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 // 37 // [the Go memory model]: https://go.dev/ref/mem 38 type Map struct { 39 mu Mutex 40 41 // read contains the portion of the map's contents that are safe for 42 // concurrent access (with or without mu held). 43 // 44 // The read field itself is always safe to load, but must only be stored with 45 // mu held. 46 // 47 // Entries stored in read may be updated concurrently without mu, but updating 48 // a previously-expunged entry requires that the entry be copied to the dirty 49 // map and unexpunged with mu held. 50 read atomic.Pointer[readOnly] 51 52 // dirty contains the portion of the map's contents that require mu to be 53 // held. To ensure that the dirty map can be promoted to the read map quickly, 54 // it also includes all of the non-expunged entries in the read map. 55 // 56 // Expunged entries are not stored in the dirty map. An expunged entry in the 57 // clean map must be unexpunged and added to the dirty map before a new value 58 // can be stored to it. 59 // 60 // If the dirty map is nil, the next write to the map will initialize it by 61 // making a shallow copy of the clean map, omitting stale entries. 62 dirty map[any]*entry 63 64 // misses counts the number of loads since the read map was last updated that 65 // needed to lock mu to determine whether the key was present. 66 // 67 // Once enough misses have occurred to cover the cost of copying the dirty 68 // map, the dirty map will be promoted to the read map (in the unamended 69 // state) and the next store to the map will make a new dirty copy. 70 misses int 71 } 72 73 // readOnly is an immutable struct stored atomically in the Map.read field. 74 type readOnly struct { 75 m map[any]*entry 76 amended bool // true if the dirty map contains some key not in m. 77 } 78 79 // expunged is an arbitrary pointer that marks entries which have been deleted 80 // from the dirty map. 81 var expunged = new(any) 82 83 // An entry is a slot in the map corresponding to a particular key. 84 type entry struct { 85 // p points to the interface{} value stored for the entry. 86 // 87 // If p == nil, the entry has been deleted, and either m.dirty == nil or 88 // m.dirty[key] is e. 89 // 90 // If p == expunged, the entry has been deleted, m.dirty != nil, and the entry 91 // is missing from m.dirty. 92 // 93 // Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty 94 // != nil, in m.dirty[key]. 95 // 96 // An entry can be deleted by atomic replacement with nil: when m.dirty is 97 // next created, it will atomically replace nil with expunged and leave 98 // m.dirty[key] unset. 99 // 100 // An entry's associated value can be updated by atomic replacement, provided 101 // p != expunged. If p == expunged, an entry's associated value can be updated 102 // only after first setting m.dirty[key] = e so that lookups using the dirty 103 // map find the entry. 104 p atomic.Pointer[any] 105 } 106 107 func newEntry(i any) *entry { 108 e := &entry{} 109 e.p.Store(&i) 110 return e 111 } 112 113 func (m *Map) loadReadOnly() readOnly { 114 if p := m.read.Load(); p != nil { 115 return *p 116 } 117 return readOnly{} 118 } 119 120 // Load returns the value stored in the map for a key, or nil if no 121 // value is present. 122 // The ok result indicates whether value was found in the map. 123 func (m *Map) Load(key any) (value any, ok bool) { 124 read := m.loadReadOnly() 125 e, ok := read.m[key] 126 if !ok && read.amended { 127 m.mu.Lock() 128 // Avoid reporting a spurious miss if m.dirty got promoted while we were 129 // blocked on m.mu. (If further loads of the same key will not miss, it's 130 // not worth copying the dirty map for this key.) 131 read = m.loadReadOnly() 132 e, ok = read.m[key] 133 if !ok && read.amended { 134 e, ok = m.dirty[key] 135 // Regardless of whether the entry was present, record a miss: this key 136 // will take the slow path until the dirty map is promoted to the read 137 // map. 138 m.missLocked() 139 } 140 m.mu.Unlock() 141 } 142 if !ok { 143 return nil, false 144 } 145 return e.load() 146 } 147 148 func (e *entry) load() (value any, ok bool) { 149 p := e.p.Load() 150 if p == nil || p == expunged { 151 return nil, false 152 } 153 return *p, true 154 } 155 156 // Store sets the value for a key. 157 func (m *Map) Store(key, value any) { 158 _, _ = m.Swap(key, value) 159 } 160 161 // Clear deletes all the entries, resulting in an empty Map. 162 func (m *Map) Clear() { 163 read := m.loadReadOnly() 164 if len(read.m) == 0 && !read.amended { 165 // Avoid allocating a new readOnly when the map is already clear. 166 return 167 } 168 169 m.mu.Lock() 170 defer m.mu.Unlock() 171 172 read = m.loadReadOnly() 173 if len(read.m) > 0 || read.amended { 174 m.read.Store(&readOnly{}) 175 } 176 177 clear(m.dirty) 178 // Don't immediately promote the newly-cleared dirty map on the next operation. 179 m.misses = 0 180 } 181 182 // tryCompareAndSwap compare the entry with the given old value and swaps 183 // it with a new value if the entry is equal to the old value, and the entry 184 // has not been expunged. 185 // 186 // If the entry is expunged, tryCompareAndSwap returns false and leaves 187 // the entry unchanged. 188 func (e *entry) tryCompareAndSwap(old, new any) bool { 189 p := e.p.Load() 190 if p == nil || p == expunged || *p != old { 191 return false 192 } 193 194 // Copy the interface after the first load to make this method more amenable 195 // to escape analysis: if the comparison fails from the start, we shouldn't 196 // bother heap-allocating an interface value to store. 197 nc := new 198 for { 199 if e.p.CompareAndSwap(p, &nc) { 200 return true 201 } 202 p = e.p.Load() 203 if p == nil || p == expunged || *p != old { 204 return false 205 } 206 } 207 } 208 209 // unexpungeLocked ensures that the entry is not marked as expunged. 210 // 211 // If the entry was previously expunged, it must be added to the dirty map 212 // before m.mu is unlocked. 213 func (e *entry) unexpungeLocked() (wasExpunged bool) { 214 return e.p.CompareAndSwap(expunged, nil) 215 } 216 217 // swapLocked unconditionally swaps a value into the entry. 218 // 219 // The entry must be known not to be expunged. 220 func (e *entry) swapLocked(i *any) *any { 221 return e.p.Swap(i) 222 } 223 224 // LoadOrStore returns the existing value for the key if present. 225 // Otherwise, it stores and returns the given value. 226 // The loaded result is true if the value was loaded, false if stored. 227 func (m *Map) LoadOrStore(key, value any) (actual any, loaded bool) { 228 // Avoid locking if it's a clean hit. 229 read := m.loadReadOnly() 230 if e, ok := read.m[key]; ok { 231 actual, loaded, ok := e.tryLoadOrStore(value) 232 if ok { 233 return actual, loaded 234 } 235 } 236 237 m.mu.Lock() 238 read = m.loadReadOnly() 239 if e, ok := read.m[key]; ok { 240 if e.unexpungeLocked() { 241 m.dirty[key] = e 242 } 243 actual, loaded, _ = e.tryLoadOrStore(value) 244 } else if e, ok := m.dirty[key]; ok { 245 actual, loaded, _ = e.tryLoadOrStore(value) 246 m.missLocked() 247 } else { 248 if !read.amended { 249 // We're adding the first new key to the dirty map. 250 // Make sure it is allocated and mark the read-only map as incomplete. 251 m.dirtyLocked() 252 m.read.Store(&readOnly{m: read.m, amended: true}) 253 } 254 m.dirty[key] = newEntry(value) 255 actual, loaded = value, false 256 } 257 m.mu.Unlock() 258 259 return actual, loaded 260 } 261 262 // tryLoadOrStore atomically loads or stores a value if the entry is not 263 // expunged. 264 // 265 // If the entry is expunged, tryLoadOrStore leaves the entry unchanged and 266 // returns with ok==false. 