1 /*------------------------------------------------------------------------
2 Junction: Concurrent data structures in C++
3 Copyright (c) 2016 Jeff Preshing
5 Distributed under the Simplified BSD License.
6 Original location: https://github.com/preshing/junction
8 This software is distributed WITHOUT ANY WARRANTY; without even the
9 implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
10 See the LICENSE file for more information.
11 ------------------------------------------------------------------------*/
13 #ifndef JUNCTION_CONCURRENTMAP_LINEAR_H
14 #define JUNCTION_CONCURRENTMAP_LINEAR_H
16 #include <junction/Core.h>
17 #include <junction/details/Linear.h>
18 #include <junction/QSBR.h>
19 #include <turf/Heap.h>
20 #include <turf/Trace.h>
24 TURF_TRACE_DECLARE(ConcurrentMap_Linear, 17)
26 template <typename K, typename V, class KT = DefaultKeyTraits<K>, class VT = DefaultValueTraits<V> >
27 class ConcurrentMap_Linear {
32 typedef VT ValueTraits;
33 typedef typename turf::util::BestFit<Key>::Unsigned Hash;
34 typedef details::Linear<ConcurrentMap_Linear> Details;
37 turf::Atomic<typename Details::Table*> m_root;
40 ConcurrentMap_Linear(ureg capacity = Details::InitialSize) : m_root(Details::Table::create(capacity)) {
43 ~ConcurrentMap_Linear() {
44 typename Details::Table* table = m_root.loadNonatomic();
48 // publishTableMigration() is called by exactly one thread from Details::TableMigration::run()
49 // after all the threads participating in the migration have completed their work.
50 void publishTableMigration(typename Details::TableMigration* migration) {
51 // There are no racing calls to this function.
52 typename Details::Table* oldRoot = m_root.loadNonatomic();
53 m_root.store(migration->m_destination, turf::Release);
54 TURF_ASSERT(oldRoot == migration->getSources()[0].table);
55 // Caller will GC the TableMigration and the source table.
58 // A Mutator represents a known cell in the hash table.
59 // It's meant for manipulations within a temporary function scope.
60 // Obviously you must not call QSBR::Update while holding a Mutator.
61 // Any operation that modifies the table (exchangeValue, eraseValue)
62 // may be forced to follow a redirected cell, which changes the Mutator itself.
63 // Note that even if the Mutator was constructed from an existing cell,
64 // exchangeValue() can still trigger a resize if the existing cell was previously marked deleted,
65 // or if another thread deletes the key between the two steps.
68 friend class ConcurrentMap_Linear;
70 ConcurrentMap_Linear& m_map;
71 typename Details::Table* m_table;
72 typename Details::Cell* m_cell;
75 // Constructor: Find existing cell
76 Mutator(ConcurrentMap_Linear& map, Key key, bool) : m_map(map), m_value(Value(ValueTraits::NullValue)) {
77 TURF_TRACE(ConcurrentMap_Linear, 0, "[Mutator] find constructor called", uptr(0), uptr(key));
78 Hash hash = KeyTraits::hash(key);
80 m_table = m_map.m_root.load(turf::Consume);
81 m_cell = Details::find(hash, m_table);
84 m_value = m_cell->value.load(turf::Consume);
85 if (m_value != Value(ValueTraits::Redirect))
86 return; // Found an existing value
87 // We've encountered a Redirect value. Help finish the migration.
88 TURF_TRACE(ConcurrentMap_Linear, 1, "[Mutator] find was redirected", uptr(m_table), 0);
89 m_table->jobCoordinator.participate();
90 // Try again using the latest root.
94 // Constructor: Insert cell
95 Mutator(ConcurrentMap_Linear& map, Key key) : m_map(map), m_value(Value(ValueTraits::NullValue)) {
96 TURF_TRACE(ConcurrentMap_Linear, 2, "[Mutator] insertOrFind constructor called", uptr(0), uptr(key));
97 Hash hash = KeyTraits::hash(key);
98 bool mustDouble = false;
100 m_table = m_map.m_root.load(turf::Consume);
101 switch (Details::insertOrFind(hash, m_table, m_cell)) { // Modifies m_cell
102 case Details::InsertResult_InsertedNew: {
103 // We've inserted a new cell. Don't load m_cell->value.
106 case Details::InsertResult_AlreadyFound: {
107 // The hash was already found in the table.
108 m_value = m_cell->value.load(turf::Consume);
109 if (m_value == Value(ValueTraits::Redirect)) {
110 // We've encountered a Redirect value.
