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. With
the compiler optimization and inline helpers, it should provides the
same efficient code generation as using atomic_try_cmpxchg() or
atomic_cmpxchg() correctly.
Except it's not! Because of 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:
// Uses the latest value regardlessly, same as atomic_cmpxchg() in C.
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
Attempting to write an x86 try_cmpxchg_exclusive() for Rust use only,
because the Rust function takes a `&mut` for old pointer, which must be
exclusive to the function, therefore it's unsafe to use some shared
pointer. But maybe I'm missing something?
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
rust/kernel/sync/atomic/generic.rs | 151 +++++++++++++++++++++++++++++
1 file changed, 151 insertions(+)
diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
index 204da38e2691..bfccc4336c75 100644
--- a/rust/kernel/sync/atomic/generic.rs
+++ b/rust/kernel/sync/atomic/generic.rs
@@ -251,3 +251,154 @@ pub fn store<Ordering: ReleaseOrRelaxed>(&self, v: T, _: Ordering) {
};
}
}
+
+impl<T: AllowAtomic> Atomic<T>
+where
+ T::Repr: AtomicHasXchgOps,
+{
+ /// Atomic exchange.
+ ///
+ /// # Examples
+ ///
+ /// ```rust
+ /// 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));
+ /// ```
+ #[inline(always)]
+ pub fn xchg<Ordering: All>(&self, v: T, _: Ordering) -> T {
+ let v = T::into_repr(v);
+ let a = self.as_ptr().cast::<T::Repr>();
+
+ // SAFETY:
+ // - For calling the atomic_xchg*() function:
+ // - `self.as_ptr()` is a valid pointer, and per the safety requirement of `AllocAtomic`,
+ // a `*mut T` is a valid `*mut T::Repr`. Therefore `a` is a valid pointer,
+ // - per the type invariants, the following atomic operation won't cause data races.
+ // - For extra safety requirement of usage on pointers returned by `self.as_ptr():
+ // - atomic operations are used here.
+ let ret = unsafe {
+ match Ordering::ORDER {
+ OrderingDesc::Full => T::Repr::atomic_xchg(a, v),
+ OrderingDesc::Acquire => T::Repr::atomic_xchg_acquire(a, v),
+ OrderingDesc::Release => T::Repr::atomic_xchg_release(a, v),
+ OrderingDesc::Relaxed => T::Repr::atomic_xchg_relaxed(a, v),
+ }
+ };
+
+ T::from_repr(ret)
+ }
+
+ /// Atomic compare and exchange.
+ ///
+ /// Compare: The comparison is done via the byte level comparison between the atomic variables
+ /// with the `old` value.
+ ///
+ /// Ordering: A failed compare and exchange does provide anything, the read part of a failed
+ /// cmpxchg should be treated as a relaxed read.
+ ///
+ /// Returns `Ok(value)` if cmpxchg succeeds, and `value` is guaranteed to be equal to `old`,
+ /// otherwise returns `Err(value)`, and `value` is the value of the atomic variable when cmpxchg
+ /// was happening.
+ ///
+ /// # Examples
+ ///
+ /// ```rust
+ /// 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));
+ /// ```
+ #[inline(always)]
+ pub fn cmpxchg<Ordering: All>(&self, mut old: T, new: T, o: Ordering) -> Result<T, T> {
+ if self.try_cmpxchg(&mut old, new, o) {
+ Ok(old)
+ } else {
+ Err(old)
+ }
+ }
+
+ /// Atomic compare and exchange and returns whether the operation succeeds.
+ ///
+ /// "Compare" and "Ordering" part are the same as [`Atomic::cmpxchg`].
+ ///
+ /// Returns `true` means the cmpxchg succeeds otherwise returns `false` with `old` updated to
+ /// the value of the atomic variable when cmpxchg was happening.
+ #[inline(always)]
+ fn try_cmpxchg<Ordering: All>(&self, old: &mut T, new: T, _: Ordering) -> bool {
+ let old = (old as *mut T).cast::<T::Repr>();
+ let new = T::into_repr(new);
+ let a = self.0.get().cast::<T::Repr>();
+
+ // SAFETY:
+ // - For calling the atomic_try_cmpchg*() function:
+ // - `self.as_ptr()` is a valid pointer, and per the safety requirement of `AllocAtomic`,
+ // a `*mut T` is a valid `*mut T::Repr`. Therefore `a` is a valid pointer,
+ // - per the type invariants, the following atomic operation won't cause data races.
+ // - `old` is a valid pointer to write because it comes from a mutable reference.
+ // - For extra safety requirement of usage on pointers returned by `self.as_ptr():
+ // - atomic operations are used here.
+ unsafe {
+ match Ordering::ORDER {
+ OrderingDesc::Full => T::Repr::atomic_try_cmpxchg(a, old, new),
+ OrderingDesc::Acquire => T::Repr::atomic_try_cmpxchg_acquire(a, old, new),
+ OrderingDesc::Release => T::Repr::atomic_try_cmpxchg_release(a, old, new),
+ OrderingDesc::Relaxed => T::Repr::atomic_try_cmpxchg_relaxed(a, old, new),
+ }
+ }
+ }
+
+ /// Atomic compare and exchange and return the [`Result`].
+ ///
+ /// "Compare" and "Ordering" part are the same as [`Atomic::cmpxchg`].
+ ///
+ /// Returns `Ok(value)` if cmpxchg succeeds, and `value` is guaranteed to be equal to `old`,
+ /// otherwise returns `Err(value)`, and `value` is the value of the atomic variable when cmpxchg
+ /// was happening.
+ ///
+ /// # Examples
+ ///
+ /// ```rust
+ /// use kernel::sync::atomic::{Atomic, Acquire, Relaxed};
+ ///
+ /// let x = Atomic::new(42i32);
+ ///
+ /// assert!(x.compare_exchange(52, 64, Acquire).is_err());
+ /// assert_eq!(42, x.load(Relaxed));
+ ///
+ /// assert!(x.compare_exchange(42, 64, Acquire).is_ok());
+ /// assert_eq!(64, x.load(Relaxed));
+ /// ```
+ #[inline(always)]
+ pub fn compare_exchange<Ordering: All>(&self, mut old: T, new: T, o: Ordering) -> Result<T, T> {
+ if self.try_cmpxchg(&mut old, new, o) {
+ Ok(old)
+ } else {
+ Err(old)
+ }
+ }
+}
--
2.45.2
