Code:
/ 4.0 / 4.0 / untmp / DEVDIV_TFS / Dev10 / Releases / RTMRel / ndp / clr / src / BCL / System / Collections / Concurrent / ConcurrentStack.cs / 1305376 / ConcurrentStack.cs
#pragma warning disable 0420
// ==++==
//
// Copyright (c) Microsoft Corporation. All rights reserved.
//
// ==--==
// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
//
// ConcurrentStack.cs
//
// [....]
//
// A lock-free, concurrent stack 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.Serialization;
using System.Security;
using System.Security.Permissions;
using System.Threading;
namespace System.Collections.Concurrent
{
// A stack that uses CAS operations internally to maintain thread-safety in a lock-free
// manner. Attempting to push or pop concurrently from the stack will not trigger waiting,
// although some optimistic concurrency and retry is used, possibly leading to lack of
// fairness and/or livelock. The stack uses spinning and backoff to add some randomization,
// in hopes of statistically decreasing the possibility of livelock.
//
// Note that we currently allocate a new node on every push. This avoids having to worry
// about potential ABA issues, since the CLR GC ensures that a memory address cannot be
// reused before all references to it have died.
///
/// Represents a thread-safe last-in, first-out collection of objects.
///
/// Specifies the type of elements in the stack.
///
/// All public and protected members of
[DebuggerDisplay("Count = {Count}")]
[DebuggerTypeProxy(typeof(SystemCollectionsConcurrent_ProducerConsumerCollectionDebugView<>))]
[HostProtection(Synchronization = true, ExternalThreading = true)]
[Serializable]
public class ConcurrentStack : IProducerConsumerCollection
{
///
/// A simple (internal) node type used to store elements of concurrent stacks and queues.
///
private class Node
{
internal T m_value; // Value of the node.
internal Node m_next; // Next pointer.
///
/// Constructs a new node with the specified value and no next node.
///
/// The value of the node.
internal Node(T value)
{
m_value = value;
m_next = null;
}
}
[NonSerialized]
private volatile Node m_head; // The stack is a singly linked list, and only remembers the head.
private T[] m_serializationArray; // Used for custom serialization.
private const int BACKOFF_MAX_YIELDS = 8; // Arbitrary number to cap backoff.
///
/// Initializes a new instance of the
public ConcurrentStack()
{
}
///
/// Initializes a new instance of the
/// The collection whose elements are copied to the new The
public ConcurrentStack(IEnumerable collection)
{
if (collection == null)
{
throw new ArgumentNullException("collection");
}
InitializeFromCollection(collection);
}
///
/// Initializes the contents of the stack from an existing collection.
///
/// A collection from which to copy elements.
private void InitializeFromCollection(IEnumerable collection)
{
// We just copy the contents of the collection to our stack.
Node lastNode = null;
foreach (T element in collection)
{
Node newNode = new Node(element);
newNode.m_next = lastNode;
lastNode = newNode;
}
m_head = lastNode;
}
///
/// Get the data array to be serialized
///
[OnSerializing]
private void OnSerializing(StreamingContext context)
{
// save the data into the serialization array to be saved
m_serializationArray = ToArray();
}
///
/// Construct the stack from a previously seiralized one
///
[OnDeserialized]
private void OnDeserialized(StreamingContext context)
{
Contract.Assert(m_serializationArray != null);
// Add the elements to our stack. We need to add them from head-to-tail, to
// preserve the original ordering of the stack before serialization.
Node prevNode = null;
Node head = null;
for (int i = 0; i < m_serializationArray.Length; i++)
{
Node currNode = new Node(m_serializationArray[i]);
if (prevNode == null)
{
head = currNode;
}
else
{
prevNode.m_next = currNode;
}
prevNode = currNode;
}
m_head = head;
m_serializationArray = null;
}
///
/// Gets a value that indicates whether the
/// true if the
///
/// For determining whether the collection contains any items, use of this property is recommended
/// rather than retrieving the number of items from the
public bool IsEmpty
{
// Checks whether the stack is empty. Clearly the answer may be out of date even prior to
// the function returning (i.e. if another thread concurrently adds to the stack). It does
// guarantee, however, that, if another thread does not mutate the stack, a subsequent call
// to TryPop will return true -- i.e. it will also read the stack as non-empty.
get { return m_head == null; }
}
///
/// Gets the number of elements contained in the
/// The number of elements contained in the
///
/// For determining whether the collection contains any items, use of the
public int Count
{
// Counts the number of entries in the stack. This is an O(n) operation. The answer may be out
// of date before returning, but guarantees to return a count that was once valid. Conceptually,
// the implementation snaps a copy of the list and then counts the entries, though physically
// this is not what actually happens.
get
{
int count = 0;
// Just whip through the list and tally up the number of nodes. We rely on the fact that
// node next pointers are immutable after being enqueued for the first time, even as
// they are being dequeued. If we ever changed this (e.g. to pool nodes somehow),
// we'd need to revisit this implementation.
