From e41c100460126d8cb044f11db73b52d6b80f9309 Mon Sep 17 00:00:00 2001 From: Lars Knoll Date: Fri, 20 Nov 2020 13:51:16 +0100 Subject: Fix crash in concurrent disconnect This does not fix all data races that we have in the system yet. One major issue is the virtual disconnectNotify(), that can be called from any thread and thus is inherently problematic, as it can collide with the object getting destroyed at the same time in another thread. Task-number: QTBUG-88248 Change-Id: I9d841eb363b7e4f0de1657aeb8f5340d0fd55190 Reviewed-by: Andrei Golubev Reviewed-by: Thiago Macieira (cherry picked from commit 71b4d4f150bc3c904a5aceec37513ddc3cd1c150) Reviewed-by: Qt Cherry-pick Bot --- src/corelib/kernel/qobject.cpp | 47 ++++++++++++++--------- src/corelib/kernel/qobject_p.h | 16 ++++++-- tests/auto/other/qobjectrace/tst_qobjectrace.cpp | 49 ++++++++++++++++++++++++ 3 files changed, 91 insertions(+), 21 deletions(-) diff --git a/src/corelib/kernel/qobject.cpp b/src/corelib/kernel/qobject.cpp index 33afcaaa7e..326fba5be2 100644 --- a/src/corelib/kernel/qobject.cpp +++ b/src/corelib/kernel/qobject.cpp @@ -390,8 +390,14 @@ void QObjectPrivate::ConnectionData::removeConnection(QObjectPrivate::Connection Q_ASSERT(c != orphaned.loadRelaxed()); // add c to orphanedConnections - c->nextInOrphanList = orphaned.loadRelaxed(); - orphaned.storeRelaxed(c); + Connection *o = nullptr; + /* No ABA issue here: When adding a node, we only care about the list head, it doesn't + * matter if the tail changes. + */ + do { + o = orphaned.loadRelaxed(); + c->nextInOrphanList = o; + } while (!orphaned.testAndSetRelease(o, c)); #ifndef QT_NO_DEBUG found = false; @@ -420,8 +426,7 @@ void QObjectPrivate::ConnectionData::cleanOrphanedConnectionsImpl(QObject *sende // Since ref == 1, no activate() is in process since we locked the mutex. That implies, // that nothing can reference the orphaned connection objects anymore and they can // be safely deleted - c = orphaned.loadRelaxed(); - orphaned.storeRelaxed(nullptr); + c = orphaned.fetchAndStoreRelaxed(nullptr); } if (c) { // Deleting c might run arbitrary user code, so we must not hold the lock @@ -1027,7 +1032,7 @@ QObject::~QObject() QBasicMutex *m = signalSlotLock(c->receiver.loadRelaxed()); bool needToUnlock = QOrderedMutexLocker::relock(signalSlotMutex, m); - if (c->receiver.loadAcquire()) { + if (c == connectionList.first.loadAcquire() && c->receiver.loadAcquire()) { cd->removeConnection(c); Q_ASSERT(connectionList.first.loadRelaxed() != c); } @@ -1068,11 +1073,11 @@ QObject::~QObject() if (needToUnlock) m->unlock(); - if (slotObj) { - locker.unlock(); + locker.unlock(); + if (slotObj) slotObj->destroyIfLastRef(); - locker.relock(); - } + senderData->cleanOrphanedConnections(sender); + locker.relock(); } // invalidate all connections on the object and make sure @@ -4978,11 +4983,11 @@ QMetaObject::Connection QObjectPrivate::connectImpl(const QObject *sender, int s bool QObject::disconnect(const QMetaObject::Connection &connection) { QObjectPrivate::Connection *c = static_cast(connection.d_ptr); + if (!c) + return false; const bool disconnected = QObjectPrivate::disconnect(c); - if (disconnected) { - const_cast(connection).d_ptr = nullptr; - c->deref(); // has been removed from the QMetaObject::Connection object - } + const_cast(connection).d_ptr = nullptr; + c->deref(); // has been removed from the QMetaObject::Connection object return disconnected; } @@ -5176,12 +5181,18 @@ bool QObjectPrivate::disconnect(QObjectPrivate::Connection *c) connections = QObjectPrivate::get(c->sender)->connections.loadRelaxed(); Q_ASSERT(connections); connections->removeConnection(c); - } - - connections->cleanOrphanedConnections(c->sender); - c->sender->disconnectNotify(QMetaObjectPrivate::signal(c->sender->metaObject(), - c->signal_index)); + c->sender->disconnectNotify(QMetaObjectPrivate::signal(c->sender->metaObject(), c->signal_index)); + // We must not hold the receiver mutex, else we risk dead-locking; we also only need the sender mutex + // It is however vital to hold the senderMutex before calling cleanOrphanedConnections, as otherwise + // another thread might modify/delete the connection + if (receiverMutex != senderMutex) { + receiverMutex->unlock(); + } + connections->cleanOrphanedConnections(c->sender, ConnectionData::AlreadyLockedAndTemporarilyReleasingLock); + senderMutex->unlock(); // now both sender and receiver mutex have been manually unlocked + locker.dismiss(); // so we dismiss the QOrderedMutexLocker + } return true; } diff --git a/src/corelib/kernel/qobject_p.h b/src/corelib/kernel/qobject_p.h index 8dd29e05ce..4d5543d34a 100644 --- a/src/corelib/kernel/qobject_p.h +++ b/src/corelib/kernel/qobject_p.h @@ -257,7 +257,10 @@ public: ~ConnectionData() { - deleteOrphaned(orphaned.loadRelaxed()); + Q_ASSERT(ref.loadRelaxed() == 0); + auto *c = orphaned.fetchAndStoreRelaxed(nullptr); + if (c) + deleteOrphaned(c); SignalVector *v = signalVector.loadRelaxed(); if (v) free(v); @@ -304,8 +307,15 @@ public: signalVector.storeRelaxed(newVector); if (vector) { - vector->nextInOrphanList = orphaned.loadRelaxed(); - orphaned.storeRelaxed(ConnectionOrSignalVector::fromSignalVector(vector)); + Connection *o = nullptr; + /* No ABA issue here: When adding a node, we only care about the list head, it doesn't + * matter if the tail changes. + */ + do { + o = orphaned.loadRelaxed(); + vector->nextInOrphanList = o; + } while (!orphaned.testAndSetRelease(o, ConnectionOrSignalVector::fromSignalVector(vector))); + } } int signalVectorCount() const diff --git a/tests/auto/other/qobjectrace/tst_qobjectrace.cpp b/tests/auto/other/qobjectrace/tst_qobjectrace.cpp index ac33fa3ec3..093b4d2476 100644 --- a/tests/auto/other/qobjectrace/tst_qobjectrace.cpp +++ b/tests/auto/other/qobjectrace/tst_qobjectrace.cpp @@ -49,6 +49,7 @@ private slots: void destroyRace(); void blockingQueuedDestroyRace(); void disconnectRace(); + void disconnectRace2(); }; class RaceObject : public QObject @@ -562,5 +563,53 @@ void tst_QObjectRace::disconnectRace() QCOMPARE(countedStructObjectsCount.loadRelaxed(), 0u); } +void tst_QObjectRace::disconnectRace2() +{ + enum { IterationCount = 100, ConnectionCount = 100, YieldCount = 100 }; + + QAtomicPointer ptr; + QSemaphore createSemaphore(0); + QSemaphore proceedSemaphore(0); + + std::unique_ptr t1(QThread::create([&]() { + for (int i = 0; i < IterationCount; ++i) { + MyObject sender; + ptr.storeRelease(&sender); + createSemaphore.release(); + proceedSemaphore.acquire(); + ptr.storeRelaxed(nullptr); + for (int i = 0; i < YieldCount; ++i) + QThread::yieldCurrentThread(); + } + })); + t1->start(); + + + std::unique_ptr t2(QThread::create([&]() { + auto connections = std::make_unique(ConnectionCount); + for (int i = 0; i < IterationCount; ++i) { + MyObject receiver; + MyObject *sender = nullptr; + + createSemaphore.acquire(); + + while (!(sender = ptr.loadAcquire())) + ; + + for (int i = 0; i < ConnectionCount; ++i) + connections[i] = QObject::connect(sender, &MyObject::signal1, &receiver, &MyObject::slot1); + + proceedSemaphore.release(); + + for (int i = 0; i < ConnectionCount; ++i) + QObject::disconnect(connections[i]); + } + })); + t2->start(); + + t1->wait(); + t2->wait(); +} + QTEST_MAIN(tst_QObjectRace) #include "tst_qobjectrace.moc" -- cgit v1.2.3