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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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import java.util.concurrent.atomic.AtomicReference; |
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/** |
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* An unbounded {@linkplain TransferQueue} based on linked nodes. |
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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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private static final long serialVersionUID = -3223113410248163686L; |
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/* |
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* This class extends the approach used in FIFO-mode |
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* SynchronousQueues. See the internal documentation, as well as |
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* the PPoPP 2006 paper "Scalable Synchronous Queues" by Scherer, |
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* Lea & Scott |
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* (http://www.cs.rice.edu/~wns1/papers/2006-PPoPP-SQ.pdf) |
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* *** Overview of Dual Queues with Slack *** |
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* |
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* The main extension is to provide different Wait modes for the |
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* main "xfer" method that puts or takes items. These don't |
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* impact the basic dual-queue logic, but instead control whether |
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* or how threads block upon insertion of request or data nodes |
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* into the dual queue. It also uses slightly different |
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* conventions for tracking whether nodes are off-list or |
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* cancelled. |
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*/ |
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|
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// Wait modes for xfer method |
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static final int NOWAIT = 0; |
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static final int TIMEOUT = 1; |
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static final int WAIT = 2; |
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|
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/** The number of CPUs, for spin control */ |
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static final int NCPUS = Runtime.getRuntime().availableProcessors(); |
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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 embedded 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 item can never again change. We |
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* may thus arrange that the linked list of them contains a prefix |
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* of zero or more matched nodes, followed by a suffix of zero or |
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* more unmatched nodes. (Note that we allow both the prefix and |
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* suffix to be zero length, which in turn means that we do not |
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* use a dummy header.) If we were not concerned with either time |
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* or space efficiency, we could correctly perform enqueue and |
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* dequeue operations by traversing from a pointer to the initial |
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* node; CASing the item of the first unmatched node on match and |
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* CASing the next field of the trailing node on appends. While |
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* this would be a terrible idea in itself, it does have the |
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* benefit of not requiring ANY atomic updates on head/tail |
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* 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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* that reflects the tradeoff of sometimes require extra traversal |
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* steps to locate the first and/or last unmatched nodes, versus |
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* the reduced overhead and contention of fewer updates to queue |
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* pointers. For example, a possible snapshot of 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. |
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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 releasing another thread) to be read-only, |
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* thus not introducing any further contention. As described |
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* below, we implement this by performing slack maintenance |
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* 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. During traversals, threads may sometimes |
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* shortcut the "next" link path from the current "head" node to |
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* be closer to the currently known first unmatched node. Again, |
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* this 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 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 internal nodes (due to timed out or interrupted |
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* waits, or calls to remove or Iterator.remove) uses a scheme |
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* roughly similar to that in Scherer, Lea, and Scott |
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* SynchronousQueue. Given a predecessor, we can unsplice any node |
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* except the (actual) tail of the queue. To avoid build-up of |
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* cancelled trailing nodes, upon a request to remove a trailing |
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* node, it is placed in field "cleanMe" to be unspliced later. |
