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jsr166 |
1.1 |
/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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*/ |
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package java.util.concurrent; |
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import java.util.AbstractQueue; |
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import java.util.Collection; |
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jsr166 |
1.5 |
import java.util.ConcurrentModificationException; |
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jsr166 |
1.1 |
import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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jsr166 |
1.5 |
import java.util.Queue; |
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jsr166 |
1.1 |
import java.util.concurrent.locks.LockSupport; |
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/** |
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jsr166 |
1.6 |
* An unbounded {@link TransferQueue} based on linked nodes. |
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jsr166 |
1.1 |
* This queue orders elements FIFO (first-in-first-out) with respect |
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* to any given producer. The <em>head</em> of the queue is that |
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* element that has been on the queue the longest time for some |
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* producer. The <em>tail</em> of the queue is that element that has |
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* been on the queue the shortest time for some producer. |
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* |
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* <p>Beware that, unlike in most collections, the {@code size} |
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* method is <em>NOT</em> a constant-time operation. Because of the |
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* asynchronous nature of these queues, determining the current number |
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* of elements requires a traversal of the elements. |
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* |
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* <p>This class and its iterator implement all of the |
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* <em>optional</em> methods of the {@link Collection} and {@link |
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* Iterator} interfaces. |
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* |
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* <p>Memory consistency effects: As with other concurrent |
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* collections, actions in a thread prior to placing an object into a |
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* {@code LinkedTransferQueue} |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* actions subsequent to the access or removal of that element from |
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* the {@code LinkedTransferQueue} in another thread. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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* @since 1.7 |
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* @author Doug Lea |
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* @param <E> the type of elements held in this collection |
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*/ |
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public class LinkedTransferQueue<E> extends AbstractQueue<E> |
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implements TransferQueue<E>, java.io.Serializable { |
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private static final long serialVersionUID = -3223113410248163686L; |
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/* |
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jsr166 |
1.8 |
* *** Overview of Dual Queues with Slack *** |
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jsr166 |
1.1 |
* |
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jsr166 |
1.8 |
* Dual Queues, introduced by Scherer and Scott |
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* (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are |
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* (linked) queues in which nodes may represent either data or |
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* requests. When a thread tries to enqueue a data node, but |
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* encounters a request node, it instead "matches" and removes it; |
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* and vice versa for enqueuing requests. Blocking Dual Queues |
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* arrange that threads enqueuing unmatched requests block until |
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* other threads provide the match. Dual Synchronous Queues (see |
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* Scherer, Lea, & Scott |
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* http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf) |
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* additionally arrange that threads enqueuing unmatched data also |
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* block. Dual Transfer Queues support all of these modes, as |
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* dictated by callers. |
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* |
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* A FIFO dual queue may be implemented using a variation of the |
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* Michael & Scott (M&S) lock-free queue algorithm |
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* (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf). |
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* It maintains two pointer fields, "head", pointing to a |
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* (matched) node that in turn points to the first actual |
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* (unmatched) queue node (or null if empty); and "tail" that |
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* points to the last node on the queue (or again null if |
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* empty). For example, here is a possible queue with four data |
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* elements: |
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* |
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* head tail |
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* | | |
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* v v |
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* M -> U -> U -> U -> U |
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* |
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* The M&S queue algorithm is known to be prone to scalability and |
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* overhead limitations when maintaining (via CAS) these head and |
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* tail pointers. This has led to the development of |
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* contention-reducing variants such as elimination arrays (see |
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* Moir et al http://portal.acm.org/citation.cfm?id=1074013) and |
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* optimistic back pointers (see Ladan-Mozes & Shavit |
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* http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf). |
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* However, the nature of dual queues enables a simpler tactic for |
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* improving M&S-style implementations when dual-ness is needed. |
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* |
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* In a dual queue, each node must atomically maintain its match |
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* status. While there are other possible variants, we implement |
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* this here as: for a data-mode node, matching entails CASing an |
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* "item" field from a non-null data value to null upon match, and |
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* vice-versa for request nodes, CASing from null to a data |
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* value. (Note that the linearization properties of this style of |
