[RFC PATCH v2 3/4] rust: rcu: Introduce RcuFreeBox

[Boqun Feng <boqun@kernel.org>]

The current RcuBox will call the `drop()` function after a grace period
inside an RCU callback. This suffices for maintaining a RCU-protected
object:

  RcuBox::drop():
    call_rcu(
      |..| { // <- call back after one grace period.
        T::drop(); // <- call the destructor of the inner object.
      }
    )

However, to support a different RCU usage pattern as below we need to
extend RcuBox:

1. clean up the object, and unshare it from future RCU readers.
2. wait for an RCU grace period.
3. no other RCU readers, we can free the memory.

An `RcuFreeBox<T: RcuFreeSafe>` is introduced to provide support for
this:

  RcuFreeBox::drop():
    T::drop_before_gp(); // clean up and ushare.
    kfree_call_rcu(..);  // free it after one grace period.

Signed-off-by: Boqun Feng <boqun@kernel.org>
---
 rust/kernel/sync/rcu.rs         | 34 ++++++++++++++++
 rust/kernel/sync/rcu/rcu_box.rs | 72 +++++++++++++++++++++++++++++++--
 2 files changed, 102 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs
index 42f6bbc83f71..e781a5044de9 100644
--- a/rust/kernel/sync/rcu.rs
+++ b/rust/kernel/sync/rcu.rs
@@ -4,6 +4,8 @@
 //!
 //! C header: [`include/linux/rcupdate.h`](srctree/include/linux/rcupdate.h)
 
+use core::pin::Pin;
+
 use crate::{
     bindings,
     types::{
@@ -18,6 +20,8 @@
 pub use self::rcu_box::RcuKVBox;
 pub use self::rcu_box::RcuVBox;
 
+pub use self::rcu_box::RcuFreeBox;
+
 /// Evidence that the RCU read side lock is held on the current thread/CPU.
 ///
 /// The type is explicitly not `Send` because this property is per-thread/CPU.
@@ -100,3 +104,33 @@ pub trait ForeignOwnableRcu: ForeignOwnable {
     /// [`from_foreign`]: ForeignOwnable::from_foreign
     unsafe fn rcu_borrow<'a>(ptr: *mut ffi::c_void) -> Self::RcuBorrowed<'a>;
 }
+
+/// Declares a struct is safe to free after a grace period if all readers are guarded by RCU.
+///
+/// # Safety
+///
+/// Implementation must guarantee `drop_before_gp()` makes sure no future RCU reader will access
+/// any part of [`Self`], as a result, after `drop_before_gp()` return + one grace period, no RCU
+/// reader will be on the object, and it's safe to free it.
+///
+/// Notes for implementators: implementing this trait in general requires `Self` being a
+/// [`UnsafePinned`], i.e. a `&mut Self` is not a noalias reference if `Self` has non-trivial
+/// `drop()` function.
+pub unsafe trait RcuFreeSafe {
+    /// Clean up `Self` and make it ready to be RCU freed.
+    fn drop_before_gp(self: Pin<&mut Self>);
+}
+
+macro_rules! impl_not_drop {
+    ($($t:ty, )*) => {
+        // SAFETY: Dropping `T` has no side effect means `T` is always ready to be freed. And an
+        // empty `drop_before_gp()` suffices.
+        $(unsafe impl RcuFreeSafe for $t {
+            fn drop_before_gp(self: Pin<&mut Self>) {
+                $crate::const_assert!(!core::mem::needs_drop::<$t>());
+            }
+        })*
+    }
+}
+
+impl_not_drop! {i8,u8,i16,u16,i32,u32,isize,usize,i64,u64,}
diff --git a/rust/kernel/sync/rcu/rcu_box.rs b/rust/kernel/sync/rcu/rcu_box.rs
index cb1fd422480a..ab4e9b2a4444 100644
--- a/rust/kernel/sync/rcu/rcu_box.rs
+++ b/rust/kernel/sync/rcu/rcu_box.rs
@@ -6,6 +6,7 @@
 
 use core::{
     marker::PhantomData,
+    mem::ManuallyDrop,
     ops::Deref,
     ptr::NonNull, //
 };
@@ -29,17 +30,18 @@
 
 use super::{
     ForeignOwnableRcu,
-    Guard, //
+    Guard,
+    RcuFreeSafe, //
 };
 
