[PATCH v4 3/5] rust: convert `Arc` to use `Refcount`

[Gary Guo <gary@kernel.org>]

From: Gary Guo <gary@garyguo.net>

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

Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/kernel/sync/arc.rs      | 45 +++++++++++-------------------------
 rust/kernel/sync/refcount.rs |  8 +++++++
 2 files changed, 22 insertions(+), 31 deletions(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 09a471fea4da..fc63ded00f6e 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::InPlaceInit,
+    sync::Refcount,
     try_init,
-    types::{ForeignOwnable, Opaque},
+    types::ForeignOwnable,
 };
 use core::{
     alloc::Layout,
@@ -144,7 +144,7 @@ pub struct Arc<T: ?Sized> {
 #[pin_data]
 #[repr(C)]
 pub struct ArcInner<T: ?Sized> {
-    refcount: Opaque<bindings::refcount_t>,
+    refcount: Refcount,
     data: T,
 }
 
@@ -156,7 +156,7 @@ 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
@@ -228,8 +228,7 @@ 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,
         };
 
@@ -347,18 +346,13 @@ 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 this = ManuallyDrop::new(this);
         // SAFETY: We own a refcount, so the pointer is still valid.
-        let refcount = unsafe { this.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)) };
+        if refcount.dec_and_test() {
+            refcount.set(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
@@ -428,14 +422,10 @@ fn as_ref(&self) -> &T {
 
 impl<T: ?Sized> Clone for Arc<T> {
     fn clone(&self) -> Self {
-        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
-        // safe to dereference it.
-        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
-
-        // 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(refcount) };
+        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) }
@@ -444,16 +434,10 @@ 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.
+        let is_zero = unsafe { self.ptr.as_ref().refcount.dec_and_test() };
         if is_zero {
             // The count reached zero, we must free the memory.
             //
@@ -747,8 +731,7 @@ 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 <- pin_init::uninit::<T, AllocError>(),
             }? AllocError),
             flags,
diff --git a/rust/kernel/sync/refcount.rs b/rust/kernel/sync/refcount.rs
index a0fc22f6d645..8e7b9b0c1979 100644
--- a/rust/kernel/sync/refcount.rs
+++ b/rust/kernel/sync/refcount.rs
@@ -71,6 +71,14 @@ pub fn dec(&self) {
     /// must come after.
     ///
     /// Returns true if the resulting refcount is 0, false otherwise.
+    ///
+    /// # Notes
+    ///
+    /// A common pattern of using `Refcount` is to free memory when the reference count reaches
+    /// zero. This means that the reference to `Refcount` could become invalid after calling this
+    /// function. This is fine as long as the reference to `Refcount` is no longer used when this
+    /// function returns `false`. It is not necessary to use raw pointers in this scenario, see
+    /// https://github.com/rust-lang/rust/issues/55005.
     #[inline]
     #[must_use = "use `dec` instead if you do not need to test if it is 0"]
     pub fn dec_and_test(&self) -> bool {
-- 
2.49.0