Rollup merge of #106836 - ibraheemdev:sync-sender-spin, r=Amanieu

Remove optimistic spinning from `mpsc::SyncSender`

Per #106701 (comment).
Closes #106804

r? `@Amanieu`

Author: matthiaskrgr

library/std/src/sync/mpmc/array.rs

@@ -319,19 +319,10 @@ impl<T> Channel<T> {
     ) -> Result<(), SendTimeoutError<T>> {
         let token = &mut Token::default();
         loop {
-            // Try sending a message several times.
-            let backoff = Backoff::new();
-            loop {
-                if self.start_send(token) {
-                    let res = unsafe { self.write(token, msg) };
-                    return res.map_err(SendTimeoutError::Disconnected);
-                }
-
-                if backoff.is_completed() {
-                    break;
-                } else {
-                    backoff.spin_light();
-                }
+            // Try sending a message.
+            if self.start_send(token) {
+                let res = unsafe { self.write(token, msg) };
+                return res.map_err(SendTimeoutError::Disconnected);
             }
 
             if let Some(d) = deadline {
@@ -379,6 +370,7 @@ impl<T> Channel<T> {
     pub(crate) fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
         let token = &mut Token::default();
         loop {
+            // Try receiving a message.
             if self.start_recv(token) {
                 let res = unsafe { self.read(token) };
                 return res.map_err(|_| RecvTimeoutError::Disconnected);

library/std/src/sync/mpmc/utils.rs

@@ -105,10 +105,8 @@ impl Backoff {
 
     /// Backs off using lightweight spinning.
     ///
-    /// This method should be used for:
-    ///     - Retrying an operation because another thread made progress. i.e. on CAS failure.
-    ///     - Waiting for an operation to complete by spinning optimistically for a few iterations
-    ///     before falling back to parking the thread (see `Backoff::is_completed`).
+    /// This method should be used for retrying an operation because another thread made
+    /// progress. i.e. on CAS failure.
     #[inline]
     pub fn spin_light(&self) {
         let step = self.step.get().min(SPIN_LIMIT);
@@ -134,10 +132,4 @@ impl Backoff {
 
         self.step.set(self.step.get() + 1);
     }
-
-    /// Returns `true` if quadratic backoff has completed and parking the thread is advised.
-    #[inline]
-    pub fn is_completed(&self) -> bool {
-        self.step.get() > SPIN_LIMIT
-    }
 }