111 TURF_TRACE(ConcurrentMap_Linear, 3, "[Mutator] insertOrFind was redirected", uptr(m_table), uptr(m_value));
112 break; // Help finish the migration.
114 return; // Found an existing value
116 case Details::InsertResult_Overflow: {
117 Details::beginTableMigration(m_map, m_table, mustDouble);
121 // A migration has been started (either by us, or another thread). Participate until it's complete.
122 m_table->jobCoordinator.participate();
123 // If we still overflow after this, avoid an infinite loop by forcing the next table to double.
125 // Try again using the latest root.
130 Value getValue() const {
131 // Return previously loaded value. Don't load it again.
132 return Value(m_value);
135 Value exchangeValue(Value desired) {
136 TURF_ASSERT(desired != Value(ValueTraits::NullValue));
137 TURF_ASSERT(desired != Value(ValueTraits::Redirect));
138 TURF_ASSERT(m_cell); // Cell must have been found or inserted
139 TURF_TRACE(ConcurrentMap_Linear, 4, "[Mutator::exchangeValue] called", uptr(m_table), uptr(m_value));
140 bool mustDouble = false;
142 Value oldValue = m_value;
143 if (m_cell->value.compareExchangeStrong(m_value, desired, turf::ConsumeRelease)) {
144 // Exchange was successful. Return previous value.
145 TURF_TRACE(ConcurrentMap_Linear, 5, "[Mutator::exchangeValue] exchanged Value", uptr(m_value), uptr(desired));
146 Value result = m_value;
147 m_value = desired; // Leave the mutator in a valid state
150 // The CAS failed and m_value has been updated with the latest value.
151 if (m_value != Value(ValueTraits::Redirect)) {
152 TURF_TRACE(ConcurrentMap_Linear, 6, "[Mutator::exchangeValue] detected race to write value", uptr(m_table),
154 if (oldValue == Value(ValueTraits::NullValue) && m_value != Value(ValueTraits::NullValue)) {
155 TURF_TRACE(ConcurrentMap_Linear, 7, "[Mutator::exchangeValue] racing write inserted new value",
156 uptr(m_table), uptr(m_value));
158 // There was a racing write (or erase) to this cell.
159 // Pretend we exchanged with ourselves, and just let the racing write win.
162 // We've encountered a Redirect value. Help finish the migration.
163 TURF_TRACE(ConcurrentMap_Linear, 8, "[Mutator::exchangeValue] was redirected", uptr(m_table), uptr(m_value));
164 Hash hash = m_cell->hash.load(turf::Relaxed);
166 // Help complete the migration.
167 m_table->jobCoordinator.participate();
168 // Try again in the new table.
169 m_table = m_map.m_root.load(turf::Consume);
170 m_value = Value(ValueTraits::NullValue);
171 switch (Details::insertOrFind(hash, m_table, m_cell)) { // Modifies m_cell
172 case Details::InsertResult_AlreadyFound:
173 m_value = m_cell->value.load(turf::Consume);
174 if (m_value == Value(ValueTraits::Redirect)) {
175 TURF_TRACE(ConcurrentMap_Linear, 9, "[Mutator::exchangeValue] was re-redirected", uptr(m_table),
180 case Details::InsertResult_InsertedNew:
182 case Details::InsertResult_Overflow:
183 TURF_TRACE(ConcurrentMap_Linear, 10, "[Mutator::exchangeValue] overflow after redirect", uptr(m_table), 0);
184 Details::beginTableMigration(m_map, m_table, mustDouble);
187 // If we still overflow after this, avoid an infinite loop by forcing the next table to double.
189 // We were redirected... again
192 // Try again in the new table.
196 void assignValue(Value desired) {
197 exchangeValue(desired);
201 TURF_ASSERT(m_cell); // Cell must have been found or inserted
202 TURF_TRACE(ConcurrentMap_Linear, 11, "[Mutator::eraseValue] called", uptr(m_table), m_cell - m_table->getCells());
204 if (m_value == Value(ValueTraits::NullValue))
205 return Value(m_value);
206 TURF_ASSERT(m_cell); // m_value is non-NullValue, therefore cell must have been found or inserted.
207 if (m_cell->value.compareExchangeStrong(m_value, Value(ValueTraits::NullValue), turf::Consume)) {
208 // Exchange was successful and a non-NULL value was erased and returned by reference in m_value.
209 TURF_ASSERT(m_value != ValueTraits::NullValue); // Implied by the test at the start of the loop.
210 Value result = m_value;
211 m_value = Value(ValueTraits::NullValue); // Leave the mutator in a valid state
214 // The CAS failed and m_value has been updated with the latest value.
