Recursively read-locking a QReadWriteLock fails if other thread is waiting for write lock

#include <QThread>
#include <QReadWriteLock>
#include <QDebug>

#include <unistd.h>

class RecursiveLock : public QReadWriteLock
{
public:
    RecursiveLock() : QReadWriteLock(QReadWriteLock::Recursive) {}
};

RecursiveLock lock;

class ReadLockThread : public QThread
{
    Q_OBJECT
protected:
    void run()
    {
        QReadLocker lock1(&lock);
        qDebug() << "Read lock 1";
        sleep(5);
        {
            QReadLocker lock2(&lock);
            qDebug() << "Read lock 2";
        }
    }
};

int main(int argc, char *argv[])
{
    ReadLockThread t;
    t.start();
    sleep(2);
    QWriteLocker lock3(&lock);
    qDebug() << "Write lock";
}

#include "main.moc"

Thread 2 (Thread 0x7f8939d18700 (LWP 4630)):
#0  pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
No locals.
#1  0x00007f893d9767fb in QWaitCondition::wait(QMutex*, unsigned long) () from /home/ulf/Qt/5.5/gcc_64/lib/libQt5Core.so.5
No symbol table info available.
#2  0x00007f893d96ec4c in QReadWriteLock::lockForRead() () from /home/ulf/Qt/5.5/gcc_64/lib/libQt5Core.so.5
No symbol table info available.
#3  0x00000000004018bb in QReadLocker::relock (this=0x7f8939d17e30) at ../../Qt/5.5/gcc_64/include/QtCore/qreadwritelock.h:98
No locals.
#4  0x0000000000401942 in QReadLocker::QReadLocker (this=0x7f8939d17e30, areadWriteLock=0x602ca8 <lock>) at ../../Qt/5.5/gcc_64/include/QtCore/qreadwritelock.h:117
No locals.
#5  0x0000000000401be5 in ReadLockThread::run (this=0x7ffc31e8f2e0) at ../deadlock/main.cpp:58
        lock2 = {q_val = 6302888}
        lock1 = {q_val = 6302889}
        __PRETTY_FUNCTION__ = "virtual void ReadLockThread::run()"
#6  0x00007f893d9757df in ?? () from /home/ulf/Qt/5.5/gcc_64/lib/libQt5Core.so.5
No symbol table info available.
#7  0x00007f893d6bd0a4 in start_thread (arg=0x7f8939d18700) at pthread_create.c:309
        __res = <optimized out>
        pd = 0x7f8939d18700
        now = <optimized out>
        unwind_buf = {cancel_jmp_buf = {{jmp_buf = {140227357280000, 7339065152721156614, 1, 140227429761120, 7, 140227357280000, -7292476246588098042, -7292486643519423994}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x0, 0x0}, data = {prev = 0x0, cleanup = 0x0, canceltype = 0}}}
        not_first_call = <optimized out>
        pagesize_m1 = <optimized out>
        sp = <optimized out>
        freesize = <optimized out>
        __PRETTY_FUNCTION__ = "start_thread"
#8  0x00007f893cbd004d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111
No locals.

Thread 1 (Thread 0x7f893e206740 (LWP 4622)):
#0  pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
No locals.
#1  0x00007f893d9767fb in QWaitCondition::wait(QMutex*, unsigned long) () from /home/ulf/Qt/5.5/gcc_64/lib/libQt5Core.so.5
No symbol table info available.
#2  0x00007f893d96ee46 in QReadWriteLock::lockForWrite() () from /home/ulf/Qt/5.5/gcc_64/lib/libQt5Core.so.5
No symbol table info available.
#3  0x00000000004019eb in QWriteLocker::relock (this=0x7ffc31e8f2d0) at ../../Qt/5.5/gcc_64/include/QtCore/qreadwritelock.h:142
No locals.
#4  0x0000000000401a72 in QWriteLocker::QWriteLocker (this=0x7ffc31e8f2d0, areadWriteLock=0x602ca8 <lock>) at ../../Qt/5.5/gcc_64/include/QtCore/qreadwritelock.h:162
No locals.
#5  0x00000000004014c3 in main (argc=1, argv=0x7ffc31e8f418) at ../deadlock/main.cpp:69
        t = {<QThread> = {<No data fields>}, static staticMetaObject = {d = {superdata = 0x7f893e00e2c0 <QThread::staticMetaObject>, stringdata = 0x401fc0 <qt_meta_stringdata_ReadLockThread>, data = 0x402000 <qt_meta_data_ReadLockThread>, static_metacall = 0x401576 <ReadLockThread::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}}
        lock3 = {q_val = 6302888}
        __PRETTY_FUNCTION__ = "int main(int, char**)"