Source file src/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 ) 10 11 // Map is like a Go map[any]any but is safe for concurrent use 12 // by multiple goroutines without additional locking or coordination. 13 // Loads, stores, and deletes run in amortized constant time. 14 // 15 // The Map type is specialized. Most code should use a plain Go map instead, 16 // with separate locking or coordination, for better type safety and to make it 17 // easier to maintain other invariants along with the map content. 18 // 19 // The Map type is optimized for two common use cases: (1) when the entry for a given 20 // key is only ever written once but read many times, as in caches that only grow, 21 // or (2) when multiple goroutines read, write, and overwrite entries for disjoint 22 // sets of keys. In these two cases, use of a Map may significantly reduce lock 23 // contention compared to a Go map paired with a separate [Mutex] or [RWMutex]. 24 // 25 // The zero Map is empty and ready for use. A Map must not be copied after first use. 26 // 27 // In the terminology of the Go memory model, Map arranges that a write operation 28 // “synchronizes before” any read operation that observes the effect of the write, where 29 // read and write operations are defined as follows. 30 // [Map.Load], [Map.LoadAndDelete], [Map.LoadOrStore], [Map.Swap], [Map.CompareAndSwap], 31 // and [Map.CompareAndDelete] are read operations; 32 // [Map.Delete], [Map.LoadAndDelete], [Map.Store], and [Map.Swap] are write operations; 33 // [Map.LoadOrStore] is a write operation when it returns loaded set to false; 34 // [Map.CompareAndSwap] is a write operation when it returns swapped set to true; 35 // and [Map.CompareAndDelete] is a write operation when it returns deleted set to true. 36 type Map struct { 37 mu Mutex 38 39 // read contains the portion of the map's contents that are safe for 40 // concurrent access (with or without mu held). 41 // 42 // The read field itself is always safe to load, but must only be stored with 43 // mu held. 44 // 45 // Entries stored in read may be updated concurrently without mu, but updating 46 // a previously-expunged entry requires that the entry be copied to the dirty 47 // map and unexpunged with mu held. 48 read atomic.Pointer[readOnly] 49 50 // dirty contains the portion of the map's contents that require mu to be 51 // held. To ensure that the dirty map can be promoted to the read map quickly, 52 // it also includes all of the non-expunged entries in the read map. 53 // 54 // Expunged entries are not stored in the dirty map. An expunged entry in the 55 // clean map must be unexpunged and added to the dirty map before a new value 56 // can be stored to it. 57 // 58 // If the dirty map is nil, the next write to the map will initialize it by 59 // making a shallow copy of the clean map, omitting stale entries. 60 dirty map[any]*entry 61 62 // misses counts the number of loads since the read map was last updated that 63 // needed to lock mu to determine whether the key was present. 64 // 65 // Once enough misses have occurred to cover the cost of copying the dirty 66 // map, the dirty map will be promoted to the read map (in the unamended 67 // state) and the next store to the map will make a new dirty copy. 68 misses int 69 } 70 71 // readOnly is an immutable struct stored atomically in the Map.read field. 72 type readOnly struct { 73 m map[any]*entry 74 amended bool // true if the dirty map contains some key not in m. 75 } 76 77 // expunged is an arbitrary pointer that marks entries which have been deleted 78 // from the dirty map. 79 var expunged = new(any) 80 81 // An entry is a slot in the map corresponding to a particular key. 82 type entry struct { 83 // p points to the interface{} value stored for the entry. 84 // 85 // If p == nil, the entry has been deleted, and either m.dirty == nil or 86 // m.dirty[key] is e. 87 // 88 // If p == expunged, the entry has been deleted, m.dirty != nil, and the entry 89 // is missing from m.dirty. 90 // 91 // Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty 92 // != nil, in m.dirty[key]. 93 // 94 // An entry can be deleted by atomic replacement with nil: when m.dirty is 95 // next created, it will atomically replace nil with expunged and leave 96 // m.dirty[key] unset. 