rust: block: convert `block::mq` to use `Refcount`

Currently there's a custom reference counting in `block::mq`, which uses
`AtomicU64` Rust atomics, and this type doesn't exist on some 32-bit
architectures. We cannot just change it to use 32-bit atomics, because
doing so will make it vulnerable to refcount overflow. So switch it to
use the kernel refcount `kernel::sync::Refcount` instead.

There is an operation needed by `block::mq`, atomically decreasing
refcount from 2 to 0, which is not available through refcount.h, so
I exposed `Refcount::as_atomic` which allows accessing the refcount
directly.

[boqun: Adopt the LKMM atomic API]
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Elle Rhumsaa <elle@weathered-steel.dev>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Tested-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/r/20250723233312.3304339-5-gary@kernel.org

1 files changed, 15 insertions, 0 deletions

diff --git a/rust/kernel/sync/refcount.rs b/rust/kernel/sync/refcount.rs
index cc1a80ae7ae9..19236a5bccde 100644
--- a/rust/kernel/sync/refcount.rs
+++ b/rust/kernel/sync/refcount.rs
@@ -5,6 +5,7 @@
 //! C header: [`include/linux/refcount.h`](srctree/include/linux/refcount.h)
 
 use crate::build_assert;
+use crate::sync::atomic::Atomic;
 use crate::types::Opaque;
 
 /// Atomic reference counter.
@@ -34,6 +35,20 @@ impl Refcount {
         self.0.get()
     }
 
+    /// Get the underlying atomic counter that backs the refcount.
+    ///
+    /// NOTE: Usage of this function is discouraged as it can circumvent the protections offered by
+    /// `refcount.h`. If there is no way to achieve the result using APIs in `refcount.h`, then
+    /// this function can be used. Otherwise consider adding a binding for the required API.
+    #[inline]
+    pub fn as_atomic(&self) -> &Atomic<i32> {
+        let ptr = self.0.get().cast();
+        // SAFETY: `refcount_t` is a transparent wrapper of `atomic_t`, which is an atomic 32-bit
+        // integer that is layout-wise compatible with `Atomic<i32>`. All values are valid for
+        // `refcount_t`, despite some of the values being considered saturated and "bad".
+        unsafe { &*ptr }
+    }
+
     /// Set a refcount's value.
     #[inline]
     pub fn set(&self, value: i32) {