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 //go:build !goexperiment.synchashtriemap 6 7 package sync 8 9 import ( 10 "sync/atomic" 11 ) 12 13 // Map is like a Go map[any]any but is safe for concurrent use 14 // by multiple goroutines without additional locking or coordination. 15 // Loads, stores, and deletes run in amortized constant time. 16 // 17 // The Map type is specialized. Most code should use a plain Go map instead, 18 // with separate locking or coordination, for better type safety and to make it 19 // easier to maintain other invariants along with the map content. 20 // 21 // The Map type is optimized for two common use cases: (1) when the entry for a given 22 // key is only ever written once but read many times, as in caches that only grow, 23 // or (2) when multiple goroutines read, write, and overwrite entries for disjoint 24 // sets of keys. In these two cases, use of a Map may significantly reduce lock 25 // contention compared to a Go map paired with a separate [Mutex] or [RWMutex]. 26 // 27 // The zero Map is empty and ready for use. A Map must not be copied after first use. 28 // 29 // In the terminology of [the Go memory model], Map arranges that a write operation 30 // “synchronizes before” any read operation that observes the effect of the write, where 31 // read and write operations are defined as follows. 32 // [Map.Load], [Map.LoadAndDelete], [Map.LoadOrStore], [Map.Swap], [Map.CompareAndSwap], 33 // and [Map.CompareAndDelete] are read operations; 34 // [Map.Delete], [Map.LoadAndDelete], [Map.Store], and [Map.Swap] are write operations; 35 // [Map.LoadOrStore] is a write operation when it returns loaded set to false; 36 // [Map.CompareAndSwap] is a write operation when it returns swapped set to true; 37 // and [Map.CompareAndDelete] is a write operation when it returns deleted set to true. 38 // 39 // [the Go memory model]: https://go.dev/ref/mem 40 type Map struct { 41 _ noCopy 42 43 mu Mutex 44 45 // read contains the portion of the map's contents that are safe for 46 // concurrent access (with or without mu held). 47 // 48 // The read field itself is always safe to load, but must only be stored with 49 // mu held. 50 // 51 // Entries stored in read may be updated concurrently without mu, but updating 52 // a previously-expunged entry requires that the entry be copied to the dirty 53 // map and unexpunged with mu held. 54 read atomic.Pointer[readOnly] 55 56 // dirty contains the portion of the map's contents that require mu to be 57 // held. To ensure that the dirty map can be promoted to the read map quickly, 58 // it also includes all of the non-expunged entries in the read map. 59 // 60 // Expunged entries are not stored in the dirty map. An expunged entry in the 61 // clean map must be unexpunged and added to the dirty map before a new value 62 // can be stored to it. 63 // 64 // If the dirty map is nil, the next write to the map will initialize it by 65 // making a shallow copy of the clean map, omitting stale entries. 66 dirty map[any]*entry 67 68 // misses counts the number of loads since the read map was last updated that 69 // needed to lock mu to determine whether the key was present. 70 // 71 // Once enough misses have occurred to cover the cost of copying the dirty 72 // map, the dirty map will be promoted to the read map (in the unamended 73 // state) and the next store to the map will make a new dirty copy. 74 misses int 75 } 76 77 // readOnly is an immutable struct stored atomically in the Map.read field. 78 type readOnly struct { 79 m map[any]*entry 80 amended bool // true if the dirty map contains some key not in m. 81 } 82 83 // expunged is an arbitrary pointer that marks entries which have been deleted 84 // from the dirty map. 85 var expunged = new(any) 86 87 // An entry is a slot in the map corresponding to a particular key. 88 type entry struct { 89 // p points to the interface{} value stored for the entry. 90 // 91 // If p == nil, the entry has been deleted, and either m.dirty == nil or 92 // m.dirty[key] is e. 93 // 94 // If p == expunged, the entry has been deleted, m.dirty != nil, and the entry 95 // is missing from m.dirty. 96 // 97 // Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty 98 // != nil, in m.dirty[key]. 