libgo/go/sync/map.go

   1 // Copyright 2016 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 sync
   6
   7 import (
   8         "sync/atomic"
   9         "unsafe"
  10 )
  11
  12 // Map is like a Go map[interface{}]interface{} but is safe for concurrent use
  13 // by multiple goroutines without additional locking or coordination.
  14 // Loads, stores, and deletes run in amortized constant time.
  15 //
  16 // The Map type is specialized. Most code should use a plain Go map instead,
  17 // with separate locking or coordination, for better type safety and to make it
  18 // easier to maintain other invariants along with the map content.
  19 //
  20 // The Map type is optimized for two common use cases: (1) when the entry for a given
  21 // key is only ever written once but read many times, as in caches that only grow,
  22 // or (2) when multiple goroutines read, write, and overwrite entries for disjoint
  23 // sets of keys. In these two cases, use of a Map may significantly reduce lock
  24 // contention compared to a Go map paired with a separate Mutex or RWMutex.
  25 //
  26 // The zero Map is empty and ready for use. A Map must not be copied after first use.
  27 type Map struct {
  28         mu Mutex
  29
  30         // read contains the portion of the map's contents that are safe for
  31         // concurrent access (with or without mu held).
  32         //
  33         // The read field itself is always safe to load, but must only be stored with
  34         // mu held.
  35         //
  36         // Entries stored in read may be updated concurrently without mu, but updating
  37         // a previously-expunged entry requires that the entry be copied to the dirty
  38         // map and unexpunged with mu held.
  39         read atomic.Value // readOnly
  40
  41         // dirty contains the portion of the map's contents that require mu to be
  42         // held. To ensure that the dirty map can be promoted to the read map quickly,
  43         // it also includes all of the non-expunged entries in the read map.
  44         //
  45         // Expunged entries are not stored in the dirty map. An expunged entry in the
  46         // clean map must be unexpunged and added to the dirty map before a new value
  47         // can be stored to it.
  48         //
  49         // If the dirty map is nil, the next write to the map will initialize it by
  50         // making a shallow copy of the clean map, omitting stale entries.
  51         dirty map[any]*entry
  52
  53         // misses counts the number of loads since the read map was last updated that
  54         // needed to lock mu to determine whether the key was present.
  55         //
  56         // Once enough misses have occurred to cover the cost of copying the dirty
  57         // map, the dirty map will be promoted to the read map (in the unamended
  58         // state) and the next store to the map will make a new dirty copy.
  59         misses int
  60 }
  61
  62 // readOnly is an immutable struct stored atomically in the Map.read field.
  63 type readOnly struct {
  64         m       map[any]*entry
  65         amended bool // true if the dirty map contains some key not in m.
  66 }
  67
  68 // expunged is an arbitrary pointer that marks entries which have been deleted
  69 // from the dirty map.
  70 var expunged = unsafe.Pointer(new(any))
  71
  72 // An entry is a slot in the map corresponding to a particular key.
  73 type entry struct {
  74         // p points to the interface{} value stored for the entry.
  75         //
  76         // If p == nil, the entry has been deleted, and either m.dirty == nil or
  77         // m.dirty[key] is e.
  78         //
  79         // If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
  80         // is missing from m.dirty.
  81         //
  82         // Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
  83         // != nil, in m.dirty[key].
  84         //
  85         // An entry can be deleted by atomic replacement with nil: when m.dirty is
  86         // next created, it will atomically replace nil with expunged and leave
  87         // m.dirty[key] unset.
  88         //
  89         // An entry's associated value can be updated by atomic replacement, provided
  90         // p != expunged. If p == expunged, an entry's associated value can be updated
  91         // only after first setting m.dirty[key] = e so that lookups using the dirty
  92         // map find the entry.
  93         p unsafe.Pointer // *interface{}
  94 }
  95
  96 func newEntry(i any) *entry {
  97         return &entry{p: unsafe.Pointer(&i)}
  98 }
  99
 100 // Load returns the value stored in the map for a key, or nil if no
 101 // value is present.
 102 // The ok result indicates whether value was found in the map.
 103 func (m *Map) Load(key any) (value any, ok bool) {
 104         read, _ := m.read.Load().(readOnly)
 105         e, ok := read.m[key]
 106         if !ok && read.amended {
 107                 m.mu.Lock()
 108                 // Avoid reporting a spurious miss if m.dirty got promoted while we were
 109                 // blocked on m.mu. (If further loads of the same key will not miss, it's
 110                 // not worth copying the dirty map for this key.)
 111                 read, _ = m.read.Load().(readOnly)
 112                 e, ok = read.m[key]
 113                 if !ok && read.amended {
 114                         e, ok = m.dirty[key]
 115                         // Regardless of whether the entry was present, record a miss: this key
 116                         // will take the slow path until the dirty map is promoted to the read
 117                         // map.
