vb.net - 如何在 VB.net 中指定 volatile 的等效项？

Question

我正在尝试编写用于消息传递的调用队列的无锁版本。这不是为了任何严肃的事情，只是为了学习线程。

我相对确定我的代码是正确的，除非指令被重新排序或在寄存器中完成。我知道我可以使用内存屏障来停止重新排序，但如何确保值立即写入内存？

Public Class CallQueue
    Private first As New Node(Nothing) 'owned by consumer'
    Private last As Node = first 'owned by producers'
    Private Class Node
        Public ReadOnly action As Action
        Public [next] As Node
        Public Sub New(ByVal action As Action)
            Me.action = action
        End Sub
    End Class

    Private _running As Integer
    Private Function TryAcquireConsumer() As Boolean
        Threading.Thread.MemoryBarrier()

        'Dont bother acquiring if there are no items to consume'
        'This unsafe check is alright because enqueuers call this method, so we never end up with a non-empty idle queue'
        If first.next Is Nothing Then Return False

        Threading.Thread.MemoryBarrier()

        'Try to acquire'
        Return Threading.Interlocked.Exchange(_running, 1) = 0
    End Function
    Private Function TryReleaseConsumer() As Boolean
        Do
            Threading.Thread.MemoryBarrier()

            'Dont release while there are still things to consume'
            If first.next IsNot Nothing Then Return False

            Threading.Thread.MemoryBarrier()

            'Release'
            _running = 0

            Threading.Thread.MemoryBarrier()

            'It is possible that a new item was queued between the first.next check and releasing'
            'Therefore it is necessary to check if we can re-acquire in order to guarantee we dont leave a non-empty queue idle'
            If Not TryAcquireConsumer() Then Return True
        Loop
    End Function

    Public Sub QueueAction(ByVal action As Action)
        'Enqueue'
        'Essentially, this works because each node is returned by InterLocked.Exchange *exactly once*'
        'Each node has its .next property set exactly once, and also each node is targeted by .next exactly once, so they end up forming a valid tail'
        Dim n = New Node(action)
        Threading.Interlocked.Exchange(last, n).next = n

        'Start the consumer thread if it is not already running'
        If TryAcquireConsumer() Then
            Call New Threading.Thread(Sub() Consume()).Start()
        End If
    End Sub
    Private Sub Consume()
        'Run until queue is empty'
        Do Until TryReleaseConsumer()
            first = first.next
            Call first.action()
        Loop
    End Sub
End Class

Answer 1

没有等同于 C# 的 volatile VB.NET 中的关键字。相反，通常推荐的是使用 MemoryBarrier .辅助方法也可以写成:

Function VolatileRead(Of T)(ByRef Address As T) As T
    VolatileRead = Address
    Threading.Thread.MemoryBarrier()
End Function

Sub VolatileWrite(Of T)(ByRef Address As T, ByVal Value As T)
    Threading.Thread.MemoryBarrier()
    Address = Value
End Sub

还有一个有用的博客 post关于这个问题。