How to: Enable Thread-Tracking Mode in SpinLock
System.Threading.SpinLock is a low-level mutual exclusion lock that you can use for scenarios that have very short wait times. SpinLock is not re-entrant. After a thread enters the lock, it must exit the lock correctly before it can enter again. Typically, any attempt to re-enter the lock would cause deadlock, and deadlocks can be very difficult to debug. As an aid to development, System.Threading.SpinLock supports a thread-tracking mode that causes an exception to be thrown when a thread attempts to re-enter a lock that it already holds. This lets you more easily locate the point at which the lock was not exited correctly. You can turn on thread-tracking mode by using the SpinLock constructor that takes a Boolean input parameter, and passing in an argument of true. After you complete the development and testing phases, turn off thread-tracking mode for better performance.
The following example demonstrates thread-tracking mode. The lines that correctly exit the lock are commented out to simulate a coding error that causes one of the following results:
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An exception is thrown if the SpinLock was created by using an argument of true (True in Visual Basic).
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Deadlock if the SpinLock was created by using an argument of false (False in Visual Basic).
using System; using System.Collections.Generic; using System.Collections.Concurrent; using System.Diagnostics; using System.Text; using System.Threading; using System.Threading.Tasks; namespace SpinLockDemo { // C# public class SpinLockTest { // Specify true to enable thread tracking. This will cause // exception to be thrown when the first thread attempts to reenter the lock. // Specify false to cause deadlock due to coding error below. private static SpinLock _spinLock = new SpinLock(true); static void Main() { Parallel.Invoke( () => DoWork(), () => DoWork(), () => DoWork(), () => DoWork() ); Console.WriteLine("Press any key."); Console.ReadKey(); } public static void DoWork() { StringBuilder sb = new StringBuilder(); for (int i = 0; i < 100; i++) { bool lockTaken = false; try { _spinLock.Enter(ref lockTaken); // do work here protected by the lock Thread.SpinWait(50000); sb.Append(Thread.CurrentThread.ManagedThreadId); sb.Append(" Entered-"); } catch (LockRecursionException ex) { Console.WriteLine("Thread {0} attempted to reenter the lock", Thread.CurrentThread.ManagedThreadId); throw; } finally { // INTENTIONAL CODING ERROR TO DEMONSTRATE THREAD TRACKING! // UNCOMMENT THE LINES FOR CORRECT SPINLOCK BEHAVIOR // Commenting out these lines causes the same thread // to attempt to reenter the lock. If the SpinLock was // created with thread tracking enabled, the exception // is thrown. Otherwise the spinlock deadlocks. if (lockTaken) { // _spinLock.Exit(false); // sb.Append("Exited "); } } // Output for diagnostic display. if(i % 4 != 0) Console.Write(sb.ToString()); else Console.WriteLine(sb.ToString()); sb.Clear(); } } } }
