libgo: update to go1.9

    
    Reviewed-on: https://go-review.googlesource.com/63753

From-SVN: r252767

19 files changed, 1148 insertions, 79 deletions

diff --git a/libgo/go/sync/atomic/atomic_test.go b/libgo/go/sync/atomic/atomic_test.go
index 6d0831c3f9d..17baccb4683 100644
--- a/libgo/go/sync/atomic/atomic_test.go
+++ b/libgo/go/sync/atomic/atomic_test.go
@@ -953,16 +953,20 @@ func hammerSwapUint64(addr *uint64, count int) {
 	}
 }
 
+const arch32 = unsafe.Sizeof(uintptr(0)) == 4
+
 func hammerSwapUintptr64(uaddr *uint64, count int) {
 	// only safe when uintptr is 64-bit.
 	// not called on 32-bit systems.
-	addr := (*uintptr)(unsafe.Pointer(uaddr))
-	seed := int(uintptr(unsafe.Pointer(&count)))
-	for i := 0; i < count; i++ {
-		new := uintptr(seed+i)<<32 | uintptr(seed+i)<<32>>32
-		old := SwapUintptr(addr, new)
-		if old>>32 != old<<32>>32 {
-			panic(fmt.Sprintf("SwapUintptr is not atomic: %v", old))
+	if !arch32 {
+		addr := (*uintptr)(unsafe.Pointer(uaddr))
+		seed := int(uintptr(unsafe.Pointer(&count)))
+		for i := 0; i < count; i++ {
+			new := uintptr(seed+i)<<32 | uintptr(seed+i)<<32>>32
+			old := SwapUintptr(addr, new)
+			if old>>32 != old<<32>>32 {
+				panic(fmt.Sprintf("SwapUintptr is not atomic: %v", old))
+			}
 		}
 	}
 }
@@ -1116,8 +1120,6 @@ func hammerStoreLoadUint64(t *testing.T, paddr unsafe.Pointer) {
 
 func hammerStoreLoadUintptr(t *testing.T, paddr unsafe.Pointer) {
 	addr := (*uintptr)(paddr)
-	var test64 uint64 = 1 << 50
-	arch32 := uintptr(test64) == 0
 	v := LoadUintptr(addr)
 	new := v
 	if arch32 {
@@ -1144,8 +1146,6 @@ func hammerStoreLoadUintptr(t *testing.T, paddr unsafe.Pointer) {
 
 func hammerStoreLoadPointer(t *testing.T, paddr unsafe.Pointer) {
 	addr := (*unsafe.Pointer)(paddr)
-	var test64 uint64 = 1 << 50
-	arch32 := uintptr(test64) == 0
 	v := uintptr(LoadPointer(addr))
 	new := v
 	if arch32 {
@@ -1398,7 +1398,7 @@ func TestUnaligned64(t *testing.T) {
 
 	switch runtime.GOARCH {
 	default:
-		if unsafe.Sizeof(int(0)) != 4 {
+		if !arch32 {
 			t.Skip("test only runs on 32-bit systems")
 		}
 	case "amd64p32":
diff --git a/libgo/go/sync/atomic/doc.go b/libgo/go/sync/atomic/doc.go
index 302ff43070a..7c007d7a150 100644
--- a/libgo/go/sync/atomic/doc.go
+++ b/libgo/go/sync/atomic/doc.go
@@ -48,8 +48,8 @@ import (
 // On non-Linux ARM, the 64-bit functions use instructions unavailable before the ARMv6k core.
 //
 // On both ARM and x86-32, it is the caller's responsibility to arrange for 64-bit
-// alignment of 64-bit words accessed atomically. The first word in a global
-// variable or in an allocated struct or slice can be relied upon to be
+// alignment of 64-bit words accessed atomically. The first word in a
+// variable or in an allocated struct, array, or slice can be relied upon to be
 // 64-bit aligned.
 
 // SwapInt32 atomically stores new into *addr and returns the previous *addr value.
diff --git a/libgo/go/sync/atomic/value.go b/libgo/go/sync/atomic/value.go
index 30abf726344..1fc1f681f20 100644
--- a/libgo/go/sync/atomic/value.go
+++ b/libgo/go/sync/atomic/value.go
@@ -9,7 +9,6 @@ import (
 )
 
