预习内容：unsafe atomic
整体的逻辑的一个简单解释：
1.读写分离，其实内部也是加锁，使用分离的方式细化锁（只对写加锁）的作用范围，提升效率。
读：采用原子操作形式，保留一份数据；
写：也保留一份数据，大于等于读的数据，在一定条件下同步到读的数据，详细看代码实现。
数据指向：采用unsafe的指针稍微节省空间
这是从网上找的图片，描述了大概的流程
image.png
这里就是在源码上加了自己的注释，自己的注释以 @recode：开头
只要看Load和Store两个方法即可，其他的都是i嵌套的方法。
// Copyright 2016 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 sync
import (
"sync/atomic"
"unsafe"
)
// Map is like a Go map[interface{}]interface{} but is safe for concurrent use
// by multiple goroutines without additional locking or coordination.
// Loads, stores, and deletes run in amortized constant time.
//
// The Map type is specialized. Most code should use a plain Go map instead,
// with separate locking or coordination, for better type safety and to make it
// easier to maintain other invariants along with the map content.
//
// The Map type is optimized for two common use cases: (1) when the entry for a given
// key is only ever written once but read many times, as in caches that only grow,
// or (2) when multiple goroutines read, write, and overwrite entries for disjoint
// sets of keys. In these two cases, use of a Map may significantly reduce lock
// contention compared to a Go map paired with a separate Mutex or RWMutex.
//
// The zero Map is empty and ready for use. A Map must not be copied after first use.
type Map struct {
mu Mutex // @recode:给写操作准备的锁
// read contains the portion of the map's contents that are safe for
// concurrent access (with or without mu held).
//
// The read field itself is always safe to load, but must only be stored with
// mu held.
//
// Entries stored in read may be updated concurrently without mu, but updating
// a previously-expunged entry requires that the entry be copied to the dirty
// map and unexpunged with mu held.
read atomic.Value // readOnly @recode:原子操作保证读的准确
// dirty contains the portion of the map's contents that require mu to be
// held. To ensure that the dirty map can be promoted to the read map quickly,
// it also includes all of the non-expunged entries in the read map.
//
// Expunged entries are not stored in the dirty map. An expunged entry in the
// clean map must be unexpunged and added to the dirty map before a new value
// can be stored to it.
//
// If the dirty map is nil, the next write to the map will initialize it by
// making a shallow copy of the clean map, omitting stale entries.
dirty map[interface{}]*entry // @recode:直接反馈的意思是脏数据，是写操作的入口，符合一定条件后，同步到read
// misses counts the number of loads since the read map was last updated that
// needed to lock mu to determine whether the key was present.
//
// Once enough misses have occurred to cover the cost of copying the dirty
// map, the dirty map will be promoted to the read map (in the unamended
// state) and the next store to the map will make a new dirty copy.
misses int // @recode:意思是记录read读的时候找不到对应值的次数，达到脏数据的长度的次数后，归零，并把脏数据（写入的数据）同步到read中
}
// readOnly is an immutable struct stored atomically in the Map.read field.
type readOnly struct {
m map[interface{}]*entry // 保存读的总数据
amended bool // true if the dirty map contains some key not in m.
// @recode:amended表示脏数据中的集合和这里的不同（可能有增加和删除的操作，还没有同步到read中）
}
// expunged is an arbitrary pointer that marks entries which have been deleted
// from the dirty map.
// @recode:表示这个内容已经被删除，就是key对应的内容已经没了，和没有这个key稍微不同，但是类似
var expunged = unsafe.Pointer(new(interface{}))
// An entry is a slot in the map corresponding to a particular key.// @recode:这个是存储的实际的内容值，内部用指针的形式存，可以节省空间
type entry struct { // @recode:保存数据的结构，使用unsafe.Pointer结构指针保存
// p points to the interface{} value stored for the entry.
//
// If p == nil, the entry has been deleted and m.dirty == nil.
//
// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
// is missing from m.dirty.
//
// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
// != nil, in m.dirty[key].
//
// An entry can be deleted by atomic replacement with nil: when m.dirty is
// next created, it will atomically replace nil with expunged and leave
// m.dirty[key] unset.
//
// An entry's associated value can be updated by atomic replacement, provided
// p != expunged. If p == expunged, an entry's associated value can be updated
// only after first setting m.dirty[key] = e so that lookups using the dirty
// map find the entry.
p unsafe.Pointer // *interface{}
}
func newEntry(i interface{}) *entry {
return &entry{p: unsafe.Pointer(&i)}
}
// Load returns the value stored in the map for a key, or nil if no
// value is present.
