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Details

Type: Bug
Resolution: Fixed
Priority: P2: Important
Fix Version/s: 6.6.2, 6.7.0 Beta1, 6.8.0 FF
Affects Version/s: 6.0, 6.5.3, dev
Component/s: Core: Threads
Labels:
None

Commits:
59e21a536 (dev), f89d80c90 (6.7), d5528cdec (6.6)

Description

To reproduce:

Obtain a future from QtConcurrent::run()
Add a continuation with a context object that belongs to the main thread
Call QFuture::waitForFinished on the main thread

Expected result:

3: waitForFinished completes

Actual result:

3: waitForFinished deadlocks occasionally

Reason for deadlock

QFuture::waitForFinished() calls QFutureInterfaceBase::reportFinished() which locks a QMutex. Further down the callstack, T QFutureInterface<T>::takeResult() again locks the same mutex inside the call to Q_ASSERT(isValid());

     ntdll.dll!00007ffeefaf2e54()    Unknown
     ntdll.dll!00007ffeefa839c5()    Unknown
     ntdll.dll!00007ffeefa9f9bc()    Unknown
     ntdll.dll!00007ffeefa9ba83()    Unknown
     KernelBase.dll!00007ffeed0f2773()    Unknown
     Qt6Cored.dll!QtWindowsFutex::futexWait<QBasicAtomicPointer<QMutexPrivate>>(QBasicAtomicPointer<QMutexPrivate> & futex, QMutexPrivate * expectedValue) Line 36    C++
     Qt6Cored.dll!QBasicMutex::lockInternal() Line 650    C++
     Qt6Cored.dll!QBasicMutex::lock() Line 43    C++
     Qt6Cored.dll!QMutexLocker<QMutex>::QMutexLocker<QMutex>(QMutex * mutex) Line 224    C++
     Qt6Cored.dll!QFutureInterfaceBase::isValid() Line 326    C++
     ffmpegmediaplugind.dll!QFutureInterface<QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::takeResult() Line 421    C++
     ffmpegmediaplugind.dll!QFuture<QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::takeResult<QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>,void>() Line 117    C++
     ffmpegmediaplugind.dll!QtPrivate::Continuation<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>,void,QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::fulfillVoidPromise() Line 642    C++
     ffmpegmediaplugind.dll!QtPrivate::Continuation<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>,void,QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::runFunction() Line 460    C++
     ffmpegmediaplugind.dll!QtPrivate::SyncContinuation<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>,void,QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::runImpl() Line 346    C++
     ffmpegmediaplugind.dll!QtPrivate::Continuation<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>,void,QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::execute() Line 492    C++
     ffmpegmediaplugind.dll!QtPrivate::Continuation<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>,void,QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::create::__l2::<lambda_1>::operator()() Line 622    C++
     ffmpegmediaplugind.dll!QtPrivate::FunctorCall<QtPrivate::IndexesList<>,QtPrivate::List<>,void,`QtPrivate::Continuation<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>,void,QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::create<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>>'::`2'::<lambda_1>>::call(QtPrivate::Continuation<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>,void,QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::create::__l2::<lambda_1> & f, void * * arg) Line 137    C++
     ffmpegmediaplugind.dll!QtPrivate::Functor<`QtPrivate::Continuation<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>,void,QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::create<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>>'::`2'::<lambda_1>,0>::call<QtPrivate::List<>,void>(QtPrivate::Continuation<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>,void,QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::create::__l2::<lambda_1> & f, void * __formal, void * * arg) Line 340    C++
