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#pragma warning disable 0420
// ==++==
//
// Copyright (c) Microsoft Corporation. All rights reserved.
//
// ==--==
// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
//
// ConcurrentQueue.cs
//
// <OWNER>Microsoft</OWNER>
//
// A lock-free, concurrent queue primitive, and its associated debugger view type.
//
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.Contracts;
using System.Runtime.ConstrainedExecution;
using System.Runtime.InteropServices;
using System.Runtime.Serialization;
using System.Security;
using System.Security.Permissions;
using System.Threading;
namespace System.Collections.Concurrent
{
/// <summary>
/// Represents a thread-safe first-in, first-out collection of objects.
/// </summary>
/// <typeparam name="T">Specifies the type of elements in the queue.</typeparam>
/// <remarks>
/// All public and protected members of <see cref="ConcurrentQueue{T}"/> are thread-safe and may be used
/// concurrently from multiple threads.
/// </remarks>
[ComVisible(false)]
[DebuggerDisplay("Count = {Count}")]
[DebuggerTypeProxy(typeof(SystemCollectionsConcurrent_ProducerConsumerCollectionDebugView<>))]
[HostProtection(Synchronization = true, ExternalThreading = true)]
[Serializable]
public class ConcurrentQueue<T> : IProducerConsumerCollection<T>, IReadOnlyCollection<T>
{
//fields of ConcurrentQueue
[NonSerialized]
private volatile Segment m_head;
[NonSerialized]
private volatile Segment m_tail;
private T[] m_serializationArray; // Used for custom serialization.
private const int SEGMENT_SIZE = 32;
//number of snapshot takers, GetEnumerator(), ToList() and ToArray() operations take snapshot.
[NonSerialized]
internal volatile int m_numSnapshotTakers = 0;
/// <summary>
/// Initializes a new instance of the <see cref="ConcurrentQueue{T}"/> class.
/// </summary>
public ConcurrentQueue()
{
m_head = m_tail = new Segment(0, this);
}
/// <summary>
/// Initializes the contents of the queue from an existing collection.
/// </summary>
/// <param name="collection">A collection from which to copy elements.</param>
private void InitializeFromCollection(IEnumerable<T> collection)
{
Segment localTail = new Segment(0, this);//use this local variable to avoid the extra volatile read/write. this is safe because it is only called from ctor
m_head = localTail;
int index = 0;
foreach (T element in collection)
{
Contract.Assert(index >= 0 && index < SEGMENT_SIZE);
localTail.UnsafeAdd(element);
index++;
if (index >= SEGMENT_SIZE)
{
localTail = localTail.UnsafeGrow();
index = 0;
}
}
m_tail = localTail;
}
/// <summary>
/// Initializes a new instance of the <see cref="ConcurrentQueue{T}"/>
/// class that contains elements copied from the specified collection
/// </summary>
/// <param name="collection">The collection whose elements are copied to the new <see
/// cref="ConcurrentQueue{T}"/>.</param>
/// <exception cref="T:System.ArgumentNullException">The <paramref name="collection"/> argument is
/// null.</exception>
public ConcurrentQueue(IEnumerable<T> collection)
{
if (collection == null)
{
throw new ArgumentNullException("collection");
}
InitializeFromCollection(collection);
}
/// <summary>
/// Get the data array to be serialized
/// </summary>
[OnSerializing]
private void OnSerializing(StreamingContext context)
{
// save the data into the serialization array to be saved
m_serializationArray = ToArray();
}
/// <summary>
/// Construct the queue from a previously seiralized one
/// </summary>
[OnDeserialized]
private void OnDeserialized(StreamingContext context)
{
Contract.Assert(m_serializationArray != null);
InitializeFromCollection(m_serializationArray);
m_serializationArray = null;
}
/// <summary>
/// Copies the elements of the <see cref="T:System.Collections.ICollection"/> to an <see
/// cref="T:System.Array"/>, starting at a particular
/// <see cref="T:System.Array"/> index.