for (Node curr = m_head; curr != null; curr = curr.m_next)
{
count++; //we don't handle overflow, to be consistent with existing generic collection types in CLR
}
return count;
}
}
///
/// Gets a value indicating whether access to the
/// true if access to the
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; }
}
///
/// Gets an object that can be used to synchronize access to the
/// The SyncRoot property is not supported
object ICollection.SyncRoot
{
get
{
throw new NotSupportedException(Environment.GetResourceString("ConcurrentCollection_SyncRoot_NotSupported"));
}
}
///
/// Removes all objects from the
public void Clear()
{
// Clear the list by setting the head to null. We don't need to use an atomic
// operation for this: anybody who is mutating the head by pushing or popping
// will need to use an atomic operation to guarantee they serialize and don't
// overwrite our setting of the head to null.
m_head = null;
}
///
/// Copies the elements of the
/// The one-dimensional
///
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);
}
///
/// Copies the
/// The one-dimensional
/// Inserts an object at the top of the
/// The object to push onto the
/// Inserts multiple objects at the top of the
/// The objects to push onto the
/// When adding multiple items to the stack, using PushRange is a more efficient
/// mechanism than using
public void PushRange(T[] items)
{
if (items == null)
{
throw new ArgumentNullException("items");
}
PushRange(items, 0, items.Length);
}
///
/// Inserts multiple objects at the top of the
/// The objects to push onto the
/// When adding multiple items to the stack, using PushRange is a more efficient
/// mechanism than using
public void PushRange(T[] items, int startIndex, int count)
{
ValidatePushPopRangeInput(items, startIndex, count);
// No op if the count is zero
if (count == 0)
return;
Node head, tail;
head = tail = new Node(items[startIndex]);
for (int i = startIndex + 1; i < startIndex + count; i++)
{
Node node = new Node(items[i]);
node.m_next = head;
head = node;
}
tail.m_next = m_head;
if (Interlocked.CompareExchange(ref m_head, head, tail.m_next) == tail.m_next)
{
return;
}
// If we failed, go to the slow path and loop around until we succeed.
PushCore(head, tail);
}
///
/// Push one or many nodes into the stack, if head and tails are equal then push one node to the stack other wise push the list between head
/// and tail to the stack
///
/// The head pointer to the new list
/// The tail pointer to the new list
private void PushCore(Node head, Node tail)
{
SpinWait spin = new SpinWait();
// Keep trying to CAS the exising head with the new node until we succeed.
do
{
spin.SpinOnce();
// Reread the head and link our new node.
tail.m_next = m_head;
}
while (Interlocked.CompareExchange(
ref m_head, head, tail.m_next) != tail.m_next);
#if !FEATURE_PAL
if (CDSCollectionETWBCLProvider.Log.IsEnabled())
{
CDSCollectionETWBCLProvider.Log.ConcurrentStack_FastPushFailed(spin.Count);
}
#endif //!FEATURE_PAL
}
///
/// Local helper function to validate the Pop Push range methods input
///
private void ValidatePushPopRangeInput(T[] items, int startIndex, int count)
{
if (items == null)
{
throw new ArgumentNullException("items");
}
if (count < 0)
{
throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ConcurrentStack_PushPopRange_CountOutOfRange"));
}
int length = items.Length;
if (startIndex >= length || startIndex < 0)
{
throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ConcurrentStack_PushPopRange_StartOutOfRange"));
}
if (length - count < startIndex) //instead of (startIndex + count > items.Length) to prevent overflow
{
throw new ArgumentException(Environment.GetResourceString("ConcurrentStack_PushPopRange_InvalidCount"));
}
}
///
/// Attempts to add an object to the
/// The object to add to the true if the object was added successfully; otherwise, false.
/// For
bool IProducerConsumerCollection.TryAdd(T item)
{
Push(item);
return true;
}
///
/// Attempts to return an object from the top of the
/// When this method returns, true if and object was returned successfully; otherwise, false.
public bool TryPeek(out T result)
{
Node head = m_head;
// If the stack is empty, return false; else return the element and true.
if (head == null)
{
result = default(T);
return false;
}
else
{
result = head.m_value;
return true;
}
}
///
/// Attempts to pop and return the object at the top of the
///
/// When this method returns, if the operation was successful, true if an element was removed and returned from the top of the
public bool TryPop(out T result)
{
Node head = m_head;
//stack is empty
if (head == null)
{
result = default(T);
return false;
}
if (Interlocked.CompareExchange(ref m_head, head.m_next, head) == head)
{
result = head.m_value;
return true;
}
// Fall through to the slow path.
