[PATCH v1] rust: rcu: Add abstraction for call_rcu()

[Philipp Stanner <phasta@kernel.org>]

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 <phasta@kernel.org>
---
 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