source: trunk/src/corelib/thread/qmutex.cpp@ 192

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#include "qplatformdefs.h"
#include "qmutex.h"

#ifndef QT_NO_THREAD
#include "qatomic.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
    \ingroup environment
    \mainclass

    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 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()
{
    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.fetchAndAddAcquire(1) == 0;
        if (!isLocked) {
#ifndef QT_NO_DEBUG
            if (d->owner == self)
                qWarning("QMutex::lock: Deadlock detected in thread %ld",
                         long(d->owner));
#endif

            // didn't get the lock, wait for it
            isLocked = d->wait();
            Q_ASSERT_X(isLocked, "QMutex::lock",
                       "Internal error, infinite wait has timed out.");

            // don't need to wait for the lock anymore
            d->contenders.deref();
        }

        d->owner = self;
        ++d->count;
        Q_ASSERT_X(d->count != 0, "QMutex::lock", "Overflow in recursion counter");
        return;
    }

#ifndef QT_NO_DEBUG
    self = QThread::currentThreadId();
#endif

    bool isLocked = d->contenders == 0 && d->contenders.testAndSetAcquire(0, 1);
    if (!isLocked) {
        int spinCount = 0;
        int lastSpinCount = d->lastSpinCount;

        enum { AdditionalSpins = 20, SpinCountPenalizationDivisor = 4 };
        const int maximumSpinCount = lastSpinCount + AdditionalSpins;

        do {
            if (spinCount++ > maximumSpinCount) {
                // puts("spinning useless, sleeping");
                isLocked = d->contenders.fetchAndAddAcquire(1) == 0;
                if (!isLocked) {
#ifndef QT_NO_DEBUG
                    if (d->owner == self)
                        qWarning("QMutex::lock: Deadlock detected in thread %ld",
                                 long(d->owner));
#endif

                    // didn't get the lock, wait for it
                    isLocked = d->wait();
                    Q_ASSERT_X(isLocked, "QMutex::lock",
                               "Internal error, infinite wait has timed out.");

                    // don't need to wait for the lock anymore
                    d->contenders.deref();
                }
                // decrease the lastSpinCount since we didn't actually get the lock by spinning
                spinCount = -d->lastSpinCount / SpinCountPenalizationDivisor;
                break;
            }

            isLocked = d->contenders == 0 && d->contenders.testAndSetAcquire(0, 1);
        } while (!isLocked);

        // adjust the last spin lock count
        lastSpinCount = d->lastSpinCount;
        d->lastSpinCount = spinCount >= 0
                           ? qMax(lastSpinCount, spinCount)
                           : lastSpinCount + spinCount;
    }

#ifndef QT_NO_DEBUG
    d->owner = self;
#endif
}

/*!
    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()
{
    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 == 0 && 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;
    }

#ifndef QT_NO_DEBUG
    self = QThread::currentThreadId();
#endif
    bool isLocked = d->contenders == 0 && 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;
    }
#ifndef QT_NO_DEBUG
    d->owner = self;
#endif
    return isLocked;
}

/*! \overload

    Attempts to lock the mutex. This function returns true if the lock
    was obtained; otherwise it returns false. If another thread has