/// </summary>
/// <param name="array">The one-dimensional <see cref="T:System.Array">Array</see> that is the
/// destination of the elements copied from the
/// <see cref="T:System.Collections.Concurrent.ConcurrentBag"/>. The <see
/// cref="T:System.Array">Array</see> must have zero-based indexing.</param>
/// <param name="index">The zero-based index in <paramref name="array"/> at which copying
/// begins.</param>
/// <exception cref="ArgumentNullException"><paramref name="array"/> is a null reference (Nothing in
/// Visual Basic).</exception>
/// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is less than
/// zero.</exception>
/// <exception cref="ArgumentException">
/// <paramref name="array"/> is multidimensional. -or-
/// <paramref name="array"/> does not have zero-based indexing. -or-
/// <paramref name="index"/> is equal to or greater than the length of the <paramref name="array"/>
/// -or- The number of elements in the source <see cref="T:System.Collections.ICollection"/> is
/// greater than the available space from <paramref name="index"/> to the end of the destination
/// <paramref name="array"/>. -or- The type of the source <see
/// cref="T:System.Collections.ICollection"/> cannot be cast automatically to the type of the
/// destination <paramref name="array"/>.
/// </exception>
void ICollection.CopyTo(Array array, int index)
{
// Validate arguments.
if (array == null)
{
throw new ArgumentNullException("array");
}
// We must be careful not to corrupt the array, so we will first accumulate an
// internal list of elements that we will then copy to the array. This requires
// some extra allocation, but is necessary since we don't know up front whether
// the array is sufficiently large to hold the stack's contents.
((ICollection)ToList()).CopyTo(array, index);
}
/// <summary>
/// Gets a value indicating whether access to the <see cref="T:System.Collections.ICollection"/> is
/// synchronized with the SyncRoot.
/// </summary>
/// <value>true if access to the <see cref="T:System.Collections.ICollection"/> is synchronized
/// with the SyncRoot; otherwise, false. For <see cref="ConcurrentQueue{T}"/>, this property always
/// returns false.</value>
bool ICollection.IsSynchronized
{
// Gets a value indicating whether access to this collection is synchronized. Always returns
// false. The reason is subtle. While access is in face thread safe, it's not the case that
// locking on the SyncRoot would have prevented concurrent pushes and pops, as this property
// would typically indicate; that's because we internally use CAS operations vs. true locks.
get { return false; }
}
/// <summary>
/// Gets an object that can be used to synchronize access to the <see
/// cref="T:System.Collections.ICollection"/>. This property is not supported.
/// </summary>
/// <exception cref="T:System.NotSupportedException">The SyncRoot property is not supported.</exception>
object ICollection.SyncRoot
{
get
{
throw new NotSupportedException(Environment.GetResourceString("ConcurrentCollection_SyncRoot_NotSupported"));
}
}
/// <summary>
/// Returns an enumerator that iterates through a collection.
/// </summary>
/// <returns>An <see cref="T:System.Collections.IEnumerator"/> that can be used to iterate through the collection.</returns>
IEnumerator IEnumerable.GetEnumerator()
{
return ((IEnumerable<T>)this).GetEnumerator();
}
/// <summary>
/// Attempts to add an object to the <see
/// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>.
/// </summary>
/// <param name="item">The object to add to the <see
/// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>. The value can be a null
/// reference (Nothing in Visual Basic) for reference types.
/// </param>
/// <returns>true if the object was added successfully; otherwise, false.</returns>
/// <remarks>For <see cref="ConcurrentQueue{T}"/>, this operation will always add the object to the
/// end of the <see cref="ConcurrentQueue{T}"/>
/// and return true.</remarks>
bool IProducerConsumerCollection<T>.TryAdd(T item)
{
Enqueue(item);
return true;
}
/// <summary>
/// Attempts to remove and return an object from the <see
/// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>.
/// </summary>
/// <param name="item">
/// When this method returns, if the operation was successful, <paramref name="item"/> contains the
/// object removed. If no object was available to be removed, the value is unspecified.
