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Message-Id: <20250714053656.66712-6-boqun.feng@gmail.com>
Date: Sun, 13 Jul 2025 22:36:52 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: linux-kernel@...r.kernel.org,
rust-for-linux@...r.kernel.org,
lkmm@...ts.linux.dev,
linux-arch@...r.kernel.org
Cc: "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" <lossin@...nel.org>,
"Andreas Hindborg" <a.hindborg@...nel.org>,
"Alice Ryhl" <aliceryhl@...gle.com>,
"Trevor Gross" <tmgross@...ch.edu>,
"Danilo Krummrich" <dakr@...nel.org>,
"Will Deacon" <will@...nel.org>,
"Peter Zijlstra" <peterz@...radead.org>,
"Mark Rutland" <mark.rutland@....com>,
"Wedson Almeida Filho" <wedsonaf@...il.com>,
"Viresh Kumar" <viresh.kumar@...aro.org>,
"Lyude Paul" <lyude@...hat.com>,
"Ingo Molnar" <mingo@...nel.org>,
"Mitchell Levy" <levymitchell0@...il.com>,
"Paul E. McKenney" <paulmck@...nel.org>,
"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
"Linus Torvalds" <torvalds@...ux-foundation.org>,
"Thomas Gleixner" <tglx@...utronix.de>,
Alan Stern <stern@...land.harvard.edu>
Subject: [PATCH v7 5/9] rust: sync: atomic: Add atomic {cmp,}xchg operations
xchg() and cmpxchg() are basic operations on atomic. Provide these based
on C APIs.
Note that cmpxchg() use the similar function signature as
compare_exchange() in Rust std: returning a `Result`, `Ok(old)` means
the operation succeeds and `Err(old)` means the operation fails.
Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---
rust/kernel/sync/atomic/generic.rs | 181 ++++++++++++++++++++++++++++-
1 file changed, 180 insertions(+), 1 deletion(-)
diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
index b3e07328d857..4e45d594d8ef 100644
--- a/rust/kernel/sync/atomic/generic.rs
+++ b/rust/kernel/sync/atomic/generic.rs
@@ -2,7 +2,7 @@
//! Generic atomic primitives.
-use super::ops::{AtomicHasBasicOps, AtomicImpl};
+use super::ops::{AtomicHasBasicOps, AtomicHasXchgOps, AtomicImpl};
use super::{ordering, ordering::OrderingType};
use crate::build_error;
use core::cell::UnsafeCell;
@@ -283,3 +283,182 @@ pub fn store<Ordering: ordering::ReleaseOrRelaxed>(&self, v: T, _: Ordering) {
};
}
}
+
+impl<T: AllowAtomic> Atomic<T>
+where
+ T::Repr: AtomicHasXchgOps,
+{
+ /// Atomic exchange.
+ ///
+ /// Atomically updates `*self` to `v` and returns the old value of `*self`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::sync::atomic::{Atomic, Acquire, Relaxed};
+ ///
+ /// let x = Atomic::new(42);
+ ///
+ /// assert_eq!(42, x.xchg(52, Acquire));
+ /// assert_eq!(52, x.load(Relaxed));
+ /// ```
+ #[doc(alias("atomic_xchg", "atomic64_xchg", "swap"))]
+ #[inline(always)]
+ pub fn xchg<Ordering: ordering::Any>(&self, v: T, _: Ordering) -> T {
+ let v = into_repr(v);
+ // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is a valid
+ // pointer of `T::Repr` for reads and valid for writes of values transmutable to `T`.
+ let a = self.as_ptr().cast::<T::Repr>();
+
+ // `*self` remains valid after `atomic_xchg*()` because `v` is transmutable to `T`.
+ //
+ // SAFETY:
+ // - `a` is aligned to `align_of::<T::Repr>()` because of the safety requirement of
+ // `AllowAtomic` and the guarantee of `Atomic::as_ptr()`.
+ // - `a` is a valid pointer per the CAST justification above.
+ let ret = unsafe {
+ match Ordering::TYPE {
+ OrderingType::Full => T::Repr::atomic_xchg(a, v),
+ OrderingType::Acquire => T::Repr::atomic_xchg_acquire(a, v),
+ OrderingType::Release => T::Repr::atomic_xchg_release(a, v),
+ OrderingType::Relaxed => T::Repr::atomic_xchg_relaxed(a, v),
+ }
+ };
+
+ // SAFETY: `v` comes from reading `a` which was derived from `self.as_ptr()` which points
+ // at a valid `T`.
+ unsafe { from_repr(ret) }
+ }
+
+ /// Atomic compare and exchange.
+ ///
+ /// If `*self` == `old`, atomically updates `*self` to `new`. Otherwise, `*self` is not
+ /// modified.
+ ///
+ /// Compare: The comparison is done via the byte level comparison between `*self` and `old`.
+ ///
+ /// Ordering: When succeeds, provides the corresponding ordering as the `Ordering` type
+ /// parameter indicates, and a failed one doesn't provide any ordering, the load part of a
+ /// failed cmpxchg is a [`Relaxed`] load.