 // A Value provides an atomic load and store of a consistently typed value.
-// Values can be created as part of other data structures.
 // The zero value for a Value returns nil from Load.
 // Once Store has been called, a Value must not be copied.
 //
diff --git a/libgo/go/sync/cond.go b/libgo/go/sync/cond.go
index c070d9d84ef..14e2f6b24d4 100644
--- a/libgo/go/sync/cond.go
+++ b/libgo/go/sync/cond.go
@@ -17,7 +17,6 @@ import (
 // which must be held when changing the condition and
 // when calling the Wait method.
 //
-// A Cond can be created as part of other structures.
 // A Cond must not be copied after first use.
 type Cond struct {
 	noCopy noCopy
diff --git a/libgo/go/sync/export_test.go b/libgo/go/sync/export_test.go
index 6ed38dad89d..669076efad3 100644
--- a/libgo/go/sync/export_test.go
+++ b/libgo/go/sync/export_test.go
@@ -7,3 +7,5 @@ package sync
 // Export for testing.
 var Runtime_Semacquire = runtime_Semacquire
 var Runtime_Semrelease = runtime_Semrelease
+var Runtime_procPin = runtime_procPin
+var Runtime_procUnpin = runtime_procUnpin
diff --git a/libgo/go/sync/map.go b/libgo/go/sync/map.go
new file mode 100644
index 00000000000..083f4a563f8
--- /dev/null
+++ b/libgo/go/sync/map.go
@@ -0,0 +1,375 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sync
+
+import (
+	"sync/atomic"
+	"unsafe"
+)
+
+// Map is a concurrent map with amortized-constant-time loads, stores, and deletes.
+// It is safe for multiple goroutines to call a Map's methods concurrently.
+//
+// It is optimized for use in concurrent loops with keys that are
+// stable over time, and either few steady-state stores, or stores
+// localized to one goroutine per key.
+//
+// For use cases that do not share these attributes, it will likely have
+// comparable or worse performance and worse type safety than an ordinary
+// map paired with a read-write mutex.
+//
+// The zero Map is valid and empty.
+//
+// A Map must not be copied after first use.
+type Map struct {
+	mu Mutex
+
+	// read contains the portion of the map's contents that are safe for
+	// concurrent access (with or without mu held).
+	//
+	// The read field itself is always safe to load, but must only be stored with
+	// mu held.
+	//
+	// Entries stored in read may be updated concurrently without mu, but updating
+	// a previously-expunged entry requires that the entry be copied to the dirty
+	// map and unexpunged with mu held.
+	read atomic.Value // readOnly
+
+	// dirty contains the portion of the map's contents that require mu to be
+	// held. To ensure that the dirty map can be promoted to the read map quickly,
+	// it also includes all of the non-expunged entries in the read map.
+	//
+	// Expunged entries are not stored in the dirty map. An expunged entry in the
+	// clean map must be unexpunged and added to the dirty map before a new value
+	// can be stored to it.
+	//
+	// If the dirty map is nil, the next write to the map will initialize it by
+	// making a shallow copy of the clean map, omitting stale entries.
+	dirty map[interface{}]*entry
+
+	// misses counts the number of loads since the read map was last updated that
+	// needed to lock mu to determine whether the key was present.
+	//
+	// Once enough misses have occurred to cover the cost of copying the dirty
+	// map, the dirty map will be promoted to the read map (in the unamended
+	// state) and the next store to the map will make a new dirty copy.
+	misses int
+}
+
+// readOnly is an immutable struct stored atomically in the Map.read field.
+type readOnly struct {
+	m       map[interface{}]*entry
+	amended bool // true if the dirty map contains some key not in m.
+}
+
+// expunged is an arbitrary pointer that marks entries which have been deleted
+// from the dirty map.
+var expunged = unsafe.Pointer(new(interface{}))
+
+// An entry is a slot in the map corresponding to a particular key.
+type entry struct {
+	// p points to the interface{} value stored for the entry.
+	//
+	// If p == nil, the entry has been deleted and m.dirty == nil.
+	//
+	// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
+	// is missing from m.dirty.
+	//
+	// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
+	// != nil, in m.dirty[key].
+	//
+	// An entry can be deleted by atomic replacement with nil: when m.dirty is
+	// next created, it will atomically replace nil with expunged and leave
+	// m.dirty[key] unset.
+	//
+	// An entry's associated value can be updated by atomic replacement, provided
+	// p != expunged. If p == expunged, an entry's associated value can be updated
+	// only after first setting m.dirty[key] = e so that lookups using the dirty
+	// map find the entry.
+	p unsafe.Pointer // *interface{}
+}
+
+func newEntry(i interface{}) *entry {
+	return &entry{p: unsafe.Pointer(&i)}
+}
+
+// Load returns the value stored in the map for a key, or nil if no
+// value is present.
+// The ok result indicates whether value was found in the map.
+func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
+	read, _ := m.read.Load().(readOnly)
+	e, ok := read.m[key]
+	if !ok && read.amended {
+		m.mu.Lock()
+		// Avoid reporting a spurious miss if m.dirty got promoted while we were
+		// blocked on m.mu. (If further loads of the same key will not miss, it's
+		// not worth copying the dirty map for this key.)
+		read, _ = m.read.Load().(readOnly)
+		e, ok = read.m[key]
+		if !ok && read.amended {
+			e, ok = m.dirty[key]
+			// Regardless of whether the entry was present, record a miss: this key
+			// will take the slow path until the dirty map is promoted to the read
+			// map.
+			m.missLocked()
+		}
+		m.mu.Unlock()
+	}
+	if !ok {
+		return nil, false
+	}
+	return e.load()
+}
+
+func (e *entry) load() (value interface{}, ok bool) {
+	p := atomic.LoadPointer(&e.p)
+	if p == nil || p == expunged {
+		return nil, false
+	}
+	return *(*interface{})(p), true
+}
+
+// Store sets the value for a key.
+func (m *Map) Store(key, value interface{}) {
+	read, _ := m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok && e.tryStore(&value) {
+		return
+	}
+
+	m.mu.Lock()
+	read, _ = m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok {
+		if e.unexpungeLocked() {
+			// The entry was previously expunged, which implies that there is a
+			// non-nil dirty map and this entry is not in it.
+			m.dirty[key] = e
+		}
+		e.storeLocked(&value)
+	} else if e, ok := m.dirty[key]; ok {
+		e.storeLocked(&value)
+	} else {
+		if !read.amended {
+			// We're adding the first new key to the dirty map.
+			// Make sure it is allocated and mark the read-only map as incomplete.
+			m.dirtyLocked()
+			m.read.Store(readOnly{m: read.m, amended: true})
+		}
+		m.dirty[key] = newEntry(value)
+	}
+	m.mu.Unlock()
+}
+
+// tryStore stores a value if the entry has not been expunged.
+//
+// If the entry is expunged, tryStore returns false and leaves the entry
+// unchanged.
+func (e *entry) tryStore(i *interface{}) bool {
+	p := atomic.LoadPointer(&e.p)
+	if p == expunged {
+		return false
+	}
+	for {
+		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
+			return true
+		}
+		p = atomic.LoadPointer(&e.p)
+		if p == expunged {
+			return false
+		}
+	}
+}
+
+// unexpungeLocked ensures that the entry is not marked as expunged.
+//
+// If the entry was previously expunged, it must be added to the dirty map
+// before m.mu is unlocked.
+func (e *entry) unexpungeLocked() (wasExpunged bool) {
+	return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
+}
+
+// storeLocked unconditionally stores a value to the entry.
+//
+// The entry must be known not to be expunged.
+func (e *entry) storeLocked(i *interface{}) {
+	atomic.StorePointer(&e.p, unsafe.Pointer(i))
+}
+
+// LoadOrStore returns the existing value for the key if present.
+// Otherwise, it stores and returns the given value.
+// The loaded result is true if the value was loaded, false if stored.
+func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
+	// Avoid locking if it's a clean hit.
+	read, _ := m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok {
+		actual, loaded, ok := e.tryLoadOrStore(value)
+		if ok {
+			return actual, loaded
+		}
+	}
+
+	m.mu.Lock()
+	read, _ = m.read.Load().(readOnly)
+	if e, ok := read.m[key]; ok {