/// </param>
/// <returns>true if an element was removed and returned succesfully; otherwise, false.</returns>
/// <remarks>For <see cref="ConcurrentQueue{T}"/>, this operation will attempt to remove the object
/// from the beginning of the <see cref="ConcurrentQueue{T}"/>.
/// </remarks>
bool IProducerConsumerCollection<T>.TryTake(out T item)
{
return TryDequeue(out item);
}
/// <summary>
/// Gets a value that indicates whether the <see cref="ConcurrentQueue{T}"/> is empty.
/// </summary>
/// <value>true if the <see cref="ConcurrentQueue{T}"/> is empty; otherwise, false.</value>
/// <remarks>
/// For determining whether the collection contains any items, use of this property is recommended
/// rather than retrieving the number of items from the <see cref="Count"/> property and comparing it
/// to 0. However, as this collection is intended to be accessed concurrently, it may be the case
/// that another thread will modify the collection after <see cref="IsEmpty"/> returns, thus invalidating
/// the result.
/// </remarks>
public bool IsEmpty
{
get
{
Segment head = m_head;
if (!head.IsEmpty)
//fast route 1:
//if current head is not empty, then queue is not empty
return false;
else if (head.Next == null)
//fast route 2:
//if current head is empty and it's the last segment
//then queue is empty
return true;
else
//slow route:
//current head is empty and it is NOT the last segment,
//it means another thread is growing new segment
{
SpinWait spin = new SpinWait();
while (head.IsEmpty)
{
if (head.Next == null)
return true;
spin.SpinOnce();
head = m_head;
}
return false;
}
}
}
/// <summary>
/// Copies the elements stored in the <see cref="ConcurrentQueue{T}"/> to a new array.
/// </summary>
/// <returns>A new array containing a snapshot of elements copied from the <see
/// cref="ConcurrentQueue{T}"/>.</returns>
public T[] ToArray()
{
return ToList().ToArray();
}
/// <summary>
/// Copies the <see cref="ConcurrentQueue{T}"/> elements to a new <see
/// cref="T:System.Collections.Generic.List{T}"/>.
/// </summary>
/// <returns>A new <see cref="T:System.Collections.Generic.List{T}"/> containing a snapshot of
/// elements copied from the <see cref="ConcurrentQueue{T}"/>.</returns>
private List<T> ToList()
{
// Increments the number of active snapshot takers. This increment must happen before the snapshot is
// taken. At the same time, Decrement must happen after list copying is over. Only in this way, can it
// eliminate race condition when Segment.TryRemove() checks whether m_numSnapshotTakers == 0.
Interlocked.Increment(ref m_numSnapshotTakers);
List<T> list = new List<T>();
try
{
//store head and tail positions in buffer,
Segment head, tail;
int headLow, tailHigh;
GetHeadTailPositions(out head, out tail, out headLow, out tailHigh);
if (head == tail)
{
head.AddToList(list, headLow, tailHigh);
}
else
{
head.AddToList(list, headLow, SEGMENT_SIZE - 1);
Segment curr = head.Next;
while (curr != tail)
{
curr.AddToList(list, 0, SEGMENT_SIZE - 1);
curr = curr.Next;
}
//Add tail segment
tail.AddToList(list, 0, tailHigh);
}
}
finally
{
// This Decrement must happen after copying is over.
Interlocked.Decrement(ref m_numSnapshotTakers);
}
return list;
}
/// <summary>
/// Store the position of the current head and tail positions.
/// </summary>
/// <param name="head">return the head segment</param>
/// <param name="tail">return the tail segment</param>
/// <param name="headLow">return the head offset, value range [0, SEGMENT_SIZE]</param>
/// <param name="tailHigh">return the tail offset, value range [-1, SEGMENT_SIZE-1]</param>
private void GetHeadTailPositions(out Segment head, out Segment tail,
out int headLow, out int tailHigh)
{
head = m_head;
tail = m_tail;
headLow = head.Low;
tailHigh = tail.High;
SpinWait spin = new SpinWait();
//we loop until the observed values are stable and sensible.
