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Source file src/sync/map.go

Documentation: sync

     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[interface{}]*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[interface{}]*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(interface{}))
    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 interface{}) *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 interface{}) (value interface{}, 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 interface{}, ok bool) {
   129  	p := atomic.LoadPointer(&e.p)
   130  	if p == nil || p == expunged {
   131  		return nil, false
   132  	}
   133  	return *(*interface{})(p), true
   134  }
   135  
   136  // Store sets the value for a key.
   137  func (m *Map) Store(key, value interface{}) {
   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 *interface{}) 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 *interface{}) {
   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 interface{}) (actual interface{}, 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 interface{}) (actual interface{}, 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 *(*interface{})(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 *(*interface{})(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 interface{}) (value interface{}, 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 interface{}) {
   294  	m.LoadAndDelete(key)
   295  }
   296  
   297  func (e *entry) delete() (value interface{}, 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 *(*interface{})(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, Range may reflect any mapping for that key
   315  // from any point during the Range call.
   316  //
   317  // Range may be O(N) with the number of elements in the map even if f returns
   318  // false after a constant number of calls.
   319  func (m *Map) Range(f func(key, value interface{}) bool) {
   320  	// We need to be able to iterate over all of the keys that were already
   321  	// present at the start of the call to Range.
   322  	// If read.amended is false, then read.m satisfies that property without
   323  	// requiring us to hold m.mu for a long time.
   324  	read, _ := m.read.Load().(readOnly)
   325  	if read.amended {
   326  		// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
   327  		// (assuming the caller does not break out early), so a call to Range
   328  		// amortizes an entire copy of the map: we can promote the dirty copy
   329  		// immediately!
   330  		m.mu.Lock()
   331  		read, _ = m.read.Load().(readOnly)
   332  		if read.amended {
   333  			read = readOnly{m: m.dirty}
   334  			m.read.Store(read)
   335  			m.dirty = nil
   336  			m.misses = 0
   337  		}
   338  		m.mu.Unlock()
   339  	}
   340  
   341  	for k, e := range read.m {
   342  		v, ok := e.load()
   343  		if !ok {
   344  			continue
   345  		}
   346  		if !f(k, v) {
   347  			break
   348  		}
   349  	}
   350  }
   351  
   352  func (m *Map) missLocked() {
   353  	m.misses++
   354  	if m.misses < len(m.dirty) {
   355  		return
   356  	}
   357  	m.read.Store(readOnly{m: m.dirty})
   358  	m.dirty = nil
   359  	m.misses = 0
   360  }
   361  
   362  func (m *Map) dirtyLocked() {
   363  	if m.dirty != nil {
   364  		return
   365  	}
   366  
   367  	read, _ := m.read.Load().(readOnly)
   368  	m.dirty = make(map[interface{}]*entry, len(read.m))
   369  	for k, e := range read.m {
   370  		if !e.tryExpungeLocked() {
   371  			m.dirty[k] = e
   372  		}
   373  	}
   374  }
   375  
   376  func (e *entry) tryExpungeLocked() (isExpunged bool) {
   377  	p := atomic.LoadPointer(&e.p)
   378  	for p == nil {
   379  		if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
   380  			return true
   381  		}
   382  		p = atomic.LoadPointer(&e.p)
   383  	}
   384  	return p == expunged
   385  }
   386  

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