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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 target |
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* slack of two. 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 max slack at one. Avoiding tracking |
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* explicit counts across situations 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 value, path short-circuiting is rarely |
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* worthwhile. However, it is used (in awaitMatch) immediately |
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* before a waiting thread starts to block, as a final bit of |
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* helping at a point when contention with others is extremely |
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* unlikely (since if other threads that could release it are |
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* operating, then the current thread wouldn't be blocking). |
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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 nine separate methods. |
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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 retry loops until the slack is at most two. Traversals |
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* also check if the initial head is now off-list, in which |
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* case they 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, try to append a new node |
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* to the list (or if head was null, establish the first |
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* node). Nodes can be appended only if their predecessors are |
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* either already matched or are of the same mode. If we detect |
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* otherwise, then a new node with opposite mode must have been |
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* appended during traversal, so must restart at phase 1. The |
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* traversal and update steps are otherwise similar to phase 1: |
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* Retrying upon CAS misses and checking for staleness. In |
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* particular, if a self-link is encountered, then we can |
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* safely jump to a node on the list by continuing the |
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* 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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* 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 much smaller (1/4) spins for nodes |
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* that are not known to be front but whose predecessors have |
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* not 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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* spins, reflecting the longer code path lengths necessary to |
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* release them under contention. |
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*/ |
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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; |
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|
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/** |
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* The number of times to spin (with on average one randomly |
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* interspersed call to Thread.yield) on multiprocessor before |
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* blocking when a node is apparently the first waiter in the |
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* queue. See above for explanation. Must be a power of two. The |
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* value is empirically derived -- it works pretty well across a |
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* variety of processors, numbers of CPUs, and OSes. |
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*/ |
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private static final int FRONT_SPINS = 1 << 7; |
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|
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/** |
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* The number of times to spin before blocking when a node is |
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* preceded by another node that is apparently spinning. |
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*/ |
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private static final int CHAINED_SPINS = FRONT_SPINS >>> 2; |
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|
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/** |
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* Queue nodes. Uses Object, not E for items to allow forgetting |
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* them after use. Relies heavily on Unsafe mechanics to minimize |
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* unecessary ordering constraints: Writes that intrinsically |
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* precede or follow CASes use simple relaxed forms. Other |
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* cleanups use releasing/lazy writes. |
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*/ |
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static final class Node { |
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final boolean isData; // false if this is a request node |
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volatile Object item; // initially nonnull if isData; CASed to match |
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volatile Node next; |
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volatile Thread waiter; // null until waiting |
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|
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/** |
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* The number of times to spin before blocking in timed waits. |
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* The value is empirically derived -- it works well across a |
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* variety of processors and OSes. Empirically, the best value |