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* queue are easy to verify -- elements are made available by |
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* linking, and unavailable by matching.) Compared to plain M&S |
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* queues, this property of dual queues requires one additional |
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* successful atomic operation per enq/deq pair. But it also |
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* enables lower cost variants of queue maintenance mechanics. (A |
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* variation of this idea applies even for non-dual queues that |
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* support deletion of interior elements, such as |
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* j.u.c.ConcurrentLinkedQueue.) |
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* |
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* Once a node is matched, its match status can never again |
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* change. We may thus arrange that the linked list of them |
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* contain a prefix of zero or more matched nodes, followed by a |
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* suffix of zero or more unmatched nodes. (Note that we allow |
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* both the prefix and suffix to be zero length, which in turn |
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* means that we do not use a dummy header.) If we were not |
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* concerned with either time or space efficiency, we could |
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* correctly perform enqueue and dequeue operations by traversing |
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* from a pointer to the initial node; CASing the item of the |
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* first unmatched node on match and CASing the next field of the |
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* trailing node on appends. (Plus some special-casing when |
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* initially empty). While this would be a terrible idea in |
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* itself, it does have the benefit of not requiring ANY atomic |
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* updates on head/tail fields. |
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* |
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* We introduce here an approach that lies between the extremes of |
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* never versus always updating queue (head and tail) pointers. |
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* This offers a tradeoff between sometimes requiring extra |
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* traversal steps to locate the first and/or last unmatched |
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* nodes, versus the reduced overhead and contention of fewer |
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* updates to queue pointers. For example, a possible snapshot of |
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* a queue is: |
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* |
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* head tail |
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* | | |
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* v v |
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* M -> M -> U -> U -> U -> U |
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* |
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* The best value for this "slack" (the targeted maximum distance |
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* between the value of "head" and the first unmatched node, and |
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* similarly for "tail") is an empirical matter. We have found |
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* that using very small constants in the range of 1-3 work best |
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* over a range of platforms. Larger values introduce increasing |
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* costs of cache misses and risks of long traversal chains, while |
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* smaller values increase CAS contention and overhead. |
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* |
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* Dual queues with slack differ from plain M&S dual queues by |
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* virtue of only sometimes updating head or tail pointers when |
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* matching, appending, or even traversing nodes; in order to |
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* maintain a targeted slack. The idea of "sometimes" may be |
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* operationalized in several ways. The simplest is to use a |
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* per-operation counter incremented on each traversal step, and |
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* to try (via CAS) to update the associated queue pointer |
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* whenever the count exceeds a threshold. Another, that requires |
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* more overhead, is to use random number generators to update |
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* with a given probability per traversal step. |
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* |
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* In any strategy along these lines, because CASes updating |
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* fields may fail, the actual slack may exceed targeted |
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* slack. However, they may be retried at any time to maintain |
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* targets. Even when using very small slack values, this |
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* approach works well for dual queues because it allows all |
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* operations up to the point of matching or appending an item |
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* (hence potentially allowing progress by another thread) to be |
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* read-only, thus not introducing any further contention. As |
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* described below, we implement this by performing slack |
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* maintenance retries only after these points. |
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* |
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* As an accompaniment to such techniques, traversal overhead can |
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* be further reduced without increasing contention of head |
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* pointer updates: Threads may sometimes shortcut the "next" link |
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* path from the current "head" node to be closer to the currently |
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* known first unmatched node, and similarly for tail. Again, this |
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* may be triggered with using thresholds or randomization. |
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* |
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* These ideas must be further extended to avoid unbounded amounts |
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* of costly-to-reclaim garbage caused by the sequential "next" |
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* links of nodes starting at old forgotten head nodes: As first |
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* described in detail by Boehm |
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* (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC |
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* delays noticing that any arbitrarily old node has become |
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* garbage, all newer dead nodes will also be unreclaimed. |
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* (Similar issues arise in non-GC environments.) To cope with |
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* this in our implementation, upon CASing to advance the head |
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* pointer, we set the "next" link of the previous head to point |
