From 46d9b07deece6e0d5feed6fd2799ca0af7cd35ee Mon Sep 17 00:00:00 2001
From: David Faure <david.faure@kdab.com>
Date: Wed, 9 Apr 2025 14:24:04 +0200
Subject: [PATCH] QPointer: fix data race on ExternalRefCountData

The creation of the ExternalRefCountData was published with
testAndSetOrdered() but the loadRelaxed() would only load the pointer
value, not the effects of the constructor.

Pick-to: 6.5
Fixes: QTBUG-135640
Change-Id: I3acbc51e763e8a291be3f7036e0d9cd3965a2ce8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
(cherry picked from commit 253f34082f526ff1ffd9eaefac73cc9aa616ab2a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit df454f51f884004c4e65386b690c565d60af3630)
---
 src/corelib/tools/qsharedpointer.cpp          |  2 +-
 .../corelib/kernel/qpointer/tst_qpointer.cpp  | 37 +++++++++++++++++++
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/src/corelib/tools/qsharedpointer.cpp b/src/corelib/tools/qsharedpointer.cpp
index 47634c2e74d..32e22869ad2 100644
--- a/src/corelib/tools/qsharedpointer.cpp
+++ b/src/corelib/tools/qsharedpointer.cpp
@@ -1444,7 +1444,7 @@ QtSharedPointer::ExternalRefCountData *QtSharedPointer::ExternalRefCountData::ge
     QObjectPrivate *d = QObjectPrivate::get(const_cast<QObject *>(obj));
     Q_ASSERT_X(!d->wasDeleted, "QWeakPointer", "Detected QWeakPointer creation in a QObject being deleted");
 
-    ExternalRefCountData *that = d->sharedRefcount.loadRelaxed();
+    ExternalRefCountData *that = d->sharedRefcount.loadAcquire();
     if (that) {
         that->weakref.ref();
         return that;
diff --git a/tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp b/tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp
index fa95fe85f7b..7968aa6fcb4 100644
--- a/tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp
+++ b/tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp
@@ -6,6 +6,7 @@
 
 #include <QtCore/private/qobject_p.h>
 #include <QRunnable>
+#include <QSemaphore>
 #include <QThreadPool>
 
 #include <QPointer>
@@ -34,6 +35,7 @@ private slots:
     void disconnect();
     void castDuringDestruction();
     void threadSafety();
+    void raceCondition();
 
     void qvariantCast();
     void constPointer();
@@ -529,6 +531,41 @@ void tst_QPointer::threadSafety()
     owner.wait();
 }
 
+void tst_QPointer::raceCondition()
+{
+    const int NUM_THREADS = 20;
+    const int ITERATIONS_PER_THREAD = 10;
+
+    QSemaphore startSemaphore;
+
+    QObject targetObject;
+
+    std::vector<std::unique_ptr<QThread>> threads;
+    threads.reserve(NUM_THREADS);
+
+    for (int i = 0; i < NUM_THREADS; ++i) {
+        QThread *thread =
+                QThread::create([&startSemaphore, &targetObject, ITERATIONS_PER_THREAD]() {
+                    startSemaphore.acquire();
+
+                    for (int j = 0; j < ITERATIONS_PER_THREAD; ++j) {
+                        QPointer<QObject> pointer(&targetObject);
+                        Q_UNUSED(pointer);
+                    }
+                });
+
+        threads.emplace_back(thread);
+        thread->start();
+    }
+
+    QTest::qWait(100);
+    startSemaphore.release(NUM_THREADS);
+
+    for (const auto &thread : threads) {
+        QVERIFY(thread->wait(30000));
+    }
+}
+
 void tst_QPointer::qvariantCast()
 {
     QFile file;