From c49cf341090b53d2afa4dc7c8007ddeefbb3b37f Mon Sep 17 00:00:00 2001 From: Andreas Hindborg Date: Tue, 3 Mar 2026 12:16:59 -0800 Subject: rust: sync: atomic: Add fetch_sub() Add `Atomic::fetch_sub()` with implementation and documentation in line with existing `Atomic::fetch_add()` implementation. Signed-off-by: Andreas Hindborg Signed-off-by: Boqun Feng Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Alice Ryhl Link: https://patch.msgid.link/20260220-atomic-sub-v3-1-e63cbed1d2aa@kernel.org Link: https://patch.msgid.link/20260303201701.12204-12-boqun@kernel.org --- rust/kernel/sync/atomic.rs | 43 +++++++++++++++++++++++++++++++++++++ rust/kernel/sync/atomic/internal.rs | 5 +++++ 2 files changed, 48 insertions(+) (limited to 'rust/kernel') diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs index 1bb1fc2be177..545a8d37ba78 100644 --- a/rust/kernel/sync/atomic.rs +++ b/rust/kernel/sync/atomic.rs @@ -577,6 +577,49 @@ where // SAFETY: `ret` comes from reading `self.0`, which is a valid `T` per type invariants. unsafe { from_repr(ret) } } + + /// Atomic fetch and subtract. + /// + /// Atomically updates `*self` to `(*self).wrapping_sub(v)`, and returns the value of `*self` + /// before the update. + /// + /// # Examples + /// + /// ``` + /// use kernel::sync::atomic::{Atomic, Acquire, Full, Relaxed}; + /// + /// let x = Atomic::new(42); + /// assert_eq!(42, x.load(Relaxed)); + /// assert_eq!(42, x.fetch_sub(12, Acquire)); + /// assert_eq!(30, x.load(Relaxed)); + /// + /// let x = Atomic::new(42); + /// assert_eq!(42, x.load(Relaxed)); + /// assert_eq!(42, x.fetch_sub(12, Full)); + /// assert_eq!(30, x.load(Relaxed)); + /// ``` + #[inline(always)] + pub fn fetch_sub(&self, v: Rhs, _: Ordering) -> T + where + // Types that support addition also support subtraction. + T: AtomicAdd, + { + let v = T::rhs_into_delta(v); + + // INVARIANT: `self.0` is a valid `T` after `atomic_fetch_sub*()` due to safety requirement + // of `AtomicAdd`. + let ret = { + match Ordering::TYPE { + OrderingType::Full => T::Repr::atomic_fetch_sub(&self.0, v), + OrderingType::Acquire => T::Repr::atomic_fetch_sub_acquire(&self.0, v), + OrderingType::Release => T::Repr::atomic_fetch_sub_release(&self.0, v), + OrderingType::Relaxed => T::Repr::atomic_fetch_sub_relaxed(&self.0, v), + } + }; + + // SAFETY: `ret` comes from reading `self.0`, which is a valid `T` per type invariants. + unsafe { from_repr(ret) } + } } #[cfg(any(CONFIG_X86_64, CONFIG_UML, CONFIG_ARM, CONFIG_ARM64))] diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs index e301db4eaf91..b762dbdf6d18 100644 --- a/rust/kernel/sync/atomic/internal.rs +++ b/rust/kernel/sync/atomic/internal.rs @@ -340,5 +340,10 @@ declare_and_impl_atomic_methods!( // SAFETY: `a.as_ptr()` is valid and properly aligned. unsafe { bindings::#call(v, a.as_ptr().cast()) } } + + fn fetch_sub[acquire, release, relaxed](a: &AtomicRepr, v: Self::Delta) -> Self { + // SAFETY: `a.as_ptr()` guarantees the returned pointer is valid and properly aligned. + unsafe { bindings::#call(v, a.as_ptr().cast()) } + } } ); -- cgit v1.2.3