mcs/class/corlib/System.Threading/SpinLock.cs

   1 // SpinLock.cs
   2 //
   3 // Copyright (c) 2008 Jérémie "Garuma" Laval
   4 //
   5 // Permission is hereby granted, free of charge, to any person obtaining a copy
   6 // of this software and associated documentation files (the "Software"), to deal
   7 // in the Software without restriction, including without limitation the rights
   8 // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   9 // copies of the Software, and to permit persons to whom the Software is
  10 // furnished to do so, subject to the following conditions:
  11 //
  12 // The above copyright notice and this permission notice shall be included in
  13 // all copies or substantial portions of the Software.
  14 //
  15 // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  16 // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  17 // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  18 // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  19 // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  20 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  21 // THE SOFTWARE.
  22 //
  23 //
  24
  25 using System;
  26 using System.Runtime.ConstrainedExecution;
  27 using System.Runtime.InteropServices;
  28 using System.Runtime.CompilerServices;
  29
  30 #if NET_4_0 || BOOTSTRAP_NET_4_0
  31
  32 namespace System.Threading
  33 {
  34         [StructLayout(LayoutKind.Explicit)]
  35         internal struct TicketType {
  36                 [FieldOffset(0)]
  37                 public long TotalValue;
  38                 [FieldOffset(0)]
  39                 public int Value;
  40                 [FieldOffset(4)]
  41                 public int Users;
  42         }
  43
  44         // Implement the ticket SpinLock algorithm described on http://locklessinc.com/articles/locks/
  45         // This lock is usable on both endianness
  46         // TODO: some 32 bits platform apparently doesn't support CAS with 64 bits value
  47         public struct SpinLock
  48         {
  49                 TicketType ticket;
  50
  51                 int threadWhoTookLock;
  52                 readonly bool isThreadOwnerTrackingEnabled;
  53
  54                 static Watch sw = Watch.StartNew ();
  55
  56                 public bool IsThreadOwnerTrackingEnabled {
  57                         get {
  58                                 return isThreadOwnerTrackingEnabled;
  59                         }
  60                 }
  61
  62                 public bool IsHeld {
  63                         get {
  64                                 // No need for barrier here
  65                                 long totalValue = ticket.TotalValue;
  66                                 return (totalValue >> 32) != (totalValue & 0xFFFFFFFF);
  67                         }
  68                 }
  69
  70                 public bool IsHeldByCurrentThread {
  71                         get {
  72                                 if (isThreadOwnerTrackingEnabled)
  73                                         return IsHeld && Thread.CurrentThread.ManagedThreadId == threadWhoTookLock;
  74                                 else
  75                                         return IsHeld;
  76                         }
  77                 }
  78
  79                 public SpinLock (bool trackId)
  80                 {
  81                         this.isThreadOwnerTrackingEnabled = trackId;
  82                         this.threadWhoTookLock = 0;
  83                         this.ticket = new TicketType ();
  84                 }
  85
  86                 [MonoTODO ("Not safe against async exceptions"]
  87                 public void Enter (ref bool lockTaken)
  88                 {
  89                         if (lockTaken)
  90                                 throw new ArgumentException ("lockTaken", "lockTaken must be initialized to false");
  91                         if (isThreadOwnerTrackingEnabled && IsHeldByCurrentThread)
  92                                 throw new LockRecursionException ();
  93
  94                         /* The current ticket algorithm, even though it's a thing of beauty, doesn't make it easy to
  95                          * hand back ticket that have been taken in the case of an asynchronous exception and naively
  96                          * fixing it bloat a code that should be kept simple. A straightforward possibility is to wrap
  97                          * the whole thing in a finally block but due to the while loop a number of bad things can
  98                          * happen, thus for the moment the code is left as is in the spirit of "better breaking fast,
  99                          * than later in a weird way".
 100                          */
 101                         int slot = Interlocked.Increment (ref ticket.Users) - 1;
 102
 103                         SpinWait wait = new SpinWait ();
 104                         while (slot != ticket.Value)
 105                                 wait.SpinOnce ();
 106
 107                         lockTaken = true;
 108                         threadWhoTookLock = Thread.CurrentThread.ManagedThreadId;
 109                 }
 110
 111                 public void TryEnter (ref bool lockTaken)
 112                 {
 113                         TryEnter (0, ref lockTaken);
 114                 }
 115
 116                 public void TryEnter (TimeSpan timeout, ref bool lockTaken)
 117                 {
 118                         TryEnter ((int)timeout.TotalMilliseconds, ref lockTaken);
 119                 }
 120
 121                 public void TryEnter (int milliSeconds, ref bool lockTaken)
 122                 {
 123                         if (milliSeconds < -1)
 124                                 throw new ArgumentOutOfRangeException ("milliSeconds", "millisecondsTimeout is a negative number other than -1");
 125                         if (lockTaken)
 126                                 throw new ArgumentException ("lockTaken", "lockTaken must be initialized to false");
 127                         if (isThreadOwnerTrackingEnabled && IsHeldByCurrentThread)
 128                                 throw new LockRecursionException ();
 129
 130                         long start = milliSeconds == -1 ? 0 : sw.ElapsedMilliseconds;
 131                         bool stop = false;
 132
 133                         do {
 134                                 long u = ticket.Users;
 135                                 long totalValue = (u << 32) | u;
 136                                 long newTotalValue
 137                                         = BitConverter.IsLittleEndian ? (u << 32) | (u + 1) : ((u + 1) << 32) | u;
 138
 139                                 RuntimeHelpers.PrepareConstrainedRegions ();
 140                                 try {}
 141                                 finally {
 142                                         lockTaken = Interlocked.CompareExchange (ref ticket.TotalValue, newTotalValue, totalValue) == totalValue;
 143
 144                                         if (lockTaken) {
 145                                                 threadWhoTookLock = Thread.CurrentThread.ManagedThreadId;
 146                                                 stop = true;
 147                                         }
 148                                 }
 149                 } while (!stop && (milliSeconds == -1 || (sw.ElapsedMilliseconds - start) < milliSeconds));
 150                 }
 151
 152                 public void Exit ()
 153                 {
 154                         Exit (false);
 155                 }
 156
 157                 public void Exit (bool flushReleaseWrites)
 158                 {
 159                         RuntimeHelpers.PrepareConstrainedRegions ();
 160                         try {}
 161                         finally {
 162                                 if (isThreadOwnerTrackingEnabled && !IsHeldByCurrentThread)
 163                                         throw new SynchronizationLockException ("Current thread is not the owner of this lock");
 164
 165                                 threadWhoTookLock = int.MinValue;
 166                                 // Fast path
 167                                 if (flushReleaseWrites)
 168                                         Interlocked.Increment (ref ticket.Value);
 169                                 else
 170                                         ticket.Value++;
 171                         }
 172                 }
 173         }
 174
 175         // Wraps a SpinLock in a reference when we need to pass
 176         // around the lock
 177         internal class SpinLockWrapper
 178         {
 179                 public SpinLock Lock = new SpinLock (false);
 180         }
 181 }
 182 #endif