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* only to itself; thus limiting the length of connected dead lists. |
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* (We also take similar care to wipe out possibly garbage |
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* retaining values held in other Node fields.) However, doing so |
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* adds some further complexity to traversal: If any "next" |
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* pointer links to itself, it indicates that the current thread |
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* has lagged behind a head-update, and so the traversal must |
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* continue from the "head". Traversals trying to find the |
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* current tail starting from "tail" may also encounter |
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* self-links, in which case they also continue at "head". |
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* |
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* It is tempting in slack-based scheme to not even use CAS for |
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* updates (similarly to Ladan-Mozes & Shavit). However, this |
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* cannot be done for head updates under the above link-forgetting |
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* mechanics because an update may leave head at a detached node. |
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* And while direct writes are possible for tail updates, they |
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* increase the risk of long retraversals, and hence long garbage |
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* chains, which can be much more costly than is worthwhile |
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* considering that the cost difference of performing a CAS vs |
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* write is smaller when they are not triggered on each operation |
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* (especially considering that writes and CASes equally require |
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* additional GC bookkeeping ("write barriers") that are sometimes |
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* more costly than the writes themselves because of contention). |
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* |
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* Removal of interior nodes (due to timed out or interrupted |
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* waits, or calls to remove(x) or Iterator.remove) can use a |
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* scheme roughly similar to that described in Scherer, Lea, and |
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* Scott's SynchronousQueue. Given a predecessor, we can unsplice |
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* any node except the (actual) tail of the queue. To avoid |
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* build-up of cancelled trailing nodes, upon a request to remove |
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* a trailing node, it is placed in field "cleanMe" to be |
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* unspliced upon the next call to unsplice any other node. |
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* Situations needing such mechanics are not common but do occur |
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* in practice; for example when an unbounded series of short |
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* timed calls to poll repeatedly time out but never otherwise |
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* fall off the list because of an untimed call to take at the |
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* front of the queue. Note that maintaining field cleanMe does |
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* not otherwise much impact garbage retention even if never |
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* cleared by some other call because the held node will |
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* eventually either directly or indirectly lead to a self-link |
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* once off the list. |
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* |
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* *** Overview of implementation *** |
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* |
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* We use a threshold-based approach to updates, with a slack |
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* threshold of two -- that is, we update head/tail when the |
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* current pointer appears to be two or more steps away from the |
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* first/last node. The slack value is hard-wired: a path greater |
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* than one is naturally implemented by checking equality of |
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* traversal pointers except when the list has only one element, |
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* in which case we keep slack threshold at one. Avoiding tracking |
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* explicit counts across method calls slightly simplifies an |
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* already-messy implementation. Using randomization would |
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* probably work better if there were a low-quality dirt-cheap |
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* per-thread one available, but even ThreadLocalRandom is too |
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* heavy for these purposes. |
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* |
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* With such a small slack threshold value, it is rarely |
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* worthwhile to augment this with path short-circuiting; i.e., |
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* unsplicing nodes between head and the first unmatched node, or |
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* similarly for tail, rather than advancing head or tail |
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* proper. However, it is used (in awaitMatch) immediately before |
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* a waiting thread starts to block, as a final bit of helping at |
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* a point when contention with others is extremely unlikely |
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* (since if other threads that could release it are operating, |
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* then the current thread wouldn't be blocking). |
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* |
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* We allow both the head and tail fields to be null before any |
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* nodes are enqueued; initializing upon first append. This |
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* simplifies some other logic, as well as providing more |
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* efficient explicit control paths instead of letting JVMs insert |
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* implicit NullPointerExceptions when they are null. While not |
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* currently fully implemented, we also leave open the possibility |
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* of re-nulling these fields when empty (which is complicated to |
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* arrange, for little benefit.) |
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* |
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* All enqueue/dequeue operations are handled by the single method |
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* "xfer" with parameters indicating whether to act as some form |
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* of offer, put, poll, take, or transfer (each possibly with |
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* timeout). The relative complexity of using one monolithic |
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* method outweighs the code bulk and maintenance problems of |
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* using separate methods for each case. |
