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Please review the following information to ** ensure the GNU General Public License version 3.0 requirements will be ** met: http://www.gnu.org/copyleft/gpl.html. ** ** ** $QT_END_LICENSE$ ** ****************************************************************************/ #include "qplatformdefs.h" #include "qmutex.h" #include #ifndef QT_NO_THREAD #include "qatomic.h" #include "qelapsedtimer.h" #include "qthread.h" #include "qmutex_p.h" QT_BEGIN_NAMESPACE /*! \class QMutex \brief The QMutex class provides access serialization between threads. \threadsafe \ingroup thread The purpose of a QMutex is to protect an object, data structure or section of code so that only one thread can access it at a time (this is similar to the Java \c synchronized keyword). It is usually best to use a mutex with a QMutexLocker since this makes it easy to ensure that locking and unlocking are performed consistently. For example, say there is a method that prints a message to the user on two lines: \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 0 If these two methods are called in succession, the following happens: \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 1 If these two methods are called simultaneously from two threads then the following sequence could result: \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 2 If we add a mutex, we should get the result we want: \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 3 Then only one thread can modify \c number at any given time and the result is correct. This is a trivial example, of course, but applies to any other case where things need to happen in a particular sequence. When you call lock() in a thread, other threads that try to call lock() in the same place will block until the thread that got the lock calls unlock(). A non-blocking alternative to lock() is tryLock(). \sa QMutexLocker, QReadWriteLock, QSemaphore, QWaitCondition */ /*! \enum QMutex::RecursionMode \value Recursive In this mode, a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of unlock() calls have been made. \value NonRecursive In this mode, a thread may only lock a mutex once. \sa QMutex() */ /*! Constructs a new mutex. The mutex is created in an unlocked state. If \a mode is QMutex::Recursive, a thread can lock the same mutex multiple times and the mutex won't be unlocked until a corresponding number of unlock() calls have been made. The default is QMutex::NonRecursive. \sa lock(), unlock() */ QMutex::QMutex(RecursionMode mode) : d(new QMutexPrivate(mode)) { } /*! Destroys the mutex. \warning Destroying a locked mutex may result in undefined behavior. */ QMutex::~QMutex() { delete static_cast(d); } /*! Locks the mutex. If another thread has locked the mutex then this call will block until that thread has unlocked it. Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a \l{QMutex::Recursive}{recursive mutex}. If this mutex is a \l{QMutex::NonRecursive}{non-recursive mutex}, this function will \e dead-lock when the mutex is locked recursively. \sa unlock() */ void QMutex::lock() { QMutexPrivate *d = static_cast(this->d); Qt::HANDLE self; if (d->recursive) { self = QThread::currentThreadId(); if (d->owner == self) { ++d->count; Q_ASSERT_X(d->count != 0, "QMutex::lock", "Overflow in recursion counter"); return; } bool isLocked = d->contenders.testAndSetAcquire(0, 1); if (!isLocked) { // didn't get the lock, wait for it isLocked = d->wait(); Q_ASSERT_X(isLocked, "QMutex::lock", "Internal error, infinite wait has timed out."); } d->owner = self; ++d->count; Q_ASSERT_X(d->count != 0, "QMutex::lock", "Overflow in recursion counter"); return; } bool isLocked = d->contenders.testAndSetAcquire(0, 1); if (!isLocked) { lockInternal(); } } /*! Attempts to lock the mutex. If the lock was obtained, this function returns true. If another thread has locked the mutex, this function returns false immediately. If the lock was obtained, the mutex must be unlocked with unlock() before another thread can successfully lock it. Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a \l{QMutex::Recursive}{recursive mutex}. If this mutex is a \l{QMutex::NonRecursive}{non-recursive mutex}, this function will \e always return false when attempting to lock the mutex recursively. \sa lock(), unlock() */ bool QMutex::tryLock() { QMutexPrivate *d = static_cast(this->d); Qt::HANDLE self; if (d->recursive) { self = QThread::currentThreadId(); if (d->owner == self) { ++d->count; Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter"); return true; } bool isLocked = d->contenders.testAndSetAcquire(0, 1); if (!isLocked) { // some other thread has the mutex locked, or we tried to // recursively lock an non-recursive mutex return isLocked; } d->owner = self; ++d->count; Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter"); return isLocked; } return d->contenders.testAndSetAcquire(0, 1); } /*! \overload Attempts to lock the mutex. This function returns true if the lock was obtained; otherwise it returns false. If another thread has locked the mutex, this function will wait for at most \a timeout milliseconds for the mutex to become available. Note: Passing a negative number as the \a timeout is equivalent to calling lock(), i.e. this function will wait forever until mutex can be locked if \a timeout is negative. If the lock was obtained, the mutex must be unlocked with unlock() before another thread can successfully lock it. Calling this function multiple times on the same mutex from the same thread is allowed if this mutex is a \l{QMutex::Recursive}{recursive mutex}. If this mutex is a \l{QMutex::NonRecursive}{non-recursive mutex}, this function will \e always return false when attempting to lock the mutex recursively. \sa lock(), unlock() */ bool QMutex::tryLock(int timeout) { QMutexPrivate *d = static_cast(this->d); Qt::HANDLE self; if (d->recursive) { self = QThread::currentThreadId(); if (d->owner == self) { ++d->count; Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter"); return true; } bool isLocked = d->contenders.testAndSetAcquire(0, 1); if (!isLocked) { // didn't get the lock, wait for it isLocked = d->wait(timeout); if (!isLocked) return false; } d->owner = self; ++d->count; Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter"); return true; } return (d->contenders.testAndSetAcquire(0, 1) // didn't get the lock, wait for it || d->wait(timeout)); } /*! Unlocks the mutex. Attempting to unlock a mutex in a different thread to the one that locked it results in an error. Unlocking a mutex that is not locked results in undefined behavior. \sa lock() */ void QMutex::unlock() { QMutexPrivate *d = static_cast(this->d); if (d->recursive) { if (!--d->count) { d->owner = 0; if (!d->contenders.testAndSetRelease(1, 0)) d->wakeUp(); } } else { if (!d->contenders.testAndSetRelease(1, 0)) d->wakeUp(); } } /*! \fn bool QMutex::locked() Returns true if the mutex is locked by another thread; otherwise returns false. It is generally a bad idea to use this function, because code that uses it has a race condition. Use tryLock() and unlock() instead. \oldcode bool isLocked = mutex.locked(); \newcode bool isLocked = true; if (mutex.tryLock()) { mutex.unlock(); isLocked = false; } \endcode */ /*! \class QMutexLocker \brief The QMutexLocker class is a convenience class that simplifies locking and unlocking mutexes. \threadsafe \ingroup thread Locking and unlocking a QMutex in complex functions and statements or in exception handling code is error-prone and difficult to debug. QMutexLocker can be used in such situations to ensure that the state of the mutex is always well-defined. QMutexLocker should be created within a function where a QMutex needs to be locked. The mutex is locked when QMutexLocker is created. You can unlock and relock the mutex with \c unlock() and \c relock(). If locked, the mutex will be unlocked when the QMutexLocker is destroyed. For example, this complex function locks a QMutex upon entering the function and unlocks the mutex at all the exit points: \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 4 This example function will get more complicated as it is developed, which increases the likelihood that errors will occur. Using QMutexLocker greatly simplifies the code, and makes it more readable: \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 5 Now, the mutex will always be unlocked