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    <p>If you assign the value of <code>m_Head.Next</code> after it is read in <code>Take</code> for passing to <code>Interlocked.Exchange(ref this.m_Head, this.m_Head.Next)</code>, the pointer will be lost because the only way to access it was through <code>m_Head</code>.</p> <ul> <li><code>Take</code> reads <code>m_Head.Next</code> (<code>==null</code>)</li> <li><code>Put</code> writes <code>m_Head.Next</code> (<code>!=null</code>)</li> <li><code>Take</code> writes <code>m_Head</code> (<code>==null</code>)</li> </ul> <p><strong>Edit:</strong> This should work. I used a non-null sentinel value and <code>Interlocked.CompareExchange</code> to make sure <code>Put</code> doesn't try to reuse an entry that <code>Take</code> is already removing.</p> <p><strong>Edit 2:</strong> Tweaks to <code>Take</code>.</p> <p><strong>Edit 3:</strong> I believe I still need to add a <code>goto retry;</code> in <code>Put</code> if the identified tail is <code>Entry.Sentinel</code>.</p> <pre><code>using System; using System.Threading; #pragma warning disable 420 namespace Tychaia.Threading { public class TaskPipeline&lt;T&gt; { private int? m_InputThread; private int? m_OutputThread; private volatile Entry m_Head; private sealed class Entry { public static readonly Entry Sentinel = new Entry(default(T)); public readonly T Value; public Entry Next; public Entry(T value) { Value = value; Next = null; } } /// &lt;summary&gt; /// Creates a new TaskPipeline with the current thread being /// considered to be the input side of the pipeline. The /// output thread should call Connect(). /// &lt;/summary&gt; public TaskPipeline() { this.m_InputThread = Thread.CurrentThread.ManagedThreadId; this.m_OutputThread = null; } /// &lt;summary&gt; /// Connects the current thread as the output of the pipeline. /// &lt;/summary&gt; public void Connect() { if (this.m_OutputThread != null) throw new InvalidOperationException("TaskPipeline can only have one output thread connected."); this.m_OutputThread = Thread.CurrentThread.ManagedThreadId; } /// &lt;summary&gt; /// Puts an item into the queue to be processed. /// &lt;/summary&gt; /// &lt;param name="value"&gt;Value.&lt;/param&gt; public void Put(T value) { if (this.m_InputThread != Thread.CurrentThread.ManagedThreadId) throw new InvalidOperationException("Only the input thread may place items into TaskPipeline."); retry: // Walk the queued items until we find one that // has Next set to null. var head = this.m_Head; while (head != null) { if (head.Next != null) head = head.Next; if (head.Next == null) break; } if (head == null) { if (Interlocked.CompareExchange(ref m_Head, new Entry(value), null) != null) goto retry; } else { if (Interlocked.CompareExchange(ref head.Next, new Entry(value), null) != null) goto retry; } } /// &lt;summary&gt; /// Takes the next item from the pipeline, or blocks until an item /// is recieved. /// &lt;/summary&gt; /// &lt;returns&gt;The next item.&lt;/returns&gt; public T Take() { if (this.m_OutputThread != Thread.CurrentThread.ManagedThreadId) throw new InvalidOperationException("Only the output thread may retrieve items from TaskPipeline."); // Wait until there is an item to take. var spin = new SpinWait(); while (this.m_Head == null) spin.SpinOnce(); // Return the item and exchange the current head with // the next item, all in an atomic operation. Entry head = m_Head; retry: Entry next = head.Next; // replace m_Head.Next with a non-null sentinel to ensure Put won't try to reuse it if (Interlocked.CompareExchange(ref head.Next, Entry.Sentinel, next) != next) goto retry; m_Head = next; return head.Value; } } } </code></pre>
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    1. POIssue with threaded queue implementation in .NET
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    1. USSam Harwell
    1. USSam Harwell
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POIssue with threaded queue implementation in .NET
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    1. COOh so when `Put` sets the `Next` property in the `else` clause, and then the `head` disappears because it's pointing directly to `m_Head`?
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      1. USJune Rhodes
    2. COYes, the `else` part of `Put` is the problem location in that method - specifically when the local variable `head` in that method is the previously used `m_Head` that `Take` is about to discard.
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      1. USSam Harwell
    3. COIs there any way around this without using locks? I can't see how to represent the complex logic of traversing the linked list by only using `Interlocked`.
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      1. USJune Rhodes
 

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