 118                         m.missLocked()
 119                 }
 120                 m.mu.Unlock()
 121         }
 122         if !ok {
 123                 return nil, false
 124         }
 125         return e.load()
 126 }
 127
 128 func (e *entry) load() (value any, ok bool) {
 129         p := atomic.LoadPointer(&e.p)
 130         if p == nil || p == expunged {
 131                 return nil, false
 132         }
 133         return *(*any)(p), true
 134 }
 135
 136 // Store sets the value for a key.
 137 func (m *Map) Store(key, value any) {
 138         read, _ := m.read.Load().(readOnly)
 139         if e, ok := read.m[key]; ok && e.tryStore(&value) {
 140                 return
 141         }
 142
 143         m.mu.Lock()
 144         read, _ = m.read.Load().(readOnly)
 145         if e, ok := read.m[key]; ok {
 146                 if e.unexpungeLocked() {
 147                         // The entry was previously expunged, which implies that there is a
 148                         // non-nil dirty map and this entry is not in it.
 149                         m.dirty[key] = e
 150                 }
 151                 e.storeLocked(&value)
 152         } else if e, ok := m.dirty[key]; ok {
 153                 e.storeLocked(&value)
 154         } else {
 155                 if !read.amended {
 156                         // We're adding the first new key to the dirty map.
 157                         // Make sure it is allocated and mark the read-only map as incomplete.
 158                         m.dirtyLocked()
 159                         m.read.Store(readOnly{m: read.m, amended: true})
 160                 }
 161                 m.dirty[key] = newEntry(value)
 162         }
 163         m.mu.Unlock()
 164 }
 165
 166 // tryStore stores a value if the entry has not been expunged.
 167 //
 168 // If the entry is expunged, tryStore returns false and leaves the entry
 169 // unchanged.
 170 func (e *entry) tryStore(i *any) bool {
 171         for {
 172                 p := atomic.LoadPointer(&e.p)
 173                 if p == expunged {
 174                         return false
 175                 }
 176                 if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
 177                         return true
 178                 }
 179         }
 180 }
 181
 182 // unexpungeLocked ensures that the entry is not marked as expunged.
 183 //
 184 // If the entry was previously expunged, it must be added to the dirty map
 185 // before m.mu is unlocked.
 186 func (e *entry) unexpungeLocked() (wasExpunged bool) {
 187         return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
 188 }
 189
 190 // storeLocked unconditionally stores a value to the entry.
 191 //
 192 // The entry must be known not to be expunged.
 193 func (e *entry) storeLocked(i *any) {
 194         atomic.StorePointer(&e.p, unsafe.Pointer(i))
 195 }
 196
 197 // LoadOrStore returns the existing value for the key if present.
 198 // Otherwise, it stores and returns the given value.
 199 // The loaded result is true if the value was loaded, false if stored.
 200 func (m *Map) LoadOrStore(key, value any) (actual any, loaded bool) {
 201         // Avoid locking if it's a clean hit.
 202         read, _ := m.read.Load().(readOnly)
 203         if e, ok := read.m[key]; ok {
 204                 actual, loaded, ok := e.tryLoadOrStore(value)
 205                 if ok {
 206                         return actual, loaded
 207                 }
 208         }
 209
 210         m.mu.Lock()
 211         read, _ = m.read.Load().(readOnly)
 212         if e, ok := read.m[key]; ok {
 213                 if e.unexpungeLocked() {
 214                         m.dirty[key] = e
 215                 }
 216                 actual, loaded, _ = e.tryLoadOrStore(value)
 217         } else if e, ok := m.dirty[key]; ok {
 218                 actual, loaded, _ = e.tryLoadOrStore(value)
 219                 m.missLocked()
 220         } else {
 221                 if !read.amended {
 222                         // We're adding the first new key to the dirty map.
 223                         // Make sure it is allocated and mark the read-only map as incomplete.
 224                         m.dirtyLocked()
 225                         m.read.Store(readOnly{m: read.m, amended: true})
 226                 }
 227                 m.dirty[key] = newEntry(value)
 228                 actual, loaded = value, false
 229         }
 230         m.mu.Unlock()
 231
 232         return actual, loaded
 233 }
 234
 235 // tryLoadOrStore atomically loads or stores a value if the entry is not
 236 // expunged.
 237 //
 238 // If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
 239 // returns with ok==false.
 240 func (e *entry) tryLoadOrStore(i any) (actual any, loaded, ok bool) {
 241         p := atomic.LoadPointer(&e.p)
 242         if p == expunged {
 243                 return nil, false, false
 244         }
 245         if p != nil {
 246                 return *(*any)(p), true, true
 247         }
 248
 249         // Copy the interface after the first load to make this method more amenable
 250         // to escape analysis: if we hit the "load" path or the entry is expunged, we
 251         // shouldn't bother heap-allocating.
