Я пытаюсь написать версию очереди вызовов без блокировки, которую я использую для передачи сообщений. Это не для чего-то серьезного, просто чтобы узнать о многопоточности.
Я относительно уверен, что мой код верен, за исключением случаев, когда инструкции переупорядочены или сделаны в регистрах. Я знаю, что могу использовать барьеры памяти, чтобы прекратить переупорядочение, но как я могу обеспечить немедленную запись значений в память?
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