99 // 100 // An entry can be deleted by atomic replacement with nil: when m.dirty is 101 // next created, it will atomically replace nil with expunged and leave 102 // m.dirty[key] unset. 103 // 104 // An entry's associated value can be updated by atomic replacement, provided 105 // p != expunged. If p == expunged, an entry's associated value can be updated 106 // only after first setting m.dirty[key] = e so that lookups using the dirty 107 // map find the entry. 108 p atomic.Pointer[any] 109 } 110 111 func newEntry(i any) *entry { 112 e := &entry{} 113 e.p.Store(&i) 114 return e 115 } 116 117 func (m *Map) loadReadOnly() readOnly { 118 if p := m.read.Load(); p != nil { 119 return *p 120 } 121 return readOnly{} 122 } 123 124 // Load returns the value stored in the map for a key, or nil if no 125 // value is present. 126 // The ok result indicates whether value was found in the map. 127 func (m *Map) Load(key any) (value any, ok bool) { 128 read := m.loadReadOnly() 129 e, ok := read.m[key] 130 if !ok && read.amended { 131 m.mu.Lock() 132 // Avoid reporting a spurious miss if m.dirty got promoted while we were 133 // blocked on m.mu. (If further loads of the same key will not miss, it's 134 // not worth copying the dirty map for this key.) 135 read = m.loadReadOnly() 136 e, ok = read.m[key] 137 if !ok && read.amended { 138 e, ok = m.dirty[key] 139 // Regardless of whether the entry was present, record a miss: this key 140 // will take the slow path until the dirty map is promoted to the read 141 // map. 142 m.missLocked() 143 } 144 m.mu.Unlock() 145 } 146 if !ok { 147 return nil, false 148 } 149 return e.load() 150 } 151 152 func (e *entry) load() (value any, ok bool) { 153 p := e.p.Load() 154 if p == nil || p == expunged { 155 return nil, false 156 } 157 return *p, true 158 } 159 160 // Store sets the value for a key. 161 func (m *Map) Store(key, value any) { 162 _, _ = m.Swap(key, value) 163 } 164 165 // Clear deletes all the entries, resulting in an empty Map. 166 func (m *Map) Clear() { 167 read := m.loadReadOnly() 168 if len(read.m) == 0 && !read.amended { 169 // Avoid allocating a new readOnly when the map is already clear. 170 return 171 } 172 173 m.mu.Lock() 174 defer m.mu.Unlock() 175 176 read = m.loadReadOnly() 177 if len(read.m) > 0 || read.amended { 178 m.read.Store(&readOnly{}) 179 } 180 181 clear(m.dirty) 182 // Don't immediately promote the newly-cleared dirty map on the next operation. 183 m.misses = 0 184 } 185 186 // tryCompareAndSwap compare the entry with the given old value and swaps 187 // it with a new value if the entry is equal to the old value, and the entry 188 // has not been expunged. 189 // 190 // If the entry is expunged, tryCompareAndSwap returns false and leaves 191 // the entry unchanged. 192 func (e *entry) tryCompareAndSwap(old, new any) bool { 193 p := e.p.Load() 194 if p == nil || p == expunged || *p != old { 195 return false 196 } 197 198 // Copy the interface after the first load to make this method more amenable 199 // to escape analysis: if the comparison fails from the start, we shouldn't 200 // bother heap-allocating an interface value to store. 201 nc := new 202 for { 203 if e.p.CompareAndSwap(p, &nc) { 204 return true 205 } 206 p = e.p.Load() 207 if p == nil || p == expunged || *p != old { 208 return false 209 } 210 } 211 } 212 213 // unexpungeLocked ensures that the entry is not marked as expunged. 214 // 215 // If the entry was previously expunged, it must be added to the dirty map 216 // before m.mu is unlocked. 217 func (e *entry) unexpungeLocked() (wasExpunged bool) { 218 return e.p.CompareAndSwap(expunged, nil) 219 } 220 221 // swapLocked unconditionally swaps a value into the entry. 222 // 223 // The entry must be known not to be expunged. 224 func (e *entry) swapLocked(i *any) *any { 225 return e.p.Swap(i) 226 } 227 228 // LoadOrStore returns the existing value for the key if present. 229 // Otherwise, it stores and returns the given value. 230 // The loaded result is true if the value was loaded, false if stored. 