rust: list: add ListArcField

One way to explain what `ListArc` does is that it controls exclusive
access to the prev/next pointer field in a refcounted object. The
feature of having a special reference to a refcounted object with
exclusive access to specific fields is useful for other things, so
provide a general utility for that.

This is used by Rust Binder to keep track of which processes have a
reference to a given node. This involves an object for each process/node
pair, that is referenced by both the process and the node. For some
fields in this object, only the process's reference needs to access
them (and it needs mutable access), so Binder uses a ListArc to give the
process's reference exclusive access.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240814-linked-list-v5-10-f5f5e8075da0@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/list.rs

@@ -19,6 +19,9 @@ pub use self::impl_list_item_mod::{
 mod arc;
 pub use self::arc::{impl_list_arc_safe, AtomicTracker, ListArc, ListArcSafe, TryNewListArc};
 
+mod arc_field;
+pub use self::arc_field::{define_list_arc_field_getter, ListArcField};
+
 /// A linked list.
 ///
 /// All elements in this linked list will be [`ListArc`] references to the value. Since a value can

rust/kernel/list/arc_field.rs

@@ -0,0 +1,96 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! A field that is exclusively owned by a [`ListArc`].
+//!
+//! This can be used to have reference counted struct where one of the reference counted pointers
+//! has exclusive access to a field of the struct.
+//!
+//! [`ListArc`]: crate::list::ListArc
+
+use core::cell::UnsafeCell;
+
+/// A field owned by a specific [`ListArc`].
+///
+/// [`ListArc`]: crate::list::ListArc
+pub struct ListArcField<T, const ID: u64 = 0> {
+    value: UnsafeCell<T>,
+}
+
+// SAFETY: If the inner type is thread-safe, then it's also okay for `ListArc` to be thread-safe.
+unsafe impl<T: Send + Sync, const ID: u64> Send for ListArcField<T, ID> {}
+// SAFETY: If the inner type is thread-safe, then it's also okay for `ListArc` to be thread-safe.
+unsafe impl<T: Send + Sync, const ID: u64> Sync for ListArcField<T, ID> {}
+
+impl<T, const ID: u64> ListArcField<T, ID> {
+    /// Creates a new `ListArcField`.
+    pub fn new(value: T) -> Self {
+        Self {
+            value: UnsafeCell::new(value),
+        }
+    }
+
+    /// Access the value when we have exclusive access to the `ListArcField`.
+    ///
+    /// This allows access to the field using an `UniqueArc` instead of a `ListArc`.
+    pub fn get_mut(&mut self) -> &mut T {
+        self.value.get_mut()
+    }
+
+    /// Unsafely assert that you have shared access to the `ListArc` for this field.
+    ///
+    /// # Safety
+    ///
+    /// The caller must have shared access to the `ListArc<ID>` containing the struct with this
+    /// field for the duration of the returned reference.
+    pub unsafe fn assert_ref(&self) -> &T {
+        // SAFETY: The caller has shared access to the `ListArc`, so they also have shared access
+        // to this field.
+        unsafe { &*self.value.get() }
+    }
+
+    /// Unsafely assert that you have mutable access to the `ListArc` for this field.
+    ///
+    /// # Safety
+    ///
+    /// The caller must have mutable access to the `ListArc<ID>` containing the struct with this
+    /// field for the duration of the returned reference.
+    #[allow(clippy::mut_from_ref)]
+    pub unsafe fn assert_mut(&self) -> &mut T {
+        // SAFETY: The caller has exclusive access to the `ListArc`, so they also have exclusive
+        // access to this field.
+        unsafe { &mut *self.value.get() }
+    }
+}
+
+/// Defines getters for a [`ListArcField`].
+#[macro_export]
+macro_rules! define_list_arc_field_getter {
+    ($pub:vis fn $name:ident(&self $(<$id:tt>)?) -> &$typ:ty { $field:ident }
+     $($rest:tt)*
+    ) => {
+        $pub fn $name<'a>(self: &'a $crate::list::ListArc<Self $(, $id)?>) -> &'a $typ {
+            let field = &(&**self).$field;
+            // SAFETY: We have a shared reference to the `ListArc`.
+            unsafe { $crate::list::ListArcField::<$typ $(, $id)?>::assert_ref(field) }
+        }
+
+        $crate::list::define_list_arc_field_getter!($($rest)*);
+    };
+
+    ($pub:vis fn $name:ident(&mut self $(<$id:tt>)?) -> &mut $typ:ty { $field:ident }
+     $($rest:tt)*
+    ) => {
+        $pub fn $name<'a>(self: &'a mut $crate::list::ListArc<Self $(, $id)?>) -> &'a mut $typ {
+            let field = &(&**self).$field;
+            // SAFETY: We have a mutable reference to the `ListArc`.
+            unsafe { $crate::list::ListArcField::<$typ $(, $id)?>::assert_mut(field) }
+        }
+
+        $crate::list::define_list_arc_field_getter!($($rest)*);
+    };
+
+    () => {};
+}
+pub use define_list_arc_field_getter;