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* |
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* Operation consists of up to three phases. The first is |
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* implemented within method xfer, the second in tryAppend, and |
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* the third in method awaitMatch. |
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* |
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* 1. Try to match an existing node |
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* |
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* Starting at head, skip already-matched nodes until finding |
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* an unmatched node of opposite mode, if one exists, in which |
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* case matching it and returning, also if necessary updating |
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* head to one past the matched node (or the node itself if the |
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* list has no other unmatched nodes). If the CAS misses, then |
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* a loop retries advancing head by two steps until either |
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* success or the slack is at most two. By requiring that each |
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* attempt advances head by two (if applicable), we ensure that |
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* the slack does not grow without bound. Traversals also check |
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* if the initial head is now off-list, in which case they |
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* start at the new head. |
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* |
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* If no candidates are found and the call was untimed |
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* poll/offer, (argument "how" is NOW) return. |
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* |
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* 2. Try to append a new node (method tryAppend) |
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* |
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* Starting at current tail pointer, find the actual last node |
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* and try to append a new node (or if head was null, establish |
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* the first node). Nodes can be appended only if their |
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* predecessors are either already matched or are of the same |
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* mode. If we detect otherwise, then a new node with opposite |
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* mode must have been appended during traversal, so we must |
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* restart at phase 1. The traversal and update steps are |
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* otherwise similar to phase 1: Retrying upon CAS misses and |
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* checking for staleness. In particular, if a self-link is |
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* encountered, then we can safely jump to a node on the list |
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* by continuing the traversal at current head. |
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* |
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* On successful append, if the call was ASYNC, return. |
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* |
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* 3. Await match or cancellation (method awaitMatch) |
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* |
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* Wait for another thread to match node; instead cancelling if |
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* the current thread was interrupted or the wait timed out. On |
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* multiprocessors, we use front-of-queue spinning: If a node |
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* appears to be the first unmatched node in the queue, it |
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* spins a bit before blocking. In either case, before blocking |
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* it tries to unsplice any nodes between the current "head" |
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* and the first unmatched node. |
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* |
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* Front-of-queue spinning vastly improves performance of |
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* heavily contended queues. And so long as it is relatively |
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* brief and "quiet", spinning does not much impact performance |
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* of less-contended queues. During spins threads check their |
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* interrupt status and generate a thread-local random number |
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* to decide to occasionally perform a Thread.yield. While |
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* yield has underdefined specs, we assume that might it help, |
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* and will not hurt in limiting impact of spinning on busy |
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* systems. We also use smaller (1/2) spins for nodes that are |
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* not known to be front but whose predecessors have not |
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* blocked -- these "chained" spins avoid artifacts of |
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* front-of-queue rules which otherwise lead to alternating |
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* nodes spinning vs blocking. Further, front threads that |
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* represent phase changes (from data to request node or vice |
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* versa) compared to their predecessors receive additional |
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* chained spins, reflecting longer paths typically required to |
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* unblock threads during phase changes. |
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*/ |
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/** True if on multiprocessor */ |
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private static final boolean MP = |
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Runtime.getRuntime().availableProcessors() > 1; |
335 |
|
|
|
336 |
|
|
/** |
337 |
|
|
* The number of times to spin (with randomly interspersed calls |
338 |
|
|
* to Thread.yield) on multiprocessor before blocking when a node |
339 |
|
|
* is apparently the first waiter in the queue. See above for |
340 |
|
|
* explanation. Must be a power of two. The value is empirically |
341 |
|
|
* derived -- it works pretty well across a variety of processors, |
342 |
|
|
* numbers of CPUs, and OSes. |
343 |
|
|
*/ |
344 |
|
|
private static final int FRONT_SPINS = 1 << 7; |
345 |
|
|
|
346 |
|
|
/** |
347 |
|
|
* The number of times to spin before blocking when a node is |
348 |
|
|
* preceded by another node that is apparently spinning. Also |
349 |
|
|
* serves as an increment to FRONT_SPINS on phase changes, and as |
350 |
|
|
* base average frequency for yielding during spins. Must be a |
351 |
|
|
* power of two. |
352 |
|
|
*/ |
353 |
|
|
private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; |
354 |
|
|
|
355 |
|
|
/** |
356 |
|
|
* Queue nodes. Uses Object, not E, for items to allow forgetting |
357 |
|
|
* them after use. Relies heavily on Unsafe mechanics to minimize |
358 |
|
|
* unnecessary ordering constraints: Writes that intrinsically |
359 |
|
|
* precede or follow CASes use simple relaxed forms. Other |
360 |
|
|
* cleanups use releasing/lazy writes. |
361 |
|
|
*/ |
362 |
|
|
static final class Node<E> { |
363 |
|
|
final boolean isData; // false if this is a request node |
364 |
|
|
volatile Object item; // initially non-null if isData; CASed to match |
365 |
|
|
volatile Node<E> next; |
366 |
|
|
volatile Thread waiter; // null until waiting |
367 |
|
|
|
368 |
|
|
// CAS methods for fields |
369 |
|
|
final boolean casNext(Node<E> cmp, Node<E> val) { |
370 |
|
|
return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
