diff --git a/src/pkg/sync/cond.go b/src/pkg/sync/cond.go
index ea48f2e7a9..75494b5353 100644
--- a/src/pkg/sync/cond.go
+++ b/src/pkg/sync/cond.go
@@ -14,10 +14,26 @@ import "runtime"
 // which must be held when changing the condition and
 // when calling the Wait method.
 type Cond struct {
-	L       Locker // held while observing or changing the condition
-	m       Mutex  // held to avoid internal races
-	waiters int    // number of goroutines blocked on Wait
-	sema    *uint32
+	L Locker // held while observing or changing the condition
+	m Mutex  // held to avoid internal races
+
+	// We must be careful to make sure that when Signal
+	// releases a semaphore, the corresponding acquire is
+	// executed by a goroutine that was already waiting at
+	// the time of the call to Signal, not one that arrived later.
+	// To ensure this, we segment waiting goroutines into
+	// generations punctuated by calls to Signal.  Each call to
+	// Signal begins another generation if there are no goroutines
+	// left in older generations for it to wake.  Because of this
+	// optimization (only begin another generation if there
+	// are no older goroutines left), we only need to keep track
+	// of the two most recent generations, which we call old
+	// and new.
+	oldWaiters int     // number of waiters in old generation...
+	oldSema    *uint32 // ... waiting on this semaphore
+
+	newWaiters int     // number of waiters in new generation...
+	newSema    *uint32 // ... waiting on this semaphore
 }
 
 // NewCond returns a new Cond with Locker l.
@@ -42,11 +58,11 @@ func NewCond(l Locker) *Cond {
 //
 func (c *Cond) Wait() {
 	c.m.Lock()
-	if c.sema == nil {
-		c.sema = new(uint32)
+	if c.newSema == nil {
+		c.newSema = new(uint32)
 	}
-	s := c.sema
-	c.waiters++
+	s := c.newSema
+	c.newWaiters++
 	c.m.Unlock()
 	c.L.Unlock()
 	runtime.Semacquire(s)
@@ -59,9 +75,16 @@ func (c *Cond) Wait() {
 // during the call.
 func (c *Cond) Signal() {
 	c.m.Lock()
-	if c.waiters > 0 {
-		c.waiters--
-		runtime.Semrelease(c.sema)
+	if c.oldWaiters == 0 && c.newWaiters > 0 {
+		// Retire old generation; rename new to old.
+		c.oldWaiters = c.newWaiters
+		c.oldSema = c.newSema
+		c.newWaiters = 0
+		c.newSema = nil
+	}
+	if c.oldWaiters > 0 {
+		c.oldWaiters--
+		runtime.Semrelease(c.oldSema)
 	}
 	c.m.Unlock()
 }
@@ -72,19 +95,19 @@ func (c *Cond) Signal() {
 // during the call.
 func (c *Cond) Broadcast() {
 	c.m.Lock()
-	if c.waiters > 0 {
-		s := c.sema
-		n := c.waiters
-		for i := 0; i < n; i++ {
-			runtime.Semrelease(s)
+	// Wake both generations.
+	if c.oldWaiters > 0 {
+		for i := 0; i < c.oldWaiters; i++ {
+			runtime.Semrelease(c.oldSema)
 		}
-		// We just issued n wakeups via the semaphore s.
-		// To ensure that they wake up the existing waiters
-		// and not waiters that arrive after Broadcast returns,
-		// clear c.sema.  The next operation will allocate
-		// a new one.
-		c.sema = nil
-		c.waiters = 0
+		c.oldWaiters = 0
+	}
+	if c.newWaiters > 0 {
+		for i := 0; i < c.newWaiters; i++ {
+			runtime.Semrelease(c.newSema)
+		}
+		c.newWaiters = 0
+		c.newSema = nil
 	}
 	c.m.Unlock()
 }
diff --git a/src/pkg/sync/cond_test.go b/src/pkg/sync/cond_test.go
index 846f98bf39..cefacb184e 100644
--- a/src/pkg/sync/cond_test.go
+++ b/src/pkg/sync/cond_test.go
@@ -46,6 +46,33 @@ func TestCondSignal(t *testing.T) {
 	c.Signal()
 }
 
+func TestCondSignalGenerations(t *testing.T) {
+	var m Mutex
+	c := NewCond(&m)
+	n := 100
+	running := make(chan bool, n)
+	awake := make(chan int, n)
+	for i := 0; i < n; i++ {
+		go func(i int) {
+			m.Lock()
+			running <- true
+			c.Wait()
+			awake <- i
+			m.Unlock()
+		}(i)
+		if i > 0 {
+			a := <-awake
+			if a != i-1 {
+				t.Fatalf("wrong goroutine woke up: want %d, got %d", i-1, a)
+			}
+		}
+		<-running
+		m.Lock()
+		c.Signal()
+		m.Unlock()
+	}
+}
+
 func TestCondBroadcast(t *testing.T) {
 	var m Mutex
 	c := NewCond(&m)