when the QMutexLocker object is destroyed (when the function returns since \c locker is an auto variable). The same principle applies to code that throws and catches exceptions. An exception that is not caught in the function that has locked the mutex has no way of unlocking the mutex before the exception is passed up the stack to the calling function. QMutexLocker also provides a \c mutex() member function that returns the mutex on which the QMutexLocker is operating. This is useful for code that needs access to the mutex, such as QWaitCondition::wait(). For example: \snippet doc/src/snippets/code/src_corelib_thread_qmutex.cpp 6 \sa QReadLocker, QWriteLocker, QMutex */ /*! \fn QMutexLocker::QMutexLocker(QMutex *mutex) Constructs a QMutexLocker and locks \a mutex. The mutex will be unlocked when the QMutexLocker is destroyed. If \a mutex is zero, QMutexLocker does nothing. \sa QMutex::lock() */ /*! \fn QMutexLocker::~QMutexLocker() Destroys the QMutexLocker and unlocks the mutex that was locked in the constructor. \sa QMutex::unlock() */ /*! \fn QMutex *QMutexLocker::mutex() const Returns a pointer to the mutex that was locked in the constructor. */ /*! \fn void QMutexLocker::unlock() Unlocks this mutex locker. You can use \c relock() to lock it again. It does not need to be locked when destroyed. \sa relock() */ /*! \fn void QMutexLocker::relock() Relocks an unlocked mutex locker. \sa unlock() */ /*! \fn QMutex::QMutex(bool recursive) Use the constructor that takes a RecursionMode parameter instead. */ /*! \internal helper for lockInline() */ void QMutex::lockInternal() { QMutexPrivate *d = static_cast(this->d); if (QThread::idealThreadCount() == 1) { // don't spin on single cpu machines bool isLocked = d->wait(); Q_ASSERT_X(isLocked, "QMutex::lock", "Internal error, infinite wait has timed out."); Q_UNUSED(isLocked); return; } QElapsedTimer elapsedTimer; elapsedTimer.start(); do { qint64 spinTime = elapsedTimer.nsecsElapsed(); if (spinTime > d->maximumSpinTime) { // didn't get the lock, wait for it, since we're not going to gain anything by spinning more elapsedTimer.start(); bool isLocked = d->wait(); Q_ASSERT_X(isLocked, "QMutex::lock", "Internal error, infinite wait has timed out."); Q_UNUSED(isLocked); qint64 maximumSpinTime = d->maximumSpinTime; qint64 averageWaitTime = d->averageWaitTime; qint64 actualWaitTime = elapsedTimer.nsecsElapsed(); if (actualWaitTime < (QMutexPrivate::MaximumSpinTimeThreshold * 3 / 2)) { // measure the wait times averageWaitTime = d->averageWaitTime = qMin((averageWaitTime + actualWaitTime) / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold)); } // adjust the spin count when spinning does not benefit contention performance if ((spinTime + actualWaitTime) - qint64(QMutexPrivate::MaximumSpinTimeThreshold) >= qint64(QMutexPrivate::MaximumSpinTimeThreshold)) { // long waits, stop spinning d->maximumSpinTime = 0; } else { // allow spinning if wait times decrease, but never spin more than the average wait time (otherwise we may perform worse) d->maximumSpinTime = qBound(qint64(averageWaitTime * 3 / 2), maximumSpinTime / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold)); } return; } // be a good citizen... yielding lets something else run if there is something to run, but may also relieve memory pressure if not QThread::yieldCurrentThread(); } while (d->contenders != 0 || !d->contenders.testAndSetAcquire(0, 1)); // spinning is working, do not change the spin time (unless we are using much less time than allowed to spin) qint64 maximumSpinTime = d->maximumSpinTime; qint64 spinTime = elapsedTimer.nsecsElapsed(); if (spinTime < maximumSpinTime / 2) { // we are using much less time than we need, adjust the limit d->maximumSpinTime = qBound(qint64(d->averageWaitTime * 3 / 2), maximumSpinTime / 2, qint64(QMutexPrivate::MaximumSpinTimeThreshold)); } } /*! \internal */ void QMutex::unlockInternal() { static_cast(d)->wakeUp(); } /*! \fn QMutex::lockInline() \internal inline version of QMutex::lock() */ /*! \fn QMutex::unlockInline() \internal inline version of QMutex::unlock() */ /*! \fn QMutex::tryLockInline() \internal inline version of QMutex::tryLock() */ QT_END_NAMESPACE #endif // QT_NO_THREAD