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/* |
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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 jsr166y; |
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import java.util.concurrent.*; |
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import java.util.AbstractQueue; |
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import java.util.Collection; |
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import java.util.ConcurrentModificationException; |
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import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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import java.util.Queue; |
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import java.util.concurrent.locks.LockSupport; |
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/** |
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* An unbounded {@link TransferQueue} based on linked nodes. |
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* 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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* *** Overview of Dual Queues with Slack *** |
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* |
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* 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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jsr166 |
1.46 |
* 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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* *** 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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1.45 |
* 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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1.48 |
* explicit counts across method calls slightly simplifies an |
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1.45 |
* 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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1.67 |
* With such a small slack threshold value, it is not worthwhile |
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* to augment this with path short-circuiting (i.e., unsplicing |
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* interior nodes) except in the case of cancellation/removal (see |
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* below). |
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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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jsr166 |
1.49 |
* of re-nulling these fields when empty (which is complicated to |
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* arrange, for little benefit.) |
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1.45 |
* |
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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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1.50 |
* using separate methods for each case. |
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1.45 |
* |
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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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1.48 |
* 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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1.45 |
* |
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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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1.50 |
* 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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1.45 |
* |
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1.46 |
* On successful append, if the call was ASYNC, return. |
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1.45 |
* |
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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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1.50 |
* the current thread was interrupted or the wait timed out. On |
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1.45 |
* 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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1.50 |
* 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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1.45 |
* 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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1.50 |
* chained spins, reflecting longer paths typically required to |
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* unblock threads during phase changes. |
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1.67 |
* |
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* |
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* ** Unlinking removed interior nodes ** |
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* |
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* In addition to minimizing garbage retention via self-linking |
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* described above, we also unlink removed interior nodes. These |
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* may arise due to timed out or interrupted waits, or calls to |
