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 m.dirty == nil.
    77  	//
    78  	// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
    79  	// is missing from m.dirty.
    80  	//
    81  	// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
    82  	// != nil, in m.dirty[key].
    83  	//
    84  	// An entry can be deleted by atomic replacement with nil: when m.dirty is
    85  	// next created, it will atomically replace nil with expunged and leave
    86  	// m.dirty[key] unset.
    87  	//
    88  	// An entry's associated value can be updated by atomic replacement, provided
    89  	// p != expunged. If p == expunged, an entry's associated value can be updated
    90  	// only after first setting m.dirty[key] = e so that lookups using the dirty
    91  	// map find the entry.
    92  	p unsafe.Pointer // *interface{}
    93  }
    94  
    95  func newEntry(i interface{}) *entry {
    96  	return &entry{p: unsafe.Pointer(&i)}
    97  }
    98  
    99  // Load returns the value stored in the map for a key, or nil if no
   100  // value is present.
   101  // The ok result indicates whether value was found in the map.
   102  func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
   103  	read, _ := m.read.Load().(readOnly)
   104  	e, ok := read.m[key]
   105  	if !ok && read.amended {
   106  		m.mu.Lock()
   107  		// Avoid reporting a spurious miss if m.dirty got promoted while we were
   108  		// blocked on m.mu. (If further loads of the same key will not miss, it's
   109  		// not worth copying the dirty map for this key.)
   110  		read, _ = m.read.Load().(readOnly)
   111  		e, ok = read.m[key]
   112  		if !ok && read.amended {
   113  			e, ok = m.dirty[key]
   114  			// Regardless of whether the entry was present, record a miss: this key
   115  			// will take the slow path until the dirty map is promoted to the read
   116  			// map.
   117  			m.missLocked()
   118  		}
   119  		m.mu.Unlock()
   120  	}
   121  	if !ok {
   122  		return nil, false
   123  	}
   124  	return e.load()
   125  }
   126  
   127  func (e *entry) load() (value interface{}, ok bool) {
   128  	p := atomic.LoadPointer(&e.p)
   129  	if p == nil || p == expunged {
   130  		return nil, false
   131  	}
   132  	return *(*interface{})(p), true
   133  }
   134  
   135  // Store sets the value for a key.
   136  func (m *Map) Store(key, value interface{}) {
   137  	read, _ := m.read.Load().(readOnly)
   138  	if e, ok := read.m[key]; ok && e.tryStore(&value) {
   139  		return
   140  	}
   141  
   142  	m.mu.Lock()
   143  	read, _ = m.read.Load().(readOnly)
   144  	if e, ok := read.m[key]; ok {
   145  		if e.unexpungeLocked() {
   146  			// The entry was previously expunged, which implies that there is a
   147  			// non-nil dirty map and this entry is not in it.
   148  			m.dirty[key] = e
   149  		}
   150  		e.storeLocked(&value)
   151  	} else if e, ok := m.dirty[key]; ok {
   152  		e.storeLocked(&value)
   153  	} else {
   154  		if !read.amended {
   155  			// We're adding the first new key to the dirty map.
   156  			// Make sure it is allocated and mark the read-only map as incomplete.
   157  			m.dirtyLocked()
   158  			m.read.Store(readOnly{m: read.m, amended: true})
   159  		}
   160  		m.dirty[key] = newEntry(value)
   161  	}
   162  	m.mu.Unlock()
   163  }
   164  
   165  // tryStore stores a value if the entry has not been expunged.
   166  //
   167  // If the entry is expunged, tryStore returns false and leaves the entry
   168  // unchanged.
   169  func (e *entry) tryStore(i *interface{}) bool {
   170  	for {
   171  		p := atomic.LoadPointer(&e.p)
   172  		if p == expunged {
   173  			return false
   174  		}
   175  		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
   176  			return true
   177  		}
   178  	}
   179  }
   180  
   181  // unexpungeLocked ensures that the entry is not marked as expunged.
   182  //
   183  // If the entry was previously expunged, it must be added to the dirty map
   184  // before m.mu is unlocked.
   185  func (e *entry) unexpungeLocked() (wasExpunged bool) {
   186  	return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
   187  }
   188  
   189  // storeLocked unconditionally stores a value to the entry.
   190  //
   191  // The entry must be known not to be expunged.
   192  func (e *entry) storeLocked(i *interface{}) {
   193  	atomic.StorePointer(&e.p, unsafe.Pointer(i))
   194  }
   195  
   196  // LoadOrStore returns the existing value for the key if present.
   197  // Otherwise, it stores and returns the given value.
   198  // The loaded result is true if the value was loaded, false if stored.
   199  func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
   200  	// Avoid locking if it's a clean hit.
   201  	read, _ := m.read.Load().(readOnly)
   202  	if e, ok := read.m[key]; ok {
   203  		actual, loaded, ok := e.tryLoadOrStore(value)
   204  		if ok {
   205  			return actual, loaded
   206  		}
   207  	}
   208  
   209  	m.mu.Lock()
   210  	read, _ = m.read.Load().(readOnly)
   211  	if e, ok := read.m[key]; ok {
   212  		if e.unexpungeLocked() {
   213  			m.dirty[key] = e
   214  		}
   215  		actual, loaded, _ = e.tryLoadOrStore(value)
   216  	} else if e, ok := m.dirty[key]; ok {
   217  		actual, loaded, _ = e.tryLoadOrStore(value)
   218  		m.missLocked()
   219  	} else {
   220  		if !read.amended {
   221  			// We're adding the first new key to the dirty map.
