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_LEAPFROG_H
14 #define JUNCTION_CONCURRENTMAP_LEAPFROG_H
16 #include <junction/Core.h>
17 #include <junction/details/LeapFrog.h>
18 #include <junction/QSBR.h>
19 #include <turf/Heap.h>
20 #include <turf/Trace.h>
24 TURF_TRACE_DECLARE(ConcurrentMap_LeapFrog, 17)
26 template <typename K, typename V, class KT = DefaultKeyTraits<K>, class VT = DefaultValueTraits<V> >
27 class ConcurrentMap_LeapFrog {
32 typedef VT ValueTraits;
33 typedef typename turf::util::BestFit<Key>::Unsigned Hash;
34 typedef details::LeapFrog<ConcurrentMap_LeapFrog> Details;
37 turf::Atomic<typename Details::Table*> m_root;
40 ConcurrentMap_LeapFrog(ureg capacity = Details::InitialSize) : m_root(Details::Table::create(capacity)) {
43 ~ConcurrentMap_LeapFrog() {
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_LeapFrog;
70 ConcurrentMap_LeapFrog& m_map;
71 typename Details::Table* m_table;
72 typename Details::Cell* m_cell;
75 // Constructor: Find existing cell
76 Mutator(ConcurrentMap_LeapFrog& map, Key key, bool) : m_map(map), m_value(Value(ValueTraits::NullValue)) {
77 TURF_TRACE(ConcurrentMap_LeapFrog, 0, "[Mutator] find constructor called", uptr(m_table), 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_LeapFrog, 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_LeapFrog& map, Key key)
96 : m_map(map), m_table(map.m_root.load(turf::Consume)), m_value(Value(ValueTraits::NullValue)) {
97 TURF_TRACE(ConcurrentMap_LeapFrog, 2, "[Mutator] insert constructor called", uptr(m_table), uptr(key));
98 Hash hash = KeyTraits::hash(key);
100 m_table = m_map.m_root.load(turf::Consume);
102 switch (Details::insert(hash, m_table, m_cell, overflowIdx)) { // Modifies m_cell
103 case Details::InsertResult_InsertedNew: {
104 // We've inserted a new cell. Don't load m_cell->value.
107 case Details::InsertResult_AlreadyFound: {
108 // The hash was already found in the table.
109 m_value = m_cell->value.load(turf::Consume);
110 if (m_value == Value(ValueTraits::Redirect)) {
111 // We've encountered a Redirect value.
112 TURF_TRACE(ConcurrentMap_LeapFrog, 3, "[Mutator] insert was redirected", uptr(m_table), uptr(m_value));
113 break; // Help finish the migration.
115 return; // Found an existing value
117 case Details::InsertResult_Overflow: {
118 Details::beginTableMigration(m_map, m_table, overflowIdx);
122 // A migration has been started (either by us, or another thread). Participate until it's complete.
123 m_table->jobCoordinator.participate();
124 // Try again using the latest root.
129 Value getValue() const {
130 // Return previously loaded value. Don't load it again.
131 return Value(m_value);
134 Value exchangeValue(Value desired) {
135 TURF_ASSERT(desired != Value(ValueTraits::NullValue));
136 TURF_ASSERT(desired != Value(ValueTraits::Redirect));
137 TURF_ASSERT(m_cell); // Cell must have been found or inserted
138 TURF_TRACE(ConcurrentMap_LeapFrog, 4, "[Mutator::exchangeValue] called", uptr(m_table), uptr(m_value));
140 Value oldValue = m_value;
141 if (m_cell->value.compareExchangeStrong(m_value, desired, turf::ConsumeRelease)) {
142 // Exchange was successful. Return previous value.
143 TURF_TRACE(ConcurrentMap_LeapFrog, 5, "[Mutator::exchangeValue] exchanged Value", uptr(m_value),
145 Value result = m_value;
146 m_value = desired; // Leave the mutator in a valid state
149 // The CAS failed and m_value has been updated with the latest value.
150 if (m_value != Value(ValueTraits::Redirect)) {
151 TURF_TRACE(ConcurrentMap_LeapFrog, 6, "[Mutator::exchangeValue] detected race to write value", uptr(m_table),
153 if (oldValue == Value(ValueTraits::NullValue) && m_value != Value(ValueTraits::NullValue)) {
154 TURF_TRACE(ConcurrentMap_LeapFrog, 7, "[Mutator::exchangeValue] racing write inserted new value",
155 uptr(m_table), uptr(m_value));
157 // There was a racing write (or erase) to this cell.
158 // Pretend we exchanged with ourselves, and just let the racing write win.
161 // We've encountered a Redirect value. Help finish the migration.
162 TURF_TRACE(ConcurrentMap_LeapFrog, 8, "[Mutator::exchangeValue] was redirected", uptr(m_table), uptr(m_value));
163 Hash hash = m_cell->hash.load(turf::Relaxed);
165 // Help complete the migration.
166 m_table->jobCoordinator.participate();
167 // Try again in the new table.
168 m_table = m_map.m_root.load(turf::Consume);
169 m_value = Value(ValueTraits::NullValue);
171 switch (Details::insert(hash, m_table, m_cell, overflowIdx)) { // 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_LeapFrog, 9, "[Mutator::exchangeValue] was re-redirected", uptr(m_table),
180 case Details::InsertResult_InsertedNew:
182 case Details::InsertResult_Overflow:
183 TURF_TRACE(ConcurrentMap_LeapFrog, 10, "[Mutator::exchangeValue] overflow after redirect", uptr(m_table),
185 Details::beginTableMigration(m_map, m_table, overflowIdx);
188 // We were redirected... again
191 // Try again in the new table.