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* remove(x) or Iterator.remove. Normally, given a node that was |
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* at one time known to be the predecessor of some node s that is |
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* to be removed, we can unsplice s by CASing the next field of |
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* its predecessor if it still points to s (otherwise s must |
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* already have been removed or is now offlist). But there are two |
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* situations in which we cannot guarantee to make node s |
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* unreachable in this way: (1) If s is the trailing node of list |
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* (i.e., with null next), then it is pinned as the target node |
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* for appends, so can only be removed later when other nodes are |
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* appended. (2) We cannot necessarily unlink s given a |
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* predecessor node that is matched (including the case of being |
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jsr166 |
1.68 |
* cancelled): the predecessor may already be unspliced, in which |
328 |
|
|
* case some previous reachable node may still point to s. |
329 |
|
|
* (For further explanation see Herlihy & Shavit "The Art of |
330 |
dl |
1.67 |
* Multiprocessor Programming" chapter 9). Although, in both |
331 |
|
|
* cases, we can rule out the need for further action if either s |
332 |
|
|
* or its predecessor are (or can be made to be) at, or fall off |
333 |
|
|
* from, the head of list. |
334 |
|
|
* |
335 |
|
|
* Without taking these into account, it would be possible for an |
336 |
|
|
* unbounded number of supposedly removed nodes to remain |
337 |
|
|
* reachable. Situations leading to such buildup are uncommon but |
338 |
|
|
* can occur in practice; for example when a series of short timed |
339 |
|
|
* calls to poll repeatedly time out but never otherwise fall off |
340 |
|
|
* the list because of an untimed call to take at the front of the |
341 |
|
|
* queue. |
342 |
|
|
* |
343 |
|
|
* When these cases arise, rather than always retraversing the |
344 |
|
|
* entire list to find an actual predecessor to unlink (which |
345 |
|
|
* won't help for case (1) anyway), we record a conservative |
346 |
jsr166 |
1.69 |
* estimate of possible unsplice failures (in "sweepVotes"). We |
347 |
dl |
1.67 |
* trigger a full sweep when the estimate exceeds a threshold |
348 |
|
|
* indicating the maximum number of estimated removal failures to |
349 |
|
|
* tolerate before sweeping through, unlinking cancelled nodes |
350 |
|
|
* that were not unlinked upon initial removal. We perform sweeps |
351 |
|
|
* by the thread hitting threshold (rather than background threads |
352 |
|
|
* or by spreading work to other threads) because in the main |
353 |
|
|
* contexts in which removal occurs, the caller is already |
354 |
|
|
* timed-out, cancelled, or performing a potentially O(n) |
355 |
|
|
* operation (i.e., remove(x)), none of which are time-critical |
356 |
|
|
* enough to warrant the overhead that alternatives would impose |
357 |
|
|
* on other threads. |
358 |
|
|
* |
359 |
|
|
* Because the sweepVotes estimate is conservative, and because |
360 |
|
|
* nodes become unlinked "naturally" as they fall off the head of |
361 |
|
|
* the queue, and because we allow votes to accumulate even while |
362 |
jsr166 |
1.68 |
* sweeps are in progress, there are typically significantly fewer |
363 |
dl |
1.67 |
* such nodes than estimated. Choice of a threshold value |
364 |
|
|
* balances the likelihood of wasted effort and contention, versus |
365 |
|
|
* providing a worst-case bound on retention of interior nodes in |
366 |
|
|
* quiescent queues. The value defined below was chosen |
367 |
|
|
* empirically to balance these under various timeout scenarios. |
368 |
|
|
* |
369 |
|
|
* Note that we cannot self-link unlinked interior nodes during |
370 |
|
|
* sweeps. However, the associated garbage chains terminate when |
371 |
|
|
* some successor ultimately falls off the head of the list and is |
372 |
|
|
* self-linked. |
373 |
dl |
1.45 |
*/ |
374 |
|
|
|
375 |
|
|
/** True if on multiprocessor */ |
376 |
|
|
private static final boolean MP = |
377 |
|
|
Runtime.getRuntime().availableProcessors() > 1; |
378 |
|
|
|
379 |
|
|
/** |
380 |
dl |
1.50 |
* The number of times to spin (with randomly interspersed calls |
381 |
|
|
* to Thread.yield) on multiprocessor before blocking when a node |
382 |
|
|
* is apparently the first waiter in the queue. See above for |
383 |
|
|
* explanation. Must be a power of two. The value is empirically |
384 |
|
|
* derived -- it works pretty well across a variety of processors, |
385 |
|
|
* numbers of CPUs, and OSes. |
386 |
dl |
1.45 |
*/ |
387 |
|
|
private static final int FRONT_SPINS = 1 << 7; |
388 |
|
|
|
389 |
|
|
/** |
390 |
|
|
* The number of times to spin before blocking when a node is |
