code review 3770045: sync: Proposal for barrier implementation

src/pkg/sync/mutex.go

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 // The sync package provides basic synchronization primitives
-// such as mutual exclusion locks.  Other than the Once type,
-// most are intended for use by low-level library routines.
-// Higher-level synchronization is better done via channels
-// and communication.
+// such as mutual exclusion locks.  Other than the Once and
+// WaitGroup types, most are intended for use by low-level
+// library routines.  Higher-level synchronization is better
+// done via channels and communication.
 package sync
 
 import "runtime"
 
 func cas(val *uint32, old, new uint32) bool
 
 // A Mutex is a mutual exclusion lock.
 // Mutexes can be created as part of other structures;
 // the zero value for a Mutex is an unlocked mutex.
 type Mutex struct {
@@ skipping 32 matching lines @@
 // A locked Mutex is not associated with a particular goroutine.
 // It is allowed for one goroutine to lock a Mutex and then
 // arrange for another goroutine to unlock it.
 func (m *Mutex) Unlock() {
         if xadd(&m.key, -1) == 0 {
                 // changed from 1 to 0; no contention
                 return
         }
         runtime.Semrelease(&m.sema)
 }