+		if e.unexpungeLocked() {
+			m.dirty[key] = e
+		}
+		actual, loaded, _ = e.tryLoadOrStore(value)
+	} else if e, ok := m.dirty[key]; ok {
+		actual, loaded, _ = e.tryLoadOrStore(value)
+		m.missLocked()
+	} else {
+		if !read.amended {
+			// We're adding the first new key to the dirty map.
+			// Make sure it is allocated and mark the read-only map as incomplete.
+			m.dirtyLocked()
+			m.read.Store(readOnly{m: read.m, amended: true})
+		}
+		m.dirty[key] = newEntry(value)
+		actual, loaded = value, false
+	}
+	m.mu.Unlock()
+
+	return actual, loaded
+}
+
+// tryLoadOrStore atomically loads or stores a value if the entry is not
+// expunged.
+//
+// If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
+// returns with ok==false.
+func (e *entry) tryLoadOrStore(i interface{}) (actual interface{}, loaded, ok bool) {
+	p := atomic.LoadPointer(&e.p)
+	if p == expunged {
+		return nil, false, false
+	}
+	if p != nil {
+		return *(*interface{})(p), true, true
+	}
+
+	// Copy the interface after the first load to make this method more amenable
+	// to escape analysis: if we hit the "load" path or the entry is expunged, we
+	// shouldn't bother heap-allocating.
+	ic := i
+	for {
+		if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {
+			return i, false, true
+		}
+		p = atomic.LoadPointer(&e.p)
+		if p == expunged {
+			return nil, false, false
+		}
+		if p != nil {
+			return *(*interface{})(p), true, true
+		}
+	}
+}
+
+// Delete deletes the value for a key.
+func (m *Map) Delete(key interface{}) {
+	read, _ := m.read.Load().(readOnly)
+	e, ok := read.m[key]
+	if !ok && read.amended {
+		m.mu.Lock()
+		read, _ = m.read.Load().(readOnly)
+		e, ok = read.m[key]
+		if !ok && read.amended {
+			delete(m.dirty, key)
+		}
+		m.mu.Unlock()
+	}
+	if ok {
+		e.delete()
+	}
+}
+
+func (e *entry) delete() (hadValue bool) {
+	for {
+		p := atomic.LoadPointer(&e.p)
+		if p == nil || p == expunged {
+			return false
+		}
+		if atomic.CompareAndSwapPointer(&e.p, p, nil) {
+			return true
+		}
+	}
+}
+
+// Range calls f sequentially for each key and value present in the map.
+// If f returns false, range stops the iteration.
+//
+// Range does not necessarily correspond to any consistent snapshot of the Map's
+// contents: no key will be visited more than once, but if the value for any key
+// is stored or deleted concurrently, Range may reflect any mapping for that key
+// from any point during the Range call.
+//
+// Range may be O(N) with the number of elements in the map even if f returns
+// false after a constant number of calls.
+func (m *Map) Range(f func(key, value interface{}) bool) {
+	// We need to be able to iterate over all of the keys that were already
+	// present at the start of the call to Range.
+	// If read.amended is false, then read.m satisfies that property without
+	// requiring us to hold m.mu for a long time.
+	read, _ := m.read.Load().(readOnly)
+	if read.amended {
+		// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
+		// (assuming the caller does not break out early), so a call to Range
+		// amortizes an entire copy of the map: we can promote the dirty copy
+		// immediately!
+		m.mu.Lock()
+		read, _ = m.read.Load().(readOnly)
+		if read.amended {
+			read = readOnly{m: m.dirty}
+			m.read.Store(read)
+			m.dirty = nil
+			m.misses = 0
+		}
+		m.mu.Unlock()
+	}
+
+	for k, e := range read.m {
+		v, ok := e.load()
+		if !ok {
+			continue
+		}
+		if !f(k, v) {
+			break
+		}
+	}
+}
+
+func (m *Map) missLocked() {
+	m.misses++
+	if m.misses < len(m.dirty) {
+		return
+	}
+	m.read.Store(readOnly{m: m.dirty})
+	m.dirty = nil
+	m.misses = 0
+}
+
+func (m *Map) dirtyLocked() {
+	if m.dirty != nil {
+		return
+	}
+
+	read, _ := m.read.Load().(readOnly)
+	m.dirty = make(map[interface{}]*entry, len(read.m))
+	for k, e := range read.m {
+		if !e.tryExpungeLocked() {
+			m.dirty[k] = e
+		}
+	}
+}
+
+func (e *entry) tryExpungeLocked() (isExpunged bool) {
+	p := atomic.LoadPointer(&e.p)
+	for p == nil {
+		if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
+			return true
+		}
+		p = atomic.LoadPointer(&e.p)
+	}
+	return p == expunged
+}
diff --git a/libgo/go/sync/map_bench_test.go b/libgo/go/sync/map_bench_test.go
new file mode 100644
index 00000000000..e6a8badddba
--- /dev/null
+++ b/libgo/go/sync/map_bench_test.go
@@ -0,0 +1,215 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sync_test
+
+import (
+	"fmt"
+	"reflect"
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+type bench struct {
+	setup func(*testing.B, mapInterface)
+	perG  func(b *testing.B, pb *testing.PB, i int, m mapInterface)
+}
+
+func benchMap(b *testing.B, bench bench) {
+	for _, m := range [...]mapInterface{&DeepCopyMap{}, &RWMutexMap{}, &sync.Map{}} {
+		b.Run(fmt.Sprintf("%T", m), func(b *testing.B) {
+			m = reflect.New(reflect.TypeOf(m).Elem()).Interface().(mapInterface)
+			if bench.setup != nil {
+				bench.setup(b, m)
+			}
+
+			b.ResetTimer()
+
+			var i int64
+			b.RunParallel(func(pb *testing.PB) {
+				id := int(atomic.AddInt64(&i, 1) - 1)
+				bench.perG(b, pb, id*b.N, m)
+			})
+		})
+	}
+}
+
+func BenchmarkLoadMostlyHits(b *testing.B) {
+	const hits, misses = 1023, 1
+
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			for i := 0; i < hits; i++ {
+				m.LoadOrStore(i, i)
+			}
+			// Prime the map to get it into a steady state.
+			for i := 0; i < hits*2; i++ {
+				m.Load(i % hits)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.Load(i % (hits + misses))
+			}
+		},
+	})
+}
+
+func BenchmarkLoadMostlyMisses(b *testing.B) {
+	const hits, misses = 1, 1023
+
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			for i := 0; i < hits; i++ {
+				m.LoadOrStore(i, i)
+			}
+			// Prime the map to get it into a steady state.
+			for i := 0; i < hits*2; i++ {
+				m.Load(i % hits)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.Load(i % (hits + misses))
+			}
+		},
+	})
+}
+
+func BenchmarkLoadOrStoreBalanced(b *testing.B) {
+	const hits, misses = 128, 128
+
+	benchMap(b, bench{
+		setup: func(b *testing.B, m mapInterface) {
+			if _, ok := m.(*DeepCopyMap); ok {
+				b.Skip("DeepCopyMap has quadratic running time.")
+			}
+			for i := 0; i < hits; i++ {
+				m.LoadOrStore(i, i)
+			}
+			// Prime the map to get it into a steady state.
+			for i := 0; i < hits*2; i++ {
+				m.Load(i % hits)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				j := i % (hits + misses)
+				if j < hits {
+					if _, ok := m.LoadOrStore(j, i); !ok {
+						b.Fatalf("unexpected miss for %v", j)
+					}
+				} else {
+					if v, loaded := m.LoadOrStore(i, i); loaded {
+						b.Fatalf("failed to store %v: existing value %v", i, v)
+					}
+				}
+			}
+		},
+	})
+}
+
+func BenchmarkLoadOrStoreUnique(b *testing.B) {
+	benchMap(b, bench{
+		setup: func(b *testing.B, m mapInterface) {
+			if _, ok := m.(*DeepCopyMap); ok {
+				b.Skip("DeepCopyMap has quadratic running time.")
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.LoadOrStore(i, i)
+			}
+		},
+	})
+}
+
+func BenchmarkLoadOrStoreCollision(b *testing.B) {
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			m.LoadOrStore(0, 0)
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.LoadOrStore(0, 0)
+			}
+		},
+	})
+}
+
+func BenchmarkRange(b *testing.B) {
+	const mapSize = 1 << 10
+
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			for i := 0; i < mapSize; i++ {
+				m.Store(i, i)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.Range(func(_, _ interface{}) bool { return true })
+			}
+		},
+	})
+}
+
+// BenchmarkAdversarialAlloc tests performance when we store a new value
+// immediately whenever the map is promoted to clean and otherwise load a
+// unique, missing key.
+//
+// This forces the Load calls to always acquire the map's mutex.
+func BenchmarkAdversarialAlloc(b *testing.B) {
+	benchMap(b, bench{
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			var stores, loadsSinceStore int64
+			for ; pb.Next(); i++ {
+				m.Load(i)
+				if loadsSinceStore++; loadsSinceStore > stores {
+					m.LoadOrStore(i, stores)
+					loadsSinceStore = 0
+					stores++
+				}
+			}
+		},
+	})
+}
+
+// BenchmarkAdversarialDelete tests performance when we periodically delete
+// one key and add a different one in a large map.
+//