231 func (m *Map) LoadOrStore(key, value any) (actual any, loaded bool) { 232 // Avoid locking if it's a clean hit. 233 read := m.loadReadOnly() 234 if e, ok := read.m[key]; ok { 235 actual, loaded, ok := e.tryLoadOrStore(value) 236 if ok { 237 return actual, loaded 238 } 239 } 240 241 m.mu.Lock() 242 read = m.loadReadOnly() 243 if e, ok := read.m[key]; ok { 244 if e.unexpungeLocked() { 245 m.dirty[key] = e 246 } 247 actual, loaded, _ = e.tryLoadOrStore(value) 248 } else if e, ok := m.dirty[key]; ok { 249 actual, loaded, _ = e.tryLoadOrStore(value) 250 m.missLocked() 251 } else { 252 if !read.amended { 253 // We're adding the first new key to the dirty map. 254 // Make sure it is allocated and mark the read-only map as incomplete. 255 m.dirtyLocked() 256 m.read.Store(&readOnly{m: read.m, amended: true}) 257 } 258 m.dirty[key] = newEntry(value) 259 actual, loaded = value, false 260 } 261 m.mu.Unlock() 262 263 return actual, loaded 264 } 265 266 // tryLoadOrStore atomically loads or stores a value if the entry is not 267 // expunged. 268 // 269 // If the entry is expunged, tryLoadOrStore leaves the entry unchanged and 270 // returns with ok==false. 271 func (e *entry) tryLoadOrStore(i any) (actual any, loaded, ok bool) { 272 p := e.p.Load() 273 if p == expunged { 274 return nil, false, false 275 } 276 if p != nil { 277 return *p, true, true 278 } 279 280 // Copy the interface after the first load to make this method more amenable 281 // to escape analysis: if we hit the "load" path or the entry is expunged, we 282 // shouldn't bother heap-allocating. 283 ic := i 284 for { 285 if e.p.CompareAndSwap(nil, &ic) { 286 return i, false, true 287 } 288 p = e.p.Load() 289 if p == expunged { 290 return nil, false, false 291 } 292 if p != nil { 293 return *p, true, true 294 } 295 } 296 } 297 298 // LoadAndDelete deletes the value for a key, returning the previous value if any. 299 // The loaded result reports whether the key was present. 300 func (m *Map) LoadAndDelete(key any) (value any, loaded bool) { 301 read := m.loadReadOnly() 302 e, ok := read.m[key] 303 if !ok && read.amended { 304 m.mu.Lock() 305 read = m.loadReadOnly() 306 e, ok = read.m[key] 307 if !ok && read.amended { 308 e, ok = m.dirty[key] 309 delete(m.dirty, key) 310 // Regardless of whether the entry was present, record a miss: this key 311 // will take the slow path until the dirty map is promoted to the read 312 // map. 313 m.missLocked() 314 } 315 m.mu.Unlock() 316 } 317 if ok { 318 return e.delete() 319 } 320 return nil, false 321 } 322 323 // Delete deletes the value for a key. 324 func (m *Map) Delete(key any) { 325 m.LoadAndDelete(key) 326 } 327 328 func (e *entry) delete() (value any, ok bool) { 329 for { 330 p := e.p.Load() 331 if p == nil || p == expunged { 332 return nil, false 333 } 334 if e.p.CompareAndSwap(p, nil) { 335 return *p, true 336 } 337 } 338 } 339 340 // trySwap swaps a value if the entry has not been expunged. 341 // 342 // If the entry is expunged, trySwap returns false and leaves the entry 343 // unchanged. 344 func (e *entry) trySwap(i *any) (*any, bool) { 345 for { 346 p := e.p.Load() 347 if p == expunged { 348 return nil, false 349 } 350 if e.p.CompareAndSwap(p, i) { 351 return p, true 352 } 353 } 354 } 355 356 // Swap swaps the value for a key and returns the previous value if any. 357 // The loaded result reports whether the key was present. 358 func (m *Map) Swap(key, value any) (previous any, loaded bool) { 359 read := m.loadReadOnly() 360 if e, ok := read.m[key]; ok { 361 if v, ok := e.trySwap(&value); ok { 362 if v == nil { 363 return nil, false 364 } 365 return *v, true 366 } 367 } 368 369 m.mu.Lock() 370 read = m.loadReadOnly() 371 if e, ok := read.m[key]; ok { 372 if e.unexpungeLocked() { 373 // The entry was previously expunged, which implies that there is a 374 // non-nil dirty map and this entry is not in it. 375 m.dirty[key] = e 376 } 377 if v := e.swapLocked(&value); v != nil { 378 loaded = true 379 previous = *v 380 } 381 } else if e, ok := m.dirty[key]; ok { 382 if v := e.swapLocked(&value); v != nil { 383 loaded = true 384 previous = *v 385 } 386 } else { 387 if !read.amended { 388 // We're adding the first new key to the dirty map. 389 // Make sure it is allocated and mark the read-only map as incomplete. 