1 files changed, 20 insertions, 22 deletions

diff --git a/libgo/go/sync/mutex.go b/libgo/go/sync/mutex.go
index 9a2bb2b..da565d3 100644
--- a/libgo/go/sync/mutex.go
+++ b/libgo/go/sync/mutex.go
@@ -3,43 +3,36 @@
 // 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
+import (
+	"runtime"
+	"sync/atomic"
+)
 
 // 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 {
-	key  uint32
+	key  int32
 	sema uint32
 }
 
-// Add delta to *val, and return the new *val in a thread-safe way. If multiple
-// goroutines call xadd on the same val concurrently, the changes will be
-// serialized, and all the deltas will be added in an undefined order.
-func xadd(val *uint32, delta int32) (new uint32) {
-	for {
-		v := *val
-		nv := v + uint32(delta)
-		if cas(val, v, nv) {
-			return nv
-		}
-	}
-	panic("unreached")
+// A Locker represents an object that can be locked and unlocked.
+type Locker interface {
+	Lock()
+	Unlock()
 }
 
 // Lock locks m.
 // If the lock is already in use, the calling goroutine
 // blocks until the mutex is available.
 func (m *Mutex) Lock() {
-	if xadd(&m.key, 1) == 1 {
+	if atomic.AddInt32(&m.key, 1) == 1 {
 		// changed from 0 to 1; we hold lock
 		return
 	}
@@ -53,9 +46,14 @@ func (m *Mutex) Lock() {
 // 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 {
+	switch v := atomic.AddInt32(&m.key, -1); {
+	case v == 0:
 		// changed from 1 to 0; no contention
 		return
+	case v == -1:
+		// changed from 0 to -1: wasn't locked
+		// (or there are 4 billion goroutines waiting)
+		panic("sync: unlock of unlocked mutex")
 	}
 	runtime.Semrelease(&m.sema)
 }