+// This forces the Load calls to always acquire the map's mutex and periodically
+// makes a full copy of the map despite changing only one entry.
+func BenchmarkAdversarialDelete(b *testing.B) {
+	const mapSize = 1 << 10
+
+	benchMap(b, bench{
+		setup: func(_ *testing.B, m mapInterface) {
+			for i := 0; i < mapSize; i++ {
+				m.Store(i, i)
+			}
+		},
+
+		perG: func(b *testing.B, pb *testing.PB, i int, m mapInterface) {
+			for ; pb.Next(); i++ {
+				m.Load(i)
+
+				if i%mapSize == 0 {
+					m.Range(func(k, _ interface{}) bool {
+						m.Delete(k)
+						return false
+					})
+					m.Store(i, i)
+				}
+			}
+		},
+	})
+}
diff --git a/libgo/go/sync/map_reference_test.go b/libgo/go/sync/map_reference_test.go
new file mode 100644
index 00000000000..9f27b07c329
--- /dev/null
+++ b/libgo/go/sync/map_reference_test.go
@@ -0,0 +1,151 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sync_test
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+// This file contains reference map implementations for unit-tests.
+
+// mapInterface is the interface Map implements.
+type mapInterface interface {
+	Load(interface{}) (interface{}, bool)
+	Store(key, value interface{})
+	LoadOrStore(key, value interface{}) (actual interface{}, loaded bool)
+	Delete(interface{})
+	Range(func(key, value interface{}) (shouldContinue bool))
+}
+
+// RWMutexMap is an implementation of mapInterface using a sync.RWMutex.
+type RWMutexMap struct {
+	mu    sync.RWMutex
+	dirty map[interface{}]interface{}
+}
+
+func (m *RWMutexMap) Load(key interface{}) (value interface{}, ok bool) {
+	m.mu.RLock()
+	value, ok = m.dirty[key]
+	m.mu.RUnlock()
+	return
+}
+
+func (m *RWMutexMap) Store(key, value interface{}) {
+	m.mu.Lock()
+	if m.dirty == nil {
+		m.dirty = make(map[interface{}]interface{})
+	}
+	m.dirty[key] = value
+	m.mu.Unlock()
+}
+
+func (m *RWMutexMap) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
+	m.mu.Lock()
+	actual, loaded = m.dirty[key]
+	if !loaded {
+		actual = value
+		if m.dirty == nil {
+			m.dirty = make(map[interface{}]interface{})
+		}
+		m.dirty[key] = value
+	}
+	m.mu.Unlock()
+	return actual, loaded
+}
+
+func (m *RWMutexMap) Delete(key interface{}) {
+	m.mu.Lock()
+	delete(m.dirty, key)
+	m.mu.Unlock()
+}
+
+func (m *RWMutexMap) Range(f func(key, value interface{}) (shouldContinue bool)) {
+	m.mu.RLock()
+	keys := make([]interface{}, 0, len(m.dirty))
+	for k := range m.dirty {
+		keys = append(keys, k)
+	}
+	m.mu.RUnlock()
+
+	for _, k := range keys {
+		v, ok := m.Load(k)
+		if !ok {
+			continue
+		}
+		if !f(k, v) {
+			break
+		}
+	}
+}
+
+// DeepCopyMap is an implementation of mapInterface using a Mutex and
+// atomic.Value.  It makes deep copies of the map on every write to avoid
+// acquiring the Mutex in Load.
+type DeepCopyMap struct {
+	mu    sync.Mutex
+	clean atomic.Value
+}
+
+func (m *DeepCopyMap) Load(key interface{}) (value interface{}, ok bool) {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	value, ok = clean[key]
+	return value, ok
+}
+
+func (m *DeepCopyMap) Store(key, value interface{}) {
+	m.mu.Lock()
+	dirty := m.dirty()
+	dirty[key] = value
+	m.clean.Store(dirty)
+	m.mu.Unlock()
+}
+
+func (m *DeepCopyMap) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	actual, loaded = clean[key]
+	if loaded {
+		return actual, loaded
+	}
+
+	m.mu.Lock()
+	// Reload clean in case it changed while we were waiting on m.mu.
+	clean, _ = m.clean.Load().(map[interface{}]interface{})
+	actual, loaded = clean[key]
+	if !loaded {
+		dirty := m.dirty()
+		dirty[key] = value
+		actual = value
+		m.clean.Store(dirty)
+	}
+	m.mu.Unlock()
+	return actual, loaded
+}
+
+func (m *DeepCopyMap) Delete(key interface{}) {
+	m.mu.Lock()
+	dirty := m.dirty()
+	delete(dirty, key)
+	m.clean.Store(dirty)
+	m.mu.Unlock()
+}
+
+func (m *DeepCopyMap) Range(f func(key, value interface{}) (shouldContinue bool)) {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	for k, v := range clean {
+		if !f(k, v) {
+			break
+		}
+	}
+}
+
+func (m *DeepCopyMap) dirty() map[interface{}]interface{} {
+	clean, _ := m.clean.Load().(map[interface{}]interface{})
+	dirty := make(map[interface{}]interface{}, len(clean)+1)
+	for k, v := range clean {
+		dirty[k] = v
+	}
+	return dirty
+}
diff --git a/libgo/go/sync/map_test.go b/libgo/go/sync/map_test.go
new file mode 100644
index 00000000000..b60a1c7bede
--- /dev/null
+++ b/libgo/go/sync/map_test.go
@@ -0,0 +1,170 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sync_test
+
+import (
+	"math/rand"
+	"reflect"
+	"runtime"
+	"sync"
+	"testing"
+	"testing/quick"
+)
+
+type mapOp string
+
+const (
+	opLoad        = mapOp("Load")
+	opStore       = mapOp("Store")
+	opLoadOrStore = mapOp("LoadOrStore")
+	opDelete      = mapOp("Delete")
+)
+
+var mapOps = [...]mapOp{opLoad, opStore, opLoadOrStore, opDelete}
+
+// mapCall is a quick.Generator for calls on mapInterface.
+type mapCall struct {
+	op   mapOp
+	k, v interface{}
+}
+
+func (c mapCall) apply(m mapInterface) (interface{}, bool) {
+	switch c.op {
+	case opLoad:
+		return m.Load(c.k)
+	case opStore:
+		m.Store(c.k, c.v)
+		return nil, false
+	case opLoadOrStore:
+		return m.LoadOrStore(c.k, c.v)
+	case opDelete:
+		m.Delete(c.k)
+		return nil, false
+	default:
+		panic("invalid mapOp")
+	}
+}
+
+type mapResult struct {
+	value interface{}
+	ok    bool
+}
+
+func randValue(r *rand.Rand) interface{} {
+	b := make([]byte, r.Intn(4))
+	for i := range b {
+		b[i] = 'a' + byte(rand.Intn(26))
+	}
+	return string(b)
+}
+
+func (mapCall) Generate(r *rand.Rand, size int) reflect.Value {
+	c := mapCall{op: mapOps[rand.Intn(len(mapOps))], k: randValue(r)}
+	switch c.op {
+	case opStore, opLoadOrStore:
+		c.v = randValue(r)
+	}
+	return reflect.ValueOf(c)
+}
+
+func applyCalls(m mapInterface, calls []mapCall) (results []mapResult, final map[interface{}]interface{}) {
+	for _, c := range calls {
+		v, ok := c.apply(m)
+		results = append(results, mapResult{v, ok})
+	}
+
+	final = make(map[interface{}]interface{})
+	m.Range(func(k, v interface{}) bool {
+		final[k] = v
+		return true
+	})
+
+	return results, final
+}
+
+func applyMap(calls []mapCall) ([]mapResult, map[interface{}]interface{}) {
+	return applyCalls(new(sync.Map), calls)
+}
+
+func applyRWMutexMap(calls []mapCall) ([]mapResult, map[interface{}]interface{}) {
+	return applyCalls(new(RWMutexMap), calls)
+}
+
+func applyDeepCopyMap(calls []mapCall) ([]mapResult, map[interface{}]interface{}) {
+	return applyCalls(new(DeepCopyMap), calls)
+}
+
+func TestMapMatchesRWMutex(t *testing.T) {
+	if err := quick.CheckEqual(applyMap, applyRWMutexMap, nil); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestMapMatchesDeepCopy(t *testing.T) {
+	if err := quick.CheckEqual(applyMap, applyDeepCopyMap, nil); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestConcurrentRange(t *testing.T) {
+	const mapSize = 1 << 10
+
+	m := new(sync.Map)
+	for n := int64(1); n <= mapSize; n++ {
+		m.Store(n, int64(n))
+	}
+
+	done := make(chan struct{})
+	var wg sync.WaitGroup
+	defer func() {
+		close(done)
+		wg.Wait()
+	}()
+	for g := int64(runtime.GOMAXPROCS(0)); g > 0; g-- {
+		r := rand.New(rand.NewSource(g))
+		wg.Add(1)
+		go func(g int64) {
+			defer wg.Done()
+			for i := int64(0); ; i++ {
+				select {
+				case <-done:
+					return
+				default:
+				}
+				for n := int64(1); n < mapSize; n++ {
+					if r.Int63n(mapSize) == 0 {
+						m.Store(n, n*i*g)
+					} else {
+						m.Load(n)
+					}
+				}
+			}
+		}(g)
+	}
+
+	iters := 1 << 10
+	if testing.Short() {
+		iters = 16
+	}
+	for n := iters; n > 0; n-- {
+		seen := make(map[int64]bool, mapSize)
+
+		m.Range(func(ki, vi interface{}) bool {
+			k, v := ki.(int64), vi.(int64)
+			if v%k != 0 {
+				t.Fatalf("while Storing multiples of %v, Range saw value %v", k, v)
+			}
+			if seen[k] {
+				t.Fatalf("Range visited key %v twice", k)
+			}
+			seen[k] = true
+			return true
+		})
+
+		if len(seen) != mapSize {
+			t.Fatalf("Range visited %v elements of %v-element Map", len(seen), mapSize)
+		}
+	}
+}
diff --git a/libgo/go/sync/mutex.go b/libgo/go/sync/mutex.go
index 8c9366f4fe1..1232c629b18 100644
--- a/libgo/go/sync/mutex.go
+++ b/libgo/go/sync/mutex.go
@@ -19,8 +19,7 @@ import (
 func throw(string) // provided by runtime
 