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* seems not to vary with number of CPUs (beyond 2) so is just |
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* a constant. |
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*/ |
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static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32; |
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|
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/** |
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* The number of times to spin before blocking in untimed waits. |
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* This is greater than timed value because untimed waits spin |
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* faster since they don't need to check times on each spin. |
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*/ |
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static final int maxUntimedSpins = maxTimedSpins * 16; |
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|
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/** |
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* The number of nanoseconds for which it is faster to spin |
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* rather than to use timed park. A rough estimate suffices. |
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*/ |
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static final long spinForTimeoutThreshold = 1000L; |
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// CAS methods for fields |
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final boolean casNext(Node cmp, Node val) { |
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return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
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} |
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|
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/** |
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* Node class for LinkedTransferQueue. Opportunistically |
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* subclasses from AtomicReference to represent item. Uses Object, |
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* not E, to allow setting item to "this" after use, to avoid |
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* garbage retention. Similarly, setting the next field to this is |
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* used as sentinel that node is off list. |
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*/ |
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static final class Node<E> extends AtomicReference<Object> { |
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volatile Node<E> next; |
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volatile Thread waiter; // to control park/unpark |
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final boolean isData; |
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final boolean casItem(Object cmp, Object val) { |
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return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
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} |
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|
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Node(E item, boolean isData) { |
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super(item); |
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/** |
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* Create a new node. Uses relaxed write because item can only |
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* be seen if followed by CAS |
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*/ |
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Node(Object item, boolean isData) { |
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UNSAFE.putObject(this, itemOffset, item); // relaxed write |
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this.isData = isData; |
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} |
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|
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// Unsafe mechanics |
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/** |
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* Links node to itself to avoid garbage retention. Called |
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* only after CASing head field, so uses relaxed write. |
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*/ |
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final void forgetNext() { |
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UNSAFE.putObject(this, nextOffset, this); |
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} |
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|
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private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
364 |
< |
private static final long nextOffset = |
365 |
< |
objectFieldOffset(UNSAFE, "next", Node.class); |
363 |
> |
/** |
364 |
> |
* Sets item to self (using a releasing/lazy write) and waiter |
365 |
> |
* to null, to avoid garbage retention after extracting or |
366 |
> |
* cancelling. |
367 |
> |
*/ |
368 |
> |
final void forgetContents() { |
369 |
> |
UNSAFE.putOrderedObject(this, itemOffset, this); |
370 |
> |
UNSAFE.putOrderedObject(this, waiterOffset, null); |
371 |
> |
} |
372 |
|
|
373 |
< |
final boolean casNext(Node<E> cmp, Node<E> val) { |
374 |
< |
return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
373 |
> |
/** |
374 |
> |
* Returns true if this node has been matched, including the |
375 |
> |
* case of artificial matches due to cancellation. |
376 |
> |
*/ |
377 |
> |
final boolean isMatched() { |
378 |
> |
Object x = item; |
379 |
> |
return x == this || (x != null) != isData; |
380 |
|
} |
381 |
|
|
382 |
< |
final void clearNext() { |
383 |
< |
UNSAFE.putOrderedObject(this, nextOffset, this); |
382 |
> |
/** |
383 |
> |
* Returns true if a node with the given mode cannot be |
384 |
> |
* appended to this node because this node is unmatched and |
385 |
> |
* has opposite data mode. |
386 |
> |
*/ |
387 |
> |
final boolean cannotPrecede(boolean haveData) { |
388 |
> |
boolean d = isData; |
389 |
> |
Object x; |
390 |
> |
return d != haveData && (x = item) != this && (x != null) == d; |
391 |
|
} |
392 |
|
|
393 |
|
/** |
394 |
< |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
135 |
< |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
136 |
< |
* into a jdk. |
137 |