371 |
|
|
} |
372 |
jsr166 |
1.1 |
|
373 |
jsr166 |
1.8 |
final boolean casItem(Object cmp, Object val) { |
374 |
jsr166 |
1.9 |
assert cmp == null || cmp.getClass() != Node.class; |
375 |
jsr166 |
1.8 |
return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
376 |
|
|
} |
377 |
jsr166 |
1.1 |
|
378 |
jsr166 |
1.8 |
/** |
379 |
|
|
* Creates a new node. Uses relaxed write because item can only |
380 |
|
|
* be seen if followed by CAS. |
381 |
|
|
*/ |
382 |
|
|
Node(E item, boolean isData) { |
383 |
|
|
UNSAFE.putObject(this, itemOffset, item); // relaxed write |
384 |
|
|
this.isData = isData; |
385 |
|
|
} |
386 |
jsr166 |
1.1 |
|
387 |
jsr166 |
1.8 |
/** |
388 |
|
|
* Links node to itself to avoid garbage retention. Called |
389 |
|
|
* only after CASing head field, so uses relaxed write. |
390 |
|
|
*/ |
391 |
|
|
final void forgetNext() { |
392 |
|
|
UNSAFE.putObject(this, nextOffset, this); |
393 |
|
|
} |
394 |
jsr166 |
1.1 |
|
395 |
jsr166 |
1.8 |
/** |
396 |
|
|
* Sets item to self (using a releasing/lazy write) and waiter |
397 |
|
|
* to null, to avoid garbage retention after extracting or |
398 |
|
|
* cancelling. |
399 |
|
|
*/ |
400 |
|
|
final void forgetContents() { |
401 |
|
|
UNSAFE.putOrderedObject(this, itemOffset, this); |
402 |
|
|
UNSAFE.putOrderedObject(this, waiterOffset, null); |
403 |
|
|
} |
404 |
jsr166 |
1.1 |
|
405 |
jsr166 |
1.8 |
/** |
406 |
|
|
* Returns true if this node has been matched, including the |
407 |
|
|
* case of artificial matches due to cancellation. |
408 |
|
|
*/ |
409 |
|
|
final boolean isMatched() { |
410 |
|
|
Object x = item; |
411 |
jsr166 |
1.11 |
return (x == this) || ((x == null) == isData); |
412 |
|
|
} |
413 |
|
|
|
414 |
|
|
/** |
415 |
|
|
* Returns true if this is an unmatched request node. |
416 |
|
|
*/ |
417 |
|
|
final boolean isUnmatchedRequest() { |
418 |
|
|
return !isData && item == null; |
419 |
jsr166 |
1.8 |
} |
420 |
jsr166 |
1.1 |
|
421 |
jsr166 |
1.8 |
/** |
422 |
|
|
* Returns true if a node with the given mode cannot be |
423 |
|
|
* appended to this node because this node is unmatched and |
424 |
|
|
* has opposite data mode. |
425 |
|
|
*/ |
426 |
|
|
final boolean cannotPrecede(boolean haveData) { |
427 |
|
|
boolean d = isData; |
428 |
|
|
Object x; |
429 |
|
|
return d != haveData && (x = item) != this && (x != null) == d; |
430 |
|
|
} |
431 |
jsr166 |
1.1 |
|
432 |
jsr166 |
1.8 |
/** |
433 |
|
|
* Tries to artificially match a data node -- used by remove. |
434 |
|
|
*/ |
435 |
|
|
final boolean tryMatchData() { |
436 |
jsr166 |
1.11 |
assert isData; |
437 |
jsr166 |
1.8 |
Object x = item; |
438 |
|
|
if (x != null && x != this && casItem(x, null)) { |
439 |
|
|
LockSupport.unpark(waiter); |
440 |
|
|
return true; |
441 |
|
|
} |
442 |
|
|
return false; |
443 |
jsr166 |
1.1 |
} |
444 |
|
|
|
445 |
jsr166 |
1.4 |
// Unsafe mechanics |
446 |
|
|
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe(); |
447 |
|
|
private static final long nextOffset = |
448 |
|
|
objectFieldOffset(UNSAFE, "next", Node.class); |
449 |
jsr166 |
1.8 |
private static final long itemOffset = |
450 |
|
|
objectFieldOffset(UNSAFE, "item", Node.class); |
451 |
|
|
private static final long waiterOffset = |
452 |
|
|
objectFieldOffset(UNSAFE, "waiter", Node.class); |
453 |
jsr166 |
1.1 |
|
454 |
|
|
private static final long serialVersionUID = -3375979862319811754L; |
455 |
|
|
} |
456 |
|
|
|
457 |
jsr166 |
1.8 |
/** head of the queue; null until first enqueue */ |
458 |
|
|
transient volatile Node<E> head; |
459 |
|
|
|
460 |
|
|
/** predecessor of dangling unspliceable node */ |
461 |
|
|
private transient volatile Node<E> cleanMe; // decl here reduces contention |
462 |
jsr166 |
1.1 |
|
463 |
jsr166 |
1.8 |
/** tail of the queue; null until first append */ |
464 |
|
|
private transient volatile Node<E> tail; |
465 |
jsr166 |
1.1 |
|
466 |
jsr166 |
1.8 |
// CAS methods for fields |
467 |
|
|
private boolean casTail(Node<E> cmp, Node<E> val) { |
468 |
|
|
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
469 |
|
|
} |
470 |
jsr166 |
1.1 |
|
471 |
jsr166 |
1.8 |
private boolean casHead(Node<E> cmp, Node<E> val) { |
472 |
|
|
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
473 |
|
|
} |
474 |
jsr166 |
1.1 |
|
475 |
jsr166 |
1.8 |
private boolean casCleanMe(Node<E> cmp, Node<E> val) { |
476 |
|
|
return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val); |
477 |
|
|
} |
478 |
jsr166 |
1.1 |
|
479 |
jsr166 |
1.8 |
/* |
480 |
|
|
* Possible values for "how" argument in xfer method. Beware that |
481 |
|
|
* the order of assigned numerical values matters. |
482 |
jsr166 |
1.1 |
*/ |
483 |
jsr166 |
1.8 |
private static final int NOW = 0; // for untimed poll, tryTransfer |
484 |
|
|
private static final int ASYNC = 1; // for offer, put, add |
485 |
|
|
private static final int SYNC = 2; // for transfer, take |
486 |
|
|
private static final int TIMEOUT = 3; // for timed poll, tryTransfer |
487 |
jsr166 |
1.1 |
|
488 |
jsr166 |
1.10 |
@SuppressWarnings("unchecked") |
489 |
|
|
static <E> E cast(Object item) { |
490 |
|
|
assert item == null || item.getClass() != Node.class; |
491 |
|
|
return (E) item; |
492 |
|
|
} |
493 |
|
|
|
494 |
jsr166 |
1.1 |
/** |
495 |
jsr166 |
1.8 |
* Implements all queuing methods. See above for explanation. |
496 |
jsr166 |
1.1 |
* |
497 |
jsr166 |
1.8 |
* @param e the item or null for take |
498 |
|
|
* @param haveData true if this is a put, else a take |
499 |
|
|
* @param how NOW, ASYNC, SYNC, or TIMEOUT |
500 |
jsr166 |
1.1 |
* @param nanos timeout in nanosecs, used only if mode is TIMEOUT |
501 |
jsr166 |
1.8 |
* @return an item if matched, else e |
502 |
|
|
* @throws NullPointerException if haveData mode but e is null |
503 |
jsr166 |
1.1 |
*/ |
504 |
jsr166 |
1.8 |
private E xfer(E e, boolean haveData, int how, long nanos) { |
505 |
|
|
if (haveData && (e == null)) |
506 |
|
|
throw new NullPointerException(); |
507 |
|
|
Node<E> s = null; // the node to append, if needed |
508 |
jsr166 |
1.1 |
|
509 |
jsr166 |
1.8 |
retry: for (;;) { // restart on append race |
510 |
jsr166 |
1.1 |
|
511 |
jsr166 |
1.8 |
for (Node<E> h = head, p = h; p != null;) { |
512 |
|
|
// find & match first node |
513 |
|
|
boolean isData = p.isData; |
514 |
|
|
Object item = p.item; |
515 |
|
|
if (item != p && (item != null) == isData) { // unmatched |
516 |
|
|
if (isData == haveData) // can't match |
517 |
|
|
break; |
518 |
|
|
if (p.casItem(item, e)) { // match |
519 |
|
|
for (Node<E> q = p; q != h;) { |
520 |
|
|
Node<E> n = q.next; // update head by 2 |
521 |
|
|
if (n != null) // unless singleton |
522 |
|
|
q = n; |
523 |
|
|
if (head == h && casHead(h, q)) { |
524 |
|
|
h.forgetNext(); |
525 |
|
|
break; |
526 |
|
|
} // advance and retry |
527 |
|
|
if ((h = head) == null || |
528 |
|
|
(q = h.next) == null || !q.isMatched()) |
529 |
|
|
break; // unless slack < 2 |
530 |
|
|
} |
531 |
|
|
LockSupport.unpark(p.waiter); |
532 |
|
|
return this.<E>cast(item); |
533 |
jsr166 |
1.1 |
} |
534 |
|
|
} |
535 |
jsr166 |
1.8 |
Node<E> n = p.next; |
536 |
|
|
p = (p != n) ? n : (h = head); // Use head if p offlist |
537 |
|
|
} |
538 |
|
|
|
539 |
|
|
if (how >= ASYNC) { // No matches available |
540 |
|
|
if (s == null) |
541 |
|
|
s = new Node<E>(e, haveData); |
542 |
|
|
Node<E> pred = tryAppend(s, haveData); |
543 |
|
|
if (pred == null) |
544 |
|
|
continue retry; // lost race vs opposite mode |
545 |
|
|
if (how >= SYNC) |
546 |
|
|
return awaitMatch(s, pred, e, how, nanos); |
547 |
jsr166 |
1.1 |
} |
548 |
jsr166 |
1.8 |
return e; // not waiting |
549 |
jsr166 |
1.1 |
} |
550 |
|
|
} |
551 |
|
|
|
552 |
|
|
/** |
553 |
jsr166 |
1.8 |
* Tries to append node s as tail. |
554 |
|
|
* |
555 |
|
|
* @param s the node to append |
556 |
|
|
* @param haveData true if appending in data mode |
557 |
|
|
* @return null on failure due to losing race with append in |
558 |
|
|
* different mode, else s's predecessor, or s itself if no |
559 |
|
|
* predecessor |
560 |
jsr166 |
1.1 |
*/ |
561 |
jsr166 |
1.8 |
private Node<E> tryAppend(Node<E> s, boolean haveData) { |
562 |
|
|
for (Node<E> t = tail, p = t;;) { // move p to last node and append |
563 |
|
|
Node<E> n, u; // temps for reads of next & tail |
564 |
|
|
if (p == null && (p = head) == null) { |
565 |
|
|
if (casHead(null, s)) |
566 |
|
|
return s; // initialize |
567 |
|
|
} |
568 |
|
|
else if (p.cannotPrecede(haveData)) |
569 |
|
|
return null; // lost race vs opposite mode |
570 |
|
|
else if ((n = p.next) != null) // not last; keep traversing |
571 |
|
|
p = p != t && t != (u = tail) ? (t = u) : // stale tail |
572 |
|
|
(p != n) ? n : null; // restart if off list |
573 |
|
|
else if (!p.casNext(null, s)) |
574 |
|
|
p = p.next; // re-read on CAS failure |
575 |
|
|
else { |
576 |
|
|
if (p != t) { // update if slack now >= 2 |
577 |
|
|