 // 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.
+// The zero value for a Mutex is an unlocked mutex.
 //
 // A Mutex must not be copied after first use.
 type Mutex struct {
@@ -37,7 +36,34 @@ type Locker interface {
 const (
 	mutexLocked = 1 << iota // mutex is locked
 	mutexWoken
+	mutexStarving
 	mutexWaiterShift = iota
+
+	// Mutex fairness.
+	//
+	// Mutex can be in 2 modes of operations: normal and starvation.
+	// In normal mode waiters are queued in FIFO order, but a woken up waiter
+	// does not own the mutex and competes with new arriving goroutines over
+	// the ownership. New arriving goroutines have an advantage -- they are
+	// already running on CPU and there can be lots of them, so a woken up
+	// waiter has good chances of losing. In such case it is queued at front
+	// of the wait queue. If a waiter fails to acquire the mutex for more than 1ms,
+	// it switches mutex to the starvation mode.
+	//
+	// In starvation mode ownership of the mutex is directly handed off from
+	// the unlocking goroutine to the waiter at the front of the queue.
+	// New arriving goroutines don't try to acquire the mutex even if it appears
+	// to be unlocked, and don't try to spin. Instead they queue themselves at
+	// the tail of the wait queue.
+	//
+	// If a waiter receives ownership of the mutex and sees that either
+	// (1) it is the last waiter in the queue, or (2) it waited for less than 1 ms,
+	// it switches mutex back to normal operation mode.
+	//
+	// Normal mode has considerably better performance as a goroutine can acquire
+	// a mutex several times in a row even if there are blocked waiters.
+	// Starvation mode is important to prevent pathological cases of tail latency.
+	starvationThresholdNs = 1e6
 )
 