//This ensures that any update order by other methods can be tolerated.
while (
//if head and tail changed, retry
head != m_head || tail != m_tail
//if low and high pointers, retry
|| headLow != head.Low || tailHigh != tail.High
//if head jumps ahead of tail because of concurrent grow and dequeue, retry
|| head.m_index > tail.m_index)
{
spin.SpinOnce();
head = m_head;
tail = m_tail;
headLow = head.Low;
tailHigh = tail.High;
}
}
/// <summary>
/// Gets the number of elements contained in the <see cref="ConcurrentQueue{T}"/>.
/// </summary>
/// <value>The number of elements contained in the <see cref="ConcurrentQueue{T}"/>.</value>
/// <remarks>
/// For determining whether the collection contains any items, use of the <see cref="IsEmpty"/>
/// property is recommended rather than retrieving the number of items from the <see cref="Count"/>
/// property and comparing it to 0.
/// </remarks>
public int Count
{
get
{
//store head and tail positions in buffer,
Segment head, tail;
int headLow, tailHigh;
GetHeadTailPositions(out head, out tail, out headLow, out tailHigh);
if (head == tail)
{
return tailHigh - headLow + 1;
}
//head segment
int count = SEGMENT_SIZE - headLow;
//middle segment(s), if any, are full.
//We don't deal with overflow to be consistent with the behavior of generic types in CLR.
count += SEGMENT_SIZE * ((int)(tail.m_index - head.m_index - 1));
//tail segment
count += tailHigh + 1;
return count;
}
}
/// <summary>
/// Copies the <see cref="ConcurrentQueue{T}"/> elements to an existing one-dimensional <see
/// cref="T:System.Array">Array</see>, starting at the specified array index.
/// </summary>
/// <param name="array">The one-dimensional <see cref="T:System.Array">Array</see> that is the
/// destination of the elements copied from the
/// <see cref="ConcurrentQueue{T}"/>. The <see cref="T:System.Array">Array</see> must have zero-based
/// indexing.</param>
/// <param name="index">The zero-based index in <paramref name="array"/> at which copying
/// begins.</param>
/// <exception cref="ArgumentNullException"><paramref name="array"/> is a null reference (Nothing in
/// Visual Basic).</exception>
/// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is less than
/// zero.</exception>
/// <exception cref="ArgumentException"><paramref name="index"/> is equal to or greater than the
/// length of the <paramref name="array"/>
/// -or- The number of elements in the source <see cref="ConcurrentQueue{T}"/> is greater than the
/// available space from <paramref name="index"/> to the end of the destination <paramref
/// name="array"/>.
/// </exception>
public void CopyTo(T[] array, int index)
{
if (array == null)
{
throw new ArgumentNullException("array");
}
// We must be careful not to corrupt the array, so we will first accumulate an
// internal list of elements that we will then copy to the array. This requires
// some extra allocation, but is necessary since we don't know up front whether
// the array is sufficiently large to hold the stack's contents.
ToList().CopyTo(array, index);
}
/// <summary>
/// Returns an enumerator that iterates through the <see
/// cref="ConcurrentQueue{T}"/>.
/// </summary>
/// <returns>An enumerator for the contents of the <see
/// cref="ConcurrentQueue{T}"/>.</returns>
/// <remarks>
/// The enumeration represents a moment-in-time snapshot of the contents
/// of the queue. It does not reflect any updates to the collection after
/// <see cref="GetEnumerator"/> was called. The enumerator is safe to use
/// concurrently with reads from and writes to the queue.
/// </remarks>
public IEnumerator<T> GetEnumerator()
{
// Increments the number of active snapshot takers. This increment must happen before the snapshot is
// taken. At the same time, Decrement must happen after the enumeration is over. Only in this way, can it
// eliminate race condition when Segment.TryRemove() checks whether m_numSnapshotTakers == 0.