< |
* |
138 |
< |
* @return a sun.misc.Unsafe |
394 |
> |
* Tries to artifically match a data node -- used by remove. |
395 |
|
*/ |
396 |
< |
private static sun.misc.Unsafe getUnsafe() { |
397 |
< |
try { |
398 |
< |
return sun.misc.Unsafe.getUnsafe(); |
399 |
< |
} catch (SecurityException se) { |
400 |
< |
try { |
145 |
< |
return java.security.AccessController.doPrivileged |
146 |
< |
(new java.security |
147 |
< |
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
148 |
< |
public sun.misc.Unsafe run() throws Exception { |
149 |
< |
java.lang.reflect.Field f = sun.misc |
150 |
< |
.Unsafe.class.getDeclaredField("theUnsafe"); |
151 |
< |
f.setAccessible(true); |
152 |
< |
return (sun.misc.Unsafe) f.get(null); |
153 |
< |
}}); |
154 |
< |
} catch (java.security.PrivilegedActionException e) { |
155 |
< |
throw new RuntimeException("Could not initialize intrinsics", |
156 |
< |
e.getCause()); |
157 |
< |
} |
396 |
> |
final boolean tryMatchData() { |
397 |
> |
Object x = item; |
398 |
> |
if (x != null && x != this && casItem(x, null)) { |
399 |
> |
LockSupport.unpark(waiter); |
400 |
> |
return true; |
401 |
|
} |
402 |
+ |
return false; |
403 |
|
} |
404 |
|
|
405 |
+ |
// Unsafe mechanics |
406 |
+ |
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
407 |
+ |
private static final long nextOffset = |
408 |
+ |
objectFieldOffset(UNSAFE, "next", Node.class); |
409 |
+ |
private static final long itemOffset = |
410 |
+ |
objectFieldOffset(UNSAFE, "item", Node.class); |
411 |
+ |
private static final long waiterOffset = |
412 |
+ |
objectFieldOffset(UNSAFE, "waiter", Node.class); |
413 |
+ |
|
414 |
|
private static final long serialVersionUID = -3375979862319811754L; |
415 |
|
} |
416 |
|
|
417 |
< |
/** |
418 |
< |
* Padded version of AtomicReference used for head, tail and |
166 |
< |
* cleanMe, to alleviate contention across threads CASing one vs |
167 |
< |
* the other. |
168 |
< |
*/ |
169 |
< |
static final class PaddedAtomicReference<T> extends AtomicReference<T> { |
170 |
< |
// enough padding for 64bytes with 4byte refs |
171 |
< |
Object p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe; |
172 |
< |
PaddedAtomicReference(T r) { super(r); } |
173 |
< |
private static final long serialVersionUID = 8170090609809740854L; |
174 |
< |
} |
417 |
> |
/** head of the queue; null until first enqueue */ |
418 |
> |
private transient volatile Node head; |
419 |
|
|
420 |
+ |
/** predecessor of dangling unspliceable node */ |
421 |
+ |
private transient volatile Node cleanMe; // decl here to reduce contention |
422 |
|
|
423 |
< |
/** head of the queue */ |
424 |
< |
private transient final PaddedAtomicReference<Node<E>> head; |
423 |
> |
/** tail of the queue; null until first append */ |
424 |
> |
private transient volatile Node tail; |
425 |
|
|
426 |
< |
/** tail of the queue */ |
427 |
< |
private transient final PaddedAtomicReference<Node<E>> tail; |
426 |
> |
// CAS methods for fields |
427 |
> |
private boolean casTail(Node cmp, Node val) { |
428 |
> |
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
429 |
> |
} |
430 |
|
|
431 |
< |
/** |
432 |
< |
* Reference to a cancelled node that might not yet have been |
433 |
< |
* unlinked from queue because it was the last inserted node |
186 |
< |
* when it cancelled. |
187 |
< |
*/ |
188 |
< |
private transient final PaddedAtomicReference<Node<E>> cleanMe; |
431 |
> |
private boolean casHead(Node cmp, Node val) { |
432 |
> |
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
433 |
> |
} |
434 |
|
|
435 |
< |
/** |
436 |
< |
* Tries to cas nh as new head; if successful, unlink |
192 |
< |
* old head's next node to avoid garbage retention. |
193 |
< |
*/ |
194 |
< |
private boolean advanceHead(Node<E> h, Node<E> nh) { |
195 |
< |
if (h == head.get() && head.compareAndSet(h, nh)) { |
196 |
< |
h.clearNext(); // forget old next |
197 |
< |
return true; |
198 |
< |
} |
199 |
< |
return false; |
435 |
> |
private boolean casCleanMe(Node cmp, Node val) { |
436 |
> |
return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val); |
437 |
|
} |
438 |
|
|
439 |
+ |
/* |
440 |
+ |
* Possible values for "how" argument in xfer method. Beware that |
441 |
+ |
* the order of assigned numerical values matters. |
442 |
+ |
*/ |
443 |
+ |
private static final int NOW = 0; // for untimed poll, tryTransfer |
444 |
+ |
private static final int ASYNC = 1; // for offer, put, add |
445 |
+ |
private static final int SYNC = 2; // for transfer, take |
446 |
+ |
private static final int TIMEOUT = 3; // for timed poll, tryTransfer |
447 |
+ |
|
448 |
|
/** |
449 |
< |
* Puts or takes an item. Used for most queue operations (except |
204 |
< |
* poll() and tryTransfer()). See the similar code in |
205 |
< |
* SynchronousQueue for detailed explanation. |
449 |
> |
* Implements all queuing methods. See above for explanation. |
450 |
|
* |
451 |
< |
* @param e the item or if null, signifies that this is a take |
452 |
< |
* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT |
451 |
> |
* @param e the item or null for take |
452 |
> |
* @param haveData true if this is a put else a take |
453 |
> |
* @param how NOW, ASYNC, SYNC, or TIMEOUT |
454 |
|
* @param nanos timeout in nanosecs, used only if mode is TIMEOUT |
455 |
< |
* @return an item, or null on failure |
455 |
> |
* @return an item if matched, else e; |
456 |
> |
* @throws NullPointerException if haveData mode but e is null |
457 |
|
*/ |
458 |
< |
private E xfer(E e, int mode, long nanos) { |
459 |
< |
boolean isData = (e != null); |
460 |
< |
Node<E> s = null; |
461 |
< |
final PaddedAtomicReference<Node<E>> head = this.head; |
216 |
< |
final PaddedAtomicReference<Node<E>> tail = this.tail; |
217 |
< |
|
218 |
< |
for (;;) { |
219 |
< |
Node<E> t = tail.get(); |
220 |
< |
Node<E> h = head.get(); |
458 |
> |
private Object xfer(Object e, boolean haveData, int how, long nanos) { |
459 |
> |
if (haveData && (e == null)) |
460 |
> |
throw new NullPointerException(); |
461 |
> |
Node s = null; // the node to append, if needed |
462 |
|
|
463 |
< |
if (t != null && (t == h || t.isData == isData)) { |
223 |
< |
if (s == null) |
224 |
< |
s = new Node<E>(e, isData); |
225 |
< |
Node<E> last = t.next; |
226 |
< |
if (last != null) { |
227 |
< |
if (t == tail.get()) |
228 |
< |
tail.compareAndSet(t, last); |
229 |
< |
} |
230 |
< |
else if (t.casNext(null, s)) { |
231 |
< |
tail.compareAndSet(t, s); |
232 |
< |
return awaitFulfill(t, s, e, mode, nanos); |
233 |
< |
} |
234 |
< |
} |
463 |
> |
retry: for (;;) { // restart on append race |
464 |
|
|
465 |
< |
else if (h != null) { |
466 |
< |
Node<E> first = h.next; |
467 |
< |