 // Lock locks m.
@@ -52,41 +78,86 @@ func (m *Mutex) Lock() {
 		return
 	}
 
+	var waitStartTime int64
+	starving := false
 	awoke := false
 	iter := 0
+	old := m.state
 	for {
-		old := m.state
-		new := old | mutexLocked
-		if old&mutexLocked != 0 {
-			if runtime_canSpin(iter) {
-				// Active spinning makes sense.
-				// Try to set mutexWoken flag to inform Unlock
-				// to not wake other blocked goroutines.
-				if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
-					atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
-					awoke = true
-				}
-				runtime_doSpin()
-				iter++
-				continue
+		// Don't spin in starvation mode, ownership is handed off to waiters
+		// so we won't be able to acquire the mutex anyway.
+		if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) {
+			// Active spinning makes sense.
+			// Try to set mutexWoken flag to inform Unlock
+			// to not wake other blocked goroutines.
+			if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
+				atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
+				awoke = true
 			}
-			new = old + 1<<mutexWaiterShift
+			runtime_doSpin()
+			iter++
+			old = m.state
+			continue
+		}
+		new := old
+		// Don't try to acquire starving mutex, new arriving goroutines must queue.
+		if old&mutexStarving == 0 {
+			new |= mutexLocked
+		}
+		if old&(mutexLocked|mutexStarving) != 0 {
+			new += 1 << mutexWaiterShift
+		}
+		// The current goroutine switches mutex to starvation mode.
+		// But if the mutex is currently unlocked, don't do the switch.
+		// Unlock expects that starving mutex has waiters, which will not
+		// be true in this case.
+		if starving && old&mutexLocked != 0 {
+			new |= mutexStarving
 		}
 		if awoke {
 			// The goroutine has been woken from sleep,
 			// so we need to reset the flag in either case.
 			if new&mutexWoken == 0 {
-				throw("sync: inconsistent mutex state")
+				panic("sync: inconsistent mutex state")
 			}
 			new &^= mutexWoken
 		}
 		if atomic.CompareAndSwapInt32(&m.state, old, new) {
-			if old&mutexLocked == 0 {
+			if old&(mutexLocked|mutexStarving) == 0 {
+				break // locked the mutex with CAS
+			}
+			// If we were already waiting before, queue at the front of the queue.
+			queueLifo := waitStartTime != 0
+			if waitStartTime == 0 {
+				waitStartTime = runtime_nanotime()
+			}
+			runtime_SemacquireMutex(&m.sema, queueLifo)
+			starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
+			old = m.state
+			if old&mutexStarving != 0 {
+				// If this goroutine was woken and mutex is in starvation mode,
+				// ownership was handed off to us but mutex is in somewhat
+				// inconsistent state: mutexLocked is not set and we are still
+				// accounted as waiter. Fix that.
+				if old&(mutexLocked|mutexWoken) != 0 || old>>mutexWaiterShift == 0 {
+					panic("sync: inconsistent mutex state")
+				}
+				delta := int32(mutexLocked - 1<<mutexWaiterShift)
+				if !starving || old>>mutexWaiterShift == 1 {
+					// Exit starvation mode.
+					// Critical to do it here and consider wait time.
+					// Starvation mode is so inefficient, that two goroutines
+					// can go lock-step infinitely once they switch mutex
+					// to starvation mode.
+					delta -= mutexStarving
+				}
+				atomic.AddInt32(&m.state, delta)
 				break
 			}
-			runtime_SemacquireMutex(&m.sema)
 			awoke = true
 			iter = 0
+		} else {
+			old = m.state
 		}
 	}
 
@@ -110,22 +181,33 @@ func (m *Mutex) Unlock() {
 	// Fast path: drop lock bit.
 	new := atomic.AddInt32(&m.state, -mutexLocked)
 	if (new+mutexLocked)&mutexLocked == 0 {
-		throw("sync: unlock of unlocked mutex")
+		panic("sync: unlock of unlocked mutex")
 	}
-
-	old := new
-	for {
-		// If there are no waiters or a goroutine has already
-		// been woken or grabbed the lock, no need to wake anyone.
-		if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken) != 0 {
-			return
-		}
-		// Grab the right to wake someone.
-		new = (old - 1<<mutexWaiterShift) | mutexWoken
-		if atomic.CompareAndSwapInt32(&m.state, old, new) {
-			runtime_Semrelease(&m.sema)
-			return
+	if new&mutexStarving == 0 {
+		old := new
+		for {
+			// If there are no waiters or a goroutine has already
+			// been woken or grabbed the lock, no need to wake anyone.
+			// In starvation mode ownership is directly handed off from unlocking
+			// goroutine to the next waiter. We are not part of this chain,
+			// since we did not observe mutexStarving when we unlocked the mutex above.
+			// So get off the way.
+			if old>>mutexWaiterShift == 0 || old&(mutexLocked|mutexWoken|mutexStarving) != 0 {
+				return
+			}
+			// Grab the right to wake someone.
+			new = (old - 1<<mutexWaiterShift) | mutexWoken
+			if atomic.CompareAndSwapInt32(&m.state, old, new) {
+				runtime_Semrelease(&m.sema, false)
+				return
+			}
+			old = m.state
 		}
-		old = m.state
+	} else {
+		// Starving mode: handoff mutex ownership to the next waiter.
+		// Note: mutexLocked is not set, the waiter will set it after wakeup.
+		// But mutex is still considered locked if mutexStarving is set,
+		// so new coming goroutines won't acquire it.
+		runtime_Semrelease(&m.sema, true)
 	}
 }
diff --git a/libgo/go/sync/mutex_test.go b/libgo/go/sync/mutex_test.go
index 88dbccf3add..784471df129 100644
--- a/libgo/go/sync/mutex_test.go
+++ b/libgo/go/sync/mutex_test.go
@@ -15,12 +15,13 @@ import (
 	"strings"
 	. "sync"
 	"testing"
+	"time"
 )
 
 func HammerSemaphore(s *uint32, loops int, cdone chan bool) {
 	for i := 0; i < loops; i++ {
 		Runtime_Semacquire(s)
-		Runtime_Semrelease(s)
+		Runtime_Semrelease(s, false)
 	}
 	cdone <- true
 }
@@ -174,6 +175,38 @@ func TestMutexMisuse(t *testing.T) {
 	}
 }
 
