diff options
Diffstat (limited to 'src/sync/map.go')
| -rw-r--r-- | src/sync/map.go | 515 |
1 files changed, 515 insertions, 0 deletions
diff --git a/src/sync/map.go b/src/sync/map.go new file mode 100644 index 0000000..e8ccf58 --- /dev/null +++ b/src/sync/map.go @@ -0,0 +1,515 @@ +// 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" +) + +// 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. +// +// In the terminology of the Go memory model, Map arranges that a write operation +// “synchronizes before” any read operation that observes the effect of the write, where +// read and write operations are defined as follows. +// Load, LoadAndDelete, LoadOrStore, Swap, CompareAndSwap, and CompareAndDelete +// are read operations; Delete, LoadAndDelete, Store, and Swap are write operations; +// LoadOrStore is a write operation when it returns loaded set to false; +// CompareAndSwap is a write operation when it returns swapped set to true; +// and CompareAndDelete is a write operation when it returns deleted set to true. +type Map struct { + mu 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.Pointer[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[any]*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[any]*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 = new(any) + +// 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 either m.dirty == nil or + // m.dirty[key] is e. + // + // 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 atomic.Pointer[any] +} + +func newEntry(i any) *entry { + e := &entry{} + e.p.Store(&i) + return e +} + +func (m *Map) loadReadOnly() readOnly { + if p := m.read.Load(); p != nil { + return *p + } + return readOnly{} +} + +// 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 any) (value any, ok bool) { + read := m.loadReadOnly() + 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.loadReadOnly() + 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 any, ok bool) { + p := e.p.Load() + if p == nil || p == expunged { + return nil, false + } + return *p, true +} + +// Store sets the value for a key. +func (m *Map) Store(key, value any) { + _, _ = m.Swap(key, value) +} + +// tryCompareAndSwap compare the entry with the given old value and swaps +// it with a new value if the entry is equal to the old value, and the entry +// has not been expunged. +// +// If the entry is expunged, tryCompareAndSwap returns false and leaves +// the entry unchanged. +func (e *entry) tryCompareAndSwap(old, new any) bool { + p := e.p.Load() + if p == nil || p == expunged || *p != old { + return false + } + + // Copy the interface after the first load to make this method more amenable + // to escape analysis: if the comparison fails from the start, we shouldn't + // bother heap-allocating an interface value to store. + nc := new + for { + if e.p.CompareAndSwap(p, &nc) { + return true + } + p = e.p.Load() + if p == nil || p == expunged || *p != old { + 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 e.p.CompareAndSwap(expunged, nil) +} + +// swapLocked unconditionally swaps a value into the entry. +// +// The entry must be known not to be expunged. +func (e *entry) swapLocked(i *any) *any { + return e.p.Swap(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 any) (actual any, loaded bool) { + // Avoid locking if it's a clean hit. + read := m.loadReadOnly() + if e, ok := read.m[key]; ok { + actual, loaded, ok := e.tryLoadOrStore(value) + if ok { + return actual, loaded + } + } + + m.mu.Lock() + read = m.loadReadOnly() + 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 any) (actual any, loaded, ok bool) { + p := e.p.Load() + if p == expunged { + return nil, false, false + } + if p != nil { + return *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 e.p.CompareAndSwap(nil, &ic) { + return i, false, true + } + p = e.p.Load() + if p == expunged { + return nil, false, false + } + if p != nil { + return *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 any) (value any, loaded bool) { + read := m.loadReadOnly() + e, ok := read.m[key] + if !ok && read.amended { + m.mu.Lock() + read = m.loadReadOnly() + 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 any) { + m.LoadAndDelete(key) +} + +func (e *entry) delete() (value any, ok bool) { + for { + p := e.p.Load() + if p == nil || p == expunged { + return nil, false + } + if e.p.CompareAndSwap(p, nil) { + return *p, true + } + } +} + +// trySwap swaps a value if the entry has not been expunged. +// +// If the entry is expunged, trySwap returns false and leaves the entry +// unchanged. +func (e *entry) trySwap(i *any) (*any, bool) { + for { + p := e.p.Load() + if p == expunged { + return nil, false + } + if e.p.CompareAndSwap(p, i) { + return p, true + } + } +} + +// Swap swaps the value for a key and returns the previous value if any. +// The loaded result reports whether the key was present. +func (m *Map) Swap(key, value any) (previous any, loaded bool) { + read := m.loadReadOnly() + if e, ok := read.m[key]; ok { + if v, ok := e.trySwap(&value); ok { + if v == nil { + return nil, false + } + return *v, true + } + } + + m.mu.Lock() + read = m.loadReadOnly() + 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 + } + if v := e.swapLocked(&value); v != nil { + loaded = true + previous = *v + } + } else if e, ok := m.dirty[key]; ok { + if v := e.swapLocked(&value); v != nil { + loaded = true + previous = *v + } + } 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() + return previous, loaded +} + +// CompareAndSwap swaps the old and new values for key +// if the value stored in the map is equal to old. +// The old value must be of a comparable type. +func (m *Map) CompareAndSwap(key, old, new any) bool { + read := m.loadReadOnly() + if e, ok := read.m[key]; ok { + return e.tryCompareAndSwap(old, new) + } else if !read.amended { + return false // No existing value for key. + } + + m.mu.Lock() + defer m.mu.Unlock() + read = m.loadReadOnly() + swapped := false + if e, ok := read.m[key]; ok { + swapped = e.tryCompareAndSwap(old, new) + } else if e, ok := m.dirty[key]; ok { + swapped = e.tryCompareAndSwap(old, new) + // We needed to lock mu in order to load the entry for key, + // and the operation didn't change the set of keys in the map + // (so it would be made more efficient by promoting the dirty + // map to read-only). + // Count it as a miss so that we will eventually switch to the + // more efficient steady state. + m.missLocked() + } + return swapped +} + +// CompareAndDelete deletes the entry for key if its value is equal to old. +// The old value must be of a comparable type. +// +// If there is no current value for key in the map, CompareAndDelete +// returns false (even if the old value is the nil interface value). +func (m *Map) CompareAndDelete(key, old any) (deleted bool) { + read := m.loadReadOnly() + e, ok := read.m[key] + if !ok && read.amended { + m.mu.Lock() + read = m.loadReadOnly() + e, ok = read.m[key] + if !ok && read.amended { + e, ok = m.dirty[key] + // Don't delete key from m.dirty: we still need to do the “compare” part + // of the operation. The entry will eventually be expunged when the + // dirty map is promoted to the read map. + // + // 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() + } + for ok { + p := e.p.Load() + if p == nil || p == expunged || *p != old { + return false + } + if e.p.CompareAndSwap(p, nil) { + return true + } + } + return false +} + +// 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 (including by f), Range may reflect any +// mapping for that key from any point during the Range call. Range does not +// block other methods on the receiver; even f itself may call any method on m. +// +// 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 any) 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.loadReadOnly() + 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.loadReadOnly() + 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.loadReadOnly() + m.dirty = make(map[any]*entry, len(read.m)) + for k, e := range read.m { + if !e.tryExpungeLocked() { + m.dirty[k] = e + } + } +} + +func (e *entry) tryExpungeLocked() (isExpunged bool) { + p := e.p.Load() + for p == nil { + if e.p.CompareAndSwap(nil, expunged) { + return true + } + p = e.p.Load() + } + return p == expunged +} |