Add Ref to the sync module.

rust/kernel/file_operations.rs

@@ -13,6 +13,7 @@ use alloc::sync::Arc;
 use crate::bindings;
 use crate::c_types;
 use crate::error::{Error, KernelResult};
+use crate::sync::{Ref, RefCounted};
 use crate::user_ptr::{UserSlicePtr, UserSlicePtrReader, UserSlicePtrWriter};
 
 /// Wraps the kernel's `struct file`.
@@ -506,6 +507,16 @@ impl<T> PointerWrapper<T> for Box<T> {
     }
 }
 
+impl<T: RefCounted> PointerWrapper<T> for Ref<T> {
+    fn into_pointer(self) -> *const T {
+        Ref::into_raw(self)
+    }
+
+    unsafe fn from_pointer(ptr: *const T) -> Self {
+        Ref::from_raw(ptr as _)
+    }
+}
+
 impl<T> PointerWrapper<T> for Arc<T> {
     fn into_pointer(self) -> *const T {
         Arc::into_raw(self)

rust/kernel/sync/arc.rs

@@ -0,0 +1,185 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! A reference-counted pointer.
+//!
+//! This module implements a way for users to create reference-counted objects and pointers to
+//! them. Such a pointer automatically increments and decrements the count, and drops the
+//! underlying object when it reaches zero. It is also safe to use concurrently from multiple
+//! threads.
+//!
+//! It is different from the standard library's [`Arc`] in two ways: it does not support weak
+//! references, which allows it to be smaller -- a single pointer-sized integer; it allows users to
+//! safely increment the reference count from a single reference to the underlying object.
+//!
+//! [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
+
+use crate::KernelResult;
+use alloc::boxed::Box;
+use core::{
+    mem::ManuallyDrop,
+    ops::Deref,
+    ptr::NonNull,
+    sync::atomic::{fence, AtomicUsize, Ordering},
+};
+
+/// A reference-counted pointer to an instance of `T`.
+///
+/// The reference count is incremented when new instances of [`Ref`] are created, and decremented
+/// when they are dropped. When the count reaches zero, the underlying `T` is also dropped.
+///
+/// # Invariants
+///
+/// The value stored in [`RefCounted::get_count`] corresponds to the number of instances of [`Ref`]
+/// that point to that instance of `T`.
+pub struct Ref<T: RefCounted + ?Sized> {
+    ptr: NonNull<T>,
+}
+
+// SAFETY: It is safe to send `Ref<T>` to another thread when the underlying `T` is `Sync` because
+// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
+// `T` to be `Send` because any thread that has a `Ref<T>` may ultimately access `T` directly, for
+// example, when the reference count reaches zero and `T` is dropped.
+unsafe impl<T: RefCounted + ?Sized + Sync + Send> Send for Ref<T> {}
+
+// SAFETY: It is safe to send `&Ref<T>` to another thread when the underlying `T` is `Sync` for
+// the same reason as above. `T` needs to be `Send` as well because a thread can clone a `&Ref<T>`
+// into a `Ref<T>`, which may lead to `T` being accessed by the same reasoning as above.
+unsafe impl<T: RefCounted + ?Sized + Sync + Send> Sync for Ref<T> {}
+
+impl<T: RefCounted> Ref<T> {
+    /// Constructs a new reference counted instance of `T`.
+    pub fn try_new(contents: T) -> KernelResult<Self> {
+        let boxed = Box::try_new(contents)?;
+        boxed.get_count().count.store(1, Ordering::Relaxed);
+        let ptr = NonNull::from(boxed.deref());
+        Box::into_raw(boxed);
+        Ok(Ref { ptr })
+    }
+}
+
+impl<T: RefCounted + ?Sized> Ref<T> {
+    /// Creates a new reference-counted pointer to the given instance of `T`.
+    ///
+    /// It works by incrementing the current reference count as part of constructing the new
+    /// pointer.
+    pub fn new_from(obj: &T) -> Self {
+        let ref_count = obj.get_count();
+        let cur = ref_count.count.fetch_add(1, Ordering::Relaxed);
+        if cur == usize::MAX {
+            panic!("Reference count overflowed");
+        }