return TryPopCore(out result);
}
///
/// Attempts to pop and return multiple objects from the top of the
///
/// The The number of objects successfully popped from the top of the
///
/// When popping multiple items, if there is little contention on the stack, using
/// TryPopRange can be more efficient than using
public int TryPopRange(T[] items)
{
if (items == null)
{
throw new ArgumentNullException("items");
}
return TryPopRange(items, 0, items.Length);
}
///
/// Attempts to pop and return multiple objects from the top of the
///
/// The
/// When popping multiple items, if there is little contention on the stack, using
/// TryPopRange can be more efficient than using
public int TryPopRange(T[] items, int startIndex, int count)
{
ValidatePushPopRangeInput(items, startIndex, count);
// No op if the count is zero
if (count == 0)
return 0;
Node poppedHead;
int nodesCount = TryPopCore(count, out poppedHead);
if (nodesCount > 0)
{
CopyRemovedItems(poppedHead, items, startIndex, nodesCount);
}
return nodesCount;
}
///
/// Local helper function to Pop an item from the stack, slow path
///
/// The popped item
/// True if succeeded, false otherwise
private bool TryPopCore(out T result)
{
Node poppedNode;
if (TryPopCore(1, out poppedNode) == 1)
{
result = poppedNode.m_value;
return true;
}
result = default(T);
return false;
}
///
/// Slow path helper for TryPop. This method assumes an initial attempt to pop an element
/// has already occurred and failed, so it begins spinning right away.
///
/// The number of items to pop.
///
/// When this method returns, if the pop succeeded, contains the removed object. If no object was
/// available to be removed, the value is unspecified. This parameter is passed uninitialized.
///
/// True if an element was removed and returned; otherwise, false.
private int TryPopCore(int count, out Node poppedHead)
{
SpinWait spin = new SpinWait();
// Try to CAS the head with its current next. We stop when we succeed or
// when we notice that the stack is empty, whichever comes first.
Node head;
Node next;
int backoff = 1;
Random r = new Random(Environment.TickCount & Int32.MaxValue); // avoid the case where TickCount could return Int32.MinValue
while (true)
{
head = m_head;
// Is the stack empty?
if (head == null)
{
#if !FEATURE_PAL
if (count == 1 && CDSCollectionETWBCLProvider.Log.IsEnabled())
{
CDSCollectionETWBCLProvider.Log.ConcurrentStack_FastPopFailed(spin.Count);
}
#endif //!FEATURE_PAL
poppedHead = null;
return 0;
}
next = head;
int nodesCount = 1;
for (; nodesCount < count && next.m_next != null; nodesCount++)
{
next = next.m_next;
}
// Try to swap the new head. If we succeed, break out of the loop.
if (Interlocked.CompareExchange(ref m_head, next.m_next, head) == head)
{
#if !FEATURE_PAL
if (count == 1 && CDSCollectionETWBCLProvider.Log.IsEnabled())
{
CDSCollectionETWBCLProvider.Log.ConcurrentStack_FastPopFailed(spin.Count);
}
#endif //!FEATURE_PAL
// Return the popped Node.
poppedHead = head;
return nodesCount;
}
// We failed to CAS the new head. Spin briefly and retry.
for (int i = 0; i < backoff; i++)
{
spin.SpinOnce();
}
backoff = spin.NextSpinWillYield ? r.Next(1, BACKOFF_MAX_YIELDS) : backoff * 2;
}
}
///
/// Local helper function to copy the poped elements into a given collection
///
/// The head of the list to be copied
/// The collection to place the popped items in
/// the beginning of index of where to place the popped items
/// The number of nodes.
private void CopyRemovedItems(Node head, T[] collection, int startIndex, int nodesCount)
{
Node current = head;
for (int i = startIndex; i < startIndex + nodesCount; i++)
{
collection[i] = current.m_value;
current = current.m_next;
}
}
///
/// Attempts to remove and return an object from the
///
/// When this method returns, if the operation was successful, true if an element was removed and returned succesfully; otherwise, false.
/// For
bool IProducerConsumerCollection.TryTake(out T item)
{
return TryPop(out item);
}
///
/// Copies the items stored in the
/// A new array containing a snapshot of elements copied from the
public T[] ToArray()
{
return ToList().ToArray();
}
///
/// Returns an array containing a snapshot of the list's contents, using
/// the target list node as the head of a region in the list.
///
/// An array of the list's contents.
private List ToList()
{
List list = new List();
Node curr = m_head;
while (curr != null)
{
list.Add(curr.m_value);
curr = curr.m_next;
}
return list;
}
///
/// Returns an enumerator that iterates through the
/// An enumerator for the
///
/// The enumeration represents a moment-in-time snapshot of the contents
/// of the stack. It does not reflect any updates to the collection after
///
public IEnumerator GetEnumerator()
{
// Returns an enumerator for the stack. This effectively takes a snapshot
// of the stack's contents at the time of the call, i.e. subsequent modifications
// (pushes or pops) will not be reflected in the enumerator's contents.
//If we put yield-return here, the iterator will be lazily evaluated. As a result a snapshot of
//the stack 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
return GetEnumerator(m_head);
}
private IEnumerator GetEnumerator(Node head)
{
Node current = head;
while (current != null)
{
yield return current.m_value;
current = current.m_next;
}
}
///
/// Returns an enumerator that iterates through a collection.
///
/// An
///
/// The enumeration represents a moment-in-time snapshot of the contents of the stack. It does not
/// reflect any updates to the collection after
///
IEnumerator IEnumerable.GetEnumerator()
{
return ((IEnumerable)this).GetEnumerator();
}
}
}
// File provided for Reference Use Only by Microsoft Corporation (c) 2007.
// Copyright (c) Microsoft Corporation. All rights reserved.
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