diff --git a/sync2/semaphore.go b/sync2/semaphore.go
new file mode 100644
index 0000000..d310da7
--- /dev/null
+++ b/sync2/semaphore.go
@@ -0,0 +1,65 @@
+package sync2
+
+import (
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+func NewSemaphore(initialCount int) *Semaphore {
+	res := &Semaphore{
+		counter: int64(initialCount),
+	}
+	res.cond.L = &res.lock
+	return res
+}
+
+type Semaphore struct {
+	lock    sync.Mutex
+	cond    sync.Cond
+	counter int64
+}
+
+func (s *Semaphore) Release() {
+	s.lock.Lock()
+	s.counter += 1
+	if s.counter >= 0 {
+		s.cond.Signal()
+	}
+	s.lock.Unlock()
+}
+
+func (s *Semaphore) Acquire() {
+	s.lock.Lock()
+	for s.counter < 1 {
+		s.cond.Wait()
+	}
+	s.counter -= 1
+	s.lock.Unlock()
+}
+
+func (s *Semaphore) AcquireTimeout(timeout time.Duration) bool {
+	done := make(chan bool, 1)
+	// Gate used to communicate between the threads and decide what the result
+	// is. If the main thread decides, we have timed out, otherwise we succeed.
+	decided := new(int32)
+	go func() {
+		s.Acquire()
+		if atomic.SwapInt32(decided, 1) == 0 {
+			done <- true
+		} else {
+			// If we already decided the result, and this thread did not win
+			s.Release()
+		}
+	}()
+	select {
+	case <-done:
+		return true
+	case <-time.NewTimer(timeout).C:
+		if atomic.SwapInt32(decided, 1) == 1 {
+			// The other thread already decided the result
+			return true
+		}
+		return false
+	}
+}
diff --git a/sync2/semaphore_test.go b/sync2/semaphore_test.go
new file mode 100644
index 0000000..8c48694
--- /dev/null
+++ b/sync2/semaphore_test.go
@@ -0,0 +1,41 @@
+// Copyright 2012, Google Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sync2
+
+import (
+	"testing"
+	"time"
+)
+
+func TestSemaNoTimeout(t *testing.T) {
+	s := NewSemaphore(1)
+	s.Acquire()
+	released := false
+	go func() {
+		time.Sleep(10 * time.Millisecond)
+		released = true
+		s.Release()
+	}()
+	s.Acquire()
+	if !released {
+		t.Errorf("want true, got false")
+	}
+}
+
+func TestSemaTimeout(t *testing.T) {
+	s := NewSemaphore(1)
+	s.Acquire()
+	go func() {
+		time.Sleep(10 * time.Millisecond)
+		s.Release()
+	}()
+	if ok := s.AcquireTimeout(5 * time.Millisecond); ok {
+		t.Errorf("want false, got true")
+	}
+	time.Sleep(10 * time.Millisecond)
+	if ok := s.AcquireTimeout(5 * time.Millisecond); !ok {
+		t.Errorf("want true, got false")
+	}
+}