Interlocked.Increment(ref m_numSnapshotTakers);
// Takes a snapshot of the queue.
// A design flaw here: if a Thread.Abort() happens, we cannot decrement m_numSnapshotTakers. But we cannot
// wrap the following with a try/finally block, otherwise the decrement will happen before the yield return
// statements in the GetEnumerator (head, tail, headLow, tailHigh) method.
Segment head, tail;
int headLow, tailHigh;
GetHeadTailPositions(out head, out tail, out headLow, out tailHigh);
//If we put yield-return here, the iterator will be lazily evaluated. As a result a snapshot of
// the queue is not taken when GetEnumerator is initialized but when MoveNext() is first called.
// This is inconsistent with existing generic collections. In order to prevent it, we capture the
// value of m_head in a buffer and call out to a helper method.
//The old way of doing this was to return the ToList().GetEnumerator(), but ToList() was an
// unnecessary perfomance hit.
return GetEnumerator(head, tail, headLow, tailHigh);
}
/// <summary>
/// Helper method of GetEnumerator to seperate out yield return statement, and prevent lazy evaluation.
/// </summary>
private IEnumerator<T> GetEnumerator(Segment head, Segment tail, int headLow, int tailHigh)
{
try
{
SpinWait spin = new SpinWait();
if (head == tail)
{
for (int i = headLow; i <= tailHigh; i++)
{
// If the position is reserved by an Enqueue operation, but the value is not written into,
// spin until the value is available.
spin.Reset();
while (!head.m_state[i].m_value)
{
spin.SpinOnce();
}
yield return head.m_array[i];
}
}
else
{
//iterate on head segment
for (int i = headLow; i < SEGMENT_SIZE; i++)
{
// If the position is reserved by an Enqueue operation, but the value is not written into,
// spin until the value is available.
spin.Reset();
while (!head.m_state[i].m_value)
{
spin.SpinOnce();
}
yield return head.m_array[i];
}
//iterate on middle segments
Segment curr = head.Next;
while (curr != tail)
{
for (int i = 0; i < SEGMENT_SIZE; i++)
{
// If the position is reserved by an Enqueue operation, but the value is not written into,
// spin until the value is available.
spin.Reset();
while (!curr.m_state[i].m_value)
{
spin.SpinOnce();
}
yield return curr.m_array[i];
}
curr = curr.Next;
}
//iterate on tail segment
for (int i = 0; i <= tailHigh; i++)
{
// If the position is reserved by an Enqueue operation, but the value is not written into,
// spin until the value is available.
spin.Reset();
while (!tail.m_state[i].m_value)
{
spin.SpinOnce();
}
yield return tail.m_array[i];
}
}
}
finally
{
// This Decrement must happen after the enumeration is over.
Interlocked.Decrement(ref m_numSnapshotTakers);
}
}
/// <summary>
/// Adds an object to the end of the <see cref="ConcurrentQueue{T}"/>.
/// </summary>
/// <param name="item">The object to add to the end of the <see
/// cref="ConcurrentQueue{T}"/>. The value can be a null reference
/// (Nothing in Visual Basic) for reference types.
/// </param>
public void Enqueue(T item)
{
SpinWait spin = new SpinWait();
while (true)
{
Segment tail = m_tail;
if (tail.TryAppend(item))
return;
spin.SpinOnce();
}
}
/// <summary>
/// Attempts to remove and return the object at the beginning of the <see
/// cref="ConcurrentQueue{T}"/>.
/// </summary>
/// <param name="result">
/// When this method returns, if the operation was successful, <paramref name="result"/> contains the
/// object removed. If no object was available to be removed, the value is unspecified.
/// </param>
/// <returns>true if an element was removed and returned from the beggining of the <see
/// cref="ConcurrentQueue{T}"/>
/// succesfully; otherwise, false.</returns>
public bool TryDequeue(out T result)
{
while (!IsEmpty)
{
Segment head = m_head;
if (head.TryRemove(out result))
return true;
//since method IsEmpty spins, we don't need to spin in the while loop
}
result = default(T);
return false;
}
/// <summary>
/// Attempts to return an object from the beginning of the <see cref="ConcurrentQueue{T}"/>
/// without removing it.
