//int _count = 0;
#endregion
#region Constructor
public LockFreeQueue(NodeManager <T> nodeManager)
{
_nodeManager = nodeManager == null ? NodeManager <T> .Default : nodeManager;
// Get a single node and make the head and tail refer to it
// The node's Next field is null marking the end of the queue
_head = _nodeManager.Allocate();
_tail = _head;
}
public override void Free(LockFreeNode <T> node)
{
node.Item = default(T); // Allow early GC
// Try to make the new node be the head of the stack
do
{
// Make the new node refer to the old head
node.Next = _head.Next;
// If previous head's next == what we thought was next, change head's Next to the new node
// else, try again if another thread changed the head
}while (!InterlockedEx.IfThen(ref _head.Next, node.Next, node));
}
public bool TryDequeue(out T item)
{
item = default(T);
// Loop until we manage to advance the head, removing
// a node (if there are no nodes to dequeue, we'll exit the method instead)
for (bool dequeuedNode = false; !dequeuedNode;)
{
// make local copies of the head, the tail, and the head's Next reference
LockFreeNode <T> tempHead = _head;
LockFreeNode <T> tempTail = _tail;
LockFreeNode <T> tempHeadNext = tempHead.Next;
// If another thread changed the head, start over
Thread.MemoryBarrier(); // Make sure the value read from _head is fresh
if (tempHead != _head)
{
continue;
}
// If the head equals the tail
if (tempHead == tempTail)
{
// If the head node refers to null, then the queue is empty
if (tempHeadNext == null)
{
return(false);
}
// The head refers to the tail whose Next is not null. This means
// we have a lagging tail; update it
InterlockedEx.IfThen(ref _tail, tempTail, tempHeadNext);
continue;
}
// The head and tail nodes are different; dequeue the head node and advance the head
item = tempHeadNext.Item;
dequeuedNode = InterlockedEx.IfThen(ref _head, tempHead, tempHeadNext);
if (dequeuedNode)
{
_nodeManager.Free(tempHead);
}
}
//Interlocked.Decrement(ref _count);
return(true);
}
// If two threads call Enqueue simultaneously:
// Both threads create and initialize a new node
// The tail's Next will refer to one of these 2 new nodes
// The new new tail's Next will refer to the other of the 2 new nodes
// _tail will refer to the 1st new node appended (not the real tail)
// To fix this: Enqueue initializes by advancing _tail to the node whose Next is null
public void Enqueue(T item)
{
// Get (or allocate) a node and initialize it
LockFreeNode <T> newNode = _nodeManager.Allocate(item);
LockFreeNode <T> tempTail = null;
for (bool appendedNewNode = false; !appendedNewNode;)
{
// Get the current tail and what IT refers to
tempTail = _tail;
LockFreeNode <T> tempTailNext = tempTail.Next;
// If another thread changed the tail, start over
Thread.MemoryBarrier(); // Make sure the value read from _tail is fresh
if (tempTail != _tail)
{
continue;
}
// If the tail isn't truely the tail, fix the tail, start over
if (tempTailNext != null)
{
// This can happen if multiple threads append nodes at the same time
// A new node thinks it's the tail (Next is null) as another thread's new node
// updates the previous node's Nextthinks it's the tail's Next field may not
InterlockedEx.IfThen(ref _tail, tempTail, tempTailNext);
continue;
}
// The tail is truely the tail, try to append the new node
appendedNewNode = InterlockedEx.IfThen(ref tempTail.Next, null, newNode);
}
// When new node is sucessfully appended, make the tail refer to it
// This can fail if another thread scoots in. If this happens, our node is
// appended to the linked-list but _tail refers to another node that is not
// the tail. The next Enqueue/Dequeue call will fix _tail
InterlockedEx.IfThen(ref _tail, tempTail, newNode);
//Interlocked.Increment(ref _count);
}
