mirror of https://gitee.com/bigwinds/arangodb
125 lines
3.6 KiB
C++
125 lines
3.6 KiB
C++
////////////////////////////////////////////////////////////////////////////////
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/// DISCLAIMER
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///
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/// Copyright 2014-2017 ArangoDB GmbH, Cologne, Germany
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/// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
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///
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/// Licensed under the Apache License, Version 2.0 (the "License");
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/// you may not use this file except in compliance with the License.
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/// You may obtain a copy of the License at
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///
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/// http://www.apache.org/licenses/LICENSE-2.0
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///
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/// Unless required by applicable law or agreed to in writing, software
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/// distributed under the License is distributed on an "AS IS" BASIS,
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/// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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/// See the License for the specific language governing permissions and
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/// limitations under the License.
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///
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/// Copyright holder is ArangoDB GmbH, Cologne, Germany
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///
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/// @author Daniel H. Larkin
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////////////////////////////////////////////////////////////////////////////////
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#ifndef ARANGO_READ_WRITE_SPIN_LOCK_H
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#define ARANGO_READ_WRITE_SPIN_LOCK_H 1
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#include "Basics/Common.h"
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#include "Basics/SharedAtomic.h"
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#include "Basics/SharedCounter.h"
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#include "Basics/cpu-relax.h"
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#include <atomic>
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namespace arangodb {
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namespace basics {
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template <uint64_t stripes = 64>
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struct ReadWriteSpinLock {
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typedef std::function<uint64_t()> IdFunc;
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ReadWriteSpinLock() : _writer(false), _readers(SharedCounter<stripes>::DefaultIdFunc) {}
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ReadWriteSpinLock(IdFunc f) : _writer(false), _readers(f) {}
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ReadWriteSpinLock(ReadWriteSpinLock const& other) {
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if (this != &other) {
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_writer = other._writer;
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_readers = other._readers;
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}
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}
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bool writeLock(uint64_t maxTries = UINT64_MAX) {
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uint64_t attempts = 0;
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while (attempts < maxTries) {
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if (!_writer.load(std::memory_order_relaxed)) {
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// attempt to get read lock
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bool expected = false;
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bool success = _writer.compare_exchange_weak(expected, true,
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std::memory_order_acq_rel,
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std::memory_order_relaxed);
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if (success) {
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// write lock acquired, wait for readers to finish
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while (attempts++ < maxTries && _readers.nonZero(std::memory_order_acquire)) {
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cpu_relax();
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}
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if (attempts >= maxTries) {
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// timed out waiting for readers, release write lock
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_writer.store(false, std::memory_order_release);
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return false;
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}
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// locked!
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return true;
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}
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}
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attempts++;
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cpu_relax();
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} // too many attempts
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return false;
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}
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void writeUnlock() { _writer.store(false, std::memory_order_release); }
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bool readLock(uint64_t maxTries = UINT64_MAX) {
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uint64_t attempts = 0;
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while (attempts++ < maxTries) {
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if (!_writer.load(std::memory_order_relaxed)) {
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_readers.add(1, std::memory_order_acq_rel); // read locked
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// double check writer hasn't stepped in
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if (_writer.load(std::memory_order_acquire)) {
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// writer got the lock, go back to waiting
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_readers.sub(1, std::memory_order_release);
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} else {
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// locked!
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return true;
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}
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}
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cpu_relax();
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} // too many attempts
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return false;
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}
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void readUnlock() { _readers.sub(1, std::memory_order_release); }
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bool isLocked() const {
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return (_readers.nonZero() || _writer.load());
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}
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bool isWriteLocked() const { return _writer.load(); }
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private:
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SharedAtomic<bool> _writer;
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SharedCounter<stripes, true> _readers;
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};
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} // namespace basics
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} // namespace arangodb
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#endif
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