Atomics: reordering and memory semantics

[mentalmushroom]

I'm now trying to find my feet in lock-free programming. In particular I am curious about possible reordering for different memory semantics.

Consider the following example. There are two threads (producer and consumer). Release-acquire memory order is used for synchronization.

#include <QCoreApplication>
#include <QDebug>
#include <QtConcurrent>
 
QAtomicPointer<QString> ptr;
int data;
 
void producer()
{
    QString* p  = new QString("Hello");
    data = 42;
    ptr.storeRelease(p);
}
 
void consumer()
{
    QString* p2;
    //while (!(p2 = ptr.load())) // ?
    while (!(p2 = ptr.loadAcquire()))
        ;
    Q_ASSERT(*p2 == "Hello"); // never fires
    Q_ASSERT(data == 42); // never fires
}
 
 
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QtConcurrent::run(producer);
    QtConcurrent::run(consumer);
    return a.exec();
}

#include <QCoreApplication>
#include <QDebug>
#include <QtConcurrent>

QAtomicPointer<QString> ptr;
int data;

void producer()
{
    QString* p  = new QString("Hello");
    data = 42;
    ptr.storeRelease(p);
}

void consumer()
{
    QString* p2;
    //while (!(p2 = ptr.load())) // ?
    while (!(p2 = ptr.loadAcquire()))
        ;
    Q_ASSERT(*p2 == "Hello"); // never fires
    Q_ASSERT(data == 42); // never fires
}


int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QtConcurrent::run(producer);
    QtConcurrent::run(consumer);
    return a.exec();
}

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I'm wondering why it is necessary to use release-acquire semantics here? What might happen if we used the relaxed load in consumer?