if (t == tail.get() && first != null && |
468 |
< |
advanceHead(h, first)) { |
469 |
< |
Object x = first.get(); |
470 |
< |
if (x != first && first.compareAndSet(x, e)) { |
471 |
< |
LockSupport.unpark(first.waiter); |
472 |
< |
return isData ? e : (E) x; |
465 |
> |
for (Node h = head, p = h; p != null;) { // find & match first node |
466 |
> |
boolean isData = p.isData; |
467 |
> |
Object item = p.item; |
468 |
> |
if (item != p && (item != null) == isData) { // unmatched |
469 |
> |
if (isData == haveData) // can't match |
470 |
> |
break; |
471 |
> |
if (p.casItem(item, e)) { // match |
472 |
> |
Thread w = p.waiter; |
473 |
> |
while (p != h) { // update head |
474 |
> |
Node n = p.next; // by 2 unless singleton |
475 |
> |
if (n != null) |
476 |
> |
p = n; |
477 |
> |
if (head == h && casHead(h, p)) { |
478 |
> |
h.forgetNext(); |
479 |
> |
break; |
480 |
> |
} // advance and retry |
481 |
> |
if ((h = head) == null || |
482 |
> |
(p = h.next) == null || !p.isMatched()) |
483 |
> |
break; // unless slack < 2 |
484 |
> |
} |
485 |
> |
LockSupport.unpark(w); |
486 |
> |
return item; |
487 |
|
} |
488 |
|
} |
489 |
+ |
Node n = p.next; |
490 |
+ |
p = p != n ? n : (h = head); // Use head if p offlist |
491 |
|
} |
492 |
+ |
|
493 |
+ |
if (how >= ASYNC) { // No matches available |
494 |
+ |
if (s == null) |
495 |
+ |
s = new Node(e, haveData); |
496 |
+ |
Node pred = tryAppend(s, haveData); |
497 |
+ |
if (pred == null) |
498 |
+ |
continue retry; // lost race vs opposite mode |
499 |
+ |
if (how >= SYNC) |
500 |
+ |
return awaitMatch(pred, s, e, how, nanos); |
501 |
+ |
} |
502 |
+ |
return e; // not waiting |
503 |
|
} |
504 |
|
} |
505 |
|
|
250 |
– |
|
506 |
|
/** |
507 |
< |
* Version of xfer for poll() and tryTransfer, which |
508 |
< |
* simplifies control paths both here and in xfer. |
509 |
< |
*/ |
510 |
< |
private E fulfill(E e) { |
511 |
< |
boolean isData = (e != null); |
512 |
< |
final PaddedAtomicReference<Node<E>> head = this.head; |
513 |
< |
final PaddedAtomicReference<Node<E>> tail = this.tail; |
514 |
< |
|
515 |
< |
for (;;) { |
516 |
< |
Node<E> t = tail.get(); |
517 |
< |
Node<E> h = head.get(); |
518 |
< |
|
519 |
< |
if (t != null && (t == h || t.isData == isData)) { |
520 |
< |
Node<E> last = t.next; |
521 |
< |
if (t == tail.get()) { |
522 |
< |
if (last != null) |
523 |
< |
tail.compareAndSet(t, last); |
524 |
< |
else |
525 |
< |
return null; |
526 |
< |
} |
527 |
< |
} |
528 |
< |
else if (h != null) { |
529 |
< |
Node<E> first = h.next; |
530 |
< |
if (t == tail.get() && |
531 |
< |
first != null && |
532 |
< |
advanceHead(h, first)) { |
533 |
< |
Object x = first.get(); |
279 |
< |
if (x != first && first.compareAndSet(x, e)) { |
280 |
< |
LockSupport.unpark(first.waiter); |
281 |
< |
return isData ? e : (E) x; |
282 |
< |
} |
507 |
> |
* Tries to append node s as tail |
508 |
> |
* @param haveData true if appending in data mode |
509 |
> |
* @param s the node to append |
510 |
> |
* @return null on failure due to losing race with append in |
511 |
> |
* different mode, else s's predecessor, or s itself if no |
512 |
> |
* predecessor |
513 |
> |
*/ |
514 |
> |
private Node tryAppend(Node s, boolean haveData) { |
515 |
> |
for (Node t = tail, p = t;;) { // move p to actual tail and append |
516 |
> |
Node n, u; // temps for reads of next & tail |
517 |
> |
if (p == null && (p = head) == null) { |
518 |
> |
if (casHead(null, s)) |
519 |
> |
return s; // initialize |
520 |
> |
} |
521 |
> |
else if (p.cannotPrecede(haveData)) |
522 |
> |
return null; // lost race vs opposite mode |
523 |
> |
else if ((n = p.next) != null) // Not tail; keep traversing |
524 |
> |
p = p != t && t != (u = tail) ? (t = u) : // stale tail |
525 |
> |
p != n ? n : null; // restart if off list |
526 |
> |
else if (!p.casNext(null, s)) |
527 |
> |
p = p.next; // re-read on CAS failure |
528 |
> |
else { |
529 |
> |
if (p != t) { // Update if slack now >= 2 |
530 |
> |
while ((tail != t || !casTail(t, s)) && |
531 |
> |
(t = tail) != null && |
532 |
> |
(s = t.next) != null && // advance and retry |
533 |
> |
(s = s.next) != null && s != t); |
534 |
|
} |
535 |
+ |
return p; |
536 |
|
} |
537 |
|
} |
538 |
|
} |
539 |
|
|
540 |
|
/** |
541 |
< |
* Spins/blocks until node s is fulfilled or caller gives up, |
290 |
< |
* depending on wait mode. |
541 |
> |
* Spins/yields/blocks until node s is matched or caller gives up. |
542 |
|
* |
543 |
< |
* @param pred the predecessor of waiting node |
543 |
> |
* @param pred the predecessor of s or s or null if none |
544 |
|
* @param s the waiting node |
545 |
|
* @param e the comparison value for checking match |
546 |
< |
* @param mode mode |
546 |
> |
* @param how either SYNC or TIMEOUT |
547 |
|
* @param nanos timeout value |
548 |
< |
* @return matched item, or s if cancelled |
548 |
> |
* @return matched item, or e if unmatched on interrupt or timeout |
549 |
|
*/ |
550 |
< |
private E awaitFulfill(Node<E> pred, Node<E> s, E e, |
551 |
< |
int mode, long nanos) { |
552 |
< |
if (mode == NOWAIT) |
302 |
< |
return null; |
303 |
< |
|
304 |
< |
long lastTime = (mode == TIMEOUT) ? System.nanoTime() : 0; |
550 |
> |
private Object awaitMatch(Node pred, Node s, Object e, |
551 |
> |
int how, long nanos) { |
552 |
> |
long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L; |
553 |
|
Thread w = Thread.currentThread(); |
554 |
< |
int spins = -1; // set to desired spin count below |
554 |
> |
int spins = -1; // initialized after first item and cancel checks |
555 |
> |
ThreadLocalRandom randomYields = null; // bound if needed |
556 |
> |
|
557 |
|
for (;;) { |
558 |
< |
if (w.isInterrupted()) |
559 |
< |
s.compareAndSet(e, s); |
560 |
< |
Object x = s.get(); |
561 |
< |
if (x != e) { // Node was matched or cancelled |
562 |
< |
advanceHead(pred, s); // unlink if head |
563 |
< |
if (x == s) { // was cancelled |
564 |
< |
clean(pred, s); |
565 |
< |
return null; |
566 |
< |
} |
567 |
< |
else if (x != null) { |
568 |
< |
s.set(s); // avoid garbage retention |
569 |
< |
return (E) x; |
570 |
< |
} |
571 |
< |
else |
572 |
< |
return e; |
558 |
> |
Object item = s.item; |
559 |
> |
if (item != e) { // matched |
560 |
> |
s.forgetContents(); // avoid garbage |
561 |
> |
return item; |
562 |
> |
} |
563 |
> |
if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) && |
564 |
> |
s.casItem(e, s)) { // cancel |
565 |
> |
unsplice(pred, s); |
566 |
> |
return e; |
567 |
> |
} |
568 |
> |
|
569 |
> |
if (spins < 0) { // establish spins at/near front |
570 |
> |
if ((spins = spinsFor(pred, s.isData)) > 0) |
571 |
> |
randomYields = ThreadLocalRandom.current(); |
572 |
> |
} |
573 |