267 func (e *entry) tryLoadOrStore(i any) (actual any, loaded, ok bool) { 268 p := e.p.Load() 269 if p == expunged { 270 return nil, false, false 271 } 272 if p != nil { 273 return *p, true, true 274 } 275 276 // Copy the interface after the first load to make this method more amenable 277 // to escape analysis: if we hit the "load" path or the entry is expunged, we 278 // shouldn't bother heap-allocating. 279 ic := i 280 for { 281 if e.p.CompareAndSwap(nil, &ic) { 282 return i, false, true 283 } 284 p = e.p.Load() 285 if p == expunged { 286 return nil, false, false 287 } 288 if p != nil { 289 return *p, true, true 290 } 291 } 292 } 293 294 // LoadAndDelete deletes the value for a key, returning the previous value if any. 295 // The loaded result reports whether the key was present. 296 func (m *Map) LoadAndDelete(key any) (value any, loaded bool) { 297 read := m.loadReadOnly() 298 e, ok := read.m[key] 299 if !ok && read.amended { 300 m.mu.Lock() 301 read = m.loadReadOnly() 302 e, ok = read.m[key] 303 if !ok && read.amended { 304 e, ok = m.dirty[key] 305 delete(m.dirty, key) 306 // Regardless of whether the entry was present, record a miss: this key 307 // will take the slow path until the dirty map is promoted to the read 308 // map. 309 m.missLocked() 310 } 311 m.mu.Unlock() 312 } 313 if ok { 314 return e.delete() 315 } 316 return nil, false 317 } 318 319 // Delete deletes the value for a key. 320 func (m *Map) Delete(key any) { 321 m.LoadAndDelete(key) 322 } 323 324 func (e *entry) delete() (value any, ok bool) { 325 for { 326 p := e.p.Load() 327 if p == nil || p == expunged { 328 return nil, false 329 } 330 if e.p.CompareAndSwap(p, nil) { 331 return *p, true 332 } 333 } 334 } 335 336 // trySwap swaps a value if the entry has not been expunged. 337 // 338 // If the entry is expunged, trySwap returns false and leaves the entry 339 // unchanged. 340 func (e *entry) trySwap(i *any) (*any, bool) { 341 for { 342 p := e.p.Load() 343 if p == expunged { 344 return nil, false 345 } 346 if e.p.CompareAndSwap(p, i) { 347 return p, true 348 } 349 } 350 } 351 352 // Swap swaps the value for a key and returns the previous value if any. 353 // The loaded result reports whether the key was present. 354 func (m *Map) Swap(key, value any) (previous any, loaded bool) { 355 read := m.loadReadOnly() 356 if e, ok := read.m[key]; ok { 357 if v, ok := e.trySwap(&value); ok { 358 if v == nil { 359 return nil, false 360 } 361 return *v, true 362 } 363 } 364 365 m.mu.Lock() 366 read = m.loadReadOnly() 367 if e, ok := read.m[key]; ok { 368 if e.unexpungeLocked() { 369 // The entry was previously expunged, which implies that there is a 370 // non-nil dirty map and this entry is not in it. 371 m.dirty[key] = e 372 } 373 if v := e.swapLocked(&value); v != nil { 374 loaded = true 375 previous = *v 376 } 377 } else if e, ok := m.dirty[key]; ok { 378 if v := e.swapLocked(&value); v != nil { 379 loaded = true 380 previous = *v 381 } 382 } else { 383 if !read.amended { 384 // We're adding the first new key to the dirty map. 385 // Make sure it is allocated and mark the read-only map as incomplete. 386 m.dirtyLocked() 387 m.read.Store(&readOnly{m: read.m, amended: true}) 388 } 389 m.dirty[key] = newEntry(value) 390 } 391 m.mu.Unlock() 392 return previous, loaded 393 } 394 395 // CompareAndSwap swaps the old and new values for key 396 // if the value stored in the map is equal to old. 397 // The old value must be of a comparable type. 398 func (m *Map) CompareAndSwap(key, old, new any) (swapped bool) { 399 read := m.loadReadOnly() 400 if e, ok := read.m[key]; ok { 401 return e.tryCompareAndSwap(old, new) 402 } else if !read.amended { 403 return false // No existing value for key. 404 } 405 406 m.mu.Lock() 407 defer m.mu.Unlock() 408 read = m.loadReadOnly() 409 swapped = false 410 if e, ok := read.m[key]; ok { 411 swapped = e.tryCompareAndSwap(old, new) 412 } else if e, ok := m.dirty[key]; ok { 413 swapped = e.tryCompareAndSwap(old, new) 414 // We needed to lock mu in order to load the entry for key, 415 // and the operation didn't change the set of keys in the map 416 // (so it would be made more efficient by promoting the dirty 417 // map to read-only). 