[RFC v3 05/12] rust: sync: atomic: Add atomic {cmp,}xchg operations

[Boqun Feng <boqun.feng@gmail.com>]

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@gmail.com>
---
 rust/kernel/sync/atomic/generic.rs | 122 +++++++++++++++++++++++++++++
 1 file changed, 122 insertions(+)

diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
index 5d8bbaaf108e..73aacfac381b 100644
--- a/rust/kernel/sync/atomic/generic.rs
+++ b/rust/kernel/sync/atomic/generic.rs
@@ -252,3 +252,125 @@ 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: When succeeds, provides the corresponding ordering as the `Ordering` type
+    /// parameter indicates, and a failed one doesn't provide any ordering, 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 `AllowAtomic`,
+        //      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),
+            }
+        }
+    }
+}
-- 
2.47.1