> |
else if (spins > 0) { // spin, occasionally yield |
574 |
> |
if (randomYields.nextInt(FRONT_SPINS) == 0) |
575 |
> |
Thread.yield(); |
576 |
> |
--spins; |
577 |
|
} |
578 |
< |
if (mode == TIMEOUT) { |
578 |
> |
else if (s.waiter == null) { |
579 |
> |
shortenHeadPath(); // reduce slack before blocking |
580 |
> |
s.waiter = w; // request unpark |
581 |
> |
} |
582 |
> |
else if (how == TIMEOUT) { |
583 |
|
long now = System.nanoTime(); |
584 |
< |
nanos -= now - lastTime; |
584 |
> |
if ((nanos -= now - lastTime) > 0) |
585 |
> |
LockSupport.parkNanos(this, nanos); |
586 |
|
lastTime = now; |
328 |
– |
if (nanos <= 0) { |
329 |
– |
s.compareAndSet(e, s); // try to cancel |
330 |
– |
continue; |
331 |
– |
} |
587 |
|
} |
588 |
< |
if (spins < 0) { |
334 |
< |
Node<E> h = head.get(); // only spin if at head |
335 |
< |
spins = ((h != null && h.next == s) ? |
336 |
< |
((mode == TIMEOUT) ? |
337 |
< |
maxTimedSpins : maxUntimedSpins) : 0); |
338 |
< |
} |
339 |
< |
if (spins > 0) |
340 |
< |
--spins; |
341 |
< |
else if (s.waiter == null) |
342 |
< |
s.waiter = w; |
343 |
< |
else if (mode != TIMEOUT) { |
588 |
> |
else { |
589 |
|
LockSupport.park(this); |
590 |
< |
s.waiter = null; |
346 |
< |
spins = -1; |
347 |
< |
} |
348 |
< |
else if (nanos > spinForTimeoutThreshold) { |
349 |
< |
LockSupport.parkNanos(this, nanos); |
350 |
< |
s.waiter = null; |
351 |
< |
spins = -1; |
590 |
> |
spins = -1; // spin if front upon wakeup |
591 |
|
} |
592 |
|
} |
593 |
|
} |
594 |
|
|
595 |
|
/** |
596 |
< |
* Returns validated tail for use in cleaning methods. |
596 |
> |
* Return spin/yield value for a node with given predecessor and |
597 |
> |
* data mode. See above for explanation. |
598 |
|
*/ |
599 |
< |
private Node<E> getValidatedTail() { |
600 |
< |
for (;;) { |
601 |
< |
Node<E> h = head.get(); |
602 |
< |
Node<E> first = h.next; |
603 |
< |
if (first != null && first.next == first) { // help advance |
604 |
< |
advanceHead(h, first); |
605 |
< |
continue; |
606 |
< |
} |
607 |
< |
Node<E> t = tail.get(); |
608 |
< |
Node<E> last = t.next; |
609 |
< |
if (t == tail.get()) { |
610 |
< |
if (last != null) |
611 |
< |
tail.compareAndSet(t, last); // help advance |
612 |
< |
else |
613 |
< |
return t; |
599 |
> |
private static int spinsFor(Node pred, boolean haveData) { |
600 |
> |
if (MP && pred != null) { |
601 |
> |
boolean predData = pred.isData; |
602 |
> |
if (predData != haveData) // front and phase change |
603 |
> |
return FRONT_SPINS + (FRONT_SPINS >>> 1); |
604 |
> |
if (predData != (pred.item != null)) // probably at front |
605 |
> |
return FRONT_SPINS; |
606 |
> |
if (pred.waiter == null) // pred apparently spinning |
607 |
> |
return CHAINED_SPINS; |
608 |
> |
} |
609 |
> |
return 0; |
610 |
> |
} |
611 |
> |
|
612 |
> |
/** |
613 |
> |
* Tries (once) to unsplice nodes between head and first unmatched |
614 |
> |
* or trailing node; failing on contention. |
615 |
> |
*/ |
616 |
> |
private void shortenHeadPath() { |
617 |
> |
Node h, hn, p, q; |
618 |
> |
if ((p = h = head) != null && h.isMatched() && |
619 |
> |
(q = hn = h.next) != null) { |
620 |
> |
Node n; |
621 |
> |
while ((n = q.next) != q) { |
622 |
> |
if (n == null || !q.isMatched()) { |
623 |
> |
if (hn != q && h.next == hn) |
624 |
> |
h.casNext(hn, q); |
625 |
> |
break; |
626 |
> |
} |
627 |
> |
p = q; |
628 |
> |
q = n; |
629 |
|
} |
630 |
|
} |
631 |
|
} |
632 |
|
|
633 |
+ |
/* -------------- Traversal methods -------------- */ |
634 |
+ |
|
635 |
|
/** |
636 |
< |
* Gets rid of cancelled node s with original predecessor pred. |
637 |
< |
* |
638 |
< |
* @param pred predecessor of cancelled node |
639 |
< |
* @param s the cancelled node |
636 |
> |
* Return the first unmatched node of the given mode, or null if |
637 |
> |
* none. Used by methods isEmpty, hasWaitingConsumer. |
638 |
> |
*/ |
639 |
> |
private Node firstOfMode(boolean data) { |
640 |
> |
for (Node p = head; p != null; ) { |
641 |
> |
if (!p.isMatched()) |
642 |
> |
return p.isData == data? p : null; |
643 |
> |
Node n = p.next; |
644 |
> |
p = n != p ? n : head; |
645 |
> |
} |
646 |
> |
return null; |
647 |
> |
} |
648 |
> |
|
649 |
> |
/** |
650 |
> |
* Returns the item in the first unmatched node with isData; or |
651 |
> |
* null if none. Used by peek. |
652 |
|
*/ |
653 |
< |
private void clean(Node<E> pred, Node<E> s) { |
654 |
< |
Thread w = s.waiter; |
655 |
< |
if (w != null) { // Wake up thread |
656 |
< |
s.waiter = null; |
657 |
< |
if (w != Thread.currentThread()) |
658 |
< |
LockSupport.unpark(w); |
653 |
> |
private Object firstDataItem() { |
654 |
> |
for (Node p = head; p != null; ) { |
655 |
> |
boolean isData = p.isData; |
656 |
> |
Object item = p.item; |
657 |
> |
if (item != p && (item != null) == isData) |
658 |
> |
return isData ? item : null; |
659 |
> |
Node n = p.next; |
660 |
> |
p = n != p ? n : head; |
661 |
|
} |
662 |
+ |
return null; |
663 |
+ |
} |
664 |
+ |
|
665 |
+ |
/** |
666 |
+ |
* Traverse and count nodes of the given mode. |
667 |
+ |
* Used by methds size and getWaitingConsumerCount. |
668 |
+ |
*/ |
669 |
+ |
private int countOfMode(boolean data) { |
670 |
+ |
int count = 0; |
671 |
+ |
for (Node p = head; p != null; ) { |
672 |
+ |
if (!p.isMatched()) { |
673 |
+ |
if (p.isData != data) |
674 |
+ |
return 0; |
675 |
+ |
if (++count == Integer.MAX_VALUE) // saturated |
676 |
+ |
break; |
677 |
+ |
} |
678 |
+ |
Node n = p.next; |
679 |
+ |
if (n != p) |
680 |
+ |
p = n; |
681 |
+ |
else { |
682 |
+ |
count = 0; |
683 |
+ |
p = head; |
684 |
+ |
} |
685 |
+ |
} |
686 |
+ |
return count; |
687 |
+ |
} |
688 |
|
|
689 |
< |
if (pred == null) |
690 |
< |
return; |
689 |
> |
final class Itr implements Iterator<E> { |
690 |
> |
private Node nextNode; // next node to return item for |
691 |
> |
private Object nextItem; // the corresponding item |
692 |
> |
private Node lastRet; // last returned node, to support remove |
693 |
|
|
694 |
+ |
/** |
695 |
+ |
* Moves to next node after prev, or first node if prev null. |
696 |
+ |
*/ |
697 |
+ |
private void advance(Node prev) { |
698 |
+ |
lastRet = prev; |
699 |
+ |
Node p; |
700 |
+ |
if (prev == null || (p = prev.next) == prev) |
701 |
+ |
p = head; |
702 |
+ |
while (p != null) { |
703 |
+ |
Object item = p.item; |
704 |
+ |
if (p.isData) { |
705 |
+ |
if (item != null && item != p) { |
706 |
+ |
nextItem = item; |
707 |
+ |
nextNode = p; |
708 |
+ |
return; |
709 |
+ |
} |
710 |
+ |
} |
711 |
+ |
else if (item == null) |
712 |
+ |
break; |
713 |
+ |
Node n = p.next; |
714 |
+ |
p = n != p ? n : head; |
715 |
+ |
} |
716 |
+ |
nextNode = null; |
717 |
+ |
} |
718 |
+ |
|
719 |
+ |
Itr() { |
