rw_lock.cpp

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/** @file
 *  @author Bram de Greve (bram@cocamware.com)
 *  @author Tom De Muer (tom@cocamware.com)
 *
 *  *** 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) 2004-2024 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 
 *  others to use your version of this file under the CPAL, indicate your decision by 
 *  deleting the provisions above and replace them with the notice and other 
 *  provisions required by the GPL License. If you do not delete the provisions above,
 *  a recipient may use your version of this file under either the CPAL or the GPL.
 *  
 *  *** END LICENSE INFORMATION ***
 */
 
#include "lass_common.h"
#include "rw_lock.h"   
 
namespace lass
{
namespace util
{
 
RWLock::RWLock(int iMaxReaders):
    maxReaders_(iMaxReaders),
    spinLock_(iMaxReaders),
    writersTrying_(0)
{
}
 
RWLock::~RWLock()
{
    LASS_ASSERT(spinLock_.load(std::memory_order_relaxed) == maxReaders_);
    LASS_ASSERT(writersTrying_.load(std::memory_order_relaxed) == 0);
}
 
void RWLock::lockr()
{
    int oldSpinLock;
    do
    {
        while (writersTrying_.load() != 0)
        {
            LASS_SPIN_PAUSE;
        }
        oldSpinLock = spinLock_.load();
        LASS_ASSERT(oldSpinLock>=0);
        while (oldSpinLock <= 0)
        {
            oldSpinLock = spinLock_.load();
            LASS_SPIN_PAUSE;
        }
    }
    while (!spinLock_.compare_exchange_weak(oldSpinLock, oldSpinLock - 1));
}
 
void RWLock::lockw()
{
    writersTrying_.fetch_add(1);
    int expected;
    do
    {
        expected = maxReaders_;
        while (spinLock_.load() != expected)
        {
            LASS_SPIN_PAUSE;
        }
    }
    while (!spinLock_.compare_exchange_weak(expected, 0));
}
 
void RWLock::unlockw()
{
    int expected = 0;
    LASS_ENFORCE(spinLock_.compare_exchange_strong(expected, maxReaders_));
    writersTrying_.fetch_sub(1);
}
 
void RWLock::unlockr()
{
    [[maybe_unused]] const int oldSpinLock = spinLock_.fetch_add(1);
    LASS_ASSERT(oldSpinLock <= maxReaders_);
}
 
 
LockResult RWLock::tryLockr()
{
    int oldSpinLock;
    do
    {
        if (writersTrying_.load() != 0)
        {
            return lockBusy;
        }
        oldSpinLock = spinLock_.load();
        LASS_ASSERT(oldSpinLock >= 0);
        if (oldSpinLock <= 0)
        {
            return lockBusy;
        }
    }
    while (!spinLock_.compare_exchange_weak(oldSpinLock, oldSpinLock - 1));
    return lockSuccess;
}
 
 
LockResult RWLock::tryLockw()
{
    int expected = maxReaders_;
    writersTrying_.fetch_add(1);
    if (spinLock_.compare_exchange_strong(expected, 0))
    {
        return lockSuccess;
    }
    else
    {
        writersTrying_.fetch_sub(1);
        return lockBusy;
    }
}
 
} //namespace util
} //namespace lass