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 Value value = m_cell->value.load(turf::Consume);
85 if (value != Value(ValueTraits::Redirect)) {
86 // Found an existing value
90 // We've encountered a Redirect value. Help finish the migration.
91 TURF_TRACE(ConcurrentMap_Linear, 1, "[Mutator] find was redirected", uptr(m_table), 0);
92 m_table->jobCoordinator.participate();
93 // Try again using the latest root.
97 // Constructor: Insert cell
98 Mutator(ConcurrentMap_Linear& map, Key key) : m_map(map), m_value(Value(ValueTraits::NullValue)) {
99 TURF_TRACE(ConcurrentMap_Linear, 2, "[Mutator] insertOrFind constructor called", uptr(0), uptr(key));
100 Hash hash = KeyTraits::hash(key);
101 bool mustDouble = false;
103 m_table = m_map.m_root.load(turf::Consume);
104 switch (Details::insertOrFind(hash, m_table, m_cell)) { // Modifies m_cell
105 case Details::InsertResult_InsertedNew: {
106 // We've inserted a new cell. Don't load m_cell->value.
109 case Details::InsertResult_AlreadyFound: {
110 // The hash was already found in the table.
111 Value value = m_cell->value.load(turf::Consume);
112 if (value == Value(ValueTraits::Redirect)) {
113 // We've encountered a Redirect value.
114 TURF_TRACE(ConcurrentMap_Linear, 3, "[Mutator] insertOrFind was redirected", uptr(m_table), uptr(m_value));
115 break; // Help finish the migration.
117 // Found an existing value
121 case Details::InsertResult_Overflow: {
122 Details::beginTableMigration(m_map, m_table, mustDouble);
126 // A migration has been started (either by us, or another thread). Participate until it's complete.
127 m_table->jobCoordinator.participate();
128 // If we still overflow after this, avoid an infinite loop by forcing the next table to double.
130 // Try again using the latest root.
135 Value getValue() const {
136 // Return previously loaded value. Don't load it again.
137 return Value(m_value);
140 Value exchangeValue(Value desired) {
141 TURF_ASSERT(desired != Value(ValueTraits::NullValue));
142 TURF_ASSERT(desired != Value(ValueTraits::Redirect));
143 TURF_ASSERT(m_cell); // Cell must have been found or inserted
144 TURF_TRACE(ConcurrentMap_Linear, 4, "[Mutator::exchangeValue] called", uptr(m_table), uptr(m_value));
145 bool mustDouble = false;
147 Value oldValue = m_value;
148 if (m_cell->value.compareExchangeStrong(m_value, desired, turf::ConsumeRelease)) {
149 // Exchange was successful. Return previous value.
150 TURF_TRACE(ConcurrentMap_Linear, 5, "[Mutator::exchangeValue] exchanged Value", uptr(m_value), uptr(desired));
151 Value result = m_value;
152 m_value = desired; // Leave the mutator in a valid state
155 // The CAS failed and m_value has been updated with the latest value.
156 if (m_value != Value(ValueTraits::Redirect)) {
157 TURF_TRACE(ConcurrentMap_Linear, 6, "[Mutator::exchangeValue] detected race to write value", uptr(m_table),
159 if (oldValue == Value(ValueTraits::NullValue) && m_value != Value(ValueTraits::NullValue)) {
160 TURF_TRACE(ConcurrentMap_Linear, 7, "[Mutator::exchangeValue] racing write inserted new value",
161 uptr(m_table), uptr(m_value));
163 // There was a racing write (or erase) to this cell.
164 // Pretend we exchanged with ourselves, and just let the racing write win.
167 // We've encountered a Redirect value. Help finish the migration.
168 TURF_TRACE(ConcurrentMap_Linear, 8, "[Mutator::exchangeValue] was redirected", uptr(m_table), uptr(m_value));
169 Hash hash = m_cell->hash.load(turf::Relaxed);
171 // Help complete the migration.
172 m_table->jobCoordinator.participate();
173 // Try again in the new table.
174 m_table = m_map.m_root.load(turf::Consume);
175 m_value = Value(ValueTraits::NullValue);
176 switch (Details::insertOrFind(hash, m_table, m_cell)) { // Modifies m_cell
177 case Details::InsertResult_AlreadyFound:
178 m_value = m_cell->value.load(turf::Consume);
179 if (m_value == Value(ValueTraits::Redirect)) {
180 TURF_TRACE(ConcurrentMap_Linear, 9, "[Mutator::exchangeValue] was re-redirected", uptr(m_table),
185 case Details::InsertResult_InsertedNew:
187 case Details::InsertResult_Overflow:
188 TURF_TRACE(ConcurrentMap_Linear, 10, "[Mutator::exchangeValue] overflow after redirect", uptr(m_table), 0);
189 Details::beginTableMigration(m_map, m_table, mustDouble);
192 // If we still overflow after this, avoid an infinite loop by forcing the next table to double.
194 // We were redirected... again
197 // Try again in the new table.