/// </summary>
/// <param name="result">When this method returns, <paramref name="result"/> contains an object from
/// the beginning of the <see cref="T:System.Collections.Concurrent.ConccurrentQueue{T}"/> or an
/// unspecified value if the operation failed.</param>
/// <returns>true if and object was returned successfully; otherwise, false.</returns>
public bool TryPeek(out T result)
{
Interlocked.Increment(ref m_numSnapshotTakers);
while (!IsEmpty)
{
Segment head = m_head;
if (head.TryPeek(out result))
{
Interlocked.Decrement(ref m_numSnapshotTakers);
return true;
}
//since method IsEmpty spins, we don't need to spin in the while loop
}
result = default(T);
Interlocked.Decrement(ref m_numSnapshotTakers);
return false;
}
/// <summary>
/// private class for ConcurrentQueue.
/// a queue is a linked list of small arrays, each node is called a segment.
/// A segment contains an array, a pointer to the next segment, and m_low, m_high indices recording
/// the first and last valid elements of the array.
/// </summary>
private class Segment
{
//we define two volatile arrays: m_array and m_state. Note that the accesses to the array items
//do not get volatile treatment. But we don't need to worry about loading adjacent elements or
//store/load on adjacent elements would suffer reordering.
// - Two stores: these are at risk, but CLRv2 memory model guarantees store-release hence we are safe.
// - Two loads: because one item from two volatile arrays are accessed, the loads of the array references
// are sufficient to prevent reordering of the loads of the elements.
internal volatile T[] m_array;
// For each entry in m_array, the corresponding entry in m_state indicates whether this position contains
// a valid value. m_state is initially all false.
internal volatile VolatileBool[] m_state;
//pointer to the next segment. null if the current segment is the last segment
private volatile Segment m_next;
//We use this zero based index to track how many segments have been created for the queue, and
//to compute how many active segments are there currently.
// * The number of currently active segments is : m_tail.m_index - m_head.m_index + 1;
// * m_index is incremented with every Segment.Grow operation. We use Int64 type, and we can safely
// assume that it never overflows. To overflow, we need to do 2^63 increments, even at a rate of 4
// billion (2^32) increments per second, it takes 2^31 seconds, which is about 64 years.
internal readonly long m_index;
//indices of where the first and last valid values
// - m_low points to the position of the next element to pop from this segment, range [0, infinity)
// m_low >= SEGMENT_SIZE implies the segment is disposable
// - m_high points to the position of the latest pushed element, range [-1, infinity)
// m_high == -1 implies the segment is new and empty
// m_high >= SEGMENT_SIZE-1 means this segment is ready to grow.
// and the thread who sets m_high to SEGMENT_SIZE-1 is responsible to grow the segment
// - Math.Min(m_low, SEGMENT_SIZE) > Math.Min(m_high, SEGMENT_SIZE-1) implies segment is empty
// - initially m_low =0 and m_high=-1;
private volatile int m_low;
private volatile int m_high;
private volatile ConcurrentQueue<T> m_source;
/// <summary>
/// Create and initialize a segment with the specified index.
