rw_lock.h

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/** @file
 *  @author Bram de Greve (bramz@users.sourceforge.net)
 *  @author Tom De Muer (tomdemuer@users.sourceforge.net)
 *
 *  *** BEGIN LICENSE INFORMATION ***
 *  
 *  The contents of this file are subject to the Common Public Attribution License 
 *  Version 1.0 (the "License"); you may not use this file except in compliance with 
 *  the License. You may obtain a copy of the License at 
 *  http://lass.sourceforge.net/cpal-license. The License is based on the 
 *  Mozilla Public License Version 1.1 but Sections 14 and 15 have been added to cover 
 *  use of software over a computer network and provide for limited attribution for 
 *  the Original Developer. In addition, Exhibit A has been modified to be consistent 
 *  with Exhibit B.
 *  
 *  Software distributed under the License is distributed on an "AS IS" basis, WITHOUT 
 *  WARRANTY OF ANY KIND, either express or implied. See the License for the specific 
 *  language governing rights and limitations under the License.
 *  
 *  The Original Code is LASS - Library of Assembled Shared Sources.
 *  
 *  The Initial Developer of the Original Code is Bram de Greve and Tom De Muer.
 *  The Original Developer is the Initial Developer.
 *  
 *  All portions of the code written by the Initial Developer are:
 *  Copyright (C) 2004-2007 the Initial Developer.
 *  All Rights Reserved.
 *  
 *  Contributor(s):
 *
 *  Alternatively, the contents of this file may be used under the terms of the 
 *  GNU General Public License Version 2 or later (the GPL), in which case the 
 *  provisions of GPL are applicable instead of those above.  If you wish to allow use
 *  of your version of this file only under the terms of the GPL and not to allow 
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 *  deleting the provisions above and replace them with the notice and other 
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 *  a recipient may use your version of this file under either the CPAL or the GPL.
 *  
 *  *** END LICENSE INFORMATION ***
 */

#ifndef LASS_GUARDIAN_OF_INCLUSION_UTIL_RW_LOCK_H
#define LASS_GUARDIAN_OF_INCLUSION_UTIL_RW_LOCK_H

#include "util_common.h"
#include "atomic.h"
#include "thread.h" 

namespace lass
{
namespace util
{

namespace impl
{
} //namespace impl


/** Lean and mean synchronisation object, without OS support.
*   @ingroup Threading
*   @see RWLocker
*   @author Tom De Muer
*   @date 2006
*   
*   This lock is built upon atomic operations that are programmed in assembly.  It makes a difference
*   between reading and writing.  A maximum number of readers is allowed while only 1 writer is allowed.
*   The writer will only enter if there are no readers are reading at the same moment.  This RWLock is 
*   a blocking and spinning synchronization object.  Priority is given over writers, as soon as a writer is
*   trying to enter, only one subsequent reader will in worst case be able to enter.
*/

class RWLock : NonCopyable
{

public:
    RWLock(util::CallTraits<size_t>::TParam iMaxReaders);
    ~RWLock();

    void lockr();
    void lockw();
    void unlockr();
    void unlockw();

    const LockResult tryLockr();
    const LockResult tryLockw();

private:
    size_t maxReaders_;     /**< the maximum of simultaneous readers allowed */
    int spinLock_;
    int writersTrying_;     /**< the number of writers trying to enter */
};


RWLock::RWLock(util::CallTraits<size_t>::TParam iMaxReaders)
{
    maxReaders_ = iMaxReaders;
    spinLock_ = maxReaders_;
    writersTrying_ = 0;
}

RWLock::~RWLock()
{
    LASS_ENFORCE(spinLock_==maxReaders_);
}

void RWLock::lockr()
{
    int newSpinLock;
    int oldSpinLock;
    do
    {
        oldSpinLock = spinLock_;
        LASS_ASSERT(oldSpinLock>=0);
        newSpinLock = oldSpinLock-1;
    } while (writersTrying_!=0 || oldSpinLock==0 || !lass::util::atomicCompareAndSwap(spinLock_,oldSpinLock,newSpinLock));
}

void RWLock::lockw()
{
    lass::util::atomicIncrement(writersTrying_);
    do
    {
    } while (!lass::util::atomicCompareAndSwap<int>(spinLock_,maxReaders_,0));
}

void RWLock::unlockw()
{
    LASS_ENFORCE(spinLock_==0);
    do
    {
    } while (!lass::util::atomicCompareAndSwap<int>(spinLock_,0,maxReaders_));
    lass::util::atomicDecrement(writersTrying_);
}

void RWLock::unlockr()
{
    lass::util::atomicIncrement(spinLock_);
    LASS_ASSERT(spinLock_<=maxReaders_);
}


const LockResult RWLock::tryLockr()
{
    int oldSpinLock;
    int newSpinLock;
    do
    {
        oldSpinLock = spinLock_;
        LASS_ASSERT(spinLock_ >= 0);
        if (spinLock_ == 0)
        {
            return lockBusy;
        }
        newSpinLock = oldSpinLock - 1;
    }
    while (!atomicCompareAndSwap(spinLock_, oldSpinLock, newSpinLock));
    return lockSuccess;
}


const LockResult RWLock::tryLockw()
{
    lass::util::atomicIncrement(writersTrying_);
    do
    {
        LASS_ASSERT(spinLock_ >= 0);
        if (spinLock_ != maxReaders_)
        {
            lass::util::atomicDecrement(writersTrying_);
            return lockBusy;
        }
    }
    while (!atomicCompareAndSwap<int>(spinLock_, maxReaders_, 0));
    return lockSuccess;
}

} //namespace util
} //namespace lass


#endif

// EOF