[RFC 2/5] rust: sync: Arc: Introduces ArcInner::count()

[Boqun Feng <boqun.feng@gmail.com>]

This allows reading the current count of a refcount in an `ArcInner`.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 rust/helpers.c          | 6 ++++++
 rust/kernel/sync/arc.rs | 9 +++++++++
 2 files changed, 15 insertions(+)

diff --git a/rust/helpers.c b/rust/helpers.c
index 09a4d93f9d62..afc5f1a39fef 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -46,6 +46,12 @@ bool rust_helper_refcount_dec_and_test(refcount_t *r)
 }
 EXPORT_SYMBOL_GPL(rust_helper_refcount_dec_and_test);
 
+unsigned int rust_helper_refcount_read(refcount_t *r)
+{
+	return refcount_read(r);
+}
+EXPORT_SYMBOL_GPL(rust_helper_refcount_read);
+
 /*
  * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
  * as the Rust `usize` type, so we can use it in contexts where Rust
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index fc680a4a795c..fbfceaa3096e 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -127,6 +127,15 @@ struct ArcInner<T: ?Sized> {
     data: T,
 }
 
+impl<T: ?Sized> ArcInner<T> {
+    /// Returns the current reference count of [`ArcInner`].
+    fn count(&self) -> u32 {
+        // SAFETY: `self.refcount.get()` is always a valid pointer, and `refcount_read()` is a
+        // normal atomic read (i.e. no data race) only requiring on the address is valid.
+        unsafe { bindings::refcount_read(self.refcount.get()) }
+    }
+}
+
 // This is to allow [`Arc`] (and variants) to be used as the type of `self`.
 impl<T: ?Sized> core::ops::Receiver for Arc<T> {}
 
-- 
2.39.1