201 void assignValue(Value desired) {
202 exchangeValue(desired);
206 TURF_ASSERT(m_cell); // Cell must have been found or inserted
207 TURF_TRACE(ConcurrentMap_Linear, 11, "[Mutator::eraseValue] called", uptr(m_table), m_cell - m_table->getCells());
209 if (m_value == Value(ValueTraits::NullValue))
210 return Value(m_value);
211 TURF_ASSERT(m_cell); // m_value is non-NullValue, therefore cell must have been found or inserted.
212 if (m_cell->value.compareExchangeStrong(m_value, Value(ValueTraits::NullValue), turf::Consume)) {
213 // Exchange was successful and a non-NULL value was erased and returned by reference in m_value.
214 TURF_ASSERT(m_value != ValueTraits::NullValue); // Implied by the test at the start of the loop.
215 Value result = m_value;
216 m_value = Value(ValueTraits::NullValue); // Leave the mutator in a valid state
219 // The CAS failed and m_value has been updated with the latest value.
220 TURF_TRACE(ConcurrentMap_Linear, 12, "[Mutator::eraseValue] detected race to write value", uptr(m_table),
221 m_cell - m_table->getCells());
222 if (m_value != Value(ValueTraits::Redirect)) {
223 // There was a racing write (or erase) to this cell.
224 // Pretend we erased nothing, and just let the racing write win.
225 return Value(ValueTraits::NullValue);
227 // We've been redirected to a new table.
228 TURF_TRACE(ConcurrentMap_Linear, 13, "[Mutator::eraseValue] was redirected", uptr(m_table),
229 m_cell - m_table->getCells());
230 Hash hash = m_cell->hash.load(turf::Relaxed); // Re-fetch hash
232 // Help complete the migration.
233 m_table->jobCoordinator.participate();
234 // Try again in the new table.
235 m_table = m_map.m_root.load(turf::Consume);
236 m_cell = Details::find(hash, m_table);
238 m_value = Value(ValueTraits::NullValue);
241 m_value = m_cell->value.load(turf::Relaxed);
242 if (m_value != Value(ValueTraits::Redirect))
244 TURF_TRACE(ConcurrentMap_Linear, 14, "[Mutator::eraseValue] was re-redirected", uptr(m_table),
245 m_cell - m_table->getCells());
251 Mutator insertOrFind(Key key) {
252 return Mutator(*this, key);
255 Mutator find(Key key) {
256 return Mutator(*this, key, false);
259 // Lookup without creating a temporary Mutator.
261 Hash hash = KeyTraits::hash(key);
262 TURF_TRACE(ConcurrentMap_Linear, 15, "[get] called", uptr(this), uptr(hash));
264 typename Details::Table* table = m_root.load(turf::Consume);
265 typename Details::Cell* cell = Details::find(hash, table);
267 return Value(ValueTraits::NullValue);
268 Value value = cell->value.load(turf::Consume);
269 if (value != Value(ValueTraits::Redirect))
270 return value; // Found an existing value
271 // We've been redirected to a new table. Help with the migration.
272 TURF_TRACE(ConcurrentMap_Linear, 16, "[get] was redirected", uptr(table), uptr(cell));
273 table->jobCoordinator.participate();
274 // Try again in the new table.
278 Value assign(Key key, Value desired) {
279 Mutator iter(*this, key);
280 return iter.exchangeValue(desired);
283 Value exchange(Key key, Value desired) {
284 Mutator iter(*this, key);
285 return iter.exchangeValue(desired);
288 Value erase(Key key) {
289 Mutator iter(*this, key, false);
290 return iter.eraseValue();
293 // The easiest way to implement an Iterator is to prevent all Redirects.
294 // The currrent Iterator does that by forbidding concurrent inserts.
295 // To make it work with concurrent inserts, we'd need a way to block TableMigrations.
298 typename Details::Table* m_table;
304 Iterator(ConcurrentMap_Linear& map) {
305 // Since we've forbidden concurrent inserts (for now), nonatomic would suffice here, but let's plan ahead:
306 m_table = map.m_root.load(turf::Consume);
312 TURF_ASSERT(m_table);
313 TURF_ASSERT(isValid() || m_idx == -1); // Either the Iterator is already valid, or we've just started iterating.
314 while (++m_idx <= m_table->sizeMask) {
315 // Index still inside range of table.
316 typename Details::Cell* cell = m_table->getCells() + m_idx;
317 m_hash = cell->hash.load(turf::Relaxed);
318 if (m_hash != KeyTraits::NullHash) {
319 // Cell has been reserved.
320 m_value = cell->value.load(turf::Relaxed);
321 TURF_ASSERT(m_value != Value(ValueTraits::Redirect));
322 if (m_value != Value(ValueTraits::NullValue))
323 return; // Yield this cell.
326 // That's the end of the map.
327 m_hash = KeyTraits::NullHash;
328 m_value = Value(ValueTraits::NullValue);
331 bool isValid() const {
332 return m_value != Value(ValueTraits::NullValue);
336 TURF_ASSERT(isValid());
337 // Since we've forbidden concurrent inserts (for now), nonatomic would suffice here, but let's plan ahead:
338 return KeyTraits::dehash(m_hash);
341 Value getValue() const {
342 TURF_ASSERT(isValid());
348 } // namespace junction
350 #endif // JUNCTION_CONCURRENTMAP_LINEAR_H