720 |
+ |
advance(null); |
721 |
+ |
} |
722 |
+ |
|
723 |
+ |
public final boolean hasNext() { |
724 |
+ |
return nextNode != null; |
725 |
+ |
} |
726 |
+ |
|
727 |
+ |
public final E next() { |
728 |
+ |
Node p = nextNode; |
729 |
+ |
if (p == null) throw new NoSuchElementException(); |
730 |
+ |
Object e = nextItem; |
731 |
+ |
advance(p); |
732 |
+ |
return (E) e; |
733 |
+ |
} |
734 |
+ |
|
735 |
+ |
public final void remove() { |
736 |
+ |
Node p = lastRet; |
737 |
+ |
if (p == null) throw new IllegalStateException(); |
738 |
+ |
lastRet = null; |
739 |
+ |
findAndRemoveNode(p); |
740 |
+ |
} |
741 |
+ |
} |
742 |
+ |
|
743 |
+ |
/* -------------- Removal methods -------------- */ |
744 |
+ |
|
745 |
+ |
/** |
746 |
+ |
* Unsplices (now or later) the given deleted/cancelled node with |
747 |
+ |
* the given predecessor. |
748 |
+ |
* |
749 |
+ |
* @param pred predecessor of node to be unspliced |
750 |
+ |
* @param s the node to be unspliced |
751 |
+ |
*/ |
752 |
+ |
private void unsplice(Node pred, Node s) { |
753 |
+ |
s.forgetContents(); // clear unneeded fields |
754 |
|
/* |
755 |
|
* At any given time, exactly one node on list cannot be |
756 |
|
* deleted -- the last inserted node. To accommodate this, if |
757 |
|
* we cannot delete s, we save its predecessor as "cleanMe", |
758 |
< |
* processing the previously saved version first. At least one |
759 |
< |
* of node s or the node previously saved can always be |
758 |
> |
* processing the previously saved version first. Because only |
759 |
> |
* one node in the list can have a null next, at least one of |
760 |
> |
* node s or the node previously saved can always be |
761 |
|
* processed, so this always terminates. |
762 |
|
*/ |
763 |
< |
while (pred.next == s) { |
764 |
< |
Node<E> oldpred = reclean(); // First, help get rid of cleanMe |
765 |
< |
Node<E> t = getValidatedTail(); |
766 |
< |
if (s != t) { // If not tail, try to unsplice |
767 |
< |
Node<E> sn = s.next; // s.next == s means s already off list |
768 |
< |
if (sn == s || pred.casNext(s, sn)) |
763 |
> |
if (pred != null && pred != s) { |
764 |
> |
while (pred.next == s) { |
765 |
> |
Node oldpred = cleanMe == null? null : reclean(); |
766 |
> |
Node n = s.next; |
767 |
> |
if (n != null) { |
768 |
> |
if (n != s) |
769 |
> |
pred.casNext(s, n); |
770 |
|
break; |
771 |
+ |
} |
772 |
+ |
if (oldpred == pred || // Already saved |
773 |
+ |
(oldpred == null && casCleanMe(null, pred))) |
774 |
+ |
break; // Postpone cleaning |
775 |
|
} |
411 |
– |
else if (oldpred == pred || // Already saved |
412 |
– |
(oldpred == null && cleanMe.compareAndSet(null, pred))) |
413 |
– |
break; // Postpone cleaning |
776 |
|
} |
777 |
|
} |
778 |
|
|
779 |
|
/** |
780 |
< |
* Tries to unsplice the cancelled node held in cleanMe that was |
781 |
< |
* previously uncleanable because it was at tail. |
780 |
> |
* Tries to unsplice the deleted/cancelled node held in cleanMe |
781 |
> |
* that was previously uncleanable because it was at tail. |
782 |
|
* |
783 |
|
* @return current cleanMe node (or null) |
784 |
|
*/ |
785 |
< |
private Node<E> reclean() { |
785 |
> |
private Node reclean() { |
786 |
|
/* |
787 |
< |
* cleanMe is, or at one time was, predecessor of cancelled |
788 |
< |
* node s that was the tail so could not be unspliced. If s |
787 |
> |
* cleanMe is, or at one time was, predecessor of a cancelled |
788 |
> |
* node s that was the tail so could not be unspliced. If it |
789 |
|
* is no longer the tail, try to unsplice if necessary and |
790 |
|
* make cleanMe slot available. This differs from similar |
791 |
< |
* code in clean() because we must check that pred still |
792 |
< |
* points to a cancelled node that must be unspliced -- if |
793 |
< |
* not, we can (must) clear cleanMe without unsplicing. |
794 |
< |
* This can loop only due to contention on casNext or |
433 |
< |
* clearing cleanMe. |
791 |
> |
* code in unsplice() because we must check that pred still |
792 |
> |
* points to a matched node that can be unspliced -- if not, |
793 |
> |
* we can (must) clear cleanMe without unsplicing. This can |
794 |
> |
* loop only due to contention. |
795 |
|
*/ |
796 |
< |
Node<E> pred; |
797 |
< |
while ((pred = cleanMe.get()) != null) { |
798 |
< |
Node<E> t = getValidatedTail(); |
799 |
< |
Node<E> s = pred.next; |
800 |
< |
if (s != t) { |
801 |
< |
Node<E> sn; |
802 |
< |
if (s == null || s == pred || s.get() != s || |
803 |
< |
(sn = s.next) == s || pred.casNext(s, sn)) |
804 |
< |
cleanMe.compareAndSet(pred, null); |
796 |
> |
Node pred; |
797 |
> |
while ((pred = cleanMe) != null) { |
798 |
> |
Node s = pred.next; |
799 |
> |
Node n; |
800 |
> |
if (s == null || s == pred || !s.isMatched()) |
801 |
> |
casCleanMe(pred, null); // already gone |
802 |
> |
else if ((n = s.next) != null) { |
803 |
> |
if (n != s) |
804 |
> |
pred.casNext(s, n); |
805 |
> |
casCleanMe(pred, null); |
806 |
|
} |
807 |
< |
else // s is still tail; cannot clean |
807 |
> |
else |
808 |
|
break; |
809 |
|
} |
810 |
|
return pred; |
811 |
|
} |
812 |
|
|
813 |
|
/** |
814 |
+ |
* Main implementation of Iterator.remove(). Find |
815 |
+ |
* and unsplice the given node. |
816 |
+ |
*/ |
817 |
+ |
final void findAndRemoveNode(Node s) { |
818 |
+ |
if (s.tryMatchData()) { |
819 |
+ |
Node pred = null; |
820 |
+ |
Node p = head; |
821 |
+ |
while (p != null) { |
822 |
+ |
if (p == s) { |
823 |
+ |
unsplice(pred, p); |
824 |
+ |
break; |
825 |
+ |
} |
826 |
+ |
if (!p.isData && !p.isMatched()) |
827 |
+ |
break; |
828 |
+ |
pred = p; |
829 |
+ |
if ((p = p.next) == pred) { // stale |
830 |
+ |
pred = null; |
831 |
+ |
p = head; |
832 |
+ |
} |
833 |
+ |
} |
834 |
+ |
} |
835 |
+ |
} |
836 |
+ |
|
837 |
+ |
/** |
838 |
+ |
* Main implementation of remove(Object) |
839 |
+ |
*/ |
840 |
+ |
private boolean findAndRemove(Object e) { |
841 |
+ |
if (e != null) { |
842 |
+ |
Node pred = null; |
843 |
+ |
Node p = head; |
844 |
+ |
while (p != null) { |
845 |
+ |
Object item = p.item; |
846 |
+ |
if (p.isData) { |
847 |
+ |
if (item != null && item != p && e.equals(item) && |
848 |
+ |
p.tryMatchData()) { |
849 |
+ |
unsplice(pred, p); |
850 |
+ |
return true; |
851 |
+ |
} |
852 |
+ |
} |
853 |
+ |
else if (item == null) |
854 |
+ |
break; |
855 |
+ |
pred = p; |
856 |
+ |
if ((p = p.next) == pred) { |
857 |
+ |
pred = null; |
858 |
+ |
p = head; |
859 |
+ |
} |
860 |
+ |
} |
861 |
+ |
} |
862 |
+ |
return false; |
863 |
+ |
} |
864 |
+ |
|
865 |
+ |
|
866 |
+ |
/** |
867 |
|
* Creates an initially empty {@code LinkedTransferQueue}. |
868 |
|
*/ |
869 |
|
public LinkedTransferQueue() { |
455 |
– |
Node<E> dummy = new Node<E>(null, false); |