while ((tail != t || !casTail(t, s)) && |
578 |
|
|
(t = tail) != null && |
579 |
|
|
(s = t.next) != null && // advance and retry |
580 |
|
|
(s = s.next) != null && s != t); |
581 |
jsr166 |
1.1 |
} |
582 |
jsr166 |
1.8 |
return p; |
583 |
jsr166 |
1.1 |
} |
584 |
|
|
} |
585 |
|
|
} |
586 |
|
|
|
587 |
|
|
/** |
588 |
jsr166 |
1.8 |
* Spins/yields/blocks until node s is matched or caller gives up. |
589 |
jsr166 |
1.1 |
* |
590 |
|
|
* @param s the waiting node |
591 |
jsr166 |
1.8 |
* @param pred the predecessor of s, or s itself if it has no |
592 |
|
|
* predecessor, or null if unknown (the null case does not occur |
593 |
|
|
* in any current calls but may in possible future extensions) |
594 |
jsr166 |
1.1 |
* @param e the comparison value for checking match |
595 |
jsr166 |
1.8 |
* @param how either SYNC or TIMEOUT |
596 |
jsr166 |
1.1 |
* @param nanos timeout value |
597 |
jsr166 |
1.8 |
* @return matched item, or e if unmatched on interrupt or timeout |
598 |
jsr166 |
1.1 |
*/ |
599 |
jsr166 |
1.8 |
private E awaitMatch(Node<E> s, Node<E> pred, E e, int how, long nanos) { |
600 |
|
|
long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L; |
601 |
|
|
Thread w = Thread.currentThread(); |
602 |
|
|
int spins = -1; // initialized after first item and cancel checks |
603 |
|
|
ThreadLocalRandom randomYields = null; // bound if needed |
604 |
jsr166 |
1.1 |
|
605 |
|
|
for (;;) { |
606 |
jsr166 |
1.8 |
Object item = s.item; |
607 |
|
|
if (item != e) { // matched |
608 |
|
|
assert item != s; |
609 |
|
|
s.forgetContents(); // avoid garbage |
610 |
|
|
return this.<E>cast(item); |
611 |
|
|
} |
612 |
|
|
if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) && |
613 |
|
|
s.casItem(e, s)) { // cancel |
614 |
|
|
unsplice(pred, s); |
615 |
|
|
return e; |
616 |
|
|
} |
617 |
|
|
|
618 |
|
|
if (spins < 0) { // establish spins at/near front |
619 |
|
|
if ((spins = spinsFor(pred, s.isData)) > 0) |
620 |
|
|
randomYields = ThreadLocalRandom.current(); |
621 |
|
|
} |
622 |
|
|
else if (spins > 0) { // spin |
623 |
|
|
if (--spins == 0) |
624 |
|
|
shortenHeadPath(); // reduce slack before blocking |
625 |
|
|
else if (randomYields.nextInt(CHAINED_SPINS) == 0) |
626 |
|
|
Thread.yield(); // occasionally yield |
627 |
|
|
} |
628 |
|
|
else if (s.waiter == null) { |
629 |
|
|
s.waiter = w; // request unpark then recheck |
630 |
jsr166 |
1.1 |
} |
631 |
jsr166 |
1.8 |
else if (how == TIMEOUT) { |
632 |
jsr166 |
1.1 |
long now = System.nanoTime(); |
633 |
jsr166 |
1.8 |
if ((nanos -= now - lastTime) > 0) |
634 |
|
|
LockSupport.parkNanos(this, nanos); |
635 |
jsr166 |
1.1 |
lastTime = now; |
636 |
|
|
} |
637 |
jsr166 |
1.8 |
else { |
638 |
jsr166 |
1.1 |
LockSupport.park(this); |
639 |
|
|
s.waiter = null; |
640 |
jsr166 |
1.8 |
spins = -1; // spin if front upon wakeup |
641 |
jsr166 |
1.1 |
} |
642 |
jsr166 |
1.8 |
} |
643 |
|
|
} |
644 |
|
|
|
645 |
|
|
/** |
646 |
|
|
* Returns spin/yield value for a node with given predecessor and |
647 |
|
|
* data mode. See above for explanation. |
648 |
|
|
*/ |
649 |
|
|
private static int spinsFor(Node<?> pred, boolean haveData) { |
650 |
|
|
if (MP && pred != null) { |
651 |
|
|
if (pred.isData != haveData) // phase change |
652 |
|
|
return FRONT_SPINS + CHAINED_SPINS; |
653 |
|
|
if (pred.isMatched()) // probably at front |
654 |
|
|
return FRONT_SPINS; |
655 |
|
|
if (pred.waiter == null) // pred apparently spinning |
656 |
|
|
return CHAINED_SPINS; |
657 |
|
|
} |
658 |
|
|
return 0; |
659 |
|
|
} |
660 |
|
|
|
661 |
|
|
/** |
662 |
|
|
* Tries (once) to unsplice nodes between head and first unmatched |
663 |
|
|
* or trailing node; failing on contention. |
664 |
|
|
*/ |
665 |
|
|
private void shortenHeadPath() { |
666 |
|
|
Node<E> h, hn, p, q; |
667 |
|
|
if ((p = h = head) != null && h.isMatched() && |
668 |
|
|
(q = hn = h.next) != null) { |
669 |
|
|
Node<E> n; |
670 |
|
|
while ((n = q.next) != q) { |
671 |
|
|
if (n == null || !q.isMatched()) { |
672 |
|
|
if (hn != q && h.next == hn) |
673 |
|
|
h.casNext(hn, q); |
674 |
|
|
break; |
675 |
|
|
} |
676 |
|
|
p = q; |
677 |
|
|
q = n; |
678 |
jsr166 |
1.1 |
} |
679 |
|
|
} |
680 |
|
|
} |
681 |
|
|
|
682 |
jsr166 |
1.8 |
/* -------------- Traversal methods -------------- */ |
683 |
|
|
|
684 |
|
|
/** |
685 |
|
|
* Returns the first unmatched node of the given mode, or null if |
686 |
|
|
* none. Used by methods isEmpty, hasWaitingConsumer. |
687 |
|
|
*/ |
688 |
|
|
private Node<E> firstOfMode(boolean data) { |
689 |
|
|
for (Node<E> p = head; p != null; ) { |
690 |
|
|
if (!p.isMatched()) |
691 |
|
|
return (p.isData == data) ? p : null; |
692 |
|
|
Node<E> n = p.next; |
693 |
|
|
p = (n != p) ? n : head; |
694 |
|
|
} |
695 |
|
|
return null; |
696 |
|
|
} |
697 |
|
|
|
698 |
|
|
/** |
699 |
|
|
* Returns the item in the first unmatched node with isData; or |
700 |
|
|
* null if none. Used by peek. |
701 |
|
|
*/ |
702 |
|
|
private E firstDataItem() { |
703 |
|
|
for (Node<E> p = head; p != null; ) { |
704 |
|
|
boolean isData = p.isData; |
705 |
|
|
Object item = p.item; |
706 |
|
|
if (item != p && (item != null) == isData) |
707 |
|
|
return isData ? this.<E>cast(item) : null; |
708 |
|
|
Node<E> n = p.next; |
709 |
|
|
p = (n != p) ? n : head; |
710 |
|
|
} |
711 |
|
|
return null; |
712 |
|
|
} |
713 |
|
|
|
714 |
jsr166 |
1.1 |
/** |
715 |
jsr166 |
1.8 |
* Traverses and counts unmatched nodes of the given mode. |
716 |
|
|
* Used by methods size and getWaitingConsumerCount. |
717 |
jsr166 |
1.1 |
*/ |
718 |
jsr166 |
1.8 |
private int countOfMode(boolean data) { |
719 |
|
|
int count = 0; |
720 |
|
|
for (Node<E> p = head; p != null; ) { |
721 |
|
|
if (!p.isMatched()) { |
722 |
|
|
if (p.isData != data) |
723 |
|
|
return 0; |
724 |
|
|
if (++count == Integer.MAX_VALUE) // saturated |
725 |
|
|
break; |
726 |
|
|
} |
727 |
|
|
Node<E> n = p.next; |
728 |
|
|
if (n != p) |
729 |
|
|
p = n; |
730 |
|
|
else { |
731 |
|
|
count = 0; |
732 |
|
|
p = head; |
733 |
jsr166 |
1.1 |
} |
734 |
jsr166 |
1.8 |
} |
735 |
|
|
return count; |
736 |
|
|
} |
737 |
|
|
|
738 |
|
|
final class Itr implements Iterator<E> { |
739 |
|
|
private Node<E> nextNode; // next node to return item for |
740 |
|
|
private E nextItem; // the corresponding item |
741 |
|
|
private Node<E> lastRet; // last returned node, to support remove |
742 |
jsr166 |
1.13 |
private Node<E> lastPred; // predecessor to unlink lastRet |
743 |
jsr166 |
1.8 |
|
744 |
|
|
/** |
745 |
|
|
* Moves to next node after prev, or first node if prev null. |
746 |
|
|
*/ |
747 |
|
|
private void advance(Node<E> prev) { |
748 |
jsr166 |
1.13 |
lastPred = lastRet; |
749 |
jsr166 |
1.8 |
lastRet = prev; |
750 |
|
|
Node<E> p; |
751 |
|
|
if (prev == null || (p = prev.next) == prev) |
752 |
|
|
p = head; |
753 |
|
|
while (p != null) { |
754 |
|
|
Object item = p.item; |
755 |
|
|
if (p.isData) { |
756 |
|
|
if (item != null && item != p) { |
757 |
|
|
nextItem = LinkedTransferQueue.this.<E>cast(item); |
758 |
|
|
nextNode = p; |
759 |
|
|
return; |
760 |
|
|
} |
761 |
|
|
} |
762 |
|
|
else if (item == null) |
763 |
|
|
break; |
764 |
|
|
Node<E> n = p.next; |
765 |
|
|
p = (n != p) ? n : head; |
766 |
jsr166 |
1.1 |
} |
767 |
jsr166 |
1.8 |
nextNode = null; |
768 |
|
|
} |
769 |
|
|
|
770 |
|
|
Itr() { |
771 |
|
|
advance(null); |
772 |
|
|
} |
773 |
|
|
|
774 |
|
|
public final boolean hasNext() { |
775 |
|
|
return nextNode != null; |
776 |
|
|
} |
777 |
|
|
|
778 |
|
|
public final E next() { |
779 |
|
|
Node<E> p = nextNode; |
780 |
|
|
if (p == null) throw new NoSuchElementException(); |
781 |
|
|
E e = nextItem; |
782 |
|
|
advance(p); |
783 |
|
|
return e; |
784 |
|
|
} |
785 |
|
|
|
786 |
|
|
public final void remove() { |
787 |
|
|
Node<E> p = lastRet; |
788 |
|
|
if (p == null) throw new IllegalStateException(); |
789 |
jsr166 |
1.13 |
findAndRemoveDataNode(lastPred, p); |
790 |
jsr166 |
1.1 |
} |
791 |
|
|
} |
792 |
|
|
|
793 |
jsr166 |
1.8 |
/* -------------- Removal methods -------------- */ |
794 |
|
|
|
795 |
jsr166 |
1.1 |
/** |
796 |
jsr166 |
1.8 |
* Unsplices (now or later) the given deleted/cancelled node with |
797 |
|
|
* the given predecessor. |
798 |
jsr166 |
1.1 |
* |
799 |
jsr166 |
1.8 |
* @param pred predecessor of node to be unspliced |
800 |
|
|
* @param s the node to be unspliced |
801 |
jsr166 |
1.1 |
*/ |
802 |
jsr166 |
1.8 |
private void unsplice(Node<E> pred, Node<E> s) { |
803 |
|
|
s.forgetContents(); // clear unneeded fields |
804 |
jsr166 |
1.1 |
/* |
805 |
|
|
* At any given time, exactly one node on list cannot be |
806 |
jsr166 |
1.8 |
* unlinked -- the last inserted node. To accommodate this, if |
807 |
|
|
* we cannot unlink s, we save its predecessor as "cleanMe", |
808 |
|
|
* processing the previously saved version first. Because only |
809 |
|
|
* one node in the list can have a null next, at least one of |
810 |
|
|
