QReadWriteLock locks for reading...but why?

[Qtonimo]

Hi,
I have a big ressource and several Threads want to read the ressource simultanous. The ressource get exchanged every hour. While exchanging the read-access should be blocked. I solve this with QReadWriteLocker.
But why the Threads are locked for reading. If one thread reads a lot of data, all the other threads are locked, because of QReadLocker.

I just want to lock the threads, while exchanging...

Thanks, bye

[ChrisW67]

If you QReadWriteLock::lockForWrite() the file to write it blocks users in other threads (as documented) until you unlock().

What exactly does "exchanged" mean?

[Qtonimo]

Thanks for reply

Exchanged means renewed...

While renew() the read-access should be blocked....

void DataManager::renew(){
QWriteLocker(&lock);
QList<Data*>* data = getnewdata();
 
//clear old data
delete this->data;
 
//assign new data
this->data = data;
}

void DataManager::renew(){
QWriteLocker(&lock);
QList<Data*>* data = getnewdata();

//clear old data
delete this->data;

//assign new data
this->data = data;
}

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Simultaneous read should be possible, but QReadLocker prevent this.

[Qtonimo]

Nobody can help?
I just want to know why several Threads aren't able to read the same container at the same time?

[ChrisW67]

Active QReadLockers don't block other read locks but an open write lock will.

Here's an example showing three concurrent read locks and no blocks:

#include <QtCore>
#include <QDebug>
 
class SlowReader: public QObject {
    Q_OBJECT
public:
    SlowReader(int number, QReadWriteLock *lock, QObject *p = 0)
        : QObject(p), m_num(number), m_lock(lock)
    {
    }
 
public slots:
    void process()    {
        QReadLocker locker(m_lock); 
        qDebug() << QString("Thread %1 started").arg(m_num);
        for (int i = 0; i < 10; ++i) {
            sleep(1);
            qDebug() << QString("Thread %1 reading").arg(m_num);
        }
        qDebug() << QString("Thread %1 done").arg(m_num);
        emit finished();
    }
 
signals:
    void finished();
 
private:
    int m_num;
    QReadWriteLock *m_lock;
};
 
 
int main(int argc, char **argv) {
    QCoreApplication app(argc, argv);
 
    QReadWriteLock lock;
    for (int i= 0; i < 3; ++i) {
        QThread* thread = new QThread;
        SlowReader *worker = new SlowReader(i, &lock);
        worker->moveToThread(thread);
 
        QObject::connect(thread, SIGNAL(started()), worker, SLOT(process()));
        QObject::connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
        QObject::connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
        QObject::connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
        thread->start();
    }
 
    return app.exec();
}
 
#include "main.moc"

#include <QtCore>
#include <QDebug>

class SlowReader: public QObject {
    Q_OBJECT
public:
    SlowReader(int number, QReadWriteLock *lock, QObject *p = 0)
        : QObject(p), m_num(number), m_lock(lock)
    {
    }

public slots:
    void process()    {
        QReadLocker locker(m_lock); 
        qDebug() << QString("Thread %1 started").arg(m_num);
        for (int i = 0; i < 10; ++i) {
            sleep(1);
            qDebug() << QString("Thread %1 reading").arg(m_num);
        }
        qDebug() << QString("Thread %1 done").arg(m_num);
        emit finished();
    }

signals:
    void finished();

private:
    int m_num;
    QReadWriteLock *m_lock;
};


int main(int argc, char **argv) {
    QCoreApplication app(argc, argv);

    QReadWriteLock lock;
    for (int i= 0; i < 3; ++i) {
        QThread* thread = new QThread;
        SlowReader *worker = new SlowReader(i, &lock);
        worker->moveToThread(thread);

        QObject::connect(thread, SIGNAL(started()), worker, SLOT(process()));
        QObject::connect(worker, SIGNAL(finished()), thread, SLOT(quit()));
        QObject::connect(worker, SIGNAL(finished()), worker, SLOT(deleteLater()));
        QObject::connect(thread, SIGNAL(finished()), thread, SLOT(deleteLater()));
        thread->start();
    }

    return app.exec();
}

#include "main.moc"

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Each thread holds a read lock for the entire 10 seconds it takes to do the 10 reads.

[Qtonimo]

Thank you ChrisW67 for the test app.
It convinced me.