arikawa/internal/moreatomic/syncmod/syncmod.go

// 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 syncmod contains a clone of package sync's map.go file with unused
// methods removed and some tweaks with LoadOrStore.
package syncmod

import (
	"sync"
	"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 {
	New func() interface{}

	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
}

// 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)
}

// 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(k interface{}) (actual interface{}, loaded bool) {
	// Avoid locking if it's a clean hit.
	read, _ := m.read.Load().(readOnly)
	if e, ok := read.m[k]; ok {
		actual, loaded, ok = e.tryLoadOrStore(nil, m.New)
		if ok {
			return actual, loaded
		}
	}

	m.mu.Lock()
	read, _ = m.read.Load().(readOnly)
	if e, ok := read.m[k]; ok {
		if e.unexpungeLocked() {
			m.dirty[k] = e
		}
		actual, loaded, _ = e.tryLoadOrStore(actual, m.New)
	} else if e, ok := m.dirty[k]; ok {
		actual, loaded, _ = e.tryLoadOrStore(actual, m.New)
		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})
		}
		// This will likely allocate if the first tryLoadOrStore sees an
		// expunged value and this else branch is hit.
		if actual == nil {
			actual = m.New()
		}
		m.dirty[k] = newEntry(actual)
		loaded = 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{}, newFn func() 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
	}

	if i == nil {
		i = newFn()
	}

	// 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 i, false, false
		}
		if p != nil {
			return *(*interface{})(p), true, true
		}
	}
}

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
}