391 |
dl |
1.50 |
* preceded by another node that is apparently spinning. Also |
392 |
|
|
* serves as an increment to FRONT_SPINS on phase changes, and as |
393 |
|
|
* base average frequency for yielding during spins. Must be a |
394 |
|
|
* power of two. |
395 |
dl |
1.45 |
*/ |
396 |
dl |
1.50 |
private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; |
397 |
dl |
1.45 |
|
398 |
|
|
/** |
399 |
dl |
1.67 |
* The maximum number of estimated removal failures (sweepVotes) |
400 |
|
|
* to tolerate before sweeping through the queue unlinking |
401 |
|
|
* cancelled nodes that were not unlinked upon initial |
402 |
|
|
* removal. See above for explanation. The value must be at least |
403 |
|
|
* two to avoid useless sweeps when removing trailing nodes. |
404 |
|
|
*/ |
405 |
|
|
static final int SWEEP_THRESHOLD = 32; |
406 |
|
|
|
407 |
|
|
/** |
408 |
jsr166 |
1.46 |
* Queue nodes. Uses Object, not E, for items to allow forgetting |
409 |
dl |
1.45 |
* them after use. Relies heavily on Unsafe mechanics to minimize |
410 |
dl |
1.67 |
* unnecessary ordering constraints: Writes that are intrinsically |
411 |
|
|
* ordered wrt other accesses or CASes use simple relaxed forms. |
412 |
dl |
1.45 |
*/ |
413 |
jsr166 |
1.61 |
static final class Node { |
414 |
dl |
1.45 |
final boolean isData; // false if this is a request node |
415 |
jsr166 |
1.46 |
volatile Object item; // initially non-null if isData; CASed to match |
416 |
jsr166 |
1.61 |
volatile Node next; |
417 |
dl |
1.45 |
volatile Thread waiter; // null until waiting |
418 |
dl |
1.1 |
|
419 |
dl |
1.45 |
// CAS methods for fields |
420 |
jsr166 |
1.61 |
final boolean casNext(Node cmp, Node val) { |
421 |
dl |
1.45 |
return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
422 |
|
|
} |
423 |
dl |
1.1 |
|
424 |
dl |
1.45 |
final boolean casItem(Object cmp, Object val) { |
425 |
jsr166 |
1.55 |
assert cmp == null || cmp.getClass() != Node.class; |
426 |
dl |
1.45 |
return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
427 |
|
|
} |
428 |
dl |
1.1 |
|
429 |
dl |
1.45 |
/** |
430 |
jsr166 |
1.46 |
* Creates a new node. Uses relaxed write because item can only |
431 |
|
|
* be seen if followed by CAS. |
432 |
dl |
1.45 |
*/ |
433 |
jsr166 |
1.61 |
Node(Object item, boolean isData) { |
434 |
dl |
1.45 |
UNSAFE.putObject(this, itemOffset, item); // relaxed write |
435 |
dl |
1.1 |
this.isData = isData; |
436 |
|
|
} |
437 |
|
|
|
438 |
dl |
1.45 |
/** |
439 |
|
|
* Links node to itself to avoid garbage retention. Called |
440 |
|
|
* only after CASing head field, so uses relaxed write. |
441 |
|
|
*/ |
442 |
|
|
final void forgetNext() { |
443 |
|
|
UNSAFE.putObject(this, nextOffset, this); |
444 |
|
|
} |
445 |
jsr166 |
1.32 |
|
446 |
dl |
1.45 |
/** |
447 |
dl |
1.67 |
* Sets item to self and waiter to null, to avoid garbage |
448 |
|
|
* retention after matching or cancelling. Uses relaxed writes |
449 |
|
|
* bacause order is already constrained in the only calling |
450 |
|
|
* contexts: item is forgotten only after volatile/atomic |
451 |
|
|
* mechanics that extract items. Similarly, clearing waiter |
452 |
|
|
* follows either CAS or return from park (if ever parked; |
453 |
|
|
* else we don't care). |
454 |
dl |
1.45 |
*/ |
455 |
|
|
final void forgetContents() { |
456 |
dl |
1.67 |
UNSAFE.putObject(this, itemOffset, this); |
457 |
|
|
UNSAFE.putObject(this, waiterOffset, null); |
458 |
dl |
1.45 |
} |
459 |
jsr166 |
1.32 |
|
460 |
dl |
1.45 |
/** |
461 |
|
|
* Returns true if this node has been matched, including the |
462 |
|
|
* case of artificial matches due to cancellation. |
463 |
|
|
*/ |
464 |
|
|
final boolean isMatched() { |
465 |
|
|
Object x = item; |
466 |
jsr166 |
1.57 |
return (x == this) || ((x == null) == isData); |
467 |
dl |
1.1 |
} |
468 |
dl |
1.15 |
|
469 |
dl |
1.45 |
/** |
470 |
jsr166 |
1.58 |
* Returns true if this is an unmatched request node. |
471 |
|
|
*/ |
472 |
|
|
final boolean isUnmatchedRequest() { |
473 |
|
|
return !isData && item == null; |
474 |
|
|
} |
475 |
|
|
|
476 |
|
|
/** |
477 |
dl |
1.45 |
* Returns true if a node with the given mode cannot be |
478 |
|
|
* appended to this node because this node is unmatched and |
479 |
|
|
* has opposite data mode. |
480 |
|
|
*/ |
481 |
|
|
final boolean cannotPrecede(boolean haveData) { |
482 |
|
|
boolean d = isData; |
483 |
|
|
Object x; |
484 |
|
|
return d != haveData && (x = item) != this && (x != null) == d; |
485 |
jsr166 |
1.31 |
} |
486 |
|
|
|
487 |
|
|
/** |
488 |
jsr166 |
1.46 |
* Tries to artificially match a data node -- used by remove. |
489 |
jsr166 |
1.31 |
*/ |
490 |
dl |
1.45 |
final boolean tryMatchData() { |
491 |
jsr166 |
1.58 |
assert isData; |
492 |
dl |
1.45 |
Object x = item; |
493 |
|
|
if (x != null && x != this && casItem(x, null)) { |
494 |
|
|
LockSupport.unpark(waiter); |
495 |
|
|
return true; |
496 |
jsr166 |
1.31 |
} |
497 |
dl |
1.45 |
return false; |
498 |
dl |
1.15 |
} |
499 |
|
|
|
500 |
dl |
1.45 |
// Unsafe mechanics |
501 |
|
|
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
502 |
|
|
private static final long nextOffset = |
503 |
|
|
objectFieldOffset(UNSAFE, "next", Node.class); |
504 |
|
|
private static final long itemOffset = |
