call_rcu() can be expected to be needed by a great variety of users. This functionality is almost always used for deallocating resources after all accessors are gone. Hence, it appears reasonable to implement the abstractions in such a way that the user merely passes data, which is later (after a grace period) dropped.
In the rare cases where the user needs special action to take place, this could be achieved through implementing a custom drop() method. Implement a first minimal abstraction for call_rcu(). Signed-off-by: Philipp Stanner <[email protected]> --- rust/helpers/rcu.c | 1 + rust/kernel/sync.rs | 1 + rust/kernel/sync/rcu.rs | 89 ++++++++++++++++++++++++++++++++++++++++- 3 files changed, 90 insertions(+), 1 deletion(-) diff --git a/rust/helpers/rcu.c b/rust/helpers/rcu.c index 481274c05857..c9cfc99c93d5 100644 --- a/rust/helpers/rcu.c +++ b/rust/helpers/rcu.c @@ -1,5 +1,6 @@ // SPDX-License-Identifier: GPL-2.0 +#include <linux/types.h> /* for callback_head */ #include <linux/rcupdate.h> __rust_helper void rust_helper_rcu_read_lock(void) diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs index 993dbf2caa0e..1ddca3847b19 100644 --- a/rust/kernel/sync.rs +++ b/rust/kernel/sync.rs @@ -31,6 +31,7 @@ pub use locked_by::LockedBy; pub use refcount::Refcount; pub use set_once::SetOnce; +pub use rcu::Callback; /// Represents a lockdep class. /// diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs index a32bef6e490b..caf71fa46f5e 100644 --- a/rust/kernel/sync/rcu.rs +++ b/rust/kernel/sync/rcu.rs @@ -4,7 +4,15 @@ //! //! C header: [`include/linux/rcupdate.h`](srctree/include/linux/rcupdate.h) -use crate::{bindings, types::NotThreadSafe}; +use crate::{ + bindings, + prelude::*, + types::{ + NotThreadSafe, + Opaque, + }, + alloc::Flags, +}; /// Evidence that the RCU read side lock is held on the current thread/CPU. /// @@ -50,3 +58,82 @@ fn drop(&mut self) { pub fn read_lock() -> Guard { Guard::new() } + + +/// An RCU callback object. Carries the user's data to drop() it once a grace period ellapsed. +/// +/// This object serves to implement C's `call_rcu()` method. Since it is almost +/// always used to free a resource once a grace period ellapsed, the only thing +/// this implementation does is drop the user's data. In the rare cases in which +/// the user needs more action to take place, said actions need to be implemented +/// on the user's data via the [`Drop`] trait. +/// +/// # Examples +/// +/// ``` +/// use kernel::sync::rcu::Callback; +/// +/// struct Foo {}; +/// +/// impl Drop for Foo { +/// fn drop(&mut self) { +/// pr_info!("rcu::Foo Dropping.\n"); +/// } +/// } +/// +/// let data = Foo {}; +/// +/// let cb = Callback::new(data, GFP_KERNEL)?; +/// cb.submit(); +/// +/// Ok::<(), Error>(()) +/// ``` +#[repr(C)] +#[pin_data] +pub struct Callback<T: Send + 'static> { + /// The RCU head. Only used (and initialized) by the C backend. + #[pin] + inner: Opaque<bindings::callback_head>, + /// The user's data. This should implement [`Drop`] if the user wants specific + /// actions, besides mere deallocation, to happen. + #[pin] + data: T, +} + +impl<T: Send + 'static> Callback<T> { + /// Create a new callback. + pub fn new(data: impl PinInit<T>, flags: Flags) -> Result<Pin<KBox<Self>>> { + let cb = try_pin_init!(Self { + inner: Opaque::uninit(), // Only needed for the C backend, who will initialize it. + data <- data, + }); + + KBox::pin_init(cb, flags) + } + + extern "C" fn callback(rcu_head: *mut bindings::callback_head) { + let cb_ptr = rcu_head as *mut Self; + + // SAFETY: All [`Callback`] objects in this module are always created + // as `Pin<KBox<Self>>`. `Pin` is a transparent container. The action + // below merely serves re-creating the KBox so that it can drop properly. + let _cb = unsafe { KBox::from_raw(cb_ptr) }; + + // Self::data drops, ensuring the desired cleanup operation. + } + + fn as_raw(&self) -> *mut bindings::callback_head { + self.inner.get() + } + + /// Arm a [`Callback`]. One grace period after this function was called, + /// the callback object will be dropped. + pub fn submit(self: Pin<KBox<Self>>) { + // SAFETY: The memory is not moved by this code or the C backend. + let cb = unsafe { Pin::into_inner_unchecked(self) }; + let ptr = KBox::into_raw(cb); + // SAFETY: `ptr` was just created validly above. `Self::callback` relies + // on the RCU module / code never being unloaded. + unsafe { bindings::call_rcu((*ptr).as_raw(), Some(Self::callback)) }; + } +} -- 2.49.0