215 TURF_TRACE(ConcurrentMap_Linear, 12, "[Mutator::eraseValue] detected race to write value", uptr(m_table),
216 m_cell - m_table->getCells());
217 if (m_value != Value(ValueTraits::Redirect)) {
218 // There was a racing write (or erase) to this cell.
219 // Pretend we erased nothing, and just let the racing write win.
220 return Value(ValueTraits::NullValue);
222 // We've been redirected to a new table.
223 TURF_TRACE(ConcurrentMap_Linear, 13, "[Mutator::eraseValue] was redirected", uptr(m_table),
224 m_cell - m_table->getCells());
225 Hash hash = m_cell->hash.load(turf::Relaxed); // Re-fetch hash
227 // Help complete the migration.
228 m_table->jobCoordinator.participate();
229 // Try again in the new table.
230 m_table = m_map.m_root.load(turf::Consume);
231 m_cell = Details::find(hash, m_table);
233 m_value = Value(ValueTraits::NullValue);
236 m_value = m_cell->value.load(turf::Relaxed);
237 if (m_value != Value(ValueTraits::Redirect))
239 TURF_TRACE(ConcurrentMap_Linear, 14, "[Mutator::eraseValue] was re-redirected", uptr(m_table),
240 m_cell - m_table->getCells());
246 Mutator insertOrFind(Key key) {
247 return Mutator(*this, key);
250 Mutator find(Key key) {
251 return Mutator(*this, key, false);
254 // Lookup without creating a temporary Mutator.
256 Hash hash = KeyTraits::hash(key);
257 TURF_TRACE(ConcurrentMap_Linear, 15, "[get] called", uptr(this), uptr(hash));
259 typename Details::Table* table = m_root.load(turf::Consume);
260 typename Details::Cell* cell = Details::find(hash, table);
262 return Value(ValueTraits::NullValue);
263 Value value = cell->value.load(turf::Consume);
264 if (value != Value(ValueTraits::Redirect))
265 return value; // Found an existing value
266 // We've been redirected to a new table. Help with the migration.
267 TURF_TRACE(ConcurrentMap_Linear, 16, "[get] was redirected", uptr(table), uptr(cell));
268 table->jobCoordinator.participate();
269 // Try again in the new table.
273 Value assign(Key key, Value desired) {
274 Mutator iter(*this, key);
275 return iter.exchangeValue(desired);
278 Value exchange(Key key, Value desired) {
279 Mutator iter(*this, key);
280 return iter.exchangeValue(desired);
283 Value erase(Key key) {
284 Mutator iter(*this, key, false);
285 return iter.eraseValue();
288 // The easiest way to implement an Iterator is to prevent all Redirects.
289 // The currrent Iterator does that by forbidding concurrent inserts.
290 // To make it work with concurrent inserts, we'd need a way to block TableMigrations.
293 typename Details::Table* m_table;
299 Iterator(ConcurrentMap_Linear& map) {
300 // Since we've forbidden concurrent inserts (for now), nonatomic would suffice here, but let's plan ahead:
301 m_table = map.m_root.load(turf::Consume);
307 TURF_ASSERT(m_table);
308 TURF_ASSERT(isValid() || m_idx == -1); // Either the Iterator is already valid, or we've just started iterating.
309 while (++m_idx <= m_table->sizeMask) {
310 // Index still inside range of table.
311 typename Details::Cell* cell = m_table->getCells() + m_idx;
312 m_hash = cell->hash.load(turf::Relaxed);
313 if (m_hash != KeyTraits::NullHash) {
314 // Cell has been reserved.
315 m_value = cell->value.load(turf::Relaxed);
316 TURF_ASSERT(m_value != Value(ValueTraits::Redirect));
317 if (m_value != Value(ValueTraits::NullValue))
318 return; // Yield this cell.
321 // That's the end of the map.
322 m_hash = KeyTraits::NullHash;
323 m_value = Value(ValueTraits::NullValue);
326 bool isValid() const {
327 return m_value != Value(ValueTraits::NullValue);
331 TURF_ASSERT(isValid());
332 // Since we've forbidden concurrent inserts (for now), nonatomic would suffice here, but let's plan ahead:
333 return KeyTraits::dehash(m_hash);
336 Value getValue() const {
337 TURF_ASSERT(isValid());
343 } // namespace junction
345 #endif // JUNCTION_CONCURRENTMAP_LINEAR_H