* node s or the node previously saved can always be |
811 |
jsr166 |
1.1 |
* processed, so this always terminates. |
812 |
|
|
*/ |
813 |
jsr166 |
1.8 |
if (pred != null && pred != s) { |
814 |
|
|
while (pred.next == s) { |
815 |
|
|
Node<E> oldpred = (cleanMe == null) ? null : reclean(); |
816 |
|
|
Node<E> n = s.next; |
817 |
|
|
if (n != null) { |
818 |
|
|
if (n != s) |
819 |
|
|
pred.casNext(s, n); |
820 |
jsr166 |
1.1 |
break; |
821 |
jsr166 |
1.8 |
} |
822 |
|
|
if (oldpred == pred || // Already saved |
823 |
jsr166 |
1.12 |
((oldpred == null || oldpred.next == s) && |
824 |
|
|
casCleanMe(oldpred, pred))) { |
825 |
|
|
break; |
826 |
|
|
} |
827 |
jsr166 |
1.1 |
} |
828 |
|
|
} |
829 |
|
|
} |
830 |
|
|
|
831 |
|
|
/** |
832 |
jsr166 |
1.8 |
* Tries to unsplice the deleted/cancelled node held in cleanMe |
833 |
|
|
* that was previously uncleanable because it was at tail. |
834 |
jsr166 |
1.1 |
* |
835 |
|
|
* @return current cleanMe node (or null) |
836 |
|
|
*/ |
837 |
|
|
private Node<E> reclean() { |
838 |
|
|
/* |
839 |
jsr166 |
1.8 |
* cleanMe is, or at one time was, predecessor of a cancelled |
840 |
|
|
* node s that was the tail so could not be unspliced. If it |
841 |
jsr166 |
1.1 |
* is no longer the tail, try to unsplice if necessary and |
842 |
|
|
* make cleanMe slot available. This differs from similar |
843 |
jsr166 |
1.8 |
* code in unsplice() because we must check that pred still |
844 |
|
|
* points to a matched node that can be unspliced -- if not, |
845 |
|
|
* we can (must) clear cleanMe without unsplicing. This can |
846 |
|
|
* loop only due to contention. |
847 |
jsr166 |
1.1 |
*/ |
848 |
|
|
Node<E> pred; |
849 |
jsr166 |
1.8 |
while ((pred = cleanMe) != null) { |
850 |
jsr166 |
1.1 |
Node<E> s = pred.next; |
851 |
jsr166 |
1.8 |
Node<E> n; |
852 |
|
|
if (s == null || s == pred || !s.isMatched()) |
853 |
|
|
casCleanMe(pred, null); // already gone |
854 |
|
|
else if ((n = s.next) != null) { |
855 |
|
|
if (n != s) |
856 |
|
|
pred.casNext(s, n); |
857 |
|
|
casCleanMe(pred, null); |
858 |
jsr166 |
1.1 |
} |
859 |
jsr166 |
1.8 |
else |
860 |
jsr166 |
1.1 |
break; |
861 |
|
|
} |
862 |
|
|
return pred; |
863 |
|
|
} |
864 |
|
|
|
865 |
|
|
/** |
866 |
jsr166 |
1.8 |
* Main implementation of Iterator.remove(). Find |
867 |
jsr166 |
1.11 |
* and unsplice the given data node. |
868 |
jsr166 |
1.13 |
* @param possiblePred possible predecessor of s |
869 |
|
|
* @param s the node to remove |
870 |
jsr166 |
1.8 |
*/ |
871 |
jsr166 |
1.13 |
final void findAndRemoveDataNode(Node<E> possiblePred, Node<E> s) { |
872 |
jsr166 |
1.11 |
assert s.isData; |
873 |
jsr166 |
1.8 |
if (s.tryMatchData()) { |
874 |
jsr166 |
1.13 |
if (possiblePred != null && possiblePred.next == s) |
875 |
|
|
unsplice(possiblePred, s); // was actual predecessor |
876 |
|
|
else { |
877 |
|
|
for (Node<E> pred = null, p = head; p != null; ) { |
878 |
|
|
if (p == s) { |
879 |
|
|
unsplice(pred, p); |
880 |
|
|
break; |
881 |
|
|
} |
882 |
|
|
if (p.isUnmatchedRequest()) |
883 |
|
|
break; |
884 |
|
|
pred = p; |
885 |
|
|
if ((p = p.next) == pred) { // stale |
886 |
|
|
pred = null; |
887 |
|
|
p = head; |
888 |
|
|
} |
889 |
jsr166 |
1.8 |
} |
890 |
|
|
} |
891 |
|
|
} |
892 |
|
|
} |
893 |
|
|
|
894 |
|
|
/** |
895 |
|
|
* Main implementation of remove(Object) |
896 |
|
|
*/ |
897 |
|
|
private boolean findAndRemove(Object e) { |
898 |
|
|
if (e != null) { |
899 |
jsr166 |
1.11 |
for (Node<E> pred = null, p = head; p != null; ) { |
900 |
jsr166 |
1.8 |
Object item = p.item; |
901 |
|
|
if (p.isData) { |
902 |
|
|
if (item != null && item != p && e.equals(item) && |
903 |
|
|
p.tryMatchData()) { |
904 |
|
|
unsplice(pred, p); |
905 |
|
|
return true; |
906 |
|
|
} |
907 |
|
|
} |
908 |
|
|
else if (item == null) |
909 |
|
|
break; |
910 |
|
|
pred = p; |
911 |
jsr166 |
1.11 |
if ((p = p.next) == pred) { // stale |
912 |
jsr166 |
1.8 |
pred = null; |
913 |
|
|
p = head; |
914 |
|
|
} |
915 |
|
|
} |
916 |
|
|
} |
917 |
|
|
return false; |
918 |
|
|
} |
919 |
|
|
|
920 |
|
|
|
921 |
|
|
/** |
922 |
jsr166 |
1.1 |
* Creates an initially empty {@code LinkedTransferQueue}. |
923 |
|
|
*/ |
924 |
|
|
public LinkedTransferQueue() { |
925 |
|
|
} |
926 |
|
|
|
927 |
|
|
/** |
928 |
|
|
* Creates a {@code LinkedTransferQueue} |
929 |
|
|
* initially containing the elements of the given collection, |
930 |
|
|
* added in traversal order of the collection's iterator. |
931 |
|
|
* |
932 |
|
|
* @param c the collection of elements to initially contain |
933 |
|
|
* @throws NullPointerException if the specified collection or any |
934 |
|
|
* of its elements are null |
935 |
|
|
*/ |
936 |
|
|
public LinkedTransferQueue(Collection<? extends E> c) { |
937 |
|
|
this(); |
938 |
|
|
addAll(c); |
939 |
|
|
} |
940 |
|
|
|
941 |
jsr166 |
1.4 |
/** |
942 |
jsr166 |
1.5 |
* Inserts the specified element at the tail of this queue. |
943 |
|
|
* As the queue is unbounded, this method will never block. |
944 |
|
|
* |
945 |
|
|
* @throws NullPointerException if the specified element is null |
946 |
jsr166 |
1.4 |
*/ |
947 |
jsr166 |
1.5 |
public void put(E e) { |
948 |
jsr166 |
1.8 |
xfer(e, true, ASYNC, 0); |
949 |
jsr166 |
1.1 |
} |
950 |
|
|
|
951 |
jsr166 |
1.4 |
/** |
952 |
jsr166 |
1.5 |
* Inserts the specified element at the tail of this queue. |
953 |
|
|
* As the queue is unbounded, this method will never block or |
954 |
|
|
* return {@code false}. |
955 |
|
|
* |
956 |
|
|
* @return {@code true} (as specified by |
957 |
|
|
* {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer}) |
958 |
|
|
* @throws NullPointerException if the specified element is null |
959 |
jsr166 |
1.4 |
*/ |
960 |
jsr166 |
1.5 |
public boolean offer(E e, long timeout, TimeUnit unit) { |
961 |
jsr166 |
1.8 |
xfer(e, true, ASYNC, 0); |
962 |
|
|
return true; |
963 |
jsr166 |
1.1 |
} |
964 |
|
|
|
965 |
jsr166 |
1.4 |
/** |
966 |
jsr166 |
1.5 |
* Inserts the specified element at the tail of this queue. |
967 |
|
|
* As the queue is unbounded, this method will never return {@code false}. |
968 |
|
|
* |
969 |
|
|
* @return {@code true} (as specified by |
970 |
|
|
* {@link BlockingQueue#offer(Object) BlockingQueue.offer}) |
971 |
|
|
* @throws NullPointerException if the specified element is null |
972 |
jsr166 |
1.4 |
*/ |
973 |
jsr166 |
1.1 |
public boolean offer(E e) { |
974 |
jsr166 |
1.8 |
xfer(e, true, ASYNC, 0); |
975 |
jsr166 |
1.1 |
return true; |
976 |
|
|
} |
977 |
|
|
|
978 |
jsr166 |
1.4 |
/** |
979 |
jsr166 |
1.5 |
* Inserts the specified element at the tail of this queue. |
980 |
|
|
* As the queue is unbounded, this method will never throw |
981 |
|
|
* {@link IllegalStateException} or return {@code false}. |
982 |
|
|
* |
983 |
|
|
* @return {@code true} (as specified by {@link Collection#add}) |
984 |
|
|
* @throws NullPointerException if the specified element is null |
985 |
jsr166 |
1.4 |
*/ |
986 |
jsr166 |
1.1 |
public boolean add(E e) { |
987 |
jsr166 |
1.8 |
xfer(e, true, ASYNC, 0); |
988 |
|
|
return true; |
989 |
jsr166 |
1.5 |
} |
990 |
|
|
|
991 |
|
|
/** |
992 |
jsr166 |
1.6 |
* Transfers the element to a waiting consumer immediately, if possible. |
993 |
|
|
* |
994 |
|
|
* <p>More precisely, transfers the specified element immediately |
995 |
|
|
* if there exists a consumer already waiting to receive it (in |
996 |
|
|
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
997 |
|
|
* otherwise returning {@code false} without enqueuing the element. |
998 |
jsr166 |
1.5 |
* |
999 |
|
|
* @throws NullPointerException if the specified element is null |
1000 |
|
|
*/ |
1001 |
|
|
public boolean tryTransfer(E e) { |
1002 |
jsr166 |
1.8 |
return xfer(e, true, NOW, 0) == null; |
1003 |
jsr166 |
1.1 |
} |
1004 |
|
|
|
1005 |
jsr166 |
1.4 |
/** |
1006 |
jsr166 |
1.6 |
* Transfers the element to a consumer, waiting if necessary to do so. |
1007 |
|
|
* |
1008 |
|
|
* <p>More precisely, transfers the specified element immediately |
1009 |
|
|
* if there exists a consumer already waiting to receive it (in |
1010 |
|
|
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
1011 |
|
|
* else inserts the specified element at the tail of this queue |
1012 |
|
|
* and waits until the element is received by a consumer. |
1013 |
jsr166 |
1.5 |
* |
1014 |
|
|
* @throws NullPointerException if the specified element is null |
1015 |
jsr166 |
1.4 |
*/ |
1016 |
jsr166 |
1.1 |
public void transfer(E e) throws InterruptedException { |
1017 |
jsr166 |
1.8 |
if (xfer(e, true, SYNC, 0) != null) { |
1018 |
|
|
Thread.interrupted(); // failure possible only due to interrupt |
1019 |
jsr166 |
1.1 |
throw new InterruptedException(); |
1020 |
|
|
} |
1021 |
|
|
} |
1022 |
|
|
|
1023 |
jsr166 |
1.4 |
/** |
1024 |
jsr166 |
1.6 |
* Transfers the element to a consumer if it is possible to do so |
1025 |
|
|
* before the timeout elapses. |
1026 |
|
|
* |
1027 |
|