+ ///
+ /// Returns `Ok(value)` if cmpxchg succeeds, and `value` is guaranteed to be equal to `old`,
+ /// otherwise returns `Err(value)`, and `value` is the current value of `*self`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::sync::atomic::{Atomic, Full, Relaxed};
+ ///
+ /// let x = Atomic::new(42);
+ ///
+ /// // Checks whether cmpxchg succeeded.
+ /// let success = x.cmpxchg(52, 64, Relaxed).is_ok();
+ /// # assert!(!success);
+ ///
+ /// // Checks whether cmpxchg failed.
+ /// let failure = x.cmpxchg(52, 64, Relaxed).is_err();
+ /// # assert!(failure);
+ ///
+ /// // Uses the old value if failed, probably re-try cmpxchg.
+ /// match x.cmpxchg(52, 64, Relaxed) {
+ /// Ok(_) => { },
+ /// Err(old) => {
+ /// // do something with `old`.
+ /// # assert_eq!(old, 42);
+ /// }
+ /// }
+ ///
+ /// // Uses the latest value regardlessly, same as atomic_cmpxchg() in C.
+ /// let latest = x.cmpxchg(42, 64, Full).unwrap_or_else(|old| old);
+ /// # assert_eq!(42, latest);
+ /// assert_eq!(64, x.load(Relaxed));
+ /// ```
+ ///
+ /// [`Relaxed`]: super::ordering::Relaxed
+ #[doc(alias(
+ "atomic_cmpxchg",
+ "atomic64_cmpxchg",
+ "atomic_try_cmpxchg",
+ "atomic64_try_cmpxchg",
+ "compare_exchange"
+ ))]
+ #[inline(always)]
+ pub fn cmpxchg<Ordering: ordering::Any>(
+ &self,
+ mut old: T,
+ new: T,
+ o: Ordering,
+ ) -> Result<T, T> {
+ // Note on code generation:
+ //
+ // try_cmpxchg() is used to implement cmpxchg(), and if the helper functions are inlined,
+ // the compiler is able to figure out that branch is not needed if the users don't care
+ // about whether the operation succeeds or not. One exception is on x86, due to commit
+ // 44fe84459faf ("locking/atomic: Fix atomic_try_cmpxchg() semantics"), the
+ // atomic_try_cmpxchg() on x86 has a branch even if the caller doesn't care about the
+ // success of cmpxchg and only wants to use the old value. For example, for code like:
+ //
+ // let latest = x.cmpxchg(42, 64, Full).unwrap_or_else(|old| old);
+ //
+ // It will still generate code:
+ //
+ // movl $0x40, %ecx
+ // movl $0x34, %eax
+ // lock
+ // cmpxchgl %ecx, 0x4(%rsp)
+ // jne 1f
+ // 2:
+ // ...
+ // 1: movl %eax, %ecx
+ // jmp 2b
+ //
+ // This might be "fixed" by introducing a try_cmpxchg_exclusive() that knows the "*old"
+ // location in the C function is always safe to write.
+ if self.try_cmpxchg(&mut old, new, o) {
+ Ok(old)
+ } else {
+ Err(old)
+ }
+ }
+
+ /// Atomic compare and exchange and returns whether the operation succeeds.
+ ///
+ /// If `*self` == `old`, atomically updates `*self` to `new`. Otherwise, `*self` is not
+ /// modified, `*old` is updated to the current value of `*self`.
+ ///
+ /// "Compare" and "Ordering" part are the same as [`Atomic::cmpxchg()`].
+ ///
+ /// Returns `true` means the cmpxchg succeeds otherwise returns `false`.
+ #[inline(always)]
+ fn try_cmpxchg<Ordering: ordering::Any>(&self, old: &mut T, new: T, _: Ordering) -> bool {
+ let mut old_tmp = into_repr(*old);
+ let oldp = &raw mut old_tmp;
+ let new = into_repr(new);
+ // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is a valid
+ // pointer of `T::Repr` for reads and valid for writes of values transmutable to `T`.
+ let a = self.0.get().cast::<T::Repr>();
+
+ // `*self` remains valid after `atomic_try_cmpxchg*()` because `new` is transmutable to
+ // `T`.
+ //
+ // SAFETY:
+ // - `a` is aligned to `align_of::<T::Repr>()` because of the safety requirement of
+ // `AllowAtomic` and the guarantee of `Atomic::as_ptr()`.
+ // - `a` is a valid pointer per the CAST justification above.
+ // - `oldp` is a valid and properly aligned pointer of `T::Repr`.
+ let ret = unsafe {
+ match Ordering::TYPE {
+ OrderingType::Full => T::Repr::atomic_try_cmpxchg(a, oldp, new),
+ OrderingType::Acquire => T::Repr::atomic_try_cmpxchg_acquire(a, oldp, new),
+ OrderingType::Release => T::Repr::atomic_try_cmpxchg_release(a, oldp, new),
+ OrderingType::Relaxed => T::Repr::atomic_try_cmpxchg_relaxed(a, oldp, new),
+ }
+ };
+
+ // SAFETY: `old_tmp` comes from reading `a` which was derived from `self.as_ptr()` which
+ // points at a valid `T`
+ *old = unsafe { from_repr(old_tmp) };
+
+ ret
+ }
+}
--
2.39.5 (Apple Git-154)
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