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sync/atomic: add Value
A Value provides an atomic load and store of a consistently typed value. It's intended to be used with copy-on-write idiom (see the example). Performance: BenchmarkValueRead 50000000 21.7 ns/op BenchmarkValueRead-2 200000000 8.63 ns/op BenchmarkValueRead-4 300000000 4.33 ns/op TBR=rsc R=golang-codereviews CC=golang-codereviews https://golang.org/cl/136710045
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@ -166,3 +166,9 @@ TEXT runtime·main_main(SB),NOSPLIT,$0-0
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TEXT runtime·timenow(SB), NOSPLIT, $0-0
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JMP time·now(SB)
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TEXT sync∕atomic·runtime_procPin(SB),NOSPLIT,$0-0
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JMP sync·runtime_procPin(SB)
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TEXT sync∕atomic·runtime_procUnpin(SB),NOSPLIT,$0-0
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JMP sync·runtime_procUnpin(SB)
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17
src/sync/atomic/norace.go
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17
src/sync/atomic/norace.go
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// Copyright 2014 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build !race
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package atomic
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import "unsafe"
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const raceenabled = false
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func raceAcquire(addr unsafe.Pointer) {
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}
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func raceReleaseMerge(addr unsafe.Pointer) {
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}
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22
src/sync/atomic/race.go
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22
src/sync/atomic/race.go
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// Copyright 2014 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build race
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package atomic
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import (
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"runtime"
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"unsafe"
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)
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const raceenabled = true
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func raceAcquire(addr unsafe.Pointer) {
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runtime.RaceAcquire(addr)
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}
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func raceReleaseMerge(addr unsafe.Pointer) {
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runtime.RaceReleaseMerge(addr)
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}
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91
src/sync/atomic/value.go
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91
src/sync/atomic/value.go
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@ -0,0 +1,91 @@
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// Copyright 2014 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package atomic
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import (
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"unsafe"
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)
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// A Value provides an atomic load and store of a consistently typed value.
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// Values can be created as part of other data structures.
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// The zero value for a Value returns nil from Load.
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// Once Store has been called, a Value must not be copied.
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type Value struct {
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v interface{}
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}
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// ifaceWords is interface{} internal representation.
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type ifaceWords struct {
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typ unsafe.Pointer
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data unsafe.Pointer
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}
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// Load returns the value set by the most recent Store.
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// It returns nil if there has been no call to Store for this Value.
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func (v *Value) Load() (x interface{}) {
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vp := (*ifaceWords)(unsafe.Pointer(v))
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typ := LoadPointer(&vp.typ)
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if typ == nil || uintptr(typ) == ^uintptr(0) {
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// First store not yet completed.
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return nil
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}
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data := LoadPointer(&vp.data)
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xp := (*ifaceWords)(unsafe.Pointer(&x))
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xp.typ = typ
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xp.data = data
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if raceenabled {
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raceAcquire(unsafe.Pointer(v))
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}
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return
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}
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// Store sets the value of the Value to v.
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// All calls to Store for a given Value must use values of the same concrete type.
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// Store of an inconsistent type panics, as does Store(nil).
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func (v *Value) Store(x interface{}) {
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if x == nil {
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panic("sync/atomic: store of nil value into Value")
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}
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if raceenabled {
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raceReleaseMerge(unsafe.Pointer(v))
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}
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vp := (*ifaceWords)(unsafe.Pointer(v))
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xp := (*ifaceWords)(unsafe.Pointer(&x))
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for {
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typ := LoadPointer(&vp.typ)
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if typ == nil {
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// Attempt to start first store.
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// Disable preemption so that other goroutines can use
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// active spin wait to wait for completion; and so that
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// GC does not see the fake type accidentally.
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runtime_procPin()
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if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
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runtime_procUnpin()
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continue
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}
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// Complete first store.
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StorePointer(&vp.data, xp.data)
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StorePointer(&vp.typ, xp.typ)
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runtime_procUnpin()
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return
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}
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if uintptr(typ) == ^uintptr(0) {
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// First store in progress. Wait.
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// Since we disable preemption around the first store,
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// we can wait with active spinning.
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continue
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}
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// First store completed. Check type and overwrite data.
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if typ != xp.typ {
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panic("sync/atomic: store of inconsistently typed value into Value")
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}
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StorePointer(&vp.data, xp.data)
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return
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}
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}
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// Disable/enable preemption, implemented in runtime.
