SpinLock Structure

.NET Framework (current version)
 

Provides a mutual exclusion lock primitive where a thread trying to acquire the lock waits in a loop repeatedly checking until the lock becomes available.

Namespace:   System.Threading
Assembly:  mscorlib (in mscorlib.dll)

<ComVisibleAttribute(False)>
<HostProtectionAttribute(SecurityAction.LinkDemand, Synchronization := True,
	ExternalThreading := True)>
Public Structure SpinLock

NameDescription
System_CAPS_pubmethodSpinLock(Boolean)

Initializes a new instance of the SpinLock structure with the option to track thread IDs to improve debugging.

NameDescription
System_CAPS_pubpropertyIsHeld

Gets whether the lock is currently held by any thread.

System_CAPS_pubpropertyIsHeldByCurrentThread

Gets whether the lock is held by the current thread.

System_CAPS_pubpropertyIsThreadOwnerTrackingEnabled

Gets whether thread ownership tracking is enabled for this instance.

NameDescription
System_CAPS_pubmethodEnter(Boolean)

Acquires the lock in a reliable manner, such that even if an exception occurs within the method call, lockTaken can be examined reliably to determine whether the lock was acquired.

System_CAPS_pubmethodEquals(Object)

Indicates whether this instance and a specified object are equal.(Inherited from ValueType.)

System_CAPS_pubmethodExit()

Releases the lock.

System_CAPS_pubmethodExit(Boolean)

Releases the lock.

System_CAPS_pubmethodGetHashCode()

Returns the hash code for this instance.(Inherited from ValueType.)

System_CAPS_pubmethodGetType()

Gets the Type of the current instance.(Inherited from Object.)

System_CAPS_pubmethodToString()

Returns the fully qualified type name of this instance.(Inherited from ValueType.)

System_CAPS_pubmethodTryEnter(Boolean)

Attempts to acquire the lock in a reliable manner, such that even if an exception occurs within the method call, lockTaken can be examined reliably to determine whether the lock was acquired.

System_CAPS_pubmethodTryEnter(Int32, Boolean)

Attempts to acquire the lock in a reliable manner, such that even if an exception occurs within the method call, lockTaken can be examined reliably to determine whether the lock was acquired.

System_CAPS_pubmethodTryEnter(TimeSpan, Boolean)

Attempts to acquire the lock in a reliable manner, such that even if an exception occurs within the method call, lockTaken can be examined reliably to determine whether the lock was acquired.

For an example of how to use a Spin Lock, see How to: Use SpinLock for Low-Level Synchronization.

Spin locks can be used for leaf-level locks where the object allocation implied by using a Monitor, in size or due to garbage collection pressure, is overly expensive. A spin lock can be useful in to avoid blocking; however, if you expect a significant amount of blocking, you should probably not use spin locks due to excessive spinning. Spinning can be beneficial when locks are fine-grained and large in number (for example, a lock per node in a linked list) and also when lock hold-times are always extremely short. In general, while holding a spin lock, one should avoid any of these actions:

  • blocking,

  • calling anything that itself may block,

  • holding more than one spin lock at once,

  • making dynamically dispatched calls (interface and virtuals),

  • making statically dispatched calls into any code one doesn't own, or

  • allocating memory.

SpinLock should only be used after you have been determined that doing so will improve an application's performance. It is also important to note that SpinLock is a value type, for performance reasons. For this reason, you must be very careful not to accidentally copy a SpinLock instance, as the two instances (the original and the copy) would then be completely independent of one another, which would likely lead to erroneous behavior of the application. If a SpinLock instance must be passed around, it should be passed by reference rather than by value.

Do not store SpinLock instances in readonly fields.

The following example shows how to use a SpinLock:

Imports System.Text
Imports System.Threading
Imports System.Threading.Tasks


Module SpinLockDemo

    ' Demonstrates:
    ' Default SpinLock construction ()
    ' SpinLock.Enter(ref bool)
    ' SpinLock.Exit()
    Private Sub SpinLockSample1()
        Dim sl As New SpinLock()

        Dim sb As New StringBuilder()

        ' Action taken by each parallel job.
        ' Append to the StringBuilder 10000 times, protecting
        ' access to sb with a SpinLock.
        Dim action As Action =
            Sub()
                Dim gotLock As Boolean = False
                For i As Integer = 0 To 9999
                    gotLock = False
                    Try
                        sl.Enter(gotLock)
                        sb.Append((i Mod 10).ToString())
                    Finally
                        ' Only give up the lock if you actually acquired it
                        If gotLock Then
                            sl.[Exit]()
                        End If
                    End Try
                Next
            End Sub

