1 // Copyright 2014 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package atomic 6 7 import ( 8 "unsafe" 9 ) 10 11 // A Value provides an atomic load and store of a consistently typed value. 12 // The zero value for a Value returns nil from Load. 13 // Once Store has been called, a Value must not be copied. 14 // 15 // A Value must not be copied after first use. 16 type Value struct { 17 v interface{} 18 } 19 20 // ifaceWords is interface{} internal representation. 21 type ifaceWords struct { 22 typ unsafe.Pointer 23 data unsafe.Pointer 24 } 25 26 // Load returns the value set by the most recent Store. 27 // It returns nil if there has been no call to Store for this Value. 28 func (v *Value) Load() (x interface{}) { 29 vp := (*ifaceWords)(unsafe.Pointer(v)) 30 typ := LoadPointer(&vp.typ) 31 if typ == nil || uintptr(typ) == ^uintptr(0) { 32 // First store not yet completed. 33 return nil 34 } 35 data := LoadPointer(&vp.data) 36 xp := (*ifaceWords)(unsafe.Pointer(&x)) 37 xp.typ = typ 38 xp.data = data 39 return 40 } 41 42 // Store sets the value of the Value to x. 43 // All calls to Store for a given Value must use values of the same concrete type. 44 // Store of an inconsistent type panics, as does Store(nil). 45 func (v *Value) Store(x interface{}) { 46 if x == nil { 47 panic("sync/atomic: store of nil value into Value") 48 } 49 vp := (*ifaceWords)(unsafe.Pointer(v)) 50 xp := (*ifaceWords)(unsafe.Pointer(&x)) 51 for { 52 typ := LoadPointer(&vp.typ) 53 if typ == nil { 54 // Attempt to start first store. 55 // Disable preemption so that other goroutines can use 56 // active spin wait to wait for completion; and so that 57 // GC does not see the fake type accidentally. 58 runtime_procPin() 59 if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) { 60 runtime_procUnpin() 61 continue 62 } 63 // Complete first store. 64 StorePointer(&vp.data, xp.data) 65 StorePointer(&vp.typ, xp.typ) 66 runtime_procUnpin() 67 return 68 } 69 if uintptr(typ) == ^uintptr(0) { 70 // First store in progress. Wait. 71 // Since we disable preemption around the first store, 72 // we can wait with active spinning. 73 continue 74 } 75 // First store completed. Check type and overwrite data. 76 if typ != xp.typ { 77 panic("sync/atomic: store of inconsistently typed value into Value") 78 } 79 StorePointer(&vp.data, xp.data) 80 return 81 } 82 } 83 84 // Disable/enable preemption, implemented in runtime. 85 func runtime_procPin() 86 func runtime_procUnpin() 87