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func runtime_procPin()
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func runtime_procUnpin()
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195
src/sync/atomic/value_test.go
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195
src/sync/atomic/value_test.go
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// Copyright 2014 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package atomic_test
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import (
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"math/rand"
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"runtime"
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"sync"
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. "sync/atomic"
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"testing"
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"time"
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)
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func TestValue(t *testing.T) {
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var v Value
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if v.Load() != nil {
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t.Fatal("initial Value is not nil")
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}
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v.Store(42)
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x := v.Load()
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if xx, ok := x.(int); !ok || xx != 42 {
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t.Fatalf("wrong value: got %+v, want 42", x)
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}
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v.Store(84)
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x = v.Load()
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if xx, ok := x.(int); !ok || xx != 84 {
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t.Fatalf("wrong value: got %+v, want 84", x)
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}
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}
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func TestValueLarge(t *testing.T) {
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var v Value
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v.Store("foo")
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x := v.Load()
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if xx, ok := x.(string); !ok || xx != "foo" {
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t.Fatalf("wrong value: got %+v, want foo", x)
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}
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v.Store("barbaz")
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x = v.Load()
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if xx, ok := x.(string); !ok || xx != "barbaz" {
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t.Fatalf("wrong value: got %+v, want barbaz", x)
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}
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}
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func TestValuePanic(t *testing.T) {
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const nilErr = "sync/atomic: store of nil value into Value"
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const badErr = "sync/atomic: store of inconsistently typed value into Value"
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var v Value
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func() {
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defer func() {
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err := recover()
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if err != nilErr {
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t.Fatalf("inconsistent store panic: got '%v', want '%v'", err, nilErr)
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}
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}()
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v.Store(nil)
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}()
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v.Store(42)
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func() {
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defer func() {
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err := recover()
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if err != badErr {
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t.Fatalf("inconsistent store panic: got '%v', want '%v'", err, badErr)
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}
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}()
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v.Store("foo")
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}()
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func() {
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defer func() {
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err := recover()
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if err != nilErr {
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t.Fatalf("inconsistent store panic: got '%v', want '%v'", err, nilErr)
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}
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}()
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v.Store(nil)
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}()
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}
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func TestValueConcurrent(t *testing.T) {
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tests := [][]interface{}{
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{uint16(0), ^uint16(0), uint16(1 + 2<<8), uint16(3 + 4<<8)},
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{uint32(0), ^uint32(0), uint32(1 + 2<<16), uint32(3 + 4<<16)},
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{uint64(0), ^uint64(0), uint64(1 + 2<<32), uint64(3 + 4<<32)},
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{complex(0, 0), complex(1, 2), complex(3, 4), complex(5, 6)},
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}
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p := 4 * runtime.GOMAXPROCS(0)
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for _, test := range tests {
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var v Value
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done := make(chan bool)
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for i := 0; i < p; i++ {
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go func() {
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r := rand.New(rand.NewSource(rand.Int63()))
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loop:
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for j := 0; j < 1e5; j++ {
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x := test[r.Intn(len(test))]
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v.Store(x)
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x = v.Load()
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for _, x1 := range test {
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if x == x1 {
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continue loop
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}
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}
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t.Logf("loaded unexpected value %+v, want %+v", x, test)
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done <- false
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}
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done <- true
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}()
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}
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for i := 0; i < p; i++ {
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if !<-done {
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t.FailNow()
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}
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}
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}
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}
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func BenchmarkValueRead(b *testing.B) {
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var v Value
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v.Store(new(int))
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b.RunParallel(func(pb *testing.PB) {
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for pb.Next() {
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x := v.Load().(*int)
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if *x != 0 {
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b.Fatalf("wrong value: got %v, want 0", *x)
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}
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}
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})
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}
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// The following example shows how to use Value for periodic program config updates
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// and propagation of the changes to worker goroutines.
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func ExampleValue_config() {
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var config Value // holds current server configuration
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// Create initial config value and store into config.
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config.Store(loadConfig())
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go func() {
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// Reload config every 10 seconds
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// and update config value with the new version.
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for {
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time.Sleep(10 * time.Second)
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config.Store(loadConfig())
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}
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}()
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// Create worker goroutines that handle incoming requests
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// using the latest config value.
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for i := 0; i < 10; i++ {
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go func() {
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for r := range requests() {
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c := config.Load()
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// Handle request r using config c.
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_, _ = r, c
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}
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}()
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}
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}
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func loadConfig() map[string]string {
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return make(map[string]string)
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}
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func requests() chan int {
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return make(chan int)
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}
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// The following example shows how to maintain a scalable frequently read,
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// but infrequently updated data structure using copy-on-write idiom.
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func ExampleValue_readMostly() {
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type Map map[string]string
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var m Value
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m.Store(make(Map))
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var mu sync.Mutex // used only by writers
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// read function can be used to read the data without further synchronization
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read := func(key string) (val string) {
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m1 := m.Load().(Map)
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return m1[key]
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}
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// insert function can be used to update the data without further synchronization
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insert := func(key, val string) {
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mu.Lock() // synchronize with other potential writers
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defer mu.Unlock()
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m1 := m.Load().(Map) // load current value of the data structure
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m2 := make(Map) // create a new value
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for k, v := range m1 {
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m2[k] = v // copy all data from the current object to the new one
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}
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m2[key] = val // do the update that we need
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m.Store(m2) // atomically replace the current object with the new one
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// At this point all new readers start working with the new version.
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// The old version will be garbage collected once the existing readers
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// (if any) are done with it.
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}
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_, _ = read, insert
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}
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