/// </summary>
internal Segment(long index, ConcurrentQueue<T> source)
{
m_array = new T[SEGMENT_SIZE];
m_state = new VolatileBool[SEGMENT_SIZE]; //all initialized to false
m_high = -1;
Contract.Assert(index >= 0);
m_index = index;
m_source = source;
}
/// <summary>
/// return the next segment
/// </summary>
internal Segment Next
{
get { return m_next; }
}
/// <summary>
/// return true if the current segment is empty (doesn't have any element available to dequeue,
/// false otherwise
/// </summary>
internal bool IsEmpty
{
get { return (Low > High); }
}
/// <summary>
/// Add an element to the tail of the current segment
/// exclusively called by ConcurrentQueue.InitializedFromCollection
/// InitializeFromCollection is responsible to guaratee that there is no index overflow,
/// and there is no contention
/// </summary>
/// <param name="value"></param>
internal void UnsafeAdd(T value)
{
Contract.Assert(m_high < SEGMENT_SIZE - 1);
m_high++;
m_array[m_high] = value;
m_state[m_high].m_value = true;
}
/// <summary>
/// Create a new segment and append to the current one
/// Does not update the m_tail pointer
/// exclusively called by ConcurrentQueue.InitializedFromCollection
/// InitializeFromCollection is responsible to guaratee that there is no index overflow,
/// and there is no contention
/// </summary>
/// <returns>the reference to the new Segment</returns>
internal Segment UnsafeGrow()
{
Contract.Assert(m_high >= SEGMENT_SIZE - 1);
Segment newSegment = new Segment(m_index + 1, m_source); //m_index is Int64, we don't need to worry about overflow
m_next = newSegment;
return newSegment;
}
/// <summary>
/// Create a new segment and append to the current one
/// Update the m_tail pointer
/// This method is called when there is no contention
/// </summary>
internal void Grow()
{
//no CAS is needed, since there is no contention (other threads are blocked, busy waiting)
Segment newSegment = new Segment(m_index + 1, m_source); //m_index is Int64, we don't need to worry about overflow
m_next = newSegment;
Contract.Assert(m_source.m_tail == this);
m_source.m_tail = m_next;
}
/// <summary>
/// Try to append an element at the end of this segment.
/// </summary>
/// <param name="value">the element to append</param>
/// <param name="tail">The tail.</param>
/// <returns>true if the element is appended, false if the current segment is full</returns>
/// <remarks>if appending the specified element succeeds, and after which the segment is full,
/// then grow the segment</remarks>
internal bool TryAppend(T value)
{
//quickly check if m_high is already over the boundary, if so, bail out
if (m_high >= SEGMENT_SIZE - 1)
{
return false;
}
//Now we will use a CAS to increment m_high, and store the result in newhigh.
//Depending on how many free spots left in this segment and how many threads are doing this Increment
//at this time, the returning "newhigh" can be
// 1) < SEGMENT_SIZE - 1 : we took a spot in this segment, and not the last one, just insert the value
// 2) == SEGMENT_SIZE - 1 : we took the last spot, insert the value AND grow the segment
// 3) > SEGMENT_SIZE - 1 : we failed to reserve a spot in this segment, we return false to
// Queue.Enqueue method, telling it to try again in the next segment.
int newhigh = SEGMENT_SIZE; //initial value set to be over the boundary
//We need do Interlocked.Increment and value/state update in a finally block to ensure that they run
//without interuption. This is to prevent anything from happening between them, and another dequeue
//thread maybe spinning forever to wait for m_state[] to be true;
try
{ }
finally
{
newhigh = Interlocked.Increment(ref m_high);
if (newhigh <= SEGMENT_SIZE - 1)
{
m_array[newhigh] = value;
m_state[newhigh].m_value = true;
}
//if this thread takes up the last slot in the segment, then this thread is responsible
//to grow a new segment. Calling Grow must be in the finally block too for reliability reason:
//if thread abort during Grow, other threads will be left busy spinning forever.
if (newhigh == SEGMENT_SIZE - 1)
{
Grow();
}
}
//if newhigh <= SEGMENT_SIZE-1, it means the current thread successfully takes up a spot
return newhigh <= SEGMENT_SIZE - 1;
}
/// <summary>
/// try to remove an element from the head of current segment
/// </summary>
/// <param name="result">The result.</param>
/// <param name="head">The head.</param>
/// <returns>return false only if the current segment is empty</returns>
internal bool TryRemove(out T result)
{
SpinWait spin = new SpinWait();
int lowLocal = Low, highLocal = High;
while (lowLocal <= highLocal)
{
//try to update m_low
if (Interlocked.CompareExchange(ref m_low, lowLocal + 1, lowLocal) == lowLocal)
{
//if the specified value is not available (this spot is taken by a push operation,
// but the value is not written into yet), then spin
SpinWait spinLocal = new SpinWait();
while (!m_state[lowLocal].m_value)
{
spinLocal.SpinOnce();
}
result = m_array[lowLocal];
// If there is no other thread taking snapshot (GetEnumerator(), ToList(), etc), reset the deleted entry to null.