505 |
|
|
objectFieldOffset(UNSAFE, "item", Node.class); |
506 |
|
|
private static final long waiterOffset = |
507 |
|
|
objectFieldOffset(UNSAFE, "waiter", Node.class); |
508 |
|
|
|
509 |
jsr166 |
1.24 |
private static final long serialVersionUID = -3375979862319811754L; |
510 |
dl |
1.1 |
} |
511 |
|
|
|
512 |
dl |
1.45 |
/** head of the queue; null until first enqueue */ |
513 |
jsr166 |
1.61 |
transient volatile Node head; |
514 |
dl |
1.45 |
|
515 |
|
|
/** tail of the queue; null until first append */ |
516 |
jsr166 |
1.61 |
private transient volatile Node tail; |
517 |
dl |
1.1 |
|
518 |
dl |
1.67 |
/** The number of apparent failures to unsplice removed nodes */ |
519 |
|
|
private transient volatile int sweepVotes; |
520 |
|
|
|
521 |
dl |
1.45 |
// CAS methods for fields |
522 |
jsr166 |
1.61 |
private boolean casTail(Node cmp, Node val) { |
523 |
dl |
1.45 |
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
524 |
|
|
} |
525 |
jsr166 |
1.23 |
|
526 |
jsr166 |
1.61 |
private boolean casHead(Node cmp, Node val) { |
527 |
dl |
1.45 |
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
528 |
|
|
} |
529 |
dl |
1.1 |
|
530 |
dl |
1.67 |
private boolean casSweepVotes(int cmp, int val) { |
531 |
|
|
return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val); |
532 |
dl |
1.45 |
} |
533 |
dl |
1.1 |
|
534 |
dl |
1.45 |
/* |
535 |
jsr166 |
1.64 |
* Possible values for "how" argument in xfer method. |
536 |
dl |
1.1 |
*/ |
537 |
jsr166 |
1.65 |
private static final int NOW = 0; // for untimed poll, tryTransfer |
538 |
|
|
private static final int ASYNC = 1; // for offer, put, add |
539 |
|
|
private static final int SYNC = 2; // for transfer, take |
540 |
|
|
private static final int TIMED = 3; // for timed poll, tryTransfer |
541 |
jsr166 |
1.5 |
|
542 |
jsr166 |
1.56 |
@SuppressWarnings("unchecked") |
543 |
|
|
static <E> E cast(Object item) { |
544 |
|
|
assert item == null || item.getClass() != Node.class; |
545 |
|
|
return (E) item; |
546 |
|
|
} |
547 |
|
|
|
548 |
dl |
1.1 |
/** |
549 |
dl |
1.45 |
* Implements all queuing methods. See above for explanation. |
550 |
jsr166 |
1.17 |
* |
551 |
dl |
1.45 |
* @param e the item or null for take |
552 |
jsr166 |
1.46 |
* @param haveData true if this is a put, else a take |
553 |
jsr166 |
1.65 |
* @param how NOW, ASYNC, SYNC, or TIMED |
554 |
|
|
* @param nanos timeout in nanosecs, used only if mode is TIMED |
555 |
jsr166 |
1.46 |
* @return an item if matched, else e |
556 |
dl |
1.45 |
* @throws NullPointerException if haveData mode but e is null |
557 |
dl |
1.1 |
*/ |
558 |
jsr166 |
1.54 |
private E xfer(E e, boolean haveData, int how, long nanos) { |
559 |
dl |
1.45 |
if (haveData && (e == null)) |
560 |
|
|
throw new NullPointerException(); |
561 |
jsr166 |
1.61 |
Node s = null; // the node to append, if needed |
562 |
dl |
1.1 |
|
563 |
dl |
1.45 |
retry: for (;;) { // restart on append race |
564 |
dl |
1.1 |
|
565 |
jsr166 |
1.61 |
for (Node h = head, p = h; p != null;) { // find & match first node |
566 |
dl |
1.45 |
boolean isData = p.isData; |
567 |
|
|
Object item = p.item; |
568 |
|
|
if (item != p && (item != null) == isData) { // unmatched |
569 |
|
|
if (isData == haveData) // can't match |
570 |
|
|
break; |
571 |
|
|
if (p.casItem(item, e)) { // match |
572 |
jsr166 |
1.61 |
for (Node q = p; q != h;) { |
573 |
dl |
1.67 |
Node n = q.next; // update by 2 unless singleton |
574 |
|
|
if (head == h && casHead(h, n == null? q : n)) { |
575 |
dl |
1.45 |
h.forgetNext(); |
576 |
|
|
break; |
577 |
|
|
} // advance and retry |
578 |
|
|
if ((h = head) == null || |
579 |
dl |
1.52 |
(q = h.next) == null || !q.isMatched()) |
580 |
dl |
1.45 |
break; // unless slack < 2 |
581 |
|
|
} |
582 |
dl |
1.52 |
LockSupport.unpark(p.waiter); |
583 |
jsr166 |
1.54 |
return this.<E>cast(item); |
584 |
dl |
1.1 |
} |
585 |
|
|
} |
586 |
jsr166 |
1.61 |
Node n = p.next; |
587 |
jsr166 |
1.47 |
p = (p != n) ? n : (h = head); // Use head if p offlist |
588 |
dl |
1.45 |
} |
589 |
|
|
|
590 |
jsr166 |
1.64 |
if (how != NOW) { // No matches available |
591 |
dl |
1.45 |
if (s == null) |
592 |
jsr166 |
1.61 |
s = new Node(e, haveData); |
593 |
|
|
Node pred = tryAppend(s, haveData); |
594 |
dl |
1.45 |
if (pred == null) |
595 |
|
|
continue retry; // lost race vs opposite mode |
596 |
jsr166 |
1.64 |
if (how != ASYNC) |
597 |
jsr166 |
1.65 |
return awaitMatch(s, pred, e, (how == TIMED), nanos); |
598 |
dl |
1.1 |
} |
599 |
dl |
1.45 |
return e; // not waiting |
600 |
dl |
1.1 |
} |
601 |
|
|
} |
602 |
|
|
|
603 |
|
|
/** |
604 |
jsr166 |
1.46 |
* Tries to append node s as tail. |
605 |
|
|
* |
606 |
dl |
1.48 |
* @param s the node to append |
607 |
dl |
1.45 |
* @param haveData true if appending in data mode |
608 |
|
|
* @return null on failure due to losing race with append in |
609 |
|
|
* different mode, else s's predecessor, or s itself if no |
610 |
|
|
* predecessor |
611 |
dl |
1.1 |
*/ |
612 |
jsr166 |
1.61 |
private Node tryAppend(Node s, boolean haveData) { |
613 |
|
|
for (Node t = tail, p = t;;) { // move p to last node and append |
614 |
|
|
Node n, u; // temps for reads of next & tail |
615 |
dl |
1.45 |
if (p == null && (p = head) == null) { |
616 |