     ffmpegmediaplugind.dll!QtPrivate::QCallableObject<`QtPrivate::Continuation<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>,void,QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::create<`QFFmpeg::PlaybackEngine::setMedia'::`2'::<lambda_2>>'::`2'::<lambda_1>,QtPrivate::List<>,void>::impl(int which, QtPrivate::QSlotObjectBase * this_, QObject * r, void * * a, bool * ret) Line 570    C++
     Qt6Cored.dll!QtPrivate::QSlotObjectBase::call(QObject * r, void * * a) Line 472    C++
     Qt6Cored.dll!QtPrivate::watchContinuationImpl::__l2::<lambda_1>::operator()() Line 132    C++
     Qt6Cored.dll!QtPrivate::FunctorCall<QtPrivate::IndexesList<>,QtPrivate::List<>,void,`QtPrivate::watchContinuationImpl'::`2'::<lambda_1>>::call(QtPrivate::watchContinuationImpl::__l2::<lambda_1> & f, void * * arg) Line 137    C++
     Qt6Cored.dll!QtPrivate::Functor<`QtPrivate::watchContinuationImpl'::`2'::<lambda_1>,0>::call<QtPrivate::List<>,void>(QtPrivate::watchContinuationImpl::__l2::<lambda_1> & f, void * __formal, void * * arg) Line 340    C++
     Qt6Cored.dll!QtPrivate::QCallableObject<`QtPrivate::watchContinuationImpl'::`2'::<lambda_1>,QtPrivate::List<>,void>::impl(int which, QtPrivate::QSlotObjectBase * this_, QObject * r, void * * a, bool * ret) Line 570    C++
     Qt6Cored.dll!QtPrivate::QSlotObjectBase::call(QObject * r, void * * a) Line 472    C++
     Qt6Cored.dll!doActivate<0>(QObject * sender, int signal_index, void * * argv) Line 4039    C++
     Qt6Cored.dll!QMetaObject::activate(QObject * sender, const QMetaObject * m, int local_signal_index, void * * argv) Line 4100    C++
     Qt6Cored.dll!QBasicFutureWatcher::finished() Line 140    C++
     Qt6Cored.dll!QBasicFutureWatcher::event(QEvent * event) Line 109    C++
     Qt6Cored.dll!QCoreApplicationPrivate::notify_helper(QObject * receiver, QEvent * event) Line 1305    C++
     Qt6Cored.dll!doNotify(QObject * receiver, QEvent * event) Line 1232    C++
     Qt6Cored.dll!QCoreApplication::notify(QObject * receiver, QEvent * event) Line 1216    C++
     Qt6Guid.dll!QGuiApplication::notify(QObject * object, QEvent * event) Line 1994    C++
     Qt6Cored.dll!QCoreApplication::notifyInternal2(QObject * receiver, QEvent * event) Line 1131    C++
     Qt6Cored.dll!QCoreApplication::sendEvent(QObject * receiver, QEvent * event) Line 1573    C++
     Qt6Cored.dll!QBasicFutureWatcher::postCallOutEvent(const QFutureCallOutEvent & event) Line 73    C++
     Qt6Cored.dll!QFutureInterfaceBasePrivate::sendCallOut(const QFutureCallOutEvent & callOutEvent) Line 825    C++
     Qt6Cored.dll!QFutureInterfaceBase::reportFinished() Line 477    C++
     ffmpegmediaplugind.dll!QFutureInterface<QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::reportFinished() Line 254    C++
     ffmpegmediaplugind.dll!QtConcurrent::RunFunctionTaskBase<QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::run() Line 92    C++
     Qt6Cored.dll!QThreadPoolPrivate::stealAndRunRunnable(QRunnable * runnable) Line 380    C++
     Qt6Cored.dll!QFutureInterfaceBase::waitForFinished() Line 539    C++
     ffmpegmediaplugind.dll!QFuture<QMaybe<QFFmpeg::MediaDataHolder,QFFmpeg::MediaDataHolder::ContextError>>::waitForFinished() Line 102    C++
>    ffmpegmediaplugind.dll!QFFmpeg::PlaybackEngine::~PlaybackEngine() Line 57    C++
     ffmpegmediaplugind.dll!QFFmpeg::PlaybackEngine::`scalar deleting destructor'(unsigned int)    C++