@@ -82,9 +82,6 @@ type Graph struct {

 	imageMutex       imageMutex // protect images in driver.

 	retained         *retainedLayers

 	tarSplitDisabled bool

-

-	parentRefs      map[string]int

-	parentRefsMutex sync.Mutex

 }

 // file names for ./graph/<ID>/

@@ -115,11 +112,10 @@ func NewGraph(root string, driver graphdriver.Driver) (*Graph, error) {

 	}

 	graph := &Graph{

-		root:       abspath,

-		idIndex:    truncindex.NewTruncIndex([]string{}),

-		driver:     driver,

-		retained:   &retainedLayers{layerHolders: make(map[string]map[string]struct{})},

-		parentRefs: make(map[string]int),

+		root:     abspath,

+		idIndex:  truncindex.NewTruncIndex([]string{}),

+		driver:   driver,

+		retained: &retainedLayers{layerHolders: make(map[string]map[string]struct{})},

 	}

 	// Windows does not currently support tarsplit functionality.

@@ -287,13 +283,6 @@ func (graph *Graph) Register(img *image.Image, layerData io.Reader) (err error)

 		return err

 	}

 	graph.idIndex.Add(img.ID)

-

-	graph.parentRefsMutex.Lock()

-	if img.Parent != "" {

-		graph.parentRefs[img.Parent]++

-	}

-	graph.parentRefsMutex.Unlock()

-

 	return nil

 }

@@ -356,10 +345,6 @@ func (graph *Graph) Delete(name string) error {

 	if err != nil {

 		return err

 	}

-	img, err := graph.Get(id)

-	if err != nil {

-		return err

-	}

 	tmp, err := graph.mktemp()

 	graph.idIndex.Delete(id)

 	if err == nil {

@@ -374,16 +359,6 @@ func (graph *Graph) Delete(name string) error {

 	}

 	// Remove rootfs data from the driver

 	graph.driver.Remove(id)

-

-	graph.parentRefsMutex.Lock()

-	if img.Parent != "" {

-		graph.parentRefs[img.Parent]--

-		if graph.parentRefs[img.Parent] == 0 {

-			delete(graph.parentRefs, img.Parent)

-		}

-	}

-	graph.parentRefsMutex.Unlock()

-

 	// Remove the trashed image directory

 	return os.RemoveAll(tmp)

 }

@@ -401,11 +376,9 @@ func (graph *Graph) Map() map[string]*image.Image {

 // The walking order is undetermined.

 func (graph *Graph) walkAll(handler func(*image.Image)) {

 	graph.idIndex.Iterate(func(id string) {

-		img, err := graph.Get(id)

-		if err != nil {

+		if img, err := graph.Get(id); err != nil {

 			return

-		}

-		if handler != nil {

+		} else if handler != nil {

 			handler(img)

 		}

 	})

@@ -433,10 +406,7 @@ func (graph *Graph) ByParent() map[string][]*image.Image {

 // HasChildren returns whether the given image has any child images.

 func (graph *Graph) HasChildren(img *image.Image) bool {

-	graph.parentRefsMutex.Lock()

-	refCount := graph.parentRefs[img.ID]

-	graph.parentRefsMutex.Unlock()

-	return refCount > 0

+	return len(graph.ByParent()[img.ID]) > 0

 }

 // Retain keeps the images and layers that are in the pulling chain so that

@@ -454,14 +424,13 @@ func (graph *Graph) Release(sessionID string, layerIDs ...string) {

 // A head is an image which is not the parent of another image in the graph.

 func (graph *Graph) Heads() map[string]*image.Image {

 	heads := make(map[string]*image.Image)

+	byParent := graph.ByParent()

 	graph.walkAll(func(image *image.Image) {

 		// If it's not in the byParent lookup table, then

 		// it's not a parent -> so it's a head!

-		graph.parentRefsMutex.Lock()

-		if _, exists := graph.parentRefs[image.ID]; !exists {

+		if _, exists := byParent[image.ID]; !exists {

 			heads[image.ID] = image

 		}

-		graph.parentRefsMutex.Unlock()

 	})

 	return heads

 }