|
|
if (casHead(null, s)) |
617 |
|
|
return s; // initialize |
618 |
|
|
} |
619 |
|
|
else if (p.cannotPrecede(haveData)) |
620 |
|
|
return null; // lost race vs opposite mode |
621 |
dl |
1.48 |
else if ((n = p.next) != null) // not last; keep traversing |
622 |
dl |
1.45 |
p = p != t && t != (u = tail) ? (t = u) : // stale tail |
623 |
jsr166 |
1.47 |
(p != n) ? n : null; // restart if off list |
624 |
dl |
1.45 |
else if (!p.casNext(null, s)) |
625 |
|
|
p = p.next; // re-read on CAS failure |
626 |
|
|
else { |
627 |
dl |
1.48 |
if (p != t) { // update if slack now >= 2 |
628 |
dl |
1.45 |
while ((tail != t || !casTail(t, s)) && |
629 |
|
|
(t = tail) != null && |
630 |
|
|
(s = t.next) != null && // advance and retry |
631 |
|
|
(s = s.next) != null && s != t); |
632 |
dl |
1.1 |
} |
633 |
dl |
1.45 |
return p; |
634 |
dl |
1.1 |
} |
635 |
|
|
} |
636 |
|
|
} |
637 |
|
|
|
638 |
|
|
/** |
639 |
dl |
1.45 |
* Spins/yields/blocks until node s is matched or caller gives up. |
640 |
dl |
1.1 |
* |
641 |
|
|
* @param s the waiting node |
642 |
dl |
1.50 |
* @param pred the predecessor of s, or s itself if it has no |
643 |
|
|
* predecessor, or null if unknown (the null case does not occur |
644 |
|
|
* in any current calls but may in possible future extensions) |
645 |
dl |
1.1 |
* @param e the comparison value for checking match |
646 |
jsr166 |
1.64 |
* @param timed if true, wait only until timeout elapses |
647 |
jsr166 |
1.65 |
* @param nanos timeout in nanosecs, used only if timed is true |
648 |
dl |
1.45 |
* @return matched item, or e if unmatched on interrupt or timeout |
649 |
dl |
1.1 |
*/ |
650 |
jsr166 |
1.64 |
private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) { |
651 |
|
|
long lastTime = timed ? System.nanoTime() : 0L; |
652 |
dl |
1.45 |
Thread w = Thread.currentThread(); |
653 |
|
|
int spins = -1; // initialized after first item and cancel checks |
654 |
|
|
ThreadLocalRandom randomYields = null; // bound if needed |
655 |
dl |
1.1 |
|
656 |
|
|
for (;;) { |
657 |
dl |
1.45 |
Object item = s.item; |
658 |
|
|
if (item != e) { // matched |
659 |
jsr166 |
1.54 |
assert item != s; |
660 |
dl |
1.45 |
s.forgetContents(); // avoid garbage |
661 |
jsr166 |
1.54 |
return this.<E>cast(item); |
662 |
dl |
1.45 |
} |
663 |
jsr166 |
1.64 |
if ((w.isInterrupted() || (timed && nanos <= 0)) && |
664 |
dl |
1.67 |
s.casItem(e, s)) { // cancel |
665 |
dl |
1.45 |
unsplice(pred, s); |
666 |
|
|
return e; |
667 |
|
|
} |
668 |
|
|
|
669 |
|
|
if (spins < 0) { // establish spins at/near front |
670 |
|
|
if ((spins = spinsFor(pred, s.isData)) > 0) |
671 |
|
|
randomYields = ThreadLocalRandom.current(); |
672 |
|
|
} |
673 |
dl |
1.50 |
else if (spins > 0) { // spin |
674 |
dl |
1.67 |
--spins; |
675 |
|
|
if (randomYields.nextInt(CHAINED_SPINS) == 0) |
676 |
dl |
1.50 |
Thread.yield(); // occasionally yield |
677 |
dl |
1.45 |
} |
678 |
|
|
else if (s.waiter == null) { |
679 |
dl |
1.51 |
s.waiter = w; // request unpark then recheck |
680 |
dl |
1.1 |
} |
681 |
jsr166 |
1.64 |
else if (timed) { |
682 |
dl |
1.1 |
long now = System.nanoTime(); |
683 |
dl |
1.45 |
if ((nanos -= now - lastTime) > 0) |
684 |
|
|
LockSupport.parkNanos(this, nanos); |
685 |
dl |
1.1 |
lastTime = now; |
686 |
|
|
} |
687 |
dl |
1.45 |
else { |
688 |
dl |
1.12 |
LockSupport.park(this); |
689 |
dl |
1.1 |
} |
690 |
dl |
1.45 |
} |
691 |
|
|
} |
692 |
|
|
|
693 |
|
|
/** |
694 |
jsr166 |
1.46 |
* Returns spin/yield value for a node with given predecessor and |
695 |
dl |
1.45 |
* data mode. See above for explanation. |
696 |
|
|
*/ |
697 |
jsr166 |
1.61 |
private static int spinsFor(Node pred, boolean haveData) { |
698 |
dl |
1.45 |
if (MP && pred != null) { |
699 |
dl |
1.50 |
if (pred.isData != haveData) // phase change |
700 |
|
|
return FRONT_SPINS + CHAINED_SPINS; |
701 |
|
|
if (pred.isMatched()) // probably at front |
702 |
dl |
1.45 |
return FRONT_SPINS; |
703 |
|
|
if (pred.waiter == null) // pred apparently spinning |
704 |
|
|
return CHAINED_SPINS; |
705 |
|
|
} |
706 |
|
|
return 0; |
707 |
|
|
} |
708 |
|
|
|
709 |
|
|
/* -------------- Traversal methods -------------- */ |
710 |
|
|
|
711 |
dl |
1.1 |
/** |
712 |
jsr166 |
1.62 |
* Returns the successor of p, or the head node if p.next has been |
713 |
|
|
* linked to self, which will only be true if traversing with a |
714 |
|
|
* stale pointer that is now off the list. |
715 |
|
|
*/ |
716 |
|
|
final Node succ(Node p) { |
717 |
|
|
Node next = p.next; |
718 |
|
|
return (p == next) ? head : next; |
719 |
|
|
} |
720 |
|
|
|
721 |
|
|
/** |
722 |
jsr166 |
1.46 |
* Returns the first unmatched node of the given mode, or null if |
723 |
dl |
1.45 |
* none. Used by methods isEmpty, hasWaitingConsumer. |
724 |
dl |
1.9 |
*/ |
725 |
jsr166 |
1.62 |
private Node firstOfMode(boolean isData) { |
726 |
|
|
for (Node p = head; p != null; p = succ(p)) { |
727 |
dl |
1.45 |
if (!p.isMatched()) |
728 |
jsr166 |
1.62 |
return (p.isData == isData) ? p : null; |
729 |
dl |
1.45 |
} |
730 |
|
|
return null; |
731 |
|
|
} |
732 |
|
|
|
733 |
|
|
/** |
734 |
|
|
* Returns the item in the first unmatched node with isData; or |
735 |
jsr166 |
1.54 |
* null if none. Used by peek. |
736 |
dl |
1.45 |
*/ |
