[PATCH v2 2/3] rust: convert `Arc` to use `Refcount`

[Gary Guo]

With `Refcount` type created, `Arc` can use `Refcount` instead of
calling into FFI directly.

Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
---
 rust/kernel/sync/arc.rs | 64 ++++++++++++++++-------------------------
 1 file changed, 25 insertions(+), 39 deletions(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 9f0b04400e8e..34598ec1f977 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -8,7 +8,7 @@
 //! threads.
 //!
 //! It is different from the standard library's [`Arc`] in a few ways:
-//! 1. It is backed by the kernel's `refcount_t` type.
+//! 1. It is backed by the kernel's [`Refcount`] type.
 //! 2. It does not support weak references, which allows it to be half the size.
 //! 3. It saturates the reference count instead of aborting when it goes over a threshold.
 //! 4. It does not provide a `get_mut` method, so the ref counted object is pinned.
@@ -18,10 +18,10 @@
 
 use crate::{
     alloc::{AllocError, Flags, KBox},
-    bindings,
     init::{self, InPlaceInit, Init, PinInit},
+    sync::Refcount,
     try_init,
-    types::{ForeignOwnable, Opaque},
+    types::ForeignOwnable,
 };
 use core::{
     alloc::Layout,
@@ -141,7 +141,7 @@ pub struct Arc<T: ?Sized> {
 #[pin_data]
 #[repr(C)]
 struct ArcInner<T: ?Sized> {
-    refcount: Opaque<bindings::refcount_t>,
+    refcount: Refcount,
     data: T,
 }
 
@@ -153,7 +153,7 @@ impl<T: ?Sized> ArcInner<T> {
     /// `ptr` must have been returned by a previous call to [`Arc::into_raw`], and the `Arc` must
     /// not yet have been destroyed.
     unsafe fn container_of(ptr: *const T) -> NonNull<ArcInner<T>> {
-        let refcount_layout = Layout::new::<bindings::refcount_t>();
+        let refcount_layout = Layout::new::<Refcount>();
         // SAFETY: The caller guarantees that the pointer is valid.
         let val_layout = Layout::for_value(unsafe { &*ptr });
         // SAFETY: We're computing the layout of a real struct that existed when compiling this
@@ -203,8 +203,7 @@ impl<T> Arc<T> {
     pub fn new(contents: T, flags: Flags) -> Result<Self, AllocError> {
         // INVARIANT: The refcount is initialised to a non-zero value.
         let value = ArcInner {
-            // SAFETY: There are no safety requirements for this FFI call.
-            refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
+            refcount: Refcount::new(1),
             data: contents,
         };
 
@@ -285,7 +284,7 @@ pub fn ptr_eq(this: &Self, other: &Self) -> bool {
     /// use kernel::sync::{Arc, UniqueArc};
     ///
     /// let arc = Arc::new(42, GFP_KERNEL)?;
-    /// let unique_arc = arc.into_unique_or_drop();
+    /// let unique_arc = Arc::into_unique_or_drop(arc);
     ///
     /// // The above conversion should succeed since refcount of `arc` is 1.
     /// assert!(unique_arc.is_some());
@@ -301,35 +300,25 @@ pub fn ptr_eq(this: &Self, other: &Self) -> bool {
     /// let arc = Arc::new(42, GFP_KERNEL)?;
     /// let another = arc.clone();
     ///
-    /// let unique_arc = arc.into_unique_or_drop();
+    /// let unique_arc = Arc::into_unique_or_drop(arc);
     ///
     /// // The above conversion should fail since refcount of `arc` is >1.
     /// assert!(unique_arc.is_none());
     ///
     /// # Ok::<(), Error>(())
     /// ```
-    pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
+    pub fn into_unique_or_drop(this: Self) -> Option<Pin<UniqueArc<T>>> {
         // We will manually manage the refcount in this method, so we disable the destructor.
-        let me = ManuallyDrop::new(self);
+        let this = ManuallyDrop::new(this);
         // SAFETY: We own a refcount, so the pointer is still valid.
-        let refcount = unsafe { me.ptr.as_ref() }.refcount.get();
+        let refcount = unsafe { &this.ptr.as_ref().refcount };
 
-        // If the refcount reaches a non-zero value, then we have destroyed this `Arc` and will
-        // return without further touching the `Arc`. If the refcount reaches zero, then there are
-        // no other arcs, and we can create a `UniqueArc`.
-        //
-        // SAFETY: We own a refcount, so the pointer is not dangling.
-        let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
-        if is_zero {
-            // SAFETY: We have exclusive access to the arc, so we can perform unsynchronized
-            // accesses to the refcount.
-            unsafe { core::ptr::write(refcount, bindings::REFCOUNT_INIT(1)) };
-
-            // INVARIANT: We own the only refcount to this arc, so we may create a `UniqueArc`. We
-            // must pin the `UniqueArc` because the values was previously in an `Arc`, and they pin
-            // their values.
+        if !refcount.dec_not_one() {
+            // INVARIANT: If the refcount failed to decrement because it is 1, then we have the
+            // exclusive ownership, so we may create a `UniqueArc`. We must pin the `UniqueArc`
+            // because the values was previously in an `Arc`, and they pin their values.
             Some(Pin::from(UniqueArc {
-                inner: ManuallyDrop::into_inner(me),
+                inner: ManuallyDrop::into_inner(this),
             }))
         } else {
             None
@@ -380,10 +369,10 @@ fn as_ref(&self) -> &T {
 
 impl<T: ?Sized> Clone for Arc<T> {
     fn clone(&self) -> Self {
-        // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
+        // INVARIANT: `Refcount` saturates the refcount, so it cannot overflow to zero.
         // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
         // safe to increment the refcount.
-        unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()) };
+        unsafe { self.ptr.as_ref().refcount.inc() };
 
         // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
         unsafe { Self::from_inner(self.ptr) }
@@ -392,16 +381,14 @@ fn clone(&self) -> Self {
 
 impl<T: ?Sized> Drop for Arc<T> {
     fn drop(&mut self) {
-        // SAFETY: By the type invariant, there is necessarily a reference to the object. We cannot
-        // touch `refcount` after it's decremented to a non-zero value because another thread/CPU
-        // may concurrently decrement it to zero and free it. It is ok to have a raw pointer to
-        // freed/invalid memory as long as it is never dereferenced.
-        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
-
         // INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
         // this instance is being dropped, so the broken invariant is not observable.
-        // SAFETY: Also by the type invariant, we are allowed to decrement the refcount.
-        let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
+        // SAFETY: By the type invariant, there is necessarily a reference to the object.
+        // NOTE: we cannot touch `refcount` after it's decremented to a non-zero value because
+        // another thread/CPU may concurrently decrement it to zero and free it. However it is okay
+        // to have a transient reference to decrement the refcount, see
+        // https://github.com/rust-lang/rust/issues/55005.
+        let is_zero = unsafe { self.ptr.as_ref().refcount.dec_and_test() };
         if is_zero {
             // The count reached zero, we must free the memory.
             //
@@ -650,8 +637,7 @@ pub fn new_uninit(flags: Flags) -> Result<UniqueArc<MaybeUninit<T>>, AllocError>
         // INVARIANT: The refcount is initialised to a non-zero value.
         let inner = KBox::try_init::<AllocError>(
             try_init!(ArcInner {
-                // SAFETY: There are no safety requirements for this FFI call.
-                refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
+                refcount: Refcount::new(1),
                 data <- init::uninit::<T, AllocError>(),
             }? AllocError),
             flags,
-- 
2.47.0