195 void setValue(Value desired) {
196 exchangeValue(desired);
200 TURF_ASSERT(m_cell); // Cell must have been found or inserted
201 TURF_TRACE(ConcurrentMap_LeapFrog, 11, "[Mutator::eraseValue] called", uptr(m_table), uptr(m_cell));
203 if (m_value == Value(ValueTraits::NullValue))
204 return Value(m_value);
205 TURF_ASSERT(m_cell); // m_value is non-NullValue, therefore cell must have been found or inserted.
206 if (m_cell->value.compareExchangeStrong(m_value, Value(ValueTraits::NullValue), turf::Consume)) {
207 // Exchange was successful and a non-NULL value was erased and returned by reference in m_value.
208 TURF_ASSERT(m_value != ValueTraits::NullValue); // Implied by the test at the start of the loop.
209 Value result = m_value;
210 m_value = Value(ValueTraits::NullValue); // Leave the mutator in a valid state
213 // The CAS failed and m_value has been updated with the latest value.
214 TURF_TRACE(ConcurrentMap_LeapFrog, 12, "[Mutator::eraseValue] detected race to write value", uptr(m_table),
216 if (m_value != Value(ValueTraits::Redirect)) {
217 // There was a racing write (or erase) to this cell.
218 // Pretend we erased nothing, and just let the racing write win.
219 return Value(ValueTraits::NullValue);
221 // We've been redirected to a new table.
222 TURF_TRACE(ConcurrentMap_LeapFrog, 13, "[Mutator::eraseValue] was redirected", uptr(m_table), uptr(m_cell));
223 Hash hash = m_cell->hash.load(turf::Relaxed); // Re-fetch hash
225 // Help complete the migration.
226 m_table->jobCoordinator.participate();
227 // Try again in the new table.
228 m_table = m_map.m_root.load(turf::Consume);
229 m_cell = Details::find(hash, m_table);
231 m_value = Value(ValueTraits::NullValue);
234 m_value = m_cell->value.load(turf::Relaxed);
235 if (m_value != Value(ValueTraits::Redirect))
237 TURF_TRACE(ConcurrentMap_LeapFrog, 14, "[Mutator::eraseValue] was re-redirected", uptr(m_table),
244 Mutator insert(Key key) {
245 return Mutator(*this, key);
248 Mutator find(Key key) {
249 return Mutator(*this, key, false);
252 // Lookup without creating a temporary Mutator.
254 Hash hash = KeyTraits::hash(key);
255 TURF_TRACE(ConcurrentMap_LeapFrog, 15, "[get] called", uptr(this), uptr(hash));
257 typename Details::Table* table = m_root.load(turf::Consume);
258 typename Details::Cell* cell = Details::find(hash, table);
260 return Value(ValueTraits::NullValue);
261 Value value = cell->value.load(turf::Consume);
262 if (value != Value(ValueTraits::Redirect))
263 return value; // Found an existing value
264 // We've been redirected to a new table. Help with the migration.
265 TURF_TRACE(ConcurrentMap_LeapFrog, 16, "[get] was redirected", uptr(table), uptr(hash));
266 table->jobCoordinator.participate();
267 // Try again in the new table.
271 Value insert(Key key, Value desired) {
272 Mutator iter(*this, key);
273 return iter.exchangeValue(desired);
276 Value exchange(Key key, Value desired) {
277 Mutator iter(*this, key);
278 return iter.exchangeValue(desired);
281 Value erase(Key key) {
282 Mutator iter(*this, key, false);
283 return iter.eraseValue();
286 // The easiest way to implement an Iterator is to prevent all Redirects.
287 // The currrent Iterator does that by forbidding concurrent inserts.
288 // To make it work with concurrent inserts, we'd need a way to block TableMigrations.
291 typename Details::Table* m_table;
297 Iterator(ConcurrentMap_LeapFrog& map) {
298 // Since we've forbidden concurrent inserts (for now), nonatomic would suffice here, but let's plan ahead:
299 m_table = map.m_root.load(turf::Consume);
305 TURF_ASSERT(m_table);
306 TURF_ASSERT(isValid() || m_idx == -1); // Either the Iterator is already valid, or we've just started iterating.
307 while (++m_idx <= m_table->sizeMask) {
308 // Index still inside range of table.
309 typename Details::CellGroup* group = m_table->getCellGroups() + (m_idx >> 2);
310 typename Details::Cell* cell = group->cells + (m_idx & 3);
311 m_hash = cell->hash.load(turf::Relaxed);
312 if (m_hash != KeyTraits::NullHash) {
313 // Cell has been reserved.
314 m_value = cell->value.load(turf::Relaxed);
315 TURF_ASSERT(m_value != Value(ValueTraits::Redirect));
316 if (m_value != Value(ValueTraits::NullValue))
317 return; // Yield this cell.
320 // That's the end of the map.
321 m_hash = KeyTraits::NullHash;
322 m_value = ValueTraits::NullValue;
325 bool isValid() const {
326 return m_value != Value(ValueTraits::NullValue);
330 TURF_ASSERT(isValid());
331 // Since we've forbidden concurrent inserts (for now), nonatomic would suffice here, but let's plan ahead:
332 return KeyTraits::dehash(m_hash);
335 Value getValue() const {
336 TURF_ASSERT(isValid());
342 } // namespace junction
344 #endif // JUNCTION_CONCURRENTMAP_LEAPFROG_H