These functions require great care to be used correctly. Except for special, low-level applications, synchronization is better done with channels or the facilities of the sync package. Share memory by communicating; don’t communicate by sharing memory.
// 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{} }
// 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. returnnil } data := LoadPointer(&vp.data) xp := (*ifaceWords)(unsafe.Pointer(&x)) xp.typ = typ xp.data = data return }
// ifaceWords is interface{} internal representation. type ifaceWords struct { typ unsafe.Pointer data unsafe.Pointer }
// 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)) //old value,类型一致，只改data xp := (*ifaceWords)(unsafe.Pointer(&x)) //new value for { //原子读 typ := LoadPointer(&vp.typ) //case1 nil 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() //set 中间值 if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) { runtime_procUnpin() continue } // Complete first store. //第一次存储，既要更新data,还要更新type，这里的type作为实际的类型 StorePointer(&vp.data, xp.data) StorePointer(&vp.typ, xp.typ) runtime_procUnpin() return } //case2 ^uintptr(0) //第一阶段未完成,typ作为是否完成的标记 //第一阶段未完成，为什么会走下来？是异步的还是并发情况？ ifuintptr(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. //case3 类型不符 if typ != xp.typ { panic("sync/atomic: store of inconsistently typed value into Value") } StorePointer(&vp.data, xp.data) return } }