418 // Count it as a miss so that we will eventually switch to the 419 // more efficient steady state. 420 m.missLocked() 421 } 422 return swapped 423 } 424 425 // CompareAndDelete deletes the entry for key if its value is equal to old. 426 // The old value must be of a comparable type. 427 // 428 // If there is no current value for key in the map, CompareAndDelete 429 // returns false (even if the old value is the nil interface value). 430 func (m *Map) CompareAndDelete(key, old any) (deleted bool) { 431 read := m.loadReadOnly() 432 e, ok := read.m[key] 433 if !ok && read.amended { 434 m.mu.Lock() 435 read = m.loadReadOnly() 436 e, ok = read.m[key] 437 if !ok && read.amended { 438 e, ok = m.dirty[key] 439 // Don't delete key from m.dirty: we still need to do the “compare” part 440 // of the operation. The entry will eventually be expunged when the 441 // dirty map is promoted to the read map. 442 // 443 // Regardless of whether the entry was present, record a miss: this key 444 // will take the slow path until the dirty map is promoted to the read 445 // map. 446 m.missLocked() 447 } 448 m.mu.Unlock() 449 } 450 for ok { 451 p := e.p.Load() 452 if p == nil || p == expunged || *p != old { 453 return false 454 } 455 if e.p.CompareAndSwap(p, nil) { 456 return true 457 } 458 } 459 return false 460 } 461 462 // Range calls f sequentially for each key and value present in the map. 463 // If f returns false, range stops the iteration. 464 // 465 // Range does not necessarily correspond to any consistent snapshot of the Map's 466 // contents: no key will be visited more than once, but if the value for any key 467 // is stored or deleted concurrently (including by f), Range may reflect any 468 // mapping for that key from any point during the Range call. Range does not 469 // block other methods on the receiver; even f itself may call any method on m. 470 // 471 // Range may be O(N) with the number of elements in the map even if f returns 472 // false after a constant number of calls. 473 func (m *Map) Range(f func(key, value any) bool) { 474 // We need to be able to iterate over all of the keys that were already 475 // present at the start of the call to Range. 476 // If read.amended is false, then read.m satisfies that property without 477 // requiring us to hold m.mu for a long time. 478 read := m.loadReadOnly() 479 if read.amended { 480 // m.dirty contains keys not in read.m. Fortunately, Range is already O(N) 481 // (assuming the caller does not break out early), so a call to Range 482 // amortizes an entire copy of the map: we can promote the dirty copy 483 // immediately! 484 m.mu.Lock() 485 read = m.loadReadOnly() 486 if read.amended { 487 read = readOnly{m: m.dirty} 488 copyRead := read 489 m.read.Store(©Read) 490 m.dirty = nil 491 m.misses = 0 492 } 493 m.mu.Unlock() 494 } 495 496 for k, e := range read.m { 497 v, ok := e.load() 498 if !ok { 499 continue 500 } 501 if !f(k, v) { 502 break 503 } 504 } 505 } 506 507 func (m *Map) missLocked() { 508 m.misses++ 509 if m.misses < len(m.dirty) { 510 return 511 } 512 m.read.Store(&readOnly{m: m.dirty}) 513 m.dirty = nil 514 m.misses = 0 515 } 516 517 func (m *Map) dirtyLocked() { 518 if m.dirty != nil { 519 return 520 } 521 522 read := m.loadReadOnly() 523 m.dirty = make(map[any]*entry, len(read.m)) 524 for k, e := range read.m { 525 if !e.tryExpungeLocked() { 526 m.dirty[k] = e 527 } 528 } 529 } 530 531 func (e *entry) tryExpungeLocked() (isExpunged bool) { 532 p := e.p.Load() 533 for p == nil { 534 if e.p.CompareAndSwap(nil, expunged) { 535 return true 536 } 537 p = e.p.Load() 538 } 539 return p == expunged 540 } 541