qmutex.cpp

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#include "qplatformdefs.h"
#include "qmutex.h"
#include <qdebug.h>

#ifndef QT_NO_THREAD
#include "qatomic.h"
#include "qelapsedtimer.h"
#include "qthread.h"
#include "qmutex_p.h"

QT_BEGIN_NAMESPACE

QMutex::QMutex(RecursionMode mode)
    : d(new QMutexPrivate(mode))
{ }

QMutex::~QMutex()
{ delete static_cast<QMutexPrivate *>(d); }

void QMutex::lock()
{
    QMutexPrivate *d = static_cast<QMutexPrivate *>(this->d);
    Qt::HANDLE self;

    if (d->recursive) {
        self = QThread::currentThreadId();
        if (d->owner == self) {
            ++d->count;
            Q_ASSERT_X(d->count != 0, "QMutex::lock", "Overflow in recursion counter");
            return;
        }

        bool isLocked = d->contenders.testAndSetAcquire(0, 1);
        if (!isLocked) {
            // didn't get the lock, wait for it
            isLocked = d->wait();
            Q_ASSERT_X(isLocked, "QMutex::lock",
                       "Internal error, infinite wait has timed out.");
        }

        d->owner = self;
        ++d->count;
        Q_ASSERT_X(d->count != 0, "QMutex::lock", "Overflow in recursion counter");
        return;
    }

    bool isLocked = d->contenders.testAndSetAcquire(0, 1);
    if (!isLocked) {
        lockInternal();
    }
}

bool QMutex::tryLock()
{
    QMutexPrivate *d = static_cast<QMutexPrivate *>(this->d);
    Qt::HANDLE self;

    if (d->recursive) {
        self = QThread::currentThreadId();
        if (d->owner == self) {
            ++d->count;
            Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter");
            return true;
        }

        bool isLocked = d->contenders.testAndSetAcquire(0, 1);
        if (!isLocked) {
            // some other thread has the mutex locked, or we tried to
            // recursively lock an non-recursive mutex
            return isLocked;
        }

        d->owner = self;
        ++d->count;
        Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter");
        return isLocked;
    }

    return d->contenders.testAndSetAcquire(0, 1);
}

bool QMutex::tryLock(int timeout)
{
    QMutexPrivate *d = static_cast<QMutexPrivate *>(this->d);
    Qt::HANDLE self;

    if (d->recursive) {
        self = QThread::currentThreadId();
        if (d->owner == self) {
            ++d->count;
            Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter");
            return true;
        }

        bool isLocked = d->contenders.testAndSetAcquire(0, 1);
        if (!isLocked) {
            // didn't get the lock, wait for it
            isLocked = d->wait(timeout);
            if (!isLocked)
                return false;
        }

        d->owner = self;
        ++d->count;
        Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter");
        return true;
    }

    return (d->contenders.testAndSetAcquire(0, 1)
            // didn't get the lock, wait for it
            || d->wait(timeout));
}


void QMutex::unlock()
{
    QMutexPrivate *d = static_cast<QMutexPrivate *>(this->d);
    if (d->recursive) {
        if (!--d->count) {
            d->owner = 0;
            if (!d->contenders.testAndSetRelease(1, 0))
                d->wakeUp();
        }
    } else {
        if (!d->contenders.testAndSetRelease(1, 0))
            d->wakeUp();
    }
}

void QMutex::lockInternal()
{
    QMutexPrivate *d = static_cast<QMutexPrivate *>(this->d);

    if (QThread::idealThreadCount() == 1) {
        // don't spin on single cpu machines
        bool isLocked = d->wait();
        Q_ASSERT_X(isLocked, "QMutex::lock",
                   "Internal error, infinite wait has timed out.");
        Q_UNUSED(isLocked);
        return;
    }

    QElapsedTimer elapsedTimer;
    elapsedTimer.start();
    do {
        qint64 spinTime = elapsedTimer.nsecsElapsed();
        if (spinTime > d->maximumSpinTime) {
            // didn't get the lock, wait for it, since we're not going to gain anything by spinning more
            elapsedTimer.start();
            bool isLocked = d->wait();
            Q_ASSERT_X(isLocked, "QMutex::lock",
                       "Internal error, infinite wait has timed out.");
            Q_UNUSED(isLocked);

            qint64 maximumSpinTime = d->maximumSpinTime;
            qint64 averageWaitTime = d->averageWaitTime;
            qint64 actualWaitTime = elapsedTimer.nsecsElapsed();
            if (actualWaitTime < (QMutexPrivate::MaximumSpinTimeThreshold * 3 / 2)) {
                // measure the wait times
                averageWaitTime = d->averageWaitTime = qMin((averageWaitTime + actualWaitTime) / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold));
            }

            // adjust the spin count when spinning does not benefit contention performance
            if ((spinTime + actualWaitTime) - qint64(QMutexPrivate::MaximumSpinTimeThreshold) >= qint64(QMutexPrivate::MaximumSpinTimeThreshold)) {
                // long waits, stop spinning
                d->maximumSpinTime = 0;
            } else {
                // allow spinning if wait times decrease, but never spin more than the average wait time (otherwise we may perform worse)
                d->maximumSpinTime = qBound(qint64(averageWaitTime * 3 / 2), maximumSpinTime / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold));
            }
            return;
        }
        // be a good citizen... yielding lets something else run if there is something to run, but may also relieve memory pressure if not
        QThread::yieldCurrentThread();
    } while (d->contenders != 0 || !d->contenders.testAndSetAcquire(0, 1));

    // spinning is working, do not change the spin time (unless we are using much less time than allowed to spin)
    qint64 maximumSpinTime = d->maximumSpinTime;
    qint64 spinTime = elapsedTimer.nsecsElapsed();
    if (spinTime < maximumSpinTime / 2) {
        // we are using much less time than we need, adjust the limit
        d->maximumSpinTime = qBound(qint64(d->averageWaitTime * 3 / 2), maximumSpinTime / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold));
    }
}

void QMutex::unlockInternal()
{
    static_cast<QMutexPrivate *>(d)->wakeUp();
}

QT_END_NAMESPACE

#endif // QT_NO_THREAD