-/// A box that is freed with rcu.
+/// A box that is drop with RCU.
 ///
-/// The value must be `Send`, as rcu may drop it on another thread.
+/// 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 `A`.
-/// * This `RcuBox` holds exclusive permissions to rcu free the allocation.
+/// * This `RcuBox` holds exclusive permissions to RCU-free the allocation.
 pub struct RcuBox<T: Send, A: Allocator>(NonNull<RcuBoxInner<T>>, PhantomData<A>);
 
 /// Type alias for [`RcuBox`] with a [`Kmalloc`] allocator.
@@ -223,6 +225,56 @@ fn drop(&mut self) {
     drop(unsafe { Box::<_, A>::from_raw(box_inner) });
 }
 
+/// A box that is freed with RCU.
+///
+/// Currently we require `T` being `Send` because of an implementation limitation. In theory we can
+/// support `T` being `!Send`, since the RCU callback is only used to free the memory, not dropping
+/// `T`.
+pub struct RcuFreeBox<T: Send + RcuFreeSafe, A: Allocator>(RcuBox<ManuallyDrop<T>, A>);
+
+impl<T: Send + RcuFreeSafe, A: Allocator> RcuFreeBox<T, A> {
+    /// Create a new `RcuFreeBox`.
+    pub fn new(x: T, flags: alloc::Flags) -> Result<Self, AllocError> {
+        Ok(Self(RcuBox::new(ManuallyDrop::new(x), flags)?))
+    }
+}
+
+impl<T: Send + RcuFreeSafe, A: Allocator> Deref for RcuFreeBox<T, A> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        self.0.deref()
+    }
+}
+
+impl<T: Send + RcuFreeSafe, A: Allocator> Drop for RcuFreeBox<T, A> {
+    fn drop(&mut self) {
+        // CAST: `ManuallyDrop<RcuBoxInner<T>>` is transparent to `RcuBoxInner<T>`, and `RcuBox`
+        // owns the object per type invariants.
+        let inner: *mut RcuBoxInner<T> = self.0 .0.as_ptr().cast();
+
+        // SAFETY: Per the invariants of `RcuBox`, `inner` owns the pointed object. And we are not
+        // going to move it.
+        let pin = unsafe { Pin::new_unchecked(&mut (*inner).value) };
+
+        pin.drop_before_gp();
+
+        // `needs_drop::<ManuallyDrop>()` returns `false`, hence `kvfree_call_rcu()` will be called
+        // and free the underlying data after a grace period.
+    }
+}
+
+// Note that `T: Sync` is required since when moving an `RcuFreeBox<T, A>`, the previous owner may
+// still access `&T` for one grace period.
+//
+// SAFETY: Ownership of the `RcuFreeBox<T, A>` allows for `&T` and dropping the `T`, so `T: Send +
+// Sync` implies `RcuFreeBox<T, A>: Send`.
+unsafe impl<T: Send + Sync + RcuFreeSafe, A: Allocator> Send for RcuFreeBox<T, A> {}
+
+// SAFETY: `&RcuFreeBox<T, A>` allows for no operations other than those permitted by `&T`, so `T:
+// Sync` implies `RcuFreeBox<T, A>: Sync`.
+unsafe impl<T: Send + Sync + RcuFreeSafe, A: Allocator> Sync for RcuFreeBox<T, A> {}
+
 #[kunit_tests(rust_rcu_box)]
 mod tests {
     use super::*;
@@ -236,6 +288,12 @@ fn rcu_box_basic() -> Result {
 
         drop(rb);
 
+        let rb = RcuFreeBox::<_, alloc::allocator::Kmalloc>::new(42i32, alloc::flags::GFP_KERNEL)?;
+
+        assert_eq!(*rb, 42);
+
+        drop(rb);
+
         let rb = RcuBox::<_, alloc::allocator::Vmalloc>::new(42i32, alloc::flags::GFP_KERNEL)?;
 
         assert_eq!(*rb, 42);
@@ -243,6 +301,12 @@ fn rcu_box_basic() -> Result {
 
         drop(rb);
 
+        let rb = RcuFreeBox::<_, alloc::allocator::Vmalloc>::new(42i32, alloc::flags::GFP_KERNEL)?;
+
+        assert_eq!(*rb, 42);
+
+        drop(rb);
+
         Ok(())
     }
 }
-- 
2.50.1 (Apple Git-155)