//******************************************************************************************************//  ReaderWriterSpinLock.cs - Gbtc////  Copyright © 2012, Grid Protection Alliance.  All Rights Reserved.////  Licensed to the Grid Protection Alliance (GPA) under one or more contributor license agreements. See//  the NOTICE file distributed with this work for additional information regarding copyright ownership.//  The GPA licenses this file to you under the MIT License (MIT), the "License"; you may//  not use this file except in compliance with the License. You may obtain a copy of the License at:////      http://www.opensource.org/licenses/MIT////  Unless agreed to in writing, the subject software distributed under the License is distributed on an//  "AS-IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. Refer to the//  License for the specific language governing permissions and limitations.////  Code Modification History://  ----------------------------------------------------------------------------------------------------//  03/21/2011 - J. Ritchie Carroll//       Generated original version of source code based on Joe Duffy's Weblog articles.//  03/21/2014 - Steven E. Chisholm//       Removed the processor fanning that occurred in the readers since the overhead associated//       with the reduced contention slowed down the class and each instance of this class consumed//       a substantial amount of memory.////****************************************************************************************************** using System;using System.Threading; // We use plenty of interlocked operations on volatile fields below.  Safe.#pragma warning disable 0420 namespace GSF.Threading{    /// <summary>    /// Represents a fast, lightweight reader/writer lock that uses spinning to perform locking. No recursive acquires or    /// upgradable locks are allowed (i.e., all entered locks must be exited before entering another lock).    /// </summary>    /// <remarks>    /// This reader/writer lock uses <see cref="SpinWait"/> to spin the CPU instead of engaging event based locking. As a result it    /// should only be used in cases where lock times are expected to be very small, reads are very frequent and writes are rare.    /// If hold times for write locks can be lengthy, it will be better to use <see cref="ReaderWriterLockSlim"/> instead to avoid    /// unnecessary CPU utilization due to spinning incurred by waiting reads.    /// </remarks>    public class ReaderWriterSpinLock    {        #region [ Members ]         // Fields        private volatile int m_writer;        private volatile int m_readers;         #endregion         #region [ Properties ]         #endregion         #region [ Methods ]         /// <summary>        /// Enters the lock in write mode.        /// </summary>        /// <remarks>        /// Upon successful acquisition of a write lock, use the <c>finally</c> block of a <c>try/finally</c> statement to call <see cref="ExitWriteLock"/>.        /// One <see cref="ExitWriteLock"/> should be called for each <see cref="EnterWriteLock"/>.        /// </remarks>        public void EnterWriteLock()        {            SpinWait sw = new SpinWait();             while (true)            {                if (m_writer == 0 && Interlocked.Exchange(ref m_writer, 1) == 0)                {                    // We now hold the write lock, and prevent new readers -                    // but we must ensure no readers exist before proceeding.                    while (m_readers != 0)                        sw.SpinOnce();                     break;                }                 // We failed to take the write lock; wait a bit and retry.                sw.SpinOnce();            }        }         /// <summary>        /// Exits write mode.        /// </summary>        public void ExitWriteLock()        {            // No need for a CAS.            m_writer = 0;        }         /// <summary>        /// Enters the lock in read mode.        /// </summary>        /// <remarks>        /// Upon successful acquisition of a read lock, use the <c>finally</c> block of a <c>try/finally</c> statement to call <see cref="ExitReadLock"/>.        /// One <see cref="ExitReadLock"/> should be called for each <see cref="EnterReadLock"/>.        /// </remarks>        public void EnterReadLock()        {            SpinWait sw = new SpinWait();             // Wait until there are no writers.            while (true)            {                while (m_writer == 1)                    sw.SpinOnce();                 // Try to take the read lock.                Interlocked.Increment(ref m_readers);                 // If writer, success - proceed.                if (m_writer == 0)                    break;                 // Back off to let the writer go through.                Interlocked.Decrement(ref m_readers);            }        }         /// <summary>        /// Exits read mode.        /// </summary>        /// <exception cref="InvalidOperationException">Cannot exit read lock when there are no readers.</exception>        public void ExitReadLock()        {            // Just note that the current reader has left the lock.            Interlocked.Decrement(ref m_readers);        }         #endregion    }}