Improved barrier implementation.

src/sync/barrier.rs

@@ -27,11 +27,10 @@ impl Barrier {
         // waiting.
 
         let original_count = self.original_count.get();
+        let mut new = self.waited_count.fetch_add(1, Ordering::Acquire) + 1;
 
         loop {
-            let new = self.waited_count.fetch_add(1, Ordering::Acquire) + 1;
-
-            match Ord::cmp(&new, &original_count) {
+            let result = match Ord::cmp(&new, &original_count) {
                 cmp::Ordering::Less => {
                     // new < original_count, i.e. we were one of the threads that incremented the counter,
                     // but need to continue waiting for the last waiter to notify the others.
@@ -44,43 +43,39 @@ impl Barrier {
                         let _ = crate::sync::futex_wait(&self.waited_count, count, None);
                     }
 
-                    // When the required number of threads have called pthread_barrier_wait so waited_count
-                    // >= original_count (should never be able to exceed that value), we can safely reset
-                    // the counter to zero.
-
-                    if self.notified_count.fetch_add(1, Ordering::Relaxed) + 1 >= original_count {
-                        self.waited_count.store(0, Ordering::Relaxed);
-                    }
-
-                    return WaitResult::Waited;
+                    WaitResult::Waited
                 }
                 cmp::Ordering::Equal => {
                     // new == original_count, i.e. we were the one thread doing the last increment, and we
                     // will be responsible for waking up all other waiters.
 
-                    crate::sync::futex_wake(&self.waited_count, i32::MAX);
-
-                    if self.notified_count.fetch_add(1, Ordering::Relaxed) + 1 >= original_count {
-                        self.waited_count.store(0, Ordering::Relaxed);
-                    }
+                    crate::sync::futex_wake(&self.waited_count, original_count as i32 - 1);
 
-                    return WaitResult::NotifiedAll;
+                    WaitResult::NotifiedAll
                 }
-                // FIXME: Starvation?
                 cmp::Ordering::Greater => {
-                    let mut cached = new;
-                    while cached >= original_count {
-                        // new > original_count, i.e. we are waiting on a barrier that is already finished, but
-                        // which has not yet awoken all its waiters and re-initialized the self. The
-                        // simplest way to handle this is to wait for waited_count to return to zero, and
-                        // start over.
+                    crate::sync::futex_wait(&self.waited_count, new, None);
 
-                        crate::sync::futex_wait(&self.waited_count, cached, None);
+                    new = self.waited_count.load(Ordering::Acquire);
 
-                        cached = self.waited_count.load(Ordering::Acquire);
-                    }
+                    continue;
+                }
+            };
+
+            // When the required number of threads have called pthread_barrier_wait so waited_count
+            // >= original_count (should never be able to exceed that value), we can safely reset
+            // the counter to zero.
+
+            // TODO: Orderings
+            if self.notified_count.fetch_add(1, Ordering::AcqRel) + 1 >= original_count {
+                self.notified_count.store(0, Ordering::Release);
+                let next = self.waited_count.fetch_sub(original_count, Ordering::Release) - original_count;
+
+                if next >= original_count {
+                    let _ = crate::sync::futex_wake(&self.waited_count, original_count as i32);
                 }
             }
+            break result;
         }
     }
 }