+func TestMutexFairness(t *testing.T) {
+	var mu Mutex
+	stop := make(chan bool)
+	defer close(stop)
+	go func() {
+		for {
+			mu.Lock()
+			time.Sleep(100 * time.Microsecond)
+			mu.Unlock()
+			select {
+			case <-stop:
+				return
+			default:
+			}
+		}
+	}()
+	done := make(chan bool)
+	go func() {
+		for i := 0; i < 10; i++ {
+			time.Sleep(100 * time.Microsecond)
+			mu.Lock()
+			mu.Unlock()
+		}
+		done <- true
+	}()
+	select {
+	case <-done:
+	case <-time.After(10 * time.Second):
+		t.Fatalf("can't acquire Mutex in 10 seconds")
+	}
+}
+
 func BenchmarkMutexUncontended(b *testing.B) {
 	type PaddedMutex struct {
 		Mutex
diff --git a/libgo/go/sync/pool.go b/libgo/go/sync/pool.go
index 0acdbde096f..e54f917225b 100644
--- a/libgo/go/sync/pool.go
+++ b/libgo/go/sync/pool.go
@@ -54,11 +54,18 @@ type Pool struct {
 }
 
 // Local per-P Pool appendix.
-type poolLocal struct {
+type poolLocalInternal struct {
 	private interface{}   // Can be used only by the respective P.
 	shared  []interface{} // Can be used by any P.
 	Mutex                 // Protects shared.
-	pad     [128]byte     // Prevents false sharing.
+}
+
+type poolLocal struct {
+	poolLocalInternal
+
+	// Prevents false sharing on widespread platforms with
+	// 128 mod (cache line size) = 0 .
+	pad [128 - unsafe.Sizeof(poolLocalInternal{})%128]byte
 }
 
 // from runtime
@@ -241,7 +248,8 @@ func init() {
 }
 
 func indexLocal(l unsafe.Pointer, i int) *poolLocal {
-	return &(*[1000000]poolLocal)(l)[i]
+	lp := unsafe.Pointer(uintptr(l) + uintptr(i)*unsafe.Sizeof(poolLocal{}))
+	return (*poolLocal)(lp)
 }
 
 // Implemented in runtime.
diff --git a/libgo/go/sync/pool_test.go b/libgo/go/sync/pool_test.go
index f92e181a6b1..dad2f992e7c 100644
--- a/libgo/go/sync/pool_test.go
+++ b/libgo/go/sync/pool_test.go
@@ -23,6 +23,10 @@ func TestPool(t *testing.T) {
 	if p.Get() != nil {
 		t.Fatal("expected empty")
 	}
+
+	// Make sure that the goroutine doesn't migrate to another P
+	// between Put and Get calls.
+	Runtime_procPin()
 	p.Put("a")
 	p.Put("b")
 	if g := p.Get(); g != "a" {
@@ -34,6 +38,7 @@ func TestPool(t *testing.T) {
 	if g := p.Get(); g != nil {
 		t.Fatalf("got %#v; want nil", g)
 	}
+	Runtime_procUnpin()
 
 	p.Put("c")
 	debug.SetGCPercent(100) // to allow following GC to actually run
@@ -60,10 +65,16 @@ func TestPoolNew(t *testing.T) {
 	if v := p.Get(); v != 2 {
 		t.Fatalf("got %v; want 2", v)
 	}
+
+	// Make sure that the goroutine doesn't migrate to another P
+	// between Put and Get calls.
+	Runtime_procPin()
 	p.Put(42)
 	if v := p.Get(); v != 42 {
 		t.Fatalf("got %v; want 42", v)
 	}
+	Runtime_procUnpin()
+
 	if v := p.Get(); v != 3 {
 		t.Fatalf("got %v; want 3", v)
 	}
diff --git a/libgo/go/sync/runtime.go b/libgo/go/sync/runtime.go
index 4d22ce6b0da..be16bcc8f7b 100644
--- a/libgo/go/sync/runtime.go
+++ b/libgo/go/sync/runtime.go
@@ -14,13 +14,15 @@ import "unsafe"
 func runtime_Semacquire(s *uint32)
 
 // SemacquireMutex is like Semacquire, but for profiling contended Mutexes.
-func runtime_SemacquireMutex(*uint32)
+// If lifo is true, queue waiter at the head of wait queue.
+func runtime_SemacquireMutex(s *uint32, lifo bool)
 
 // Semrelease atomically increments *s and notifies a waiting goroutine
 // if one is blocked in Semacquire.
 // It is intended as a simple wakeup primitive for use by the synchronization
 // library and should not be used directly.
-func runtime_Semrelease(s *uint32)
+// If handoff is true, pass count directly to the first waiter.
+func runtime_Semrelease(s *uint32, handoff bool)
 
 // Approximation of notifyList in runtime/sema.go. Size and alignment must
 // agree.
@@ -57,3 +59,5 @@ func runtime_canSpin(i int) bool
 
 // runtime_doSpin does active spinning.
 func runtime_doSpin()
+
+func runtime_nanotime() int64
diff --git a/libgo/go/sync/runtime_sema_test.go b/libgo/go/sync/runtime_sema_test.go
index a2382f46554..a680847edf8 100644
--- a/libgo/go/sync/runtime_sema_test.go
+++ b/libgo/go/sync/runtime_sema_test.go
@@ -18,7 +18,7 @@ func BenchmarkSemaUncontended(b *testing.B) {
 	b.RunParallel(func(pb *testing.PB) {
 		sem := new(PaddedSem)
 		for pb.Next() {
-			Runtime_Semrelease(&sem.sem)
+			Runtime_Semrelease(&sem.sem, false)
 			Runtime_Semacquire(&sem.sem)
 		}
 	})
@@ -44,7 +44,7 @@ func benchmarkSema(b *testing.B, block, work bool) {
 	b.RunParallel(func(pb *testing.PB) {
 		foo := 0
 		for pb.Next() {
-			Runtime_Semrelease(&sem)
+			Runtime_Semrelease(&sem, false)
 			if work {
 				for i := 0; i < 100; i++ {
 					foo *= 2
@@ -54,7 +54,7 @@ func benchmarkSema(b *testing.B, block, work bool) {
 			Runtime_Semacquire(&sem)
 		}
 		_ = foo
-		Runtime_Semrelease(&sem)
+		Runtime_Semrelease(&sem, false)
 	})
 }
 
diff --git a/libgo/go/sync/rwmutex.go b/libgo/go/sync/rwmutex.go
index 71064eeeba3..cb2dfe1ad8d 100644
--- a/libgo/go/sync/rwmutex.go
+++ b/libgo/go/sync/rwmutex.go
@@ -10,18 +10,21 @@ import (
 	"unsafe"
 )
 
