Merge #617

617: Replace deprecated compare_and_swap with compare_exchange r=jeehoonkang a=taiki-e

`compare_and_swap` is deprecated in 1.50. (rust-lang/rust#79261)
This patch replaces the uses of `compare_and_swap` with `compare_exchange`.

See also the document about `compare_and_swap` -> `compare_exchange(_weak)` migration: https://doc.rust-lang.org/nightly/core/sync/atomic/struct.AtomicUsize.html#migrating-to-compare_exchange-and-compare_exchange_weak

Co-authored-by: Taiki Endo <te316e89@gmail.com>

Author: bors[bot]

crossbeam-channel/src/flavors/list.rs

@@ -231,8 +231,8 @@ impl<T> Channel<T> {
                 if self
                     .tail
                     .block
-                    .compare_and_swap(block, new, Ordering::Release)
-                    == block
+                    .compare_exchange(block, new, Ordering::Release, Ordering::Relaxed)
+                    .is_ok()
                 {
                     self.head.block.store(new, Ordering::Release);
                     block = new;

crossbeam-epoch/src/epoch.rs

@@ -102,13 +102,31 @@ impl AtomicEpoch {
 
     /// Stores a value into the atomic epoch if the current value is the same as `current`.
     ///
-    /// The return value is always the previous value. If it is equal to `current`, then the value
-    /// is updated.
+    /// The return value is a result indicating whether the new value was written and containing
+    /// the previous value. On success this value is guaranteed to be equal to `current`.
     ///
-    /// The `Ordering` argument describes the memory ordering of this operation.
+    /// `compare_exchange` takes two `Ordering` arguments to describe the memory
+    /// ordering of this operation. `success` describes the required ordering for the
+    /// read-modify-write operation that takes place if the comparison with `current` succeeds.
+    /// `failure` describes the required ordering for the load operation that takes place when
+    /// the comparison fails. Using `Acquire` as success ordering makes the store part
+    /// of this operation `Relaxed`, and using `Release` makes the successful load
+    /// `Relaxed`. The failure ordering can only be `SeqCst`, `Acquire` or `Relaxed`
+    /// and must be equivalent to or weaker than the success ordering.
     #[inline]
-    pub fn compare_and_swap(&self, current: Epoch, new: Epoch, ord: Ordering) -> Epoch {
-        let data = self.data.compare_and_swap(current.data, new.data, ord);
-        Epoch { data }
+    pub fn compare_exchange(
+        &self,
+        current: Epoch,
+        new: Epoch,
+        success: Ordering,
+        failure: Ordering,
+    ) -> Result<Epoch, Epoch> {
+        match self
+            .data
+            .compare_exchange(current.data, new.data, success, failure)
+        {
+            Ok(data) => Ok(Epoch { data }),
+            Err(data) => Err(Epoch { data }),
+        }
     }
 }

crossbeam-epoch/src/internal.rs

@@ -376,7 +376,10 @@ pub struct Local {
 // https://github.com/crossbeam-rs/crossbeam/issues/551
 #[test]
 fn local_size() {
-    assert!(core::mem::size_of::<Local>() <= 2048, "An allocation of `Local` should be <= 2048 bytes.");
+    assert!(
+        core::mem::size_of::<Local>() <= 2048,
+        "An allocation of `Local` should be <= 2048 bytes."
+    );
 }
 
 impl Local {
@@ -468,7 +471,7 @@ impl Local {
                 // a `SeqCst` fence.
                 //
                 // 1. `atomic::fence(SeqCst)`, which compiles into a `mfence` instruction.
-                // 2. `_.compare_and_swap(_, _, SeqCst)`, which compiles into a `lock cmpxchg`
+                // 2. `_.compare_exchange(_, _, SeqCst, SeqCst)`, which compiles into a `lock cmpxchg`
                 //    instruction.
                 //
                 // Both instructions have the effect of a full barrier, but benchmarks have shown
@@ -478,10 +481,13 @@ impl Local {
                 // works fine.  Using inline assembly would be a viable (and correct) alternative,
                 // but alas, that is not possible on stable Rust.
                 let current = Epoch::starting();
-                let previous = self
-                    .epoch
-                    .compare_and_swap(current, new_epoch, Ordering::SeqCst);
-                debug_assert_eq!(current, previous, "participant was expected to be unpinned");
+                let res = self.epoch.compare_exchange(
+                    current,
+                    new_epoch,
+                    Ordering::SeqCst,
+                    Ordering::SeqCst,
+                );
+                debug_assert!(res.is_ok(), "participant was expected to be unpinned");
                 // We add a compiler fence to make it less likely for LLVM to do something wrong
                 // here.  Formally, this is not enough to get rid of data races; practically,
                 // it should go a long way.

crossbeam-queue/src/seg_queue.rs

@@ -213,8 +213,8 @@ impl<T> SegQueue<T> {
                 if self
                     .tail
                     .block
-                    .compare_and_swap(block, new, Ordering::Release)
-                    == block
+                    .compare_exchange(block, new, Ordering::Release, Ordering::Relaxed)
+                    .is_ok()
                 {
                     self.head.block.store(new, Ordering::Release);
                     block = new;

crossbeam-utils/src/backoff.rs

@@ -27,7 +27,7 @@ const YIELD_LIMIT: u32 = 10;
 ///     let backoff = Backoff::new();
 ///     loop {
 ///         let val = a.load(SeqCst);
-///         if a.compare_and_swap(val, val.wrapping_mul(b), SeqCst) == val {
+///         if a.compare_exchange(val, val.wrapping_mul(b), SeqCst, SeqCst).is_ok() {
 ///             return val;
 ///         }
 ///         backoff.spin();
@@ -131,7 +131,7 @@ impl Backoff {
     ///     let backoff = Backoff::new();
     ///     loop {
     ///         let val = a.load(SeqCst);
-    ///         if a.compare_and_swap(val, val.wrapping_mul(b), SeqCst) == val {
+    ///         if a.compare_exchange(val, val.wrapping_mul(b), SeqCst, SeqCst).is_ok() {
     ///             return val;
     ///         }
     ///         backoff.spin();