syncmap/pre_go19.go - sync - Git at Google

 // 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.

 //go:build !go1.9
 // +build !go1.9

 package syncmap

 import (
 	"sync"
 	"sync/atomic"
 	"unsafe"
 )

 // Map is a concurrent map with amortized-constant-time loads, stores, and deletes.
 // It is safe for multiple goroutines to call a Map's methods concurrently.
 //
 // The zero Map is valid and empty.
 //
 // A Map must not be copied after first use.
 type Map struct {
 	mu sync.Mutex

 	// 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

 	// 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

 	// 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
 }

 // readOnly is an immutable struct stored atomically in the Map.read field.
 type readOnly struct {
 	m       map[interface{}]*entry
 	amended bool // true if the dirty map contains some key not in m.
 }

 // expunged is an arbitrary pointer that marks entries which have been deleted
 // from the dirty map.
 var expunged = unsafe.Pointer(new(interface{}))

 // An entry is a slot in the map corresponding to a particular key.
 type entry struct {
 	// 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]
 	if !ok && read.amended {
 		m.mu.Lock()
 		// 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]
 		if !ok && read.amended {
 			e, ok = 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 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) {
 		return
 	}

 	m.mu.Lock()
 	read, _ = m.read.Load().(readOnly)
 	if e, ok := read.m[key]; ok {
 		if e.unexpungeLocked() {
 			// 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)
 	} else if e, ok := m.dirty[key]; ok {
 		e.storeLocked(&value)
 	} 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)
 	}
 	m.mu.Unlock()
 }

 // tryStore stores a value if the entry has not been expunged.
 //
 // If the entry is expunged, tryStore returns false and leaves the entry
 // unchanged.
 func (e *entry) tryStore(i *interface{}) bool {
 	p := atomic.LoadPointer(&e.p)
 	if p == expunged {
 		return false
 	}
 	for {
 		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
 			return true
 		}
 		p = atomic.LoadPointer(&e.p)
 		if p == expunged {
 			return false
 		}
 	}
 }

 // 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.
 //
 // 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
 		}
 	}
 }

 // Delete deletes the value for a key.
 func (m *Map) Delete(key interface{}) {
 	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 {
 			delete(m.dirty, key)
 		}
 		m.mu.Unlock()
 	}
 	if ok {
 		e.delete()
 	}
 }

 func (e *entry) delete() (hadValue bool) {
 	for {
 		p := atomic.LoadPointer(&e.p)
 		if p == nil || p == expunged {
 			return false
 		}
 		if atomic.CompareAndSwapPointer(&e.p, p, nil) {
 			return 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
 		}
 	}
 }

 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
 }

 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
 }
	// 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.

	//go:build !go1.9
	// +build !go1.9

	package syncmap

	import (
	"sync"
	"sync/atomic"
	"unsafe"
	)

	// Map is a concurrent map with amortized-constant-time loads, stores, and deletes.
	// It is safe for multiple goroutines to call a Map's methods concurrently.
	//
	// The zero Map is valid and empty.
	//
	// A Map must not be copied after first use.
	type Map struct {
	mu sync.Mutex

	// 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

	// 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

	// 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
	}

	// readOnly is an immutable struct stored atomically in the Map.read field.
	type readOnly struct {
	m map[interface{}]*entry
	amended bool // true if the dirty map contains some key not in m.
	}

	// expunged is an arbitrary pointer that marks entries which have been deleted
	// from the dirty map.
	var expunged = unsafe.Pointer(new(interface{}))

	// An entry is a slot in the map corresponding to a particular key.
	type entry struct {
	// 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]
	if !ok && read.amended {
	m.mu.Lock()
	// 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]
	if !ok && read.amended {
	e, ok = 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 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) {
	return
	}

	m.mu.Lock()
	read, _ = m.read.Load().(readOnly)
	if e, ok := read.m[key]; ok {
	if e.unexpungeLocked() {
	// 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)
	} else if e, ok := m.dirty[key]; ok {
	e.storeLocked(&value)
	} 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)
	}
	m.mu.Unlock()
	}

	// tryStore stores a value if the entry has not been expunged.
	//
	// If the entry is expunged, tryStore returns false and leaves the entry
	// unchanged.
	func (e entry) tryStore(i interface{}) bool {
	p := atomic.LoadPointer(&e.p)
	if p == expunged {
	return false
	}
	for {
	if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
	return true
	}
	p = atomic.LoadPointer(&e.p)
	if p == expunged {
	return false
	}
	}
	}

	// 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.
	//
	// 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
	}
	}
	}

	// Delete deletes the value for a key.
	func (m *Map) Delete(key interface{}) {
	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 {
	delete(m.dirty, key)
	}
	m.mu.Unlock()
	}
	if ok {
	e.delete()
	}
	}

	func (e *entry) delete() (hadValue bool) {
	for {
	p := atomic.LoadPointer(&e.p)
	if p == nil \|\| p == expunged {
	return false
	}
	if atomic.CompareAndSwapPointer(&e.p, p, nil) {
	return 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
	}
	}
	}

	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
	}

	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
	}