737 |
jsr166 |
1.54 |
private E firstDataItem() { |
738 |
jsr166 |
1.62 |
for (Node p = head; p != null; p = succ(p)) { |
739 |
dl |
1.45 |
Object item = p.item; |
740 |
jsr166 |
1.62 |
if (p.isData) { |
741 |
|
|
if (item != null && item != p) |
742 |
|
|
return this.<E>cast(item); |
743 |
|
|
} |
744 |
|
|
else if (item == null) |
745 |
|
|
return null; |
746 |
dl |
1.45 |
} |
747 |
|
|
return null; |
748 |
|
|
} |
749 |
|
|
|
750 |
|
|
/** |
751 |
jsr166 |
1.46 |
* Traverses and counts unmatched nodes of the given mode. |
752 |
|
|
* Used by methods size and getWaitingConsumerCount. |
753 |
dl |
1.45 |
*/ |
754 |
|
|
private int countOfMode(boolean data) { |
755 |
|
|
int count = 0; |
756 |
jsr166 |
1.61 |
for (Node p = head; p != null; ) { |
757 |
dl |
1.45 |
if (!p.isMatched()) { |
758 |
|
|
if (p.isData != data) |
759 |
|
|
return 0; |
760 |
|
|
if (++count == Integer.MAX_VALUE) // saturated |
761 |
|
|
break; |
762 |
dl |
1.9 |
} |
763 |
jsr166 |
1.61 |
Node n = p.next; |
764 |
dl |
1.45 |
if (n != p) |
765 |
|
|
p = n; |
766 |
|
|
else { |
767 |
|
|
count = 0; |
768 |
|
|
p = head; |
769 |
dl |
1.9 |
} |
770 |
|
|
} |
771 |
dl |
1.45 |
return count; |
772 |
jsr166 |
1.10 |
} |
773 |
dl |
1.9 |
|
774 |
dl |
1.45 |
final class Itr implements Iterator<E> { |
775 |
jsr166 |
1.61 |
private Node nextNode; // next node to return item for |
776 |
|
|
private E nextItem; // the corresponding item |
777 |
|
|
private Node lastRet; // last returned node, to support remove |
778 |
|
|
private Node lastPred; // predecessor to unlink lastRet |
779 |
dl |
1.45 |
|
780 |
|
|
/** |
781 |
|
|
* Moves to next node after prev, or first node if prev null. |
782 |
|
|
*/ |
783 |
jsr166 |
1.61 |
private void advance(Node prev) { |
784 |
dl |
1.60 |
lastPred = lastRet; |
785 |
dl |
1.45 |
lastRet = prev; |
786 |
jsr166 |
1.62 |
for (Node p = (prev == null) ? head : succ(prev); |
787 |
|
|
p != null; p = succ(p)) { |
788 |
dl |
1.45 |
Object item = p.item; |
789 |
|
|
if (p.isData) { |
790 |
|
|
if (item != null && item != p) { |
791 |
jsr166 |
1.54 |
nextItem = LinkedTransferQueue.this.<E>cast(item); |
792 |
dl |
1.45 |
nextNode = p; |
793 |
|
|
return; |
794 |
|
|
} |
795 |
|
|
} |
796 |
|
|
else if (item == null) |
797 |
|
|
break; |
798 |
|
|
} |
799 |
|
|
nextNode = null; |
800 |
|
|
} |
801 |
|
|
|
802 |
|
|
Itr() { |
803 |
|
|
advance(null); |
804 |
|
|
} |
805 |
|
|
|
806 |
|
|
public final boolean hasNext() { |
807 |
|
|
return nextNode != null; |
808 |
|
|
} |
809 |
|
|
|
810 |
|
|
public final E next() { |
811 |
jsr166 |
1.61 |
Node p = nextNode; |
812 |
dl |
1.45 |
if (p == null) throw new NoSuchElementException(); |
813 |
jsr166 |
1.54 |
E e = nextItem; |
814 |
dl |
1.45 |
advance(p); |
815 |
jsr166 |
1.54 |
return e; |
816 |
dl |
1.45 |
} |
817 |
|
|
|
818 |
|
|
public final void remove() { |
819 |
jsr166 |
1.61 |
Node p = lastRet; |
820 |
dl |
1.45 |
if (p == null) throw new IllegalStateException(); |
821 |
dl |
1.67 |
if (p.tryMatchData()) |
822 |
|
|
unsplice(lastPred, p); |
823 |
dl |
1.45 |
} |
824 |
|
|
} |
825 |
|
|
|
826 |
|
|
/* -------------- Removal methods -------------- */ |
827 |
|
|
|
828 |
dl |
1.9 |
/** |
829 |
dl |
1.45 |
* Unsplices (now or later) the given deleted/cancelled node with |
830 |
|
|
* the given predecessor. |
831 |
jsr166 |
1.17 |
* |
832 |
dl |
1.67 |
* @param pred a node that was at one time known to be the |
833 |
|
|
* predecessor of s, or null or s itself if s is/was at head |
834 |
dl |
1.45 |
* @param s the node to be unspliced |
835 |
dl |
1.1 |
*/ |
836 |
dl |
1.67 |
final void unsplice(Node pred, Node s) { |
837 |
|
|
s.forgetContents(); // forget unneeded fields |
838 |
dl |
1.9 |
/* |
839 |
dl |
1.67 |
* See above for rationale. Briefly: if pred still points to |
840 |
|
|
* s, try to unlink s. If s cannot be unlinked, because it is |
841 |
|
|
* trailing node or pred might be unlinked, and neither pred |
842 |
|
|
* nor s are head or offlist, add to sweepVotes, and if enough |
843 |
|
|
* votes have accumulated, sweep. |
844 |
dl |
1.9 |
*/ |
845 |
dl |
1.67 |
if (pred != null && pred != s && pred.next == s) { |
846 |
|
|
Node n = s.next; |
847 |
|
|
if (n == null || |
848 |
|
|
(n != s && pred.casNext(s, n) && pred.isMatched())) { |
849 |
|
|
for (;;) { // check if at, or could be, head |
850 |
|
|
Node h = head; |
851 |
|
|
if (h == pred || h == s || h == null) |
852 |
|
|
return; // at head or list empty |
853 |
|
|
if (!h.isMatched()) |
854 |
|
|
break; |
855 |
|
|
Node hn = h.next; |
856 |
|
|
if (hn == null) |
857 |
|
|
return; // now empty |
858 |
|
|
if (hn != h && casHead(h, hn)) |
859 |
|
|
h.forgetNext(); // advance head |
860 |
dl |
1.45 |
} |
861 |
dl |
1.67 |
if (pred.next != pred && s.next != s) { // recheck if offlist |
862 |
|
|
for (;;) { // sweep now if enough votes |
863 |
|
|
int v = sweepVotes; |
864 |
|
|
if (v < SWEEP_THRESHOLD) { |
865 |
|
|
if (casSweepVotes(v, v + 1)) |
866 |
|
|
break; |
867 |
|
|
} |
868 |
|
|
else if (casSweepVotes(v, 0)) { |
869 |
|
|
sweep(); |
870 |
|
|
break; |
871 |
|
|
} |
872 |
|
|
} |
873 |
jsr166 |
1.59 |
} |
874 |
dl |
1.9 |
} |
875 |
|
|
} |
876 |
|
|
} |
877 |
jsr166 |
1.5 |
|
878 |
dl |
1.9 |
/** |
879 |
jsr166 |
1.70 |