// It is ok if after this conditional check m_numSnapshotTakers becomes > 0, because new snapshots won't include
// the deleted entry at m_array[lowLocal].
if (m_source.m_numSnapshotTakers <= 0)
{
m_array[lowLocal] = default(T); //release the reference to the object.
}
//if the current thread sets m_low to SEGMENT_SIZE, which means the current segment becomes
//disposable, then this thread is responsible to dispose this segment, and reset m_head
if (lowLocal + 1 >= SEGMENT_SIZE)
{
// Invariant: we only dispose the current m_head, not any other segment
// In usual situation, disposing a segment is simply seting m_head to m_head.m_next
// But there is one special case, where m_head and m_tail points to the same and ONLY
//segment of the queue: Another thread A is doing Enqueue and finds that it needs to grow,
//while the *current* thread is doing *this* Dequeue operation, and finds that it needs to
//dispose the current (and ONLY) segment. Then we need to wait till thread A finishes its
//Grow operation, this is the reason of having the following while loop
spinLocal = new SpinWait();
while (m_next == null)
{
spinLocal.SpinOnce();
}
Contract.Assert(m_source.m_head == this);
m_source.m_head = m_next;
}
return true;
}
else
{
//CAS failed due to contention: spin briefly and retry
spin.SpinOnce();
lowLocal = Low; highLocal = High;
}
}//end of while
result = default(T);
return false;
}
/// <summary>
/// try to peek the current segment
/// </summary>
/// <param name="result">holds the return value of the element at the head position,
/// value set to default(T) if there is no such an element</param>
/// <returns>true if there are elements in the current segment, false otherwise</returns>
internal bool TryPeek(out T result)
{
result = default(T);
int lowLocal = Low;
if (lowLocal > High)
return false;
SpinWait spin = new SpinWait();
while (!m_state[lowLocal].m_value)
{
spin.SpinOnce();
}
result = m_array[lowLocal];
return true;
}
/// <summary>
/// Adds part or all of the current segment into a List.
/// </summary>
/// <param name="list">the list to which to add</param>
/// <param name="start">the start position</param>
/// <param name="end">the end position</param>
internal void AddToList(List<T> list, int start, int end)
{
for (int i = start; i <= end; i++)
{
SpinWait spin = new SpinWait();
while (!m_state[i].m_value)
{
spin.SpinOnce();
}
list.Add(m_array[i]);
}
}
/// <summary>
/// return the position of the head of the current segment
/// Value range [0, SEGMENT_SIZE], if it's SEGMENT_SIZE, it means this segment is exhausted and thus empty
/// </summary>
internal int Low
{
get
{
return Math.Min(m_low, SEGMENT_SIZE);
}
}
/// <summary>
/// return the logical position of the tail of the current segment
/// Value range [-1, SEGMENT_SIZE-1]. When it's -1, it means this is a new segment and has no elemnet yet
/// </summary>
internal int High
{
get
{
//if m_high > SEGMENT_SIZE, it means it's out of range, we should return
//SEGMENT_SIZE-1 as the logical position
return Math.Min(m_high, SEGMENT_SIZE - 1);
}
}
}
}//end of class Segment
/// <summary>
/// A wrapper struct for volatile bool, please note the copy of the struct it self will not be volatile
/// for example this statement will not include in volatilness operation volatileBool1 = volatileBool2 the jit will copy the struct and will ignore the volatile
/// </summary>
struct VolatileBool
{
public VolatileBool(bool value)
{
m_value = value;
}
public volatile bool m_value;
}
}
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