Message-ID: <20241221183024.3929500-2-gary@garyguo.net>
Date: Sat, 21 Dec 2024 18:29:44 +0000
From: Gary Guo <gary@...yguo.net>
To: Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Trevor Gross <tmgross@...ch.edu>,
	Tamir Duberstein <tamird@...il.com>,
	Martin Rodriguez Reboredo <yakoyoku@...il.com>
Cc: Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mark Rutland <mark.rutland@....com>,
	rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/3] rust: implement `kernel::sync::Refcount`

This is a wrapping layer of `include/linux/refcount.h`. Currently the
kernel refcount has already been used in `Arc`, however it calls into
FFI directly.

Signed-off-by: Gary Guo <gary@...yguo.net>
---
 rust/helpers/refcount.c      | 10 ++++
 rust/kernel/sync.rs          |  2 +
 rust/kernel/sync/refcount.rs | 97 ++++++++++++++++++++++++++++++++++++
 3 files changed, 109 insertions(+)
 create mode 100644 rust/kernel/sync/refcount.rs

diff --git a/rust/helpers/refcount.c b/rust/helpers/refcount.c
index d6adbd2e45a1..d175898ad7b8 100644
--- a/rust/helpers/refcount.c
+++ b/rust/helpers/refcount.c
@@ -7,11 +7,21 @@ refcount_t rust_helper_REFCOUNT_INIT(int n)
 	return (refcount_t)REFCOUNT_INIT(n);
 }
 
+void rust_helper_refcount_set(refcount_t *r, int n)
+{
+	refcount_set(r, n);
+}
+
 void rust_helper_refcount_inc(refcount_t *r)
 {
 	refcount_inc(r);
 }
 
+void rust_helper_refcount_dec(refcount_t *r)
+{
+	refcount_dec(r);
+}
+
 bool rust_helper_refcount_dec_and_test(refcount_t *r)
 {
 	return refcount_dec_and_test(r);
diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index 1eab7ebf25fd..b76b04e16eac 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -12,6 +12,7 @@
 pub mod lock;
 mod locked_by;
 pub mod poll;
+mod refcount;
 
 pub use arc::{Arc, ArcBorrow, UniqueArc};
 pub use condvar::{new_condvar, CondVar, CondVarTimeoutResult};
@@ -19,6 +20,7 @@
 pub use lock::mutex::{new_mutex, Mutex};
 pub use lock::spinlock::{new_spinlock, SpinLock};
 pub use locked_by::LockedBy;
+pub use refcount::Refcount;
 
 /// Represents a lockdep class. It's a wrapper around C's `lock_class_key`.
 #[repr(transparent)]
diff --git a/rust/kernel/sync/refcount.rs b/rust/kernel/sync/refcount.rs
new file mode 100644
index 000000000000..2198b1598b60
--- /dev/null
+++ b/rust/kernel/sync/refcount.rs
@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Atomic reference counting.
+//!
+//! C header: [`include/linux/refcount.h`](srctree/include/linux/refcount.h)
+
+use crate::types::Opaque;
+use core::sync::atomic::AtomicI32;
+
+/// Atomic reference counter.
+///
+/// This type is conceptually an atomic integer, but provides saturation semantics compared to
+/// normal atomic integers. Values in the negative range when viewed as a signed integer are
+/// saturation (bad) values. For details about the saturation semantics, please refer to top of
+/// [`include/linux/refcount.h`](srctree/include/refcount.h).
+///
+/// Wraps the kernel's C `refcount_t`.
+#[repr(transparent)]
+pub struct Refcount(Opaque<bindings::refcount_t>);
+
+impl Refcount {
+    /// Construct a new [`Refcount`] from an initial value.
+    #[inline]
+    pub fn new(value: i32) -> Self {
+        // SAFETY: There are no safety requirements for this FFI call.
+        Self(Opaque::new(unsafe { bindings::REFCOUNT_INIT(value) }))
+    }
+
+    #[inline]
+    fn as_ptr(&self) -> *mut bindings::refcount_t {
+        self.0.get()
+    }
+
+    /// Set a refcount's value.
+    #[inline]
+    pub fn set(&self, value: i32) {
+        // SAFETY: `self.as_ptr()` is valid.
+        unsafe { bindings::refcount_set(self.as_ptr(), value) }
+    }
+
+    /// Increment a refcount.
+    ///
+    /// It will saturate if overflows and `WARN`. It will also `WARN` if the refcount is 0, as this
+    /// represents a possible use-after-free condition.
+    ///
+    /// Provides no memory ordering, it is assumed that caller already has a reference on the
+    /// object.
+    #[inline]
+    pub fn inc(&self) {
+        // SAFETY: self is valid.
+        unsafe { bindings::refcount_inc(self.as_ptr()) }
+    }
+
+    /// Decrement a refcount.
+    ///
+    /// It will `WARN` on underflow and fail to decrement when saturated.
+    ///
+    /// Provides release memory ordering, such that prior loads and stores are done
+    /// before.
+    #[inline]
+    pub fn dec(&self) {
+        // SAFETY: `self.as_ptr()` is valid.
+        unsafe { bindings::refcount_dec(self.as_ptr()) }
+    }
+
+    /// Decrement a refcount and test if it is 0.
+    ///
+    /// It will `WARN` on underflow and fail to decrement when saturated.
+    ///
+    /// Provides release memory ordering, such that prior loads and stores are done
+    /// before, and provides an acquire ordering on success such that memory deallocation
+    /// must come after.
+    ///
+    /// Returns true if the resulting refcount is 0, false otherwise.
+    #[inline]
+    #[must_use = "use `dec` instead you do not need to test if it is 0"]
+    pub fn dec_and_test(&self) -> bool {
+        // SAFETY: `self.as_ptr()` is valid.
+        unsafe { bindings::refcount_dec_and_test(self.as_ptr()) }
+    }
+
+    /// Decrement a refcount if it is not 1.
+    ///
+    /// Returns true if the decrement operation was successful, false otherwise.
+    #[inline]
+    #[must_use]
+    pub fn dec_not_one(&self) -> bool {
+        // SAFETY: `self.as_ptr()` is valid.
+        unsafe { bindings::refcount_dec_not_one(self.as_ptr()) }
+    }
+}
+
+// SAFETY: `refcount_t` is thread-safe.
+unsafe impl Send for Refcount {}
+
+// SAFETY: `refcount_t` is thread-safe.
+unsafe impl Sync for Refcount {}
-- 
2.47.0

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