|
* <p>More precisely, transfers the specified element immediately |
1028 |
|
|
* if there exists a consumer already waiting to receive it (in |
1029 |
|
|
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
1030 |
|
|
* else inserts the specified element at the tail of this queue |
1031 |
|
|
* and waits until the element is received by a consumer, |
1032 |
|
|
* returning {@code false} if the specified wait time elapses |
1033 |
|
|
* before the element can be transferred. |
1034 |
jsr166 |
1.5 |
* |
1035 |
|
|
* @throws NullPointerException if the specified element is null |
1036 |
jsr166 |
1.4 |
*/ |
1037 |
jsr166 |
1.1 |
public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
1038 |
|
|
throws InterruptedException { |
1039 |
jsr166 |
1.8 |
if (xfer(e, true, TIMEOUT, unit.toNanos(timeout)) == null) |
1040 |
jsr166 |
1.1 |
return true; |
1041 |
|
|
if (!Thread.interrupted()) |
1042 |
|
|
return false; |
1043 |
|
|
throw new InterruptedException(); |
1044 |
|
|
} |
1045 |
|
|
|
1046 |
|
|
public E take() throws InterruptedException { |
1047 |
jsr166 |
1.8 |
E e = xfer(null, false, SYNC, 0); |
1048 |
jsr166 |
1.1 |
if (e != null) |
1049 |
jsr166 |
1.5 |
return e; |
1050 |
jsr166 |
1.1 |
Thread.interrupted(); |
1051 |
|
|
throw new InterruptedException(); |
1052 |
|
|
} |
1053 |
|
|
|
1054 |
|
|
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
1055 |
jsr166 |
1.8 |
E e = xfer(null, false, TIMEOUT, unit.toNanos(timeout)); |
1056 |
jsr166 |
1.1 |
if (e != null || !Thread.interrupted()) |
1057 |
jsr166 |
1.5 |
return e; |
1058 |
jsr166 |
1.1 |
throw new InterruptedException(); |
1059 |
|
|
} |
1060 |
|
|
|
1061 |
|
|
public E poll() { |
1062 |
jsr166 |
1.8 |
return xfer(null, false, NOW, 0); |
1063 |
jsr166 |
1.1 |
} |
1064 |
|
|
|
1065 |
jsr166 |
1.4 |
/** |
1066 |
|
|
* @throws NullPointerException {@inheritDoc} |
1067 |
|
|
* @throws IllegalArgumentException {@inheritDoc} |
1068 |
|
|
*/ |
1069 |
jsr166 |
1.1 |
public int drainTo(Collection<? super E> c) { |
1070 |
|
|
if (c == null) |
1071 |
|
|
throw new NullPointerException(); |
1072 |
|
|
if (c == this) |
1073 |
|
|
throw new IllegalArgumentException(); |
1074 |
|
|
int n = 0; |
1075 |
|
|
E e; |
1076 |
|
|
while ( (e = poll()) != null) { |
1077 |
|
|
c.add(e); |
1078 |
|
|
++n; |
1079 |
|
|
} |
1080 |
|
|
return n; |
1081 |
|
|
} |
1082 |
|
|
|
1083 |
jsr166 |
1.4 |
/** |
1084 |
|
|
* @throws NullPointerException {@inheritDoc} |
1085 |
|
|
* @throws IllegalArgumentException {@inheritDoc} |
1086 |
|
|
*/ |
1087 |
jsr166 |
1.1 |
public int drainTo(Collection<? super E> c, int maxElements) { |
1088 |
|
|
if (c == null) |
1089 |
|
|
throw new NullPointerException(); |
1090 |
|
|
if (c == this) |
1091 |
|
|
throw new IllegalArgumentException(); |
1092 |
|
|
int n = 0; |
1093 |
|
|
E e; |
1094 |
|
|
while (n < maxElements && (e = poll()) != null) { |
1095 |
|
|
c.add(e); |
1096 |
|
|
++n; |
1097 |
|
|
} |
1098 |
|
|
return n; |
1099 |
|
|
} |
1100 |
|
|
|
1101 |
jsr166 |
1.5 |
/** |
1102 |
|
|
* Returns an iterator over the elements in this queue in proper |
1103 |
|
|
* sequence, from head to tail. |
1104 |
|
|
* |
1105 |
|
|
* <p>The returned iterator is a "weakly consistent" iterator that |
1106 |
|
|
* will never throw |
1107 |
|
|
* {@link ConcurrentModificationException ConcurrentModificationException}, |
1108 |
|
|
* and guarantees to traverse elements as they existed upon |
1109 |
|
|
* construction of the iterator, and may (but is not guaranteed |
1110 |
|
|
* to) reflect any modifications subsequent to construction. |
1111 |
|
|
* |
1112 |
|
|
* @return an iterator over the elements in this queue in proper sequence |
1113 |
|
|
*/ |
1114 |
jsr166 |
1.1 |
public Iterator<E> iterator() { |
1115 |
|
|
return new Itr(); |
1116 |
|
|
} |
1117 |
|
|
|
1118 |
|
|
public E peek() { |
1119 |
jsr166 |
1.8 |
return firstDataItem(); |
1120 |
jsr166 |
1.1 |
} |
1121 |
|
|
|
1122 |
jsr166 |
1.6 |
/** |
1123 |
|
|
* Returns {@code true} if this queue contains no elements. |
1124 |
|
|
* |
1125 |
|
|
* @return {@code true} if this queue contains no elements |
1126 |
|
|
*/ |
1127 |
jsr166 |
1.1 |
public boolean isEmpty() { |
1128 |
jsr166 |
1.8 |
return firstOfMode(true) == null; |
1129 |
jsr166 |
1.1 |
} |
1130 |
|
|
|
1131 |
|
|
public boolean hasWaitingConsumer() { |
1132 |
jsr166 |
1.8 |
return firstOfMode(false) != null; |
1133 |
jsr166 |
1.1 |
} |
1134 |
|
|
|
1135 |
|
|
/** |
1136 |
|
|
* Returns the number of elements in this queue. If this queue |
1137 |
|
|
* contains more than {@code Integer.MAX_VALUE} elements, returns |
1138 |
|
|
* {@code Integer.MAX_VALUE}. |
1139 |
|
|
* |
1140 |
|
|
* <p>Beware that, unlike in most collections, this method is |
1141 |
|
|
* <em>NOT</em> a constant-time operation. Because of the |
1142 |
|
|
* asynchronous nature of these queues, determining the current |
1143 |
|
|
* number of elements requires an O(n) traversal. |
1144 |
|
|
* |
1145 |
|
|
* @return the number of elements in this queue |
1146 |
|
|
*/ |
1147 |
|
|
public int size() { |
1148 |
jsr166 |
1.8 |
return countOfMode(true); |
1149 |
jsr166 |
1.1 |
} |
1150 |
|
|
|
1151 |
|
|
public int getWaitingConsumerCount() { |
1152 |
jsr166 |
1.8 |
return countOfMode(false); |
1153 |
jsr166 |
1.1 |
} |
1154 |
|
|
|
1155 |
jsr166 |
1.6 |
/** |
1156 |
|
|
* Removes a single instance of the specified element from this queue, |
1157 |
|
|
* if it is present. More formally, removes an element {@code e} such |
1158 |
|
|
* that {@code o.equals(e)}, if this queue contains one or more such |
1159 |
|
|
* elements. |
1160 |
|
|
* Returns {@code true} if this queue contained the specified element |
1161 |
|
|
* (or equivalently, if this queue changed as a result of the call). |
1162 |
|
|
* |
1163 |
|
|
* @param o element to be removed from this queue, if present |
1164 |
|
|
* @return {@code true} if this queue changed as a result of the call |
1165 |
|
|
*/ |
1166 |
jsr166 |
1.1 |
public boolean remove(Object o) { |
1167 |
jsr166 |
1.8 |
return findAndRemove(o); |
1168 |
jsr166 |
1.1 |
} |
1169 |
|
|
|
1170 |
|
|
/** |
1171 |
jsr166 |
1.5 |
* Always returns {@code Integer.MAX_VALUE} because a |
1172 |
|
|
* {@code LinkedTransferQueue} is not capacity constrained. |
1173 |
|
|
* |
1174 |
|
|
* @return {@code Integer.MAX_VALUE} (as specified by |
1175 |
|
|
* {@link BlockingQueue#remainingCapacity()}) |
1176 |
|
|
*/ |
1177 |
|
|
public int remainingCapacity() { |
1178 |
|
|
return Integer.MAX_VALUE; |
1179 |
|
|
} |
1180 |
|
|
|
1181 |
|
|
/** |
1182 |
jsr166 |
1.8 |
* Saves the state to a stream (that is, serializes it). |
1183 |
jsr166 |
1.1 |
* |
1184 |
|
|
* @serialData All of the elements (each an {@code E}) in |
1185 |
|
|
* the proper order, followed by a null |
1186 |
|
|
* @param s the stream |
1187 |
|
|
*/ |
1188 |
|
|
private void writeObject(java.io.ObjectOutputStream s) |
1189 |
|
|
throws java.io.IOException { |
1190 |
|
|
s.defaultWriteObject(); |
1191 |
|
|
for (E e : this) |
1192 |
|
|
s.writeObject(e); |
1193 |
|
|
// Use trailing null as sentinel |
1194 |
|
|
s.writeObject(null); |
1195 |
|
|
} |
1196 |
|
|
|
1197 |
|
|
/** |
1198 |
jsr166 |
1.8 |
* Reconstitutes the Queue instance from a stream (that is, |
1199 |
|
|
* deserializes it). |
1200 |
jsr166 |
1.1 |
* |
1201 |
|
|
* @param s the stream |
1202 |
|
|
*/ |
1203 |
|
|
private void readObject(java.io.ObjectInputStream s) |
1204 |
|
|
throws java.io.IOException, ClassNotFoundException { |
1205 |
|
|
s.defaultReadObject(); |
1206 |
|
|
for (;;) { |
1207 |
|
|
@SuppressWarnings("unchecked") E item = (E) s.readObject(); |
1208 |
|
|
if (item == null) |
1209 |
|
|
break; |
1210 |
|
|
else |
1211 |
|
|
offer(item); |
1212 |
|
|
} |
1213 |
|
|
} |
1214 |
|
|
|
1215 |
jsr166 |
1.3 |
// Unsafe mechanics |
1216 |
jsr166 |
1.1 |
|
1217 |
jsr166 |
1.3 |
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe(); |
1218 |
|
|
private static final long headOffset = |
1219 |
jsr166 |
1.4 |
objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class); |
1220 |
jsr166 |
1.3 |
private static final long tailOffset = |
1221 |
jsr166 |
1.4 |
objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class); |
1222 |
jsr166 |
1.3 |
private static final long cleanMeOffset = |
1223 |
jsr166 |
1.4 |
objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class); |
1224 |
|
|
|
1225 |
|
|
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
1226 |
|
|
String field, Class<?> klazz) { |
1227 |
jsr166 |
1.1 |
try { |
1228 |
jsr166 |
1.3 |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
1229 |
jsr166 |
1.1 |
} catch (NoSuchFieldException e) { |
1230 |
jsr166 |
1.3 |
// Convert Exception to corresponding Error |
1231 |
|
|
NoSuchFieldError error = new NoSuchFieldError(field); |
1232 |
jsr166 |
1.1 |
error.initCause(e); |
1233 |
|
|
throw error; |
1234 |
|
|
} |
1235 |
|
|
} |
1236 |
jsr166 |
1.8 |
|
1237 |
jsr166 |
1.1 |
} |