Imported Upstream version 2011.02.15upstream/2011.02.15

1 files changed, 10 insertions, 5 deletions

diff --git a/src/pkg/sync/mutex.go b/src/pkg/sync/mutex.go
index c4d82af00..2a1270b9c 100644
--- a/src/pkg/sync/mutex.go
+++ b/src/pkg/sync/mutex.go
@@ -3,10 +3,10 @@
 // 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"
@@ -53,9 +53,14 @@ 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 := xadd(&m.key, -1); {
+	case v == 0:
 		// changed from 1 to 0; no contention
 		return
+	case int32(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)
 }