 252         ic := i
 253         for {
 254                 if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {
 255                         return i, false, true
 256                 }
 257                 p = atomic.LoadPointer(&e.p)
 258                 if p == expunged {
 259                         return nil, false, false
 260                 }
 261                 if p != nil {
 262                         return *(*any)(p), true, true
 263                 }
 264         }
 265 }
 266
 267 // LoadAndDelete deletes the value for a key, returning the previous value if any.
 268 // The loaded result reports whether the key was present.
 269 func (m *Map) LoadAndDelete(key any) (value any, loaded bool) {
 270         read, _ := m.read.Load().(readOnly)
 271         e, ok := read.m[key]
 272         if !ok && read.amended {
 273                 m.mu.Lock()
 274                 read, _ = m.read.Load().(readOnly)
 275                 e, ok = read.m[key]
 276                 if !ok && read.amended {
 277                         e, ok = m.dirty[key]
 278                         delete(m.dirty, key)
 279                         // Regardless of whether the entry was present, record a miss: this key
 280                         // will take the slow path until the dirty map is promoted to the read
 281                         // map.
 282                         m.missLocked()
 283                 }
 284                 m.mu.Unlock()
 285         }
 286         if ok {
 287                 return e.delete()
 288         }
 289         return nil, false
 290 }
 291
 292 // Delete deletes the value for a key.
 293 func (m *Map) Delete(key any) {
 294         m.LoadAndDelete(key)
 295 }
 296
 297 func (e *entry) delete() (value any, ok bool) {
 298         for {
 299                 p := atomic.LoadPointer(&e.p)
 300                 if p == nil || p == expunged {
 301                         return nil, false
 302                 }
 303                 if atomic.CompareAndSwapPointer(&e.p, p, nil) {
 304                         return *(*any)(p), true
 305                 }
 306         }
 307 }
 308
 309 // Range calls f sequentially for each key and value present in the map.
 310 // If f returns false, range stops the iteration.
 311 //
 312 // Range does not necessarily correspond to any consistent snapshot of the Map's
 313 // contents: no key will be visited more than once, but if the value for any key
 314 // is stored or deleted concurrently (including by f), Range may reflect any
 315 // mapping for that key from any point during the Range call. Range does not
 316 // block other methods on the receiver; even f itself may call any method on m.
 317 //
 318 // Range may be O(N) with the number of elements in the map even if f returns
 319 // false after a constant number of calls.
 320 func (m *Map) Range(f func(key, value any) bool) {
 321         // We need to be able to iterate over all of the keys that were already
 322         // present at the start of the call to Range.
 323         // If read.amended is false, then read.m satisfies that property without
 324         // requiring us to hold m.mu for a long time.
 325         read, _ := m.read.Load().(readOnly)
 326         if read.amended {
 327                 // m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
 328                 // (assuming the caller does not break out early), so a call to Range
 329                 // amortizes an entire copy of the map: we can promote the dirty copy
 330                 // immediately!
 331                 m.mu.Lock()
 332                 read, _ = m.read.Load().(readOnly)
 333                 if read.amended {
 334                         read = readOnly{m: m.dirty}
 335                         m.read.Store(read)
 336                         m.dirty = nil
 337                         m.misses = 0
 338                 }
 339                 m.mu.Unlock()
 340         }
 341
 342         for k, e := range read.m {
 343                 v, ok := e.load()
 344                 if !ok {
 345                         continue
 346                 }
 347                 if !f(k, v) {
 348                         break
 349                 }
 350         }
 351 }
 352
 353 func (m *Map) missLocked() {
 354         m.misses++
 355         if m.misses < len(m.dirty) {
 356                 return
 357         }
 358         m.read.Store(readOnly{m: m.dirty})
 359         m.dirty = nil
 360         m.misses = 0
 361 }
 362
 363 func (m *Map) dirtyLocked() {
 364         if m.dirty != nil {
 365                 return
 366         }
 367
 368         read, _ := m.read.Load().(readOnly)
 369         m.dirty = make(map[any]*entry, len(read.m))
 370         for k, e := range read.m {
 371                 if !e.tryExpungeLocked() {
 372                         m.dirty[k] = e
 373                 }
 374         }
 375 }
 376
 377 func (e *entry) tryExpungeLocked() (isExpunged bool) {
 378         p := atomic.LoadPointer(&e.p)
 379         for p == nil {
 380                 if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
 381                         return true
 382                 }
 383                 p = atomic.LoadPointer(&e.p)
 384         }
 385         return p == expunged
 386 }