+        Self {
+            ptr: NonNull::from(obj),
+        }
+    }
+
+    /// Returns a mutable reference to `T` iff the reference count is one. Otherwise returns
+    /// [`None`].
+    pub fn get_mut(&mut self) -> Option<&mut T> {
+        // Synchronises with the decrement in `drop`.
+        if self.get_count().count.load(Ordering::Acquire) != 1 {
+            return None;
+        }
+        // SAFETY: Since there is only one reference, we know it isn't possible for another thread
+        // to concurrently call this.
+        Some(unsafe { self.ptr.as_mut() })
+    }
+
+    /// Determines if two reference-counted pointers point to the same underlying instance of `T`.
+    pub fn ptr_eq(a: &Self, b: &Self) -> bool {
+        core::ptr::eq(a.ptr.as_ptr(), b.ptr.as_ptr())
+    }
+
+    /// Deconstructs a [`Ref`] object into a raw pointer.
+    ///
+    /// It can be reconstructed once via [`Ref::from_raw`].
+    pub fn into_raw(obj: Self) -> *const T {
+        let no_drop = ManuallyDrop::new(obj);
+        no_drop.ptr.as_ptr()
+    }
+
+    /// Recreates a [`Ref`] instance previously deconstructed via [`Ref::into_raw`].
+    ///
+    /// # Safety
+    ///
+    /// `ptr` must have been returned by a previous call to [`Ref::into_raw`]. Additionally, it
+    /// can only be called once for each previous call to [``Ref::into_raw`].
+    pub unsafe fn from_raw(ptr: *const T) -> Self {
+        Ref {
+            ptr: NonNull::new(ptr as _).unwrap(),
+        }
+    }
+}
+
+impl<T: RefCounted + ?Sized> Deref for Ref<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
+        // safe to dereference it.
+        unsafe { self.ptr.as_ref() }
+    }
+}
+
+impl<T: RefCounted + ?Sized> Clone for Ref<T> {
+    fn clone(&self) -> Self {
+        Self::new_from(self)
+    }
+}
+
+impl<T: RefCounted + ?Sized> Drop for Ref<T> {
+    fn drop(&mut self) {
+        {
+            // SAFETY: By the type invariant, there is necessarily a reference to the object.
+            let obj = unsafe { self.ptr.as_ref() };
+
+            // Synchronises with the acquire below or with the acquire in `get_mut`.
+            if obj.get_count().count.fetch_sub(1, Ordering::Release) != 1 {
+                return;
+            }
+        }
+
+        // Synchronises with the release when decrementing above. This ensures that modifications
+        // from all previous threads/CPUs are visible to the underlying object's `drop`.
+        fence(Ordering::Acquire);
+
+        // The count reached zero, we must free the memory.
+        //
+        // SAFETY: The pointer was initialised from the result of `Box::into_raw`.
+        unsafe { Box::from_raw(self.ptr.as_ptr()) };
+    }
+}
+
+/// Trait for reference counted objects.
+///
+/// # Safety
+///
+/// Implementers of [`RefCounted`] must ensure that all of their constructors call
+/// [`Ref::try_new`].
+pub unsafe trait RefCounted {
+    /// Returns a pointer to the object field holds the reference count.
+    fn get_count(&self) -> &RefCount;
+}
+
+/// Holds the reference count of an object.
+///
+/// It is meant to be embedded in objects to be reference-counted, with [`RefCounted::get_count`]
+/// returning a reference to it.
+pub struct RefCount {
+    count: AtomicUsize,
+}
+
+impl RefCount {
+    /// Constructs a new instance of [`RefCount`].
+    pub fn new() -> Self {
+        Self {
+            count: AtomicUsize::new(1),
+        }
+    }
+}
+
+impl Default for RefCount {
+    fn default() -> Self {
+        Self::new()
+    }
+}

rust/kernel/sync/mod.rs

@@ -20,12 +20,14 @@
 use crate::{bindings, CStr};
 use core::pin::Pin;
 
+mod arc;
 mod condvar;
 mod guard;
 mod locked_by;
 mod mutex;
 mod spinlock;
 
+pub use arc::{Ref, RefCount, RefCounted};
 pub use condvar::CondVar;
 pub use guard::{Guard, Lock};
 pub use locked_by::LockedBy;