qorderedmutexlocker_p.h

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#ifndef QORDEREDMUTEXLOCKER_P_H
#define QORDEREDMUTEXLOCKER_P_H

//
//  W A R N I N G
//  -------------
//
// This file is not part of the Qt API.  It exists purely as an
// implementation detail.  This header file may change from version to
// version without notice, or even be removed.
//
// We mean it.
//

#include <QtCore/qmutex.h>

QT_BEGIN_NAMESPACE

/*
  Locks 2 mutexes in a defined order, avoiding a recursive lock if
  we're trying to lock the same mutex twice.
*/
class QOrderedMutexLocker
{
public:
    QOrderedMutexLocker(QMutex *m1, QMutex *m2)
        : mtx1((m1 == m2) ? m1 : (m1 < m2 ? m1 : m2)),
          mtx2((m1 == m2) ?  0 : (m1 < m2 ? m2 : m1)),
          locked(false)
    {
        relock();
    }
    ~QOrderedMutexLocker()
    {
        unlock();
    }

    void relock()
    {
        if (!locked) {
            if (mtx1) mtx1->lockInline();
            if (mtx2) mtx2->lockInline();
            locked = true;
        }
    }

    void unlock()
    {
        if (locked) {
            if (mtx1) mtx1->unlockInline();
            if (mtx2) mtx2->unlockInline();
            locked = false;
        }
    }

    static bool relock(QMutex *mtx1, QMutex *mtx2)
    {
        // mtx1 is already locked, mtx2 not... do we need to unlock and relock?
        if (mtx1 == mtx2)
            return false;
        if (mtx1 < mtx2) {
            mtx2->lockInline();
            return true;
        }
        if (!mtx2->tryLockInline()) {
            mtx1->unlock();
            mtx2->lock();
            mtx1->lock();
        }
        return true;
    }

private:
    QMutex *mtx1, *mtx2;
    bool locked;
};

QT_END_NAMESPACE

#endif