+// There is a modified copy of this file in runtime/rwmutex.go.
+// If you make any changes here, see if you should make them there.
+
 // An RWMutex is a reader/writer mutual exclusion lock.
 // The lock can be held by an arbitrary number of readers or a single writer.
-// RWMutexes can be created as part of other structures;
-// the zero value for a RWMutex is an unlocked mutex.
+// The zero value for a RWMutex is an unlocked mutex.
 //
 // An RWMutex must not be copied after first use.
 //
-// If a goroutine holds a RWMutex for reading, it must not expect this or any
-// other goroutine to be able to also take the read lock until the first read
-// lock is released. In particular, this prohibits recursive read locking.
-// This is to ensure that the lock eventually becomes available;
-// a blocked Lock call excludes new readers from acquiring the lock.
+// If a goroutine holds a RWMutex for reading and another goroutine might
+// call Lock, no goroutine should expect to be able to acquire a read lock
+// until the initial read lock is released. In particular, this prohibits
+// recursive read locking. This is to ensure that the lock eventually becomes
+// available; a blocked Lock call excludes new readers from acquiring the
+// lock.
 type RWMutex struct {
 	w           Mutex  // held if there are pending writers
 	writerSem   uint32 // semaphore for writers to wait for completing readers
@@ -33,6 +36,10 @@ type RWMutex struct {
 const rwmutexMaxReaders = 1 << 30
 
 // RLock locks rw for reading.
+//
+// It should not be used for recursive read locking; a blocked Lock
+// call excludes new readers from acquiring the lock. See the
+// documentation on the RWMutex type.
 func (rw *RWMutex) RLock() {
 	if race.Enabled {
 		_ = rw.w.state
@@ -66,7 +73,7 @@ func (rw *RWMutex) RUnlock() {
 		// A writer is pending.
 		if atomic.AddInt32(&rw.readerWait, -1) == 0 {
 			// The last reader unblocks the writer.
-			runtime_Semrelease(&rw.writerSem)
+			runtime_Semrelease(&rw.writerSem, false)
 		}
 	}
 	if race.Enabled {
@@ -119,7 +126,7 @@ func (rw *RWMutex) Unlock() {
 	}
 	// Unblock blocked readers, if any.
 	for i := 0; i < int(r); i++ {
-		runtime_Semrelease(&rw.readerSem)
+		runtime_Semrelease(&rw.readerSem, false)
 	}
 	// Allow other writers to proceed.
 	rw.w.Unlock()
diff --git a/libgo/go/sync/rwmutex_test.go b/libgo/go/sync/rwmutex_test.go
index 0436f97239c..9ee8864cebb 100644
--- a/libgo/go/sync/rwmutex_test.go
+++ b/libgo/go/sync/rwmutex_test.go
@@ -14,6 +14,9 @@ import (
 	"testing"
 )
 
+// There is a modified copy of this file in runtime/rwmutex_test.go.
+// If you make any changes here, see if you should make them there.
+
 func parallelReader(m *RWMutex, clocked, cunlock, cdone chan bool) {
 	m.RLock()
 	clocked <- true
diff --git a/libgo/go/sync/waitgroup.go b/libgo/go/sync/waitgroup.go
index b386e1fec2b..f266f7c2b97 100644
--- a/libgo/go/sync/waitgroup.go
+++ b/libgo/go/sync/waitgroup.go
@@ -91,11 +91,11 @@ func (wg *WaitGroup) Add(delta int) {
 	// Reset waiters count to 0.
 	*statep = 0
 	for ; w != 0; w-- {
-		runtime_Semrelease(&wg.sema)
+		runtime_Semrelease(&wg.sema, false)
 	}
 }
 
-// Done decrements the WaitGroup counter.
+// Done decrements the WaitGroup counter by one.
 func (wg *WaitGroup) Done() {
 	wg.Add(-1)
 }
diff --git a/libgo/go/sync/waitgroup_test.go b/libgo/go/sync/waitgroup_test.go
index 8ec34fd343b..e3e30966457 100644
--- a/libgo/go/sync/waitgroup_test.go
+++ b/libgo/go/sync/waitgroup_test.go
@@ -18,11 +18,11 @@ func testWaitGroup(t *testing.T, wg1 *WaitGroup, wg2 *WaitGroup) {
 	wg2.Add(n)
 	exited := make(chan bool, n)
 	for i := 0; i != n; i++ {
-		go func(i int) {
+		go func() {
 			wg1.Done()
 			wg2.Wait()
 			exited <- true
-		}(i)
+		}()
 	}
 	wg1.Wait()
 	for i := 0; i != n; i++ {
@@ -70,11 +70,8 @@ func TestWaitGroupMisuse(t *testing.T) {
 
 func TestWaitGroupMisuse2(t *testing.T) {
 	knownRacy(t)
-	if testing.Short() {
-		t.Skip("skipping flaky test in short mode; see issue 11443")
-	}
-	if runtime.NumCPU() <= 2 {
-		t.Skip("NumCPU<=2, skipping: this test requires parallelism")
+	if runtime.NumCPU() <= 4 {
+		t.Skip("NumCPU<=4, skipping: this test requires parallelism")
 	}
 	defer func() {
 		err := recover()
@@ -86,24 +83,37 @@ func TestWaitGroupMisuse2(t *testing.T) {
 	}()
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(4))
 	done := make(chan interface{}, 2)
-	// The detection is opportunistically, so we want it to panic
+	// The detection is opportunistic, so we want it to panic
 	// at least in one run out of a million.
 	for i := 0; i < 1e6; i++ {
 		var wg WaitGroup
+		var here uint32
 		wg.Add(1)
 		go func() {
 			defer func() {
 				done <- recover()
 			}()
+			atomic.AddUint32(&here, 1)
+			for atomic.LoadUint32(&here) != 3 {
+				// spin
+			}
 			wg.Wait()
 		}()
 		go func() {
 			defer func() {
 				done <- recover()
 			}()
+			atomic.AddUint32(&here, 1)
+			for atomic.LoadUint32(&here) != 3 {
+				// spin
+			}
 			wg.Add(1) // This is the bad guy.
 			wg.Done()
 		}()
+		atomic.AddUint32(&here, 1)
+		for atomic.LoadUint32(&here) != 3 {
+			// spin
+		}
 		wg.Done()
 		for j := 0; j < 2; j++ {
 			if err := <-done; err != nil {