97 // 98 // An entry's associated value can be updated by atomic replacement, provided 99 // p != expunged. If p == expunged, an entry's associated value can be updated 100 // only after first setting m.dirty[key] = e so that lookups using the dirty 101 // map find the entry. 102 p atomic.Pointer[any] 103 } 104 105 func newEntry(i any) *entry { 106 e := &entry{} 107 e.p.Store(&i) 108 return e 109 } 110 111 func (m *Map) loadReadOnly() readOnly { 112 if p := m.read.Load(); p != nil { 113 return *p 114 } 115 return readOnly{} 116 } 117 118 // Load returns the value stored in the map for a key, or nil if no 119 // value is present. 120 // The ok result indicates whether value was found in the map. 121 func (m *Map) Load(key any) (value any, ok bool) { 122 read := m.loadReadOnly() 123 e, ok := read.m[key] 124 if !ok && read.amended { 125 m.mu.Lock() 126 // Avoid reporting a spurious miss if m.dirty got promoted while we were 127 // blocked on m.mu. (If further loads of the same key will not miss, it's 128 // not worth copying the dirty map for this key.) 129 read = m.loadReadOnly() 130 e, ok = read.m[key] 131 if !ok && read.amended { 132 e, ok = m.dirty[key] 133 // Regardless of whether the entry was present, record a miss: this key 134 // will take the slow path until the dirty map is promoted to the read 135 // map. 136 m.missLocked() 137 } 138 m.mu.Unlock() 139 } 140 if !ok { 141 return nil, false 142 } 143 return e.load() 144 } 145 146 func (e *entry) load() (value any, ok bool) { 147 p := e.p.Load() 148 if p == nil || p == expunged { 149 return nil, false 150 } 151 return *p, true 152 } 153 154 // Store sets the value for a key. 155 func (m *Map) Store(key, value any) { 156 _, _ = m.Swap(key, value) 157 } 158 159 // Clear deletes all the entries, resulting in an empty Map. 160 func (m *Map) Clear() { 161 read := m.loadReadOnly() 162 if len(read.m) == 0 && !read.amended { 163 // Avoid allocating a new readOnly when the map is already clear. 164 return 165 } 166 167 m.mu.Lock() 168 defer m.mu.Unlock() 169 170 read = m.loadReadOnly() 171 if len(read.m) > 0 || read.amended { 172 m.read.Store(&readOnly{}) 173 } 174 175 clear(m.dirty) 176 // Don't immediately promote the newly-cleared dirty map on the next operation. 177 m.misses = 0 178 } 179 180 // tryCompareAndSwap compare the entry with the given old value and swaps 181 // it with a new value if the entry is equal to the old value, and the entry 182 // has not been expunged. 183 // 184 // If the entry is expunged, tryCompareAndSwap returns false and leaves 185 // the entry unchanged. 186 func (e *entry) tryCompareAndSwap(old, new any) bool { 187 p := e.p.Load() 188 if p == nil || p == expunged || *p != old { 189 return false 190 } 191 192 // Copy the interface after the first load to make this method more amenable 193 // to escape analysis: if the comparison fails from the start, we shouldn't 194 // bother heap-allocating an interface value to store. 195 nc := new 196 for { 197 if e.p.CompareAndSwap(p, &nc) { 198 return true 199 } 200 p = e.p.Load() 201 if p == nil || p == expunged || *p != old { 202 return false 203 } 204 } 205 } 206 207 // unexpungeLocked ensures that the entry is not marked as expunged. 208 // 209 // If the entry was previously expunged, it must be added to the dirty map 210 // before m.mu is unlocked. 211 func (e *entry) unexpungeLocked() (wasExpunged bool) { 212 return e.p.CompareAndSwap(expunged, nil) 213 } 214 215 // swapLocked unconditionally swaps a value into the entry. 216 // 217 // The entry must be known not to be expunged. 218 func (e *entry) swapLocked(i *any) *any { 219 return e.p.Swap(i) 220 } 221 222 // LoadOrStore returns the existing value for the key if present. 223 // Otherwise, it stores and returns the given value. 224 // The loaded result is true if the value was loaded, false if stored. 225 func (m *Map) LoadOrStore(key, value any) (actual any, loaded bool) { 226 // Avoid locking if it's a clean hit. 227 read := m.loadReadOnly() 228 if e, ok := read.m[key]; ok { 229 actual, loaded, ok := e.tryLoadOrStore(value) 230 if ok { 231 return actual, loaded 232 } 233 } 234 235 m.mu.Lock() 236 read = m.loadReadOnly() 237 if e, ok := read.m[key]; ok { 238 if e.unexpungeLocked() { 239 m.dirty[key] = e 240 } 241 actual, loaded, _ = e.tryLoadOrStore(value) 242 } else if e, ok := m.dirty[key]; ok { 243 actual, loaded, _ = e.tryLoadOrStore(value) 244 m.missLocked() 245 } else { 246 if !read.amended { 247 // We're adding the first new key to the dirty map. 248 // Make sure it is allocated and mark the read-only map as incomplete. 