fddeec60cb
3a449bbb69 amended the code to remove
acquiring a lock when waking up a condition variable. It is fine to
not have a lock associated when waking a condition variable; what I
misunderstood was the scope of the lock, which (and this underlines
the importance of commenting _what exactly_ a lock protects, for
each and ever lock) protected both the buffer as well as the counter
of the buffer. This made my reasoning flawed: it is necessary to keep
the lock while notifying, otherwise the counterpart could verify the
condition isn't satisfied and wait (e.g. see numUsedBytes==0), missing
the wake from the other thread (which could arrive between the check and
the wait).
Amends the previous commit.
Change-Id: If7db2d045331f1b33b976fb6bf6aa9117c41678f
Pick-to: 5.15 6.2 6.4 6.5
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
109 lines
2.3 KiB
C++
109 lines
2.3 KiB
C++
// Copyright (C) 2016 The Qt Company Ltd.
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// Copyright (C) 2022 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
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// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
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#include <QCoreApplication>
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#include <QMutex>
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#include <QMutexLocker>
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#include <QObject>
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#include <QRandomGenerator>
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#include <QThread>
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#include <QWaitCondition>
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#include <stdio.h>
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#include <stdlib.h>
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//! [0]
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constexpr int DataSize = 100000;
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constexpr int BufferSize = 8192;
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QMutex mutex; // protects the buffer and the counter
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char buffer[BufferSize];
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int numUsedBytes;
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QWaitCondition bufferNotEmpty;
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QWaitCondition bufferNotFull;
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//! [0]
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//! [1]
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class Producer : public QThread
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//! [1] //! [2]
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{
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public:
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explicit Producer(QObject *parent = nullptr)
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: QThread(parent)
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{
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}
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private:
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void run() override
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{
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for (int i = 0; i < DataSize; ++i) {
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{
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const QMutexLocker locker(&mutex);
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while (numUsedBytes == BufferSize)
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bufferNotFull.wait(&mutex);
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}
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buffer[i % BufferSize] = "ACGT"[QRandomGenerator::global()->bounded(4)];
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{
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const QMutexLocker locker(&mutex);
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++numUsedBytes;
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bufferNotEmpty.wakeAll();
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}
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}
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}
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};
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//! [2]
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//! [3]
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class Consumer : public QThread
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//! [3] //! [4]
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{
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public:
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explicit Consumer(QObject *parent = nullptr)
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: QThread(parent)
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{
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}
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private:
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void run() override
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{
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for (int i = 0; i < DataSize; ++i) {
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{
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const QMutexLocker locker(&mutex);
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while (numUsedBytes == 0)
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bufferNotEmpty.wait(&mutex);
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}
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fprintf(stderr, "%c", buffer[i % BufferSize]);
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{
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const QMutexLocker locker(&mutex);
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--numUsedBytes;
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bufferNotFull.wakeAll();
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}
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}
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fprintf(stderr, "\n");
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}
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};
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//! [4]
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//! [5]
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int main(int argc, char *argv[])
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//! [5] //! [6]
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{
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QCoreApplication app(argc, argv);
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Producer producer;
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Consumer consumer;
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producer.start();
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consumer.start();
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producer.wait();
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consumer.wait();
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return 0;
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}
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//! [6]
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