   222  			// Make sure it is allocated and mark the read-only map as incomplete.
   223  			m.dirtyLocked()
   224  			m.read.Store(readOnly{m: read.m, amended: true})
   225  		}
   226  		m.dirty[key] = newEntry(value)
   227  		actual, loaded = value, false
   228  	}
   229  	m.mu.Unlock()
   230  
   231  	return actual, loaded
   232  }
   233  
   234  // tryLoadOrStore atomically loads or stores a value if the entry is not
   235  // expunged.
   236  //
   237  // If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
   238  // returns with ok==false.
   239  func (e *entry) tryLoadOrStore(i interface{}) (actual interface{}, loaded, ok bool) {
   240  	p := atomic.LoadPointer(&e.p)
   241  	if p == expunged {
   242  		return nil, false, false
   243  	}
   244  	if p != nil {
   245  		return *(*interface{})(p), true, true
   246  	}
   247  
   248  	// Copy the interface after the first load to make this method more amenable
   249  	// to escape analysis: if we hit the "load" path or the entry is expunged, we
   250  	// shouldn't bother heap-allocating.
   251  	ic := i
   252  	for {
   253  		if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {
   254  			return i, false, true
   255  		}
   256  		p = atomic.LoadPointer(&e.p)
   257  		if p == expunged {
   258  			return nil, false, false
   259  		}
   260  		if p != nil {
   261  			return *(*interface{})(p), true, true
   262  		}
   263  	}
   264  }
   265  
   266  // Delete deletes the value for a key.
   267  func (m *Map) Delete(key interface{}) {
   268  	read, _ := m.read.Load().(readOnly)
   269  	e, ok := read.m[key]
   270  	if !ok && read.amended {
   271  		m.mu.Lock()
   272  		read, _ = m.read.Load().(readOnly)
   273  		e, ok = read.m[key]
   274  		if !ok && read.amended {
   275  			delete(m.dirty, key)
   276  		}
   277  		m.mu.Unlock()
   278  	}
   279  	if ok {
   280  		e.delete()
   281  	}
   282  }
   283  
   284  func (e *entry) delete() (hadValue bool) {
   285  	for {
   286  		p := atomic.LoadPointer(&e.p)
   287  		if p == nil || p == expunged {
   288  			return false
   289  		}
   290  		if atomic.CompareAndSwapPointer(&e.p, p, nil) {
   291  			return true
   292  		}
   293  	}
   294  }
   295  
   296  // Range calls f sequentially for each key and value present in the map.
   297  // If f returns false, range stops the iteration.
   298  //
   299  // Range does not necessarily correspond to any consistent snapshot of the Map's
   300  // contents: no key will be visited more than once, but if the value for any key
   301  // is stored or deleted concurrently, Range may reflect any mapping for that key
   302  // from any point during the Range call.
   303  //
   304  // Range may be O(N) with the number of elements in the map even if f returns
   305  // false after a constant number of calls.
   306  func (m *Map) Range(f func(key, value interface{}) bool) {
   307  	// We need to be able to iterate over all of the keys that were already
   308  	// present at the start of the call to Range.
   309  	// If read.amended is false, then read.m satisfies that property without
   310  	// requiring us to hold m.mu for a long time.
   311  	read, _ := m.read.Load().(readOnly)
   312  	if read.amended {
   313  		// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
   314  		// (assuming the caller does not break out early), so a call to Range
   315  		// amortizes an entire copy of the map: we can promote the dirty copy
   316  		// immediately!
   317  		m.mu.Lock()
   318  		read, _ = m.read.Load().(readOnly)
   319  		if read.amended {
   320  			read = readOnly{m: m.dirty}
   321  			m.read.Store(read)
   322  			m.dirty = nil
   323  			m.misses = 0
   324  		}
   325  		m.mu.Unlock()
   326  	}
   327  
   328  	for k, e := range read.m {
   329  		v, ok := e.load()
   330  		if !ok {
   331  			continue
   332  		}
   333  		if !f(k, v) {
   334  			break
   335  		}
   336  	}
   337  }
   338  
   339  func (m *Map) missLocked() {
   340  	m.misses++
   341  	if m.misses < len(m.dirty) {
   342  		return
   343  	}
   344  	m.read.Store(readOnly{m: m.dirty})
   345  	m.dirty = nil
   346  	m.misses = 0
   347  }
   348  
   349  func (m *Map) dirtyLocked() {
   350  	if m.dirty != nil {
   351  		return
   352  	}
   353  
   354  	read, _ := m.read.Load().(readOnly)
   355  	m.dirty = make(map[interface{}]*entry, len(read.m))
   356  	for k, e := range read.m {
   357  		if !e.tryExpungeLocked() {
   358  			m.dirty[k] = e
   359  		}
   360  	}
   361  }
   362  
   363  func (e *entry) tryExpungeLocked() (isExpunged bool) {
   364  	p := atomic.LoadPointer(&e.p)
   365  	for p == nil {
   366  		if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
   367  			return true
   368  		}
   369  		p = atomic.LoadPointer(&e.p)
   370  	}
   371  	return p == expunged
   372  }
   373  

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