// Copyright 2014 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 atomic
|
|
import (
|
"unsafe"
|
)
|
|
// A Value provides an atomic load and store of a consistently typed value.
|
// The zero value for a Value returns nil from Load.
|
// Once Store has been called, a Value must not be copied.
|
//
|
// A Value must not be copied after first use.
|
type Value struct {
|
v interface{}
|
}
|
|
// ifaceWords is interface{} internal representation.
|
type ifaceWords struct {
|
typ unsafe.Pointer
|
data unsafe.Pointer
|
}
|
|
// Load returns the value set by the most recent Store.
|
// It returns nil if there has been no call to Store for this Value.
|
func (v *Value) Load() (x interface{}) {
|
vp := (*ifaceWords)(unsafe.Pointer(v))
|
typ := LoadPointer(&vp.typ)
|
if typ == nil || uintptr(typ) == ^uintptr(0) {
|
// First store not yet completed.
|
return nil
|
}
|
data := LoadPointer(&vp.data)
|
xp := (*ifaceWords)(unsafe.Pointer(&x))
|
xp.typ = typ
|
xp.data = data
|
return
|
}
|
|
// Store sets the value of the Value to x.
|
// All calls to Store for a given Value must use values of the same concrete type.
|
// Store of an inconsistent type panics, as does Store(nil).
|
func (v *Value) Store(x interface{}) {
|
if x == nil {
|
panic("sync/atomic: store of nil value into Value")
|
}
|
vp := (*ifaceWords)(unsafe.Pointer(v))
|
xp := (*ifaceWords)(unsafe.Pointer(&x))
|
for {
|
typ := LoadPointer(&vp.typ)
|
if typ == nil {
|
// Attempt to start first store.
|
// Disable preemption so that other goroutines can use
|
// active spin wait to wait for completion; and so that
|
// GC does not see the fake type accidentally.
|
runtime_procPin()
|
if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
|
runtime_procUnpin()
|
continue
|
}
|
// Complete first store.
|
StorePointer(&vp.data, xp.data)
|
StorePointer(&vp.typ, xp.typ)
|
runtime_procUnpin()
|
return
|
}
|
if uintptr(typ) == ^uintptr(0) {
|
// First store in progress. Wait.
|
// Since we disable preemption around the first store,
|
// we can wait with active spinning.
|
continue
|
}
|
// First store completed. Check type and overwrite data.
|
if typ != xp.typ {
|
panic("sync/atomic: store of inconsistently typed value into Value")
|
}
|
StorePointer(&vp.data, xp.data)
|
return
|
}
|
}
|
|
// Disable/enable preemption, implemented in runtime.
|
func runtime_procPin()
|
func runtime_procUnpin()
|