390 m.dirtyLocked() 391 m.read.Store(&readOnly{m: read.m, amended: true}) 392 } 393 m.dirty[key] = newEntry(value) 394 } 395 m.mu.Unlock() 396 return previous, loaded 397 } 398 399 // CompareAndSwap swaps the old and new values for key 400 // if the value stored in the map is equal to old. 401 // The old value must be of a comparable type. 402 func (m *Map) CompareAndSwap(key, old, new any) (swapped bool) { 403 read := m.loadReadOnly() 404 if e, ok := read.m[key]; ok { 405 return e.tryCompareAndSwap(old, new) 406 } else if !read.amended { 407 return false // No existing value for key. 408 } 409 410 m.mu.Lock() 411 defer m.mu.Unlock() 412 read = m.loadReadOnly() 413 swapped = false 414 if e, ok := read.m[key]; ok { 415 swapped = e.tryCompareAndSwap(old, new) 416 } else if e, ok := m.dirty[key]; ok { 417 swapped = e.tryCompareAndSwap(old, new) 418 // We needed to lock mu in order to load the entry for key, 419 // and the operation didn't change the set of keys in the map 420 // (so it would be made more efficient by promoting the dirty 421 // map to read-only). 422 // Count it as a miss so that we will eventually switch to the 423 // more efficient steady state. 424 m.missLocked() 425 } 426 return swapped 427 } 428 429 // CompareAndDelete deletes the entry for key if its value is equal to old. 430 // The old value must be of a comparable type. 431 // 432 // If there is no current value for key in the map, CompareAndDelete 433 // returns false (even if the old value is the nil interface value). 434 func (m *Map) CompareAndDelete(key, old any) (deleted bool) { 435 read := m.loadReadOnly() 436 e, ok := read.m[key] 437 if !ok && read.amended { 438 m.mu.Lock() 439 read = m.loadReadOnly() 440 e, ok = read.m[key] 441 if !ok && read.amended { 442 e, ok = m.dirty[key] 443 // Don't delete key from m.dirty: we still need to do the “compare” part 444 // of the operation. The entry will eventually be expunged when the 445 // dirty map is promoted to the read map. 446 // 447 // Regardless of whether the entry was present, record a miss: this key 448 // will take the slow path until the dirty map is promoted to the read 449 // map. 450 m.missLocked() 451 } 452 m.mu.Unlock() 453 } 454 for ok { 455 p := e.p.Load() 456 if p == nil || p == expunged || *p != old { 457 return false 458 } 459 if e.p.CompareAndSwap(p, nil) { 460 return true 461 } 462 } 463 return false 464 } 465 466 // Range calls f sequentially for each key and value present in the map. 467 // If f returns false, range stops the iteration. 468 // 469 // Range does not necessarily correspond to any consistent snapshot of the Map's 470 // contents: no key will be visited more than once, but if the value for any key 471 // is stored or deleted concurrently (including by f), Range may reflect any 472 // mapping for that key from any point during the Range call. Range does not 473 // block other methods on the receiver; even f itself may call any method on m. 474 // 475 // Range may be O(N) with the number of elements in the map even if f returns 476 // false after a constant number of calls. 477 func (m *Map) Range(f func(key, value any) bool) { 478 // We need to be able to iterate over all of the keys that were already 479 // present at the start of the call to Range. 480 // If read.amended is false, then read.m satisfies that property without 481 // requiring us to hold m.mu for a long time. 482 read := m.loadReadOnly() 483 if read.amended { 484 // m.dirty contains keys not in read.m. Fortunately, Range is already O(N) 485 // (assuming the caller does not break out early), so a call to Range 486 // amortizes an entire copy of the map: we can promote the dirty copy 487 // immediately! 488 m.mu.Lock() 489 read = m.loadReadOnly() 490 if read.amended { 491 read = readOnly{m: m.dirty} 492 copyRead := read 493 m.read.Store(©Read) 494 m.dirty = nil 495 m.misses = 0 496 } 497 m.mu.Unlock() 498 } 499 500 for k, e := range read.m { 501 v, ok := e.load() 502 if !ok { 503 continue 504 } 505 if !f(k, v) { 506 break 507 } 508 } 509 } 510 511 func (m *Map) missLocked() { 512 m.misses++ 513 if m.misses < len(m.dirty) { 514 return 515 } 516 m.read.Store(&readOnly{m: m.dirty}) 517 m.dirty = nil 518 m.misses = 0 519 } 520 521 func (m *Map) dirtyLocked() { 522 if m.dirty != nil { 523 return 524 } 525 526 read := m.loadReadOnly() 527 m.dirty = make(map[any]*entry, len(read.m)) 528 for k, e := range read.m { 529 if !e.tryExpungeLocked() { 530 m.dirty[k] = e 531 } 532 } 533 } 534 535 func (e *entry) tryExpungeLocked() (isExpunged bool) { 536 p := e.p.Load() 537 for p == nil { 538 if e.p.CompareAndSwap(nil, expunged) { 539 return true 540 } 541 p = e.p.Load() 542 } 543 return p == expunged 544 } 545