        ' Invoke 3 concurrent instances of the action above
        Parallel.Invoke(action, action, action)

        ' Check/Show the results
        Console.WriteLine("sb.Length = {0} (should be 30000)", sb.Length)
        Console.WriteLine("number of occurrences of '5' in sb: {0} (should be 3000)", sb.ToString().Where(Function(c) (c = "5"c)).Count())
    End Sub

    ' Demonstrates:
    ' Default SpinLock constructor (tracking thread owner)
    ' SpinLock.Enter(ref bool)
    ' SpinLock.Exit() throwing exception
    ' SpinLock.IsHeld
    ' SpinLock.IsHeldByCurrentThread
    ' SpinLock.IsThreadOwnerTrackingEnabled
    Private Sub SpinLockSample2()
        ' Instantiate a SpinLock
        Dim sl As New SpinLock()

        ' These MRESs help to sequence the two jobs below
        Dim mre1 As New ManualResetEventSlim(False)
        Dim mre2 As New ManualResetEventSlim(False)
        Dim lockTaken As Boolean = False

        Dim taskA As Task = Task.Factory.StartNew(
            Sub()
                Try
                    sl.Enter(lockTaken)
                    Console.WriteLine("Task A: entered SpinLock")
                    mre1.[Set]()
                    ' Signal Task B to commence with its logic
                    ' Wait for Task B to complete its logic
                    ' (Normally, you would not want to perform such a potentially
                    ' heavyweight operation while holding a SpinLock, but we do it
                    ' here to more effectively show off SpinLock properties in
                    ' taskB.)
                    mre2.Wait()
                Finally
                    If lockTaken Then
                        sl.[Exit]()
                    End If
                End Try
            End Sub)

        Dim taskB As Task = Task.Factory.StartNew(
            Sub()
                mre1.Wait()
                ' wait for Task A to signal me
                Console.WriteLine("Task B: sl.IsHeld = {0} (should be true)", sl.IsHeld)
                Console.WriteLine("Task B: sl.IsHeldByCurrentThread = {0} (should be false)", sl.IsHeldByCurrentThread)
                Console.WriteLine("Task B: sl.IsThreadOwnerTrackingEnabled = {0} (should be true)", sl.IsThreadOwnerTrackingEnabled)

                Try
                    sl.[Exit]()
                    Console.WriteLine("Task B: Released sl, should not have been able to!")
                Catch e As Exception
                    Console.WriteLine("Task B: sl.Exit resulted in exception, as expected: {0}", e.Message)
                End Try

                ' Signal Task A to exit the SpinLock
                mre2.[Set]()
            End Sub)

        ' Wait for task completion and clean up
        Task.WaitAll(taskA, taskB)
        mre1.Dispose()
        mre2.Dispose()
    End Sub

    ' Demonstrates:
    ' SpinLock constructor(false) -- thread ownership not tracked
    Private Sub SpinLockSample3()
        ' Create SpinLock that does not track ownership/threadIDs
        Dim sl As New SpinLock(False)

        ' Used to synchronize with the Task below
        Dim mres As New ManualResetEventSlim(False)

        ' We will verify that the Task below runs on a separate thread
        Console.WriteLine("main thread id = {0}", Thread.CurrentThread.ManagedThreadId)

        ' Now enter the SpinLock.  Ordinarily, you would not want to spend so
        ' much time holding a SpinLock, but we do it here for the purpose of 
        ' demonstrating that a non-ownership-tracking SpinLock can be exited 
        ' by a different thread than that which was used to enter it.
        Dim lockTaken As Boolean = False
        sl.Enter(lockTaken)

        ' Create a separate Task
        Dim worker As Task = Task.Factory.StartNew(
            Sub()
                Console.WriteLine("worker task thread id = {0} (should be different than main thread id)", Thread.CurrentThread.ManagedThreadId)

                ' Now exit the SpinLock
                Try
                    sl.[Exit]()
                    Console.WriteLine("worker task: successfully exited SpinLock, as expected")
                Catch e As Exception
                    Console.WriteLine("worker task: unexpected failure in exiting SpinLock: {0}", e.Message)
                End Try

                ' Notify main thread to continue
                mres.[Set]()
            End Sub)

        ' Do this instead of worker.Wait(), because worker.Wait() could inline the worker Task,
        ' causing it to be run on the same thread. The purpose of this example is to show that
        ' a different thread can exit the SpinLock created (without thread tracking) on your thread.
        mres.Wait()

        ' now Wait() on worker and clean up
        worker.Wait()
        mres.Dispose()
    End Sub


End Module

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1

All members of SpinLock are thread-safe and may be used from multiple threads concurrently.

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