* Unlinks matched nodes encountered in a traversal from head. |
880 |
dl |
1.9 |
*/ |
881 |
dl |
1.67 |
private void sweep() { |
882 |
jsr166 |
1.71 |
for (Node p = head, s, n; p != null && (s = p.next) != null; ) { |
883 |
|
|
if (p == s) // stale |
884 |
|
|
p = head; |
885 |
|
|
else if (!s.isMatched()) |
886 |
|
|
p = s; |
887 |
|
|
else if ((n = s.next) == null) // trailing node is pinned |
888 |
|
|
break; |
889 |
|
|
else |
890 |
dl |
1.67 |
p.casNext(s, n); |
891 |
dl |
1.45 |
} |
892 |
|
|
} |
893 |
|
|
|
894 |
|
|
/** |
895 |
|
|
* Main implementation of remove(Object) |
896 |
|
|
*/ |
897 |
|
|
private boolean findAndRemove(Object e) { |
898 |
|
|
if (e != null) { |
899 |
jsr166 |
1.61 |
for (Node pred = null, p = head; p != null; ) { |
900 |
dl |
1.45 |
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 |
1.58 |
if ((p = p.next) == pred) { // stale |
912 |
dl |
1.45 |
pred = null; |
913 |
|
|
p = head; |
914 |
|
|
} |
915 |
|
|
} |
916 |
|
|
} |
917 |
|
|
return false; |
918 |
|
|
} |
919 |
|
|
|
920 |
|
|
|
921 |
|
|
/** |
922 |
jsr166 |
1.11 |
* Creates an initially empty {@code LinkedTransferQueue}. |
923 |
dl |
1.1 |
*/ |
924 |
|
|
public LinkedTransferQueue() { |
925 |
|
|
} |
926 |
|
|
|
927 |
|
|
/** |
928 |
jsr166 |
1.11 |
* Creates a {@code LinkedTransferQueue} |
929 |
dl |
1.1 |
* initially containing the elements of the given collection, |
930 |
|
|
* added in traversal order of the collection's iterator. |
931 |
jsr166 |
1.17 |
* |
932 |
dl |
1.1 |
* @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 |
dl |
1.7 |
this(); |
938 |
dl |
1.1 |
addAll(c); |
939 |
|
|
} |
940 |
|
|
|
941 |
jsr166 |
1.29 |
/** |
942 |
jsr166 |
1.35 |
* 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 |
1.29 |
*/ |
947 |
jsr166 |
1.35 |
public void put(E e) { |
948 |
dl |
1.45 |
xfer(e, true, ASYNC, 0); |
949 |
dl |
1.1 |
} |
950 |
|
|
|
951 |
jsr166 |
1.29 |
/** |
952 |
jsr166 |
1.35 |
* 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 |
1.29 |
*/ |
960 |
jsr166 |
1.35 |
public boolean offer(E e, long timeout, TimeUnit unit) { |
961 |
dl |
1.45 |
xfer(e, true, ASYNC, 0); |
962 |
|
|
return true; |
963 |
dl |
1.1 |
} |
964 |
|
|
|
965 |
jsr166 |
1.29 |
/** |
966 |
jsr166 |
1.35 |
* 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 |
1.29 |
*/ |
973 |
dl |
1.1 |
public boolean offer(E e) { |
974 |
dl |
1.45 |
xfer(e, true, ASYNC, 0); |
975 |
dl |
1.1 |
return true; |
976 |
|
|
} |
977 |
|
|
|
978 |
jsr166 |
1.29 |
/** |
979 |
jsr166 |
1.35 |
* Inserts the specified element at the tail of this queue. |
980 |
jsr166 |
1.37 |
* As the queue is unbounded, this method will never throw |
981 |
jsr166 |
1.35 |
* {@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 |
1.29 |
*/ |
986 |
dl |
1.15 |
public boolean add(E e) { |
987 |
dl |
1.45 |
xfer(e, true, ASYNC, 0); |
988 |
|
|
return true; |
989 |
jsr166 |
1.35 |
} |
990 |
|
|
|
991 |
|
|
/** |
992 |
jsr166 |
1.40 |
* 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 |
1.35 |
* |
999 |
|
|
* @throws NullPointerException if the specified element is null |
1000 |
|
|
*/ |
1001 |
|
|
public boolean tryTransfer(E e) { |
1002 |
dl |
1.45 |
return xfer(e, true, NOW, 0) == null; |
1003 |
dl |
1.15 |
} |
1004 |
|
|
|
1005 |
jsr166 |
1.29 |
/** |
1006 |
jsr166 |
1.40 |
* 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 |
1.35 |
* |
1014 |
|
|
* @throws NullPointerException if the specified element is null |
1015 |
jsr166 |
1.29 |
*/ |
1016 |
dl |
1.1 |
public void transfer(E e) throws InterruptedException { |
1017 |
dl |
1.45 |
if (xfer(e, true, SYNC, 0) != null) { |
1018 |
|
|
Thread.interrupted(); // failure possible only due to interrupt |
1019 |
dl |
1.1 |
throw new InterruptedException(); |
1020 |
jsr166 |
1.6 |
} |
1021 |
dl |
1.1 |
} |
1022 |
|
|
|
1023 |
jsr166 |
1.29 |
/** |
1024 |
jsr166 |
1.40 |
* 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 |
1.35 |
* |
1035 |
|
|
* @throws NullPointerException if the specified element is null |
1036 |
jsr166 |
1.29 |
*/ |
1037 |
dl |
1.1 |
public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
1038 |
|
|
throws InterruptedException { |
1039 |
jsr166 |
1.65 |
if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) |
1040 |
dl |
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 |
1.54 |
E e = xfer(null, false, SYNC, 0); |
1048 |
dl |
1.1 |
if (e != null) |
1049 |
jsr166 |
1.54 |
return e; |
1050 |
jsr166 |
1.6 |
Thread.interrupted(); |
1051 |
dl |
1.1 |
throw new InterruptedException(); |
1052 |
|
|
} |
1053 |
|
|
|
1054 |
|
|
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
1055 |
jsr166 |
1.65 |
E e = xfer(null, false, TIMED, unit.toNanos(timeout)); |
1056 |
dl |
1.1 |
if (e != null || !Thread.interrupted()) |
1057 |
jsr166 |
1.54 |
return e; |
1058 |
dl |
1.1 |
throw new InterruptedException(); |
1059 |
|
|
} |
1060 |
|
|
|
1061 |
|
|
public E poll() { |
1062 |
jsr166 |
1.54 |
return xfer(null, false, NOW, 0); |
1063 |
dl |
1.1 |
} |
1064 |
|
|
|
1065 |
jsr166 |
1.29 |
/** |
1066 |
jsr166 |
1.30 |
* @throws NullPointerException {@inheritDoc} |
1067 |
|
|
* @throws IllegalArgumentException {@inheritDoc} |
1068 |
jsr166 |
1.29 |
*/ |
1069 |
dl |
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 |
1.29 |
/** |