// The ok result indicates whether value was found in the map.
func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
read, _ := m.read.Load().(readOnly)
e, ok := read.m[key]// @recode：这里注意了，这个e要在下面赋值
if !ok && read.amended { // @recode:read中没有，并且read和脏（写）数据中的数据不同，说明在脏（写）数据部分中可能有
m.mu.Lock() // @recode:但凡有涉及到写的相关逻辑，都要加锁
// Avoid reporting a spurious miss if m.dirty got promoted while we were
// blocked on m.mu. (If further loads of the same key will not miss, it's
// not worth copying the dirty map for this key.)
read, _ = m.read.Load().(readOnly)
e, ok = read.m[key]// @recode：对e的操作，再次从read中获取并赋值，因为可能异步的情况，加锁之前可能有该值的内容，所以需要再次获取验证
if !ok && read.amended {// @recode：如果read中没有，取脏（写）数据中获取
e, ok = m.dirty[key]// @recode：对e的操作赋值
// Regardless of whether the entry was present, record a miss: this key
// will take the slow path until the dirty map is promoted to the read
// map.
m.missLocked()
}
m.mu.Unlock()
}
if !ok {// @recode:read中没有，并且和写、脏（写）数据部分一样，说明没该数据
return nil, false
}
return e.load()
}
func (e *entry) load() (value interface{}, ok bool) {
p := atomic.LoadPointer(&e.p)
if p == nil || p == expunged {
return nil, false
}
return *(*interface{})(p), true
}
// Store sets the value for a key.
func (m *Map) Store(key, value interface{}) {
read, _ := m.read.Load().(readOnly)
if e, ok := read.m[key]; ok && e.tryStore(&value) { // @recode:在read中取到这个值，说明本来就有对应key，只是替换操作，tryStore中交换
return
}
// @recode:没取到就说明是新key存储，需要上锁
m.mu.Lock()
read, _ = m.read.Load().(readOnly) // @recode:double check，因为可能是异步操作，上锁后需要重新验证
if e, ok := read.m[key]; ok { // @recode:找到该值
if e.unexpungeLocked() { // @recode: 判断内容是否为空，如果该值内容为空，内容为空可把这次的对象放进去
// The entry was previously expunged, which implies that there is a
// non-nil dirty map and this entry is not in it.
m.dirty[key] = e
}
e.storeLocked(&value) // @recode:把这次的值放入对应key下面
} else if e, ok := m.dirty[key]; ok { // @recode:如果直接在dirty中找到了，就和直接替换内容
e.storeLocked(&value)
} else { // @recode:进入这里说明是新的key
if !read.amended { // @recode:如果dirty中不比read中的多，就是说没有额外写入的情况或者删除的情况，就进入
// We're adding the first new key to the dirty map.
// Make sure it is allocated and mark the read-only map as incomplete.
m.dirtyLocked()
m.read.Store(readOnly{m: read.m, amended: true})
}
m.dirty[key] = newEntry(value) // @recode:保存一份新数据
}
m.mu.Unlock()
}
// tryStore stores a value if the entry has not been expunged.
// @recode:这个方法就是内容不为空的替换操作而已
// If the entry is expunged, tryStore returns false and leaves the entry
// unchanged.
func (e *entry) tryStore(i *interface{}) bool {
for {
p := atomic.LoadPointer(&e.p) // @recode:如果p为空，就反馈false
if p == expunged {
return false
}
if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) { // @recode:p不为空，就把新的值交换进去
return true
}
}
}
// unexpungeLocked ensures that the entry is not marked as expunged.
//
// If the entry was previously expunged, it must be added to the dirty map
// before m.mu is unlocked.
func (e *entry) unexpungeLocked() (wasExpunged bool) {
return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
}
// storeLocked unconditionally stores a value to the entry.// @recode:把值存到entry这个对象当中
//
// The entry must be known not to be expunged.
func (e *entry) storeLocked(i *interface{}) {
atomic.StorePointer(&e.p, unsafe.Pointer(i))
}
// LoadOrStore returns the existing value for the key if present.
// Otherwise, it stores and returns the given value.
// The loaded result is true if the value was loaded, false if stored.
func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
// Avoid locking if it's a clean hit.
read, _ := m.read.Load().(readOnly)
if e, ok := read.m[key]; ok {
actual, loaded, ok := e.tryLoadOrStore(value)
if ok {
return actual, loaded
}
}
m.mu.Lock()
read, _ = m.read.Load().(readOnly)
if e, ok := read.m[key]; ok {
if e.unexpungeLocked() {
m.dirty[key] = e
}
actual, loaded, _ = e.tryLoadOrStore(value)
} else if e, ok := m.dirty[key]; ok {
actual, loaded, _ = e.tryLoadOrStore(value)
m.missLocked()
} else {
if !read.amended {
// We're adding the first new key to the dirty map.