249 m.dirtyLocked() 250 m.read.Store(&readOnly{m: read.m, amended: true}) 251 } 252 m.dirty[key] = newEntry(value) 253 actual, loaded = value, false 254 } 255 m.mu.Unlock() 256 257 return actual, loaded 258 } 259 260 // tryLoadOrStore atomically loads or stores a value if the entry is not 261 // expunged. 262 // 263 // If the entry is expunged, tryLoadOrStore leaves the entry unchanged and 264 // returns with ok==false. 265 func (e *entry) tryLoadOrStore(i any) (actual any, loaded, ok bool) { 266 p := e.p.Load() 267 if p == expunged { 268 return nil, false, false 269 } 270 if p != nil { 271 return *p, true, true 272 } 273 274 // Copy the interface after the first load to make this method more amenable 275 // to escape analysis: if we hit the "load" path or the entry is expunged, we 276 // shouldn't bother heap-allocating. 277 ic := i 278 for { 279 if e.p.CompareAndSwap(nil, &ic) { 280 return i, false, true 281 } 282 p = e.p.Load() 283 if p == expunged { 284 return nil, false, false 285 } 286 if p != nil { 287 return *p, true, true 288 } 289 } 290 } 291 292 // LoadAndDelete deletes the value for a key, returning the previous value if any. 293 // The loaded result reports whether the key was present. 294 func (m *Map) LoadAndDelete(key any) (value any, loaded bool) { 295 read := m.loadReadOnly() 296 e, ok := read.m[key] 297 if !ok && read.amended { 298 m.mu.Lock() 299 read = m.loadReadOnly() 300 e, ok = read.m[key] 301 if !ok && read.amended { 302 e, ok = m.dirty[key] 303 delete(m.dirty, key) 304 // Regardless of whether the entry was present, record a miss: this key 305 // will take the slow path until the dirty map is promoted to the read 306 // map. 307 m.missLocked() 308 } 309 m.mu.Unlock() 310 } 311 if ok { 312 return e.delete() 313 } 314 return nil, false 315 } 316 317 // Delete deletes the value for a key. 318 func (m *Map) Delete(key any) { 319 m.LoadAndDelete(key) 320 } 321 322 func (e *entry) delete() (value any, ok bool) { 323 for { 324 p := e.p.Load() 325 if p == nil || p == expunged { 326 return nil, false 327 } 328 if e.p.CompareAndSwap(p, nil) { 329 return *p, true 330 } 331 } 332 } 333 334 // trySwap swaps a value if the entry has not been expunged. 335 // 336 // If the entry is expunged, trySwap returns false and leaves the entry 337 // unchanged. 338 func (e *entry) trySwap(i *any) (*any, bool) { 339 for { 340 p := e.p.Load() 341 if p == expunged { 342 return nil, false 343 } 344 if e.p.CompareAndSwap(p, i) { 345 return p, true 346 } 347 } 348 } 349 350 // Swap swaps the value for a key and returns the previous value if any. 351 // The loaded result reports whether the key was present. 352 func (m *Map) Swap(key, value any) (previous any, loaded bool) { 353 read := m.loadReadOnly() 354 if e, ok := read.m[key]; ok { 355 if v, ok := e.trySwap(&value); ok { 356 if v == nil { 357 return nil, false 358 } 359 return *v, true 360 } 361 } 362 363 m.mu.Lock() 364 read = m.loadReadOnly() 365 if e, ok := read.m[key]; ok { 366 if e.unexpungeLocked() { 367 // The entry was previously expunged, which implies that there is a 368 // non-nil dirty map and this entry is not in it. 369 m.dirty[key] = e 370 } 371 if v := e.swapLocked(&value); v != nil { 372 loaded = true 373 previous = *v 374 } 375 } else if e, ok := m.dirty[key]; ok { 376 if v := e.swapLocked(&value); v != nil { 377 loaded = true 378 previous = *v 379 } 380 } else { 381 if !read.amended { 382 // We're adding the first new key to the dirty map. 383 // Make sure it is allocated and mark the read-only map as incomplete. 