Qt的moveToThread和数据竞争

Qt moveToThread and data races

本文关键字：数据 竞争 moveToThread Qt      更新时间：2023-10-16

有一个只有一个信号和插槽的类:

#ifndef SOME_CLASS_H
#define SOME_CLASS_H
#include <QObject>
class SomeClass : public QObject
{
    Q_OBJECT
public:
    explicit SomeClass(QObject *parent = 0) : QObject(parent) {
        connect(this, &SomeClass::valueChanged, this, &SomeClass::setValue);
    }
    void emitSignal() {
        emit valueChanged();
    }
signals:
    void valueChanged();
public slots:
    void setValue() {}
};
#endif // SOME_CLASS_H

下面是程序的主文件，它定义了一个单例，创建了上面定义的类的实例，并把它放在单独的线程中:

#include <QCoreApplication>
#include <QThread>
#include <someclass.h>
class Singleton {
public:
    QThread t;
    SomeClass someClassObject;
    Singleton() {
        someClassObject.moveToThread(&t);
        t.start();
    }
    static SomeClass& getInstance() {
        static Singleton st;
        return st.someClassObject;
    }
    ~Singleton() {
        t.quit();
        t.wait();
    }
};
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    SomeClass& b = Singleton::getInstance();
    b.emitSignal();
    QThread::msleep(100);
    return a.exec();
}

问题是，当我使用一些线程检查工具(如ThreadSanitizer或Helgrind)评估这个程序时，会报告几个关于数据竞争条件的警告。ThreadSanitizer的输出示例:

==================
WARNING: ThreadSanitizer: data race (pid=3827)
  Read of size 8 at 0x7d040000f6a0 by thread T1:
    #0 void QtPrivate::FunctionPointer<void (SomeClass::*)()>::call<void, void>(void (SomeClass::*)(), SomeClass*, void**) /usr/include/qt5/QtCore/qobjectdefs_impl.h:142 (exe+0x0000000a2d1c)
    #1 QtPrivate::QSlotObject<void (SomeClass::*)(), void, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) /usr/include/qt5/QtCore/qobject_impl.h:149 (exe+0x0000000a29b4)
    #2 QObject::event(QEvent*) ??:0 (libQt5Core.so.5+0x00000029022d)
  Previous write of size 8 at 0x7d040000f6a0 by main thread:
    #0 malloc ??:0 (exe+0x0000000426e9)
    #1 QMetaObject::activate(QObject*, int, int, void**) ??:0 (libQt5Core.so.5+0x00000028ef1c)
    #2 main /tmp/project/main.cpp:31 (exe+0x0000000a19f2)
  Location is heap block of size 8 at 0x7d040000f6a0 allocated by main thread:
    #0 malloc ??:0 (exe+0x0000000426e9)
    #1 QMetaObject::activate(QObject*, int, int, void**) ??:0 (libQt5Core.so.5+0x00000028ef1c)
    #2 main /tmp/project/main.cpp:31 (exe+0x0000000a19f2)
  Thread T1 'QThread' (tid=3829, running) created by main thread at:
    #0 pthread_create ??:0 (exe+0x000000046f6b)
    #1 QThread::start(QThread::Priority) ??:0 (libQt5Core.so.5+0x0000000928b7)
    #2 Singleton::getInstance() /tmp/project/main.cpp:16 (exe+0x0000000a1b3c)
    #3 main /tmp/project/main.cpp:30 (exe+0x0000000a19bf)
SUMMARY: ThreadSanitizer: data race /usr/include/qt5/QtCore/qobjectdefs_impl.h:142 void QtPrivate::FunctionPointer<void (SomeClass::*)()>::call<void, void>(void (SomeClass::*)(), SomeClass*, void**)
==================
==================
WARNING: ThreadSanitizer: data race (pid=3827)
  Write of size 1 at 0x7d040000f6b0 by thread T1:
    #0 free ??:0 (exe+0x000000042dfb)
    #1 QMetaCallEvent::~QMetaCallEvent() ??:0 (libQt5Core.so.5+0x00000028d3a4)
  Previous write of size 8 at 0x7d040000f6b0 by main thread:
    #0 malloc ??:0 (exe+0x0000000426e9)
    #1 QMetaObject::activate(QObject*, int, int, void**) ??:0 (libQt5Core.so.5+0x00000028eefe)
    #2 main /tmp/project/main.cpp:31 (exe+0x0000000a19f2)
  Location is heap block of size 4 at 0x7d040000f6b0 allocated by main thread:
    #0 malloc ??:0 (exe+0x0000000426e9)
    #1 QMetaObject::activate(QObject*, int, int, void**) ??:0 (libQt5Core.so.5+0x00000028eefe)
    #2 main /tmp/project/main.cpp:31 (exe+0x0000000a19f2)
  Thread T1 'QThread' (tid=3829, running) created by main thread at:
    #0 pthread_create ??:0 (exe+0x000000046f6b)
    #1 QThread::start(QThread::Priority) ??:0 (libQt5Core.so.5+0x0000000928b7)
    #2 Singleton::getInstance() /tmp/project/main.cpp:16 (exe+0x0000000a1b3c)
    #3 main /tmp/project/main.cpp:30 (exe+0x0000000a19bf)
SUMMARY: ThreadSanitizer: data race ??:0 free
==================
==================
WARNING: ThreadSanitizer: data race (pid=3827)
  Write of size 8 at 0x7d180000edc0 by thread T1:
    #0 operator delete(void*) ??:0 (exe+0x00000004392b)
    #1 QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) ??:0 (libQt5Core.so.5+0x000000269e0f)
  Previous write of size 8 at 0x7d180000edc0 by main thread:
    #0 operator new(unsigned long) ??:0 (exe+0x0000000432b9)
    #1 QMetaObject::activate(QObject*, int, int, void**) ??:0 (libQt5Core.so.5+0x00000028ef94)
    #2 main /tmp/project/main.cpp:31 (exe+0x0000000a19f2)
  Location is heap block of size 88 at 0x7d180000edc0 allocated by main thread:
    #0 operator new(unsigned long) ??:0 (exe+0x0000000432b9)
    #1 QMetaObject::activate(QObject*, int, int, void**) ??:0 (libQt5Core.so.5+0x00000028ef94)
    #2 main /tmp/project/main.cpp:31 (exe+0x0000000a19f2)
  Thread T1 'QThread' (tid=3829, running) created by main thread at:
    #0 pthread_create ??:0 (exe+0x000000046f6b)
    #1 QThread::start(QThread::Priority) ??:0 (libQt5Core.so.5+0x0000000928b7)
    #2 Singleton::getInstance() /tmp/project/main.cpp:16 (exe+0x0000000a1b3c)
    #3 main /tmp/project/main.cpp:30 (exe+0x0000000a19bf)
SUMMARY: ThreadSanitizer: data race ??:0 operator delete(void*)
==================

是什么导致了这些竞态条件?

由于警告都起源于与信号，槽和事件循环相关的Qt内部，我认为检测到的竞争条件是假阳性。Helgrind是否检测到相同的竞争条件?一些人报告了Qt的Helgrind bug (http://www.kdab.com/~dfaure/helgrind.html)，最近似乎已经解决了。

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