// Make sure it is allocated and mark the read-only map as incomplete.
m.dirtyLocked()
m.read.Store(readOnly{m: read.m, amended: true})
}
m.dirty[key] = newEntry(value)
actual, loaded = value, false
}
m.mu.Unlock()
return actual, loaded
}
// tryLoadOrStore atomically loads or stores a value if the entry is not
// expunged.
//
// If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
// returns with ok==false.
func (e *entry) tryLoadOrStore(i interface{}) (actual interface{}, loaded, ok bool) {
p := atomic.LoadPointer(&e.p)
if p == expunged {
return nil, false, false
}
if p != nil {
return *(*interface{})(p), true, true
}
// Copy the interface after the first load to make this method more amenable
// to escape analysis: if we hit the "load" path or the entry is expunged, we
// shouldn't bother heap-allocating.
ic := i
for {
if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {
return i, false, true
}
p = atomic.LoadPointer(&e.p)
if p == expunged {
return nil, false, false
}
if p != nil {
return *(*interface{})(p), true, true
}
}
}
// LoadAndDelete deletes the value for a key, returning the previous value if any.
// The loaded result reports whether the key was present.
func (m *Map) LoadAndDelete(key interface{}) (value interface{}, loaded bool) {
read, _ := m.read.Load().(readOnly)
e, ok := read.m[key]
if !ok && read.amended {
m.mu.Lock()
read, _ = m.read.Load().(readOnly)
e, ok = read.m[key]
if !ok && read.amended {
e, ok = m.dirty[key]
delete(m.dirty, key)
// Regardless of whether the entry was present, record a miss: this key
// will take the slow path until the dirty map is promoted to the read
// map.
m.missLocked()
}
m.mu.Unlock()
}
if ok {
return e.delete()
}
return nil, false
}
// Delete deletes the value for a key.
func (m *Map) Delete(key interface{}) {
m.LoadAndDelete(key)
}
func (e *entry) delete() (value interface{}, ok bool) {
for {
p := atomic.LoadPointer(&e.p)
if p == nil || p == expunged {
return nil, false
}
if atomic.CompareAndSwapPointer(&e.p, p, nil) {
return *(*interface{})(p), true
}
}
}
// Range calls f sequentially for each key and value present in the map.
// If f returns false, range stops the iteration.
//
// Range does not necessarily correspond to any consistent snapshot of the Map's
// contents: no key will be visited more than once, but if the value for any key
// is stored or deleted concurrently, Range may reflect any mapping for that key
// from any point during the Range call.
//
// Range may be O(N) with the number of elements in the map even if f returns
// false after a constant number of calls.
func (m *Map) Range(f func(key, value interface{}) bool) {
// We need to be able to iterate over all of the keys that were already
// present at the start of the call to Range.
// If read.amended is false, then read.m satisfies that property without
// requiring us to hold m.mu for a long time.
read, _ := m.read.Load().(readOnly)
if read.amended {
// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
// (assuming the caller does not break out early), so a call to Range
// amortizes an entire copy of the map: we can promote the dirty copy
// immediately!
m.mu.Lock()
read, _ = m.read.Load().(readOnly)
if read.amended {
read = readOnly{m: m.dirty}
m.read.Store(read)
m.dirty = nil
m.misses = 0
}
m.mu.Unlock()
}
for k, e := range read.m {
v, ok := e.load()
if !ok {
continue
}
if !f(k, v) {
break
}
}
}
// @recode：符合一定条件后，将脏（写）数据提升到read中（就是将写的数据同步到read中），并且清空脏数据对象和misses
// @recode：提升的条件就是第二行，查找的次数和脏数据的条数一样多，比如脏数据中有二十条数据，如果查询了二十次没有该key的值，那就会触发脏数据到read的同步
func (m *Map) missLocked() {
m.misses++
if m.misses < len(m.dirty) {
return
}
m.read.Store(readOnly{m: m.dirty})
m.dirty = nil
m.misses = 0
}
// @recode：当脏（写）数据部分map为空的时候，就把read中的数据复制到写入部分中，达到读写的部分一致
func (m *Map) dirtyLocked() {
if m.dirty != nil {
return
}
read, _ := m.read.Load().(readOnly)
m.dirty = make(map[interface{}]*entry, len(read.m))
for k, e := range read.m {
if !e.tryExpungeLocked() {
m.dirty[k] = e
}
}
}
func (e *entry) tryExpungeLocked() (isExpunged bool) {
p := atomic.LoadPointer(&e.p)
for p == nil {
if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
return true
}
p = atomic.LoadPointer(&e.p)
}
return p == expunged
}