QSemaphore: fix deadlock when the woken up thread wakes up another

When the thread that got woken up by release() is supposed to release()
to wake up another thread, we were deadlocking. This happened because we
cleared the bit indicating that there was contention when the first
release(). Instead of storing a single bit, we now store the number of
threads waiting.

Task-number: QTBUG-66875
Change-Id: I72f5230ad59948f784eafffd15193873502ecba4
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 69 insertions, 0 deletions

diff --git a/tests/auto/corelib/thread/qsemaphore/tst_qsemaphore.cpp b/tests/auto/corelib/thread/qsemaphore/tst_qsemaphore.cpp
index a67ecc2471..b7134d0454 100644
--- a/tests/auto/corelib/thread/qsemaphore/tst_qsemaphore.cpp
+++ b/tests/auto/corelib/thread/qsemaphore/tst_qsemaphore.cpp
@@ -37,6 +37,8 @@ class tst_QSemaphore : public QObject
     Q_OBJECT
 private slots:
     void acquire();
+    void multiRelease();
+    void multiAcquireRelease();
     void tryAcquire();
     void tryAcquireWithTimeout_data();
     void tryAcquireWithTimeout();
@@ -149,6 +151,73 @@ void tst_QSemaphore::acquire()
     QCOMPARE(semaphore.available(), 0);
 }
 
+void tst_QSemaphore::multiRelease()
+{
+    class Thread : public QThread
+    {
+    public:
+        QSemaphore &sem;
+        Thread(QSemaphore &sem) : sem(sem) {}
+
+        void run() override
+        {
+            sem.acquire();
+        }
+    };
+
+    QSemaphore sem;
+    QVector<Thread *> threads;
+    threads.resize(4);
+
+    for (Thread *&t : threads)
+        t = new Thread(sem);
+    for (Thread *&t : threads)
+        t->start();
+
+    // wait for all threads to reach the sem.acquire() and then
+    // release them all
+    QTest::qSleep(1);
+    sem.release(threads.size());
+
+    for (Thread *&t : threads)
+        t->wait();
+    qDeleteAll(threads);
+}
+
+void tst_QSemaphore::multiAcquireRelease()
+{
+    class Thread : public QThread
+    {
+    public:
+        QSemaphore &sem;
+        Thread(QSemaphore &sem) : sem(sem) {}
+
+        void run() override
+        {
+            sem.acquire();
+            sem.release();
+        }
+    };
+
+    QSemaphore sem;
+    QVector<Thread *> threads;
+    threads.resize(4);
+
+    for (Thread *&t : threads)
+        t = new Thread(sem);
+    for (Thread *&t : threads)
+        t->start();
+
+    // wait for all threads to reach the sem.acquire() and then
+    // release them all
+    QTest::qSleep(1);
+    sem.release();
+
+    for (Thread *&t : threads)
+        t->wait();
+    qDeleteAll(threads);
+}
+
 void tst_QSemaphore::tryAcquire()
 {
     QSemaphore semaphore;