mirror of https://gitee.com/bigwinds/arangodb
116 lines
4.3 KiB
C++
116 lines
4.3 KiB
C++
////////////////////////////////////////////////////////////////////////////////
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/// DISCLAIMER
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///
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/// Copyright 2014-2016 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 Max Neunhoeffer
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////////////////////////////////////////////////////////////////////////////////
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#ifndef ARANGODB_BASICS_READ_WRITE_LOCK_CPP11_H
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#define ARANGODB_BASICS_READ_WRITE_LOCK_CPP11_H 1
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#include "Basics/Common.h"
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#include <mutex>
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#include <condition_variable>
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#include <thread>
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namespace arangodb {
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namespace basics {
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////////////////////////////////////////////////////////////////////////////////
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/// @brief read-write lock, slow but just using CPP11
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/// This class has two other advantages:
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/// (1) it is possible that a thread tries to acquire a lock even if it
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/// has it already. This is important when we are running a thread
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/// pool that works on task groups and a task group needs to acquire
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/// a lock across multiple (non-concurrent) tasks. This must work,
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/// even if tasks from different groups that fight for a lock are
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/// actually executed by the same thread! POSIX RW-locks do not have
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/// this property.
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/// (2) write locks have a preference over read locks: as long as a task
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/// wants to get a write lock, no other task can get a (new) read lock.
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/// This is necessary to avoid starvation of writers by many readers.
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/// The current implementation can starve readers, though.
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////////////////////////////////////////////////////////////////////////////////
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class ReadWriteLockCPP11 {
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public:
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ReadWriteLockCPP11() : _state(0), _wantWrite(false) {}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief locks for writing
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//////////////////////////////////////////////////////////////////////////////
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void writeLock();
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//////////////////////////////////////////////////////////////////////////////
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/// @brief locks for writing, but only tries
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//////////////////////////////////////////////////////////////////////////////
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bool tryWriteLock();
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//////////////////////////////////////////////////////////////////////////////
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/// @brief locks for reading
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//////////////////////////////////////////////////////////////////////////////
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void readLock();
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//////////////////////////////////////////////////////////////////////////////
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/// @brief locks for reading, tries only
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//////////////////////////////////////////////////////////////////////////////
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bool tryReadLock();
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//////////////////////////////////////////////////////////////////////////////
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/// @brief releases the read-lock or write-lock
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//////////////////////////////////////////////////////////////////////////////
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void unlock();
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private:
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//////////////////////////////////////////////////////////////////////////////
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/// @brief a mutex
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//////////////////////////////////////////////////////////////////////////////
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std::mutex _mut;
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//////////////////////////////////////////////////////////////////////////////
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/// @brief a condition variable to wake up threads
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//////////////////////////////////////////////////////////////////////////////
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std::condition_variable _bell;
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//////////////////////////////////////////////////////////////////////////////
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/// @brief _state, 0 means unlocked, -1 means write locked, positive means
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/// a number of read locks
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//////////////////////////////////////////////////////////////////////////////
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int _state;
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//////////////////////////////////////////////////////////////////////////////
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/// @brief _wantWrite, is set if somebody is waiting for the write lock
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//////////////////////////////////////////////////////////////////////////////
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bool _wantWrite;
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};
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}
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}
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#endif
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