github.com/ltltlt/go-source-code@v0.0.0-20190830023027-95be009773aa/sync/atomic/value.go (about)

     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)) // 将v解释为*ifaceWords(Value和ifaceWords大小相同)
    30  	typ := LoadPointer(&vp.typ)
    31  	if typ == nil || uintptr(typ) == ^uintptr(0) { // 全1代表正在进行第一次存储
    32  		// First store not yet completed.
    33  		return nil
    34  	}
    35  	data := LoadPointer(&vp.data)
    36  	xp := (*ifaceWords)(unsafe.Pointer(&x)) // 将x解释为interface{}
    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  // 一个Value中存的内容必须是相同类型的, 实际上也能实现为可存放不同类型的
    46  // 唯一竞态是两个goroutine同时Store, 可能导致类型和数据不匹配, 通过引入type的第三种状态来解决
    47  func (v *Value) Store(x interface{}) {
    48  	if x == nil {
    49  		panic("sync/atomic: store of nil value into Value")
    50  	}
    51  	vp := (*ifaceWords)(unsafe.Pointer(v))
    52  	xp := (*ifaceWords)(unsafe.Pointer(&x))
    53  	for {
    54  		typ := LoadPointer(&vp.typ)
    55  		if typ == nil {
    56  			// <- 可能在此typ被修改，所以使用CompareAndSwapPointer
    57  			// Attempt to start first store.
    58  			// Disable preemption so that other goroutines can use
    59  			// active spin wait to wait for completion; and so that
    60  			// GC does not see the fake type accidentally.
    61  			runtime_procPin()
    62  			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
    63  				runtime_procUnpin()
    64  				continue
    65  			}
    66  			// Complete first store.
    67  			StorePointer(&vp.data, xp.data)
    68  			StorePointer(&vp.typ, xp.typ)
    69  			runtime_procUnpin()
    70  			return
    71  		}
    72  		if uintptr(typ) == ^uintptr(0) {
    73  			// First store in progress. Wait.
    74  			// Since we disable preemption around the first store,
    75  			// we can wait with active spinning.
    76  			continue
    77  		}
    78  		// First store completed. Check type and overwrite data.
    79  		if typ != xp.typ {
    80  			panic("sync/atomic: store of inconsistently typed value into Value")
    81  		}
    82  		StorePointer(&vp.data, xp.data)
    83  		return
    84  	}
    85  }
    86  
    87  // Disable/enable preemption, implemented in runtime.
    88  func runtime_procPin()
    89  func runtime_procUnpin()