384 m.dirtyLocked() 385 m.read.Store(&readOnly{m: read.m, amended: true}) 386 } 387 m.dirty[key] = newEntry(value) 388 } 389 m.mu.Unlock() 390 return previous, loaded 391 } 392 393 // CompareAndSwap swaps the old and new values for key 394 // if the value stored in the map is equal to old. 395 // The old value must be of a comparable type. 396 func (m *Map) CompareAndSwap(key, old, new any) (swapped bool) { 397 read := m.loadReadOnly() 398 if e, ok := read.m[key]; ok { 399 return e.tryCompareAndSwap(old, new) 400 } else if !read.amended { 401 return false // No existing value for key. 402 } 403 404 m.mu.Lock() 405 defer m.mu.Unlock() 406 read = m.loadReadOnly() 407 swapped = false 408 if e, ok := read.m[key]; ok { 409 swapped = e.tryCompareAndSwap(old, new) 410 } else if e, ok := m.dirty[key]; ok { 411 swapped = e.tryCompareAndSwap(old, new) 412 // We needed to lock mu in order to load the entry for key, 413 // and the operation didn't change the set of keys in the map 414 // (so it would be made more efficient by promoting the dirty 415 // map to read-only). 416 // Count it as a miss so that we will eventually switch to the 417 // more efficient steady state. 418 m.missLocked() 419 } 420 return swapped 421 } 422 423 // CompareAndDelete deletes the entry for key if its value is equal to old. 424 // The old value must be of a comparable type. 425 // 426 // If there is no current value for key in the map, CompareAndDelete 427 // returns false (even if the old value is the nil interface value). 428 func (m *Map) CompareAndDelete(key, old any) (deleted bool) { 429 read := m.loadReadOnly() 430 e, ok := read.m[key] 431 if !ok && read.amended { 432 m.mu.Lock() 433 read = m.loadReadOnly() 434 e, ok = read.m[key] 435 if !ok && read.amended { 436 e, ok = m.dirty[key] 437 // Don't delete key from m.dirty: we still need to do the “compare” part 438 // of the operation. The entry will eventually be expunged when the 439 // dirty map is promoted to the read map. 440 // 441 // Regardless of whether the entry was present, record a miss: this key 442 // will take the slow path until the dirty map is promoted to the read 443 // map. 444 m.missLocked() 445 } 446 m.mu.Unlock() 447 } 448 for ok { 449 p := e.p.Load() 450 if p == nil || p == expunged || *p != old { 451 return false 452 } 453 if e.p.CompareAndSwap(p, nil) { 454 return true 455 } 456 } 457 return false 458 } 459 460 // Range calls f sequentially for each key and value present in the map. 461 // If f returns false, range stops the iteration. 462 // 463 // Range does not necessarily correspond to any consistent snapshot of the Map's 464 // contents: no key will be visited more than once, but if the value for any key 465 // is stored or deleted concurrently (including by f), Range may reflect any 466 // mapping for that key from any point during the Range call. Range does not 467 // block other methods on the receiver; even f itself may call any method on m. 468 // 469 // Range may be O(N) with the number of elements in the map even if f returns 470 // false after a constant number of calls. 471 func (m *Map) Range(f func(key, value any) bool) { 472 // We need to be able to iterate over all of the keys that were already 473 // present at the start of the call to Range. 474 // If read.amended is false, then read.m satisfies that property without 475 // requiring us to hold m.mu for a long time. 476 read := m.loadReadOnly() 477 if read.amended { 478 // m.dirty contains keys not in read.m. Fortunately, Range is already O(N) 479 // (assuming the caller does not break out early), so a call to Range 480 // amortizes an entire copy of the map: we can promote the dirty copy 481 // immediately! 482 m.mu.Lock() 483 read = m.loadReadOnly() 484 if read.amended { 485 read = readOnly{m: m.dirty} 486 copyRead := read 487 m.read.Store(©Read) 488 m.dirty = nil 489 m.misses = 0 490 } 491 m.mu.Unlock() 492 } 493 494 for k, e := range read.m { 495 v, ok := e.load() 496 if !ok { 497 continue 498 } 499 if !f(k, v) { 500 break 501 } 502 } 503 } 504 505 func (m *Map) missLocked() { 506 m.misses++ 507 if m.misses < len(m.dirty) { 508 return 509 } 510 m.read.Store(&readOnly{m: m.dirty}) 511 m.dirty = nil 512 m.misses = 0 513 } 514 515 func (m *Map) dirtyLocked() { 516 if m.dirty != nil { 517 return 518 } 519 520 read := m.loadReadOnly() 521 m.dirty = make(map[any]*entry, len(read.m)) 522 for k, e := range read.m { 523 if !e.tryExpungeLocked() { 524 m.dirty[k] = e 525 } 526 } 527 } 528 529 func (e *entry) tryExpungeLocked() (isExpunged bool) { 530 p := e.p.Load() 531 for p == nil { 532 if e.p.CompareAndSwap(nil, expunged) { 533 return true 534 } 535 p = e.p.Load() 536 } 537 return p == expunged 538 } 539