> +#include <linux/rcupdate.h> #include <linux/refcount.h> #include <linux/regulator/consumer.h> #include <linux/sched.h> diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs index a32bef6e490b..7234fe3e79ee 100644 --- a/rust/kernel/sync/rcu.rs +++ b/rust/kernel/sync/rcu.rs @@ -4,7 +4,16 @@ //! //! C header: [`include/linux/rcupdate.h`](srctree/include/linux/rcupdate.h)
-use crate::{bindings, types::NotThreadSafe}; +use crate::{ + bindings, + types::{ + ForeignOwnable, + NotThreadSafe, // + }, // +}; + +mod rcu_box; +pub use self::rcu_box::RcuBox;
/// Evidence that the RCU read side lock is held on the current thread/CPU. /// @@ -50,3 +59,23 @@ fn drop(&mut self) { pub fn read_lock() -> Guard { Guard::new() } + +/// Declares that a pointer type is rcu safe. +pub trait ForeignOwnableRcu: ForeignOwnable { + /// Type used to immutably borrow an rcu-safe value that is currently foreign-owned. + type RcuBorrowed<'a>; + + /// Borrows a foreign-owned object immutably for an rcu grace period. + /// + /// This method provides a way to access a foreign-owned rcu-safe value from Rust immutably. + /// + /// # Safety + /// + /// * The provided pointer must have been returned by a previous call to [`into_foreign`]. + /// * If [`from_foreign`] is called, then `'a` must not end after the call to `from_foreign` + /// plus one rcu grace period. + /// + /// [`into_foreign`]: ForeignOwnable::into_foreign + /// [`from_foreign`]: ForeignOwnable::from_foreign + unsafe fn rcu_borrow<'a>(ptr: *mut ffi::c_void) -> Self::RcuBorrowed<'a>; +} diff --git a/rust/kernel/sync/rcu/rcu_box.rs b/rust/kernel/sync/rcu/rcu_box.rs new file mode 100644 index 000000000000..2508fdb609ec --- /dev/null +++ b/rust/kernel/sync/rcu/rcu_box.rs @@ -0,0 +1,145 @@ +// SPDX-License-Identifier: GPL-2.0 + +// Copyright (C) 2026 Google LLC. + +//! Provides the `RcuBox` type for Rust allocations that live for a grace period. + +use core::{ops::Deref, ptr::NonNull}; + +use kernel::{ + alloc::{self, AllocError}, + bindings, + ffi::c_void, + prelude::*, + sync::rcu::{ForeignOwnableRcu, Guard}, + types::ForeignOwnable, +}; + +/// A box that is freed with rcu. +/// +/// The value must be `Send`, as rcu may drop it on another thread. +/// +/// # Invariants +/// +/// * The pointer is valid and references a pinned `RcuBoxInner<T>` allocated with `kmalloc`. +/// * This `RcuBox` holds exclusive permissions to rcu free the allocation. +pub struct RcuBox<T: Send>(NonNull<RcuBoxInner<T>>); + +struct RcuBoxInner<T> { + value: T, + rcu_head: bindings::callback_head, +} + +// Note that `T: Sync` is required since when moving an `RcuBox<T>`, the previous owner may still +// access `&T` for one grace period. +// +// SAFETY: Ownership of the `RcuBox<T>` allows for `&T` and dropping the `T`, so `T: Send + Sync` +// implies `RcuBox<T>: Send`. +unsafe impl<T: Send + Sync> Send for RcuBox<T> {} + +// SAFETY: `&RcuBox<T>` allows for no operations other than those permitted by `&T`, so `T: Sync` +// implies `RcuBox<T>: Sync`. +unsafe impl<T: Send + Sync> Sync for RcuBox<T> {} + +impl<T: Send> RcuBox<T> { + /// Create a new `RcuBox`. + pub fn new(x: T, flags: alloc::Flags) -> Result<Self, AllocError> { + let b = KBox::new( + RcuBoxInner { + value: x, + rcu_head: Default::default(), + }, + flags, + )?; + + // INVARIANT: + // * The pointer contains a valid `RcuBoxInner` allocated with `kmalloc`. + // * We just allocated it, so we own free permissions. + Ok(RcuBox(NonNull::from(KBox::leak(b)))) + } + + /// Access the value for a grace period. + pub fn with_rcu<'rcu>(&self, _read_guard: &'rcu Guard) -> &'rcu T { + // SAFETY: The `RcuBox` has not been dropped yet, so the value is valid for at least one + // grace period. + unsafe { &(*self.0.as_ptr()).value } + } +} + +impl<T: Send> Deref for RcuBox<T> { + type Target = T; + fn deref(&self) -> &T { + // SAFETY: While the `RcuBox<T>` exists, the value remains valid. + unsafe { &(*self.0.as_ptr()).value } + } +} + +// SAFETY: +// * The `RcuBoxInner<T>` was allocated with `kmalloc`. +// * `NonNull::as_ptr` returns a non-null pointer. +unsafe impl<T: Send + 'static> ForeignOwnable for RcuBox<T> { + const FOREIGN_ALIGN: usize = <KBox<RcuBoxInner<T>> as ForeignOwnable>::FOREIGN_ALIGN; + + type Borrowed<'a> = &'a T; + type BorrowedMut<'a> = &'a T; + + fn into_foreign(self) -> *mut c_void { + self.0.as_ptr().cast() + } + + unsafe fn from_foreign(ptr: *mut c_void) -> Self { + // INVARIANT: Pointer returned by `into_foreign` carries same invariants as `RcuBox<T>`. + // SAFETY: `into_foreign` never returns a null pointer. + Self(unsafe { NonNull::new_unchecked(ptr.cast()) }) + } + + unsafe fn borrow<'a>(ptr: *mut c_void) -> &'a T { + // SAFETY: Caller ensures that `'a` is short enough. + unsafe { &(*ptr.cast::<RcuBoxInner<T>>()).value } + } + + unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> &'a T { + // SAFETY: `borrow_mut` has strictly stronger preconditions than `borrow`. + unsafe { Self::borrow(ptr) } + } +} + +impl<T: Send + 'static> ForeignOwnableRcu for RcuBox<T> { + type RcuBorrowed<'a> = &'a T; + + unsafe fn rcu_borrow<'a>(ptr: *mut c_void) -> &'a T { + // SAFETY: `RcuBox::drop` can only run after `from_foreign` is called, and the value is + // valid until `RcuBox::drop` plus one grace period. + unsafe { &(*ptr.cast::<RcuBoxInner<T>>()).value } + } +} + +impl<T: Send> Drop for RcuBox<T> { + fn drop(&mut self) { + // SAFETY: The `rcu_head` field is in-bounds of a valid allocation. + let rcu_head = unsafe { &raw mut (*self.0.as_ptr()).rcu_head }; + if core::mem::needs_drop::<T>() { + // SAFETY: `rcu_head` is the `rcu_head` field of `RcuBoxInner<T>`. All users will be + // gone in an rcu grace period. This is the destructor, so we may pass ownership of the + // allocation. + unsafe { bindings::call_rcu(rcu_head, Some(drop_rcu_box::<T>)) }; + } else { + // SAFETY: All users will be gone in an rcu grace period. + unsafe { bindings::kvfree_call_rcu(rcu_head, self.0.as_ptr().cast()) }; + } + } +} + +/// Free this `RcuBoxInner<T>`. +/// +/// # Safety +/// +/// `head` references the `rcu_head` field of an `RcuBoxInner<T>` that has no references to it. +/// Ownership of the `KBox<RcuBoxInner<T>>` must be passed. +unsafe extern "C" fn drop_rcu_box<T>(head: *mut bindings::callback_head) { + // SAFETY: Caller provides a pointer to the `rcu_head` field of a `RcuBoxInner<T>`. + let box_inner = unsafe { crate::container_of!(head, RcuBoxInner<T>, rcu_head) }; + + // SAFETY: Caller ensures exclusive access and passed ownership. + drop(unsafe { KBox::from_raw(box_inner) }); +}