/****************************************************************************

**

** All rights reserved.

** Contact: Nokia Corporation (qt-info@nokia.com)

**

** This file is part of the QtCore module of the Qt Toolkit.

**

** $QT_BEGIN_LICENSE:LGPL$

** No Commercial Usage

** This file contains pre-release code and may not be distributed.

** You may use this file in accordance with the terms and conditions

** contained in the Technology Preview License Agreement accompanying

** this package.

**

** GNU Lesser General Public License Usage

** Alternatively, this file may be used under the terms of the GNU Lesser

** General Public License version 2.1 as published by the Free Software

** Foundation and appearing in the file LICENSE.LGPL included in the

** packaging of this file.  Please review the following information to

** ensure the GNU Lesser General Public License version 2.1 requirements

** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.

**

** In addition, as a special exception, Nokia gives you certain additional

** rights.  These rights are described in the Nokia Qt LGPL Exception

** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.

**

** If you have questions regarding the use of this file, please contact

** Nokia at qt-info@nokia.com.

**

**

**

**

**

**

**

**

** $QT_END_LICENSE$

**

****************************************************************************/

#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

    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

    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)

{

    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.fetchAndAddAcquire(1) == 0;

        if (!isLocked) {

            // didn't get the lock, wait for it

            isLocked = d->wait(timeout);

            // don't need to wait for the lock anymore

            d->contenders.deref();

            if (!isLocked)

                return false;

        }

        d->owner = self;

        ++d->count;

        Q_ASSERT_X(d->count != 0, "QMutex::tryLock", "Overflow in recursion counter");

        return true;

    }

#ifndef QT_NO_DEBUG

    self = QThread::currentThreadId();

#endif

    bool isLocked = d->contenders.fetchAndAddAcquire(1) == 0;

    if (!isLocked) {

        // didn't get the lock, wait for it

        isLocked = d->wait(timeout);

        // don't need to wait for the lock anymore

        d->contenders.deref();

        if (!isLocked)

            return false;

    }

#ifndef QT_NO_DEBUG

    d->owner = self;

#endif

    return true;

}

/*!

    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()

{

    Q_ASSERT_X(d->owner == QThread::currentThreadId(), "QMutex::unlock()",

               "A mutex must be unlocked in the same thread that locked it.");

    if (d->recursive) {

        if (!--d->count) {

            d->owner = 0;

            if (!d->contenders.testAndSetRelease(1, 0))

                d->wakeUp();

        }

    } else {

#ifndef QT_NO_DEBUG

        d->owner = 0;

#endif

        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.

*/

QT_END_NAMESPACE

#endif // QT_NO_THREAD