1084 |
jsr166 |
1.30 |
* @throws NullPointerException {@inheritDoc} |
1085 |
|
|
* @throws IllegalArgumentException {@inheritDoc} |
1086 |
jsr166 |
1.29 |
*/ |
1087 |
dl |
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 |
1.35 |
/** |
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 |
dl |
1.1 |
public Iterator<E> iterator() { |
1115 |
|
|
return new Itr(); |
1116 |
|
|
} |
1117 |
|
|
|
1118 |
|
|
public E peek() { |
1119 |
jsr166 |
1.54 |
return firstDataItem(); |
1120 |
dl |
1.1 |
} |
1121 |
|
|
|
1122 |
jsr166 |
1.41 |
/** |
1123 |
|
|
* Returns {@code true} if this queue contains no elements. |
1124 |
|
|
* |
1125 |
|
|
* @return {@code true} if this queue contains no elements |
1126 |
|
|
*/ |
1127 |
dl |
1.2 |
public boolean isEmpty() { |
1128 |
dl |
1.45 |
return firstOfMode(true) == null; |
1129 |
dl |
1.2 |
} |
1130 |
|
|
|
1131 |
dl |
1.1 |
public boolean hasWaitingConsumer() { |
1132 |
dl |
1.45 |
return firstOfMode(false) != null; |
1133 |
dl |
1.1 |
} |
1134 |
jsr166 |
1.5 |
|
1135 |
dl |
1.1 |
/** |
1136 |
|
|
* Returns the number of elements in this queue. If this queue |
1137 |
jsr166 |
1.11 |
* contains more than {@code Integer.MAX_VALUE} elements, returns |
1138 |
|
|
* {@code Integer.MAX_VALUE}. |
1139 |
dl |
1.1 |
* |
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 |
dl |
1.45 |
return countOfMode(true); |
1149 |
dl |
1.1 |
} |
1150 |
|
|
|
1151 |
|
|
public int getWaitingConsumerCount() { |
1152 |
dl |
1.45 |
return countOfMode(false); |
1153 |
dl |
1.1 |
} |
1154 |
|
|
|
1155 |
jsr166 |
1.42 |
/** |
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 |
dl |
1.15 |
public boolean remove(Object o) { |
1167 |
dl |
1.45 |
return findAndRemove(o); |
1168 |
dl |
1.15 |
} |
1169 |
|
|
|
1170 |
jsr166 |
1.35 |
/** |
1171 |
|
|
* 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 |
dl |
1.33 |
public int remainingCapacity() { |
1178 |
|
|
return Integer.MAX_VALUE; |
1179 |
|
|
} |
1180 |
|
|
|
1181 |
dl |
1.1 |
/** |
1182 |
jsr166 |
1.46 |
* Saves the state to a stream (that is, serializes it). |
1183 |
dl |
1.1 |
* |
1184 |
jsr166 |
1.11 |
* @serialData All of the elements (each an {@code E}) in |
1185 |
dl |
1.1 |
* 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 |
jsr166 |
1.16 |
for (E e : this) |
1192 |
|
|
s.writeObject(e); |
1193 |
dl |
1.1 |
// Use trailing null as sentinel |
1194 |
|
|
s.writeObject(null); |
1195 |
|
|
} |
1196 |
|
|
|
1197 |
|
|
/** |
1198 |
jsr166 |
1.46 |
* Reconstitutes the Queue instance from a stream (that is, |
1199 |
|
|
* deserializes it). |
1200 |
jsr166 |
1.19 |
* |
1201 |
dl |
1.1 |
* @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 |
jsr166 |
1.25 |
@SuppressWarnings("unchecked") E item = (E) s.readObject(); |
1208 |
dl |
1.1 |
if (item == null) |
1209 |
|
|
break; |
1210 |
|
|
else |
1211 |
|
|
offer(item); |
1212 |
|
|
} |
1213 |
|
|
} |
1214 |
dl |
1.7 |
|
1215 |
jsr166 |
1.28 |
// Unsafe mechanics |
1216 |
|
|
|
1217 |
|
|
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
1218 |
|
|
private static final long headOffset = |
1219 |
jsr166 |
1.31 |
objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class); |
1220 |
jsr166 |
1.28 |
private static final long tailOffset = |
1221 |
jsr166 |
1.31 |
objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class); |
1222 |
dl |
1.67 |
private static final long sweepVotesOffset = |
1223 |
|
|
objectFieldOffset(UNSAFE, "sweepVotes", LinkedTransferQueue.class); |
1224 |
jsr166 |
1.31 |
|
1225 |
|
|
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
1226 |
|
|
String field, Class<?> klazz) { |
1227 |
jsr166 |
1.28 |
try { |
1228 |
|
|
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
1229 |
|
|
} catch (NoSuchFieldException e) { |
1230 |
|
|
// Convert Exception to corresponding Error |
1231 |
|
|
NoSuchFieldError error = new NoSuchFieldError(field); |
1232 |
|
|
error.initCause(e); |
1233 |
|
|
throw error; |
1234 |
|
|
} |
1235 |
|
|
} |
1236 |
|
|
|
1237 |
jsr166 |
1.53 |
/** |
1238 |
|
|
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
1239 |
|
|
* Replace with a simple call to Unsafe.getUnsafe when integrating |
1240 |
|
|
* into a jdk. |
1241 |
|
|
* |
1242 |
|
|
* @return a sun.misc.Unsafe |
1243 |
|
|
*/ |
1244 |
jsr166 |
1.54 |
static sun.misc.Unsafe getUnsafe() { |
1245 |
jsr166 |
1.13 |
try { |
1246 |
jsr166 |
1.25 |
return sun.misc.Unsafe.getUnsafe(); |
1247 |
jsr166 |
1.13 |
} catch (SecurityException se) { |
1248 |
|
|
try { |
1249 |
|
|
return java.security.AccessController.doPrivileged |
1250 |
jsr166 |
1.28 |
(new java.security |
1251 |
|
|
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
1252 |
jsr166 |
1.25 |
public sun.misc.Unsafe run() throws Exception { |
1253 |
jsr166 |
1.28 |
java.lang.reflect.Field f = sun.misc |
1254 |
|
|
.Unsafe.class.getDeclaredField("theUnsafe"); |
1255 |
|
|
f.setAccessible(true); |
1256 |
|
|
return (sun.misc.Unsafe) f.get(null); |
1257 |
jsr166 |
1.13 |
}}); |
1258 |
|
|
} catch (java.security.PrivilegedActionException e) { |
1259 |
jsr166 |
1.25 |
throw new RuntimeException("Could not initialize intrinsics", |
1260 |
|
|
e.getCause()); |
1261 |
jsr166 |
1.13 |
} |
1262 |
|
|
} |
1263 |
|
|
} |
1264 |
dl |
1.45 |
|
1265 |
dl |
1.1 |
} |