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/**
* Copyright (C) 2019-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include "mongo/platform/atomic_word.h"
#include <utility>
namespace mongo {
/**
* A finish line that coordinates the winer of a race amongst a series of participants.
*
* The winner of the race is the first to arriveStrongly or the last to arriveWeakly (think the last
* to get disqualified).
*
* Every caller of arriveStrong/Weak can depend on the fact that only one caller will be returned a
* true value. That means that consumers of this latch may use it to coordinate access to mutable
* shared state as long as they only mutate that state when receiving a true response.
*/
class StrongWeakFinishLine {
constexpr static inline uint64_t kHighBit = 1ull << 63;
public:
explicit StrongWeakFinishLine(uint64_t n) : _state(n) {}
/**
* Returns true if this is the first call to arriveStrongly
*
* If the return value is true, a participant will know that it won the race. Note that there
* may still be other participants executing however (though none of them will return true from
* a call to arrive).
*/
bool arriveStrongly() {
if ((_state.load() & kHighBit) || (_state.fetchAndBitOr(kHighBit) & kHighBit)) {
return false;
}
return true;
}
/**
* Returns true if it is the last call to arriveWeakly
*
* If the return value is true, a participant will know that it is the last caller and that all
* other participants also called arriveWeakly.
*/
bool arriveWeakly() {
return _state.subtractAndFetch(1) == 0;
}
/**
* Returns true if someone has arrivedStrongly, or all participants have arrived weakly.
*/
bool isReady() const {
auto value = _state.load();
return (value & kHighBit) || (value == 0);
}
private:
// Theory of operation for State
//
// Initial state: num participants
//
// On strong: atomic.fetch_or(highBit)
// Concurrent calls race to set the high bit.
//
// On weak: atomic.fetch_sub(1)
// Decrements state by 1. If this reduces state to 0, there was no strong arriver.
AtomicWord<uint64_t> _state;
};
} // namespace mongo
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