456 |
– |
head = new PaddedAtomicReference<Node<E>>(dummy); |
457 |
– |
tail = new PaddedAtomicReference<Node<E>>(dummy); |
458 |
– |
cleanMe = new PaddedAtomicReference<Node<E>>(null); |
870 |
|
} |
871 |
|
|
872 |
|
/** |
890 |
|
* @throws NullPointerException if the specified element is null |
891 |
|
*/ |
892 |
|
public void put(E e) { |
893 |
< |
offer(e); |
893 |
> |
xfer(e, true, ASYNC, 0); |
894 |
|
} |
895 |
|
|
896 |
|
/** |
903 |
|
* @throws NullPointerException if the specified element is null |
904 |
|
*/ |
905 |
|
public boolean offer(E e, long timeout, TimeUnit unit) { |
906 |
< |
return offer(e); |
906 |
> |
xfer(e, true, ASYNC, 0); |
907 |
> |
return true; |
908 |
|
} |
909 |
|
|
910 |
|
/** |
916 |
|
* @throws NullPointerException if the specified element is null |
917 |
|
*/ |
918 |
|
public boolean offer(E e) { |
919 |
< |
if (e == null) throw new NullPointerException(); |
508 |
< |
xfer(e, NOWAIT, 0); |
919 |
> |
xfer(e, true, ASYNC, 0); |
920 |
|
return true; |
921 |
|
} |
922 |
|
|
929 |
|
* @throws NullPointerException if the specified element is null |
930 |
|
*/ |
931 |
|
public boolean add(E e) { |
932 |
< |
return offer(e); |
932 |
> |
xfer(e, true, ASYNC, 0); |
933 |
> |
return true; |
934 |
|
} |
935 |
|
|
936 |
|
/** |
937 |
< |
* Transfers the specified element immediately if there exists a |
938 |
< |
* consumer already waiting to receive it (in {@link #take} or |
939 |
< |
* timed {@link #poll(long,TimeUnit) poll}), otherwise |
940 |
< |
* returning {@code false} without enqueuing the element. |
937 |
> |
* Transfers the element to a waiting consumer immediately, if possible. |
938 |
> |
* |
939 |
> |
* <p>More precisely, transfers the specified element immediately |
940 |
> |
* if there exists a consumer already waiting to receive it (in |
941 |
> |
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
942 |
> |
* otherwise returning {@code false} without enqueuing the element. |
943 |
|
* |
944 |
|
* @throws NullPointerException if the specified element is null |
945 |
|
*/ |
946 |
|
public boolean tryTransfer(E e) { |
947 |
< |
if (e == null) throw new NullPointerException(); |
534 |
< |
return fulfill(e) != null; |
947 |
> |
return xfer(e, true, NOW, 0) == null; |
948 |
|
} |
949 |
|
|
950 |
|
/** |
951 |
< |
* Inserts the specified element at the tail of this queue, |
952 |
< |
* waiting if necessary for the element to be received by a |
953 |
< |
* consumer invoking {@code take} or {@code poll}. |
951 |
> |
* Transfers the element to a consumer, waiting if necessary to do so. |
952 |
> |
* |
953 |
> |
* <p>More precisely, transfers the specified element immediately |
954 |
> |
* if there exists a consumer already waiting to receive it (in |
955 |
> |
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
956 |
> |
* else inserts the specified element at the tail of this queue |
957 |
> |
* and waits until the element is received by a consumer. |
958 |
|
* |
959 |
|
* @throws NullPointerException if the specified element is null |
960 |
|
*/ |
961 |
|
public void transfer(E e) throws InterruptedException { |
962 |
< |
if (e == null) throw new NullPointerException(); |
963 |
< |
if (xfer(e, WAIT, 0) == null) { |
547 |
< |
Thread.interrupted(); |
962 |
> |
if (xfer(e, true, SYNC, 0) != null) { |
963 |
> |
Thread.interrupted(); // failure possible only due to interrupt |
964 |
|
throw new InterruptedException(); |
965 |
|
} |
966 |
|
} |
967 |
|
|
968 |
|
/** |
969 |
< |
* Inserts the specified element at the tail of this queue, |
970 |
< |
* waiting up to the specified wait time for the element to be |
971 |
< |
* received by a consumer invoking {@code take} or {@code poll}. |
969 |
> |
* Transfers the element to a consumer if it is possible to do so |
970 |
> |
* before the timeout elapses. |
971 |
> |
* |
972 |
> |
* <p>More precisely, transfers the specified element immediately |
973 |
> |
* if there exists a consumer already waiting to receive it (in |
974 |
> |
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
975 |
> |
* else inserts the specified element at the tail of this queue |
976 |
> |
* and waits until the element is received by a consumer, |
977 |
> |
* returning {@code false} if the specified wait time elapses |
978 |
> |
* before the element can be transferred. |
979 |
|
* |
980 |
|
* @throws NullPointerException if the specified element is null |
981 |
|
*/ |
982 |
|
public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
983 |
|
throws InterruptedException { |
984 |
< |
if (e == null) throw new NullPointerException(); |
562 |
< |
if (xfer(e, TIMEOUT, unit.toNanos(timeout)) != null) |
984 |
> |
if (xfer(e, true, TIMEOUT, unit.toNanos(timeout)) == null) |
985 |
|
return true; |
986 |
|
if (!Thread.interrupted()) |
987 |
|
return false; |
989 |
|
} |
990 |
|
|
991 |
|
public E take() throws InterruptedException { |
992 |
< |
E e = xfer(null, WAIT, 0); |
992 |
> |
Object e = xfer(null, false, SYNC, 0); |
993 |
|
if (e != null) |
994 |
< |
return e; |
994 |
> |
return (E)e; |
995 |
|
Thread.interrupted(); |
996 |
|
throw new InterruptedException(); |
997 |
|
} |
998 |
|
|
999 |
|
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
1000 |
< |
E e = xfer(null, TIMEOUT, unit.toNanos(timeout)); |
1000 |
> |
Object e = xfer(null, false, TIMEOUT, unit.toNanos(timeout)); |
1001 |
|
if (e != null || !Thread.interrupted()) |
1002 |
< |
return e; |
1002 |
> |
return (E)e; |
1003 |
|
throw new InterruptedException(); |
1004 |
|
} |
1005 |
|
|
1006 |
|
public E poll() { |
1007 |
< |
return fulfill(null); |
1007 |
> |
return (E)xfer(null, false, NOW, 0); |
1008 |
|
} |
1009 |
|
|
1010 |
|
/** |
1043 |
|
return n; |
1044 |
|
} |
1045 |
|
|
624 |
– |
// Traversal-based methods |
625 |
– |
|
626 |
– |
/** |
627 |
– |
* Returns head after performing any outstanding helping steps. |
628 |
– |
*/ |
629 |
– |
private Node<E> traversalHead() { |
630 |
– |
for (;;) { |
631 |
– |
Node<E> t = tail.get(); |
632 |
– |
Node<E> h = head.get(); |
633 |
– |
if (h != null && t != null) { |
634 |
– |
Node<E> last = t.next; |
635 |
– |
Node<E> first = h.next; |
636 |
– |
if (t == tail.get()) { |
637 |
– |
if (last != null) |
638 |
– |
tail.compareAndSet(t, last); |
639 |
– |
else if (first != null) { |
640 |
– |
Object x = first.get(); |
641 |
– |
if (x == first) |
642 |
– |
advanceHead(h, first); |
643 |
– |
else |
644 |
– |
return h; |
645 |
– |
} |
646 |
– |
else |
647 |
– |
return h; |
648 |
– |
} |
649 |
– |
} |
650 |
– |
reclean(); |
651 |
– |
} |
652 |
– |
} |
653 |
– |
|
1046 |
|
/** |
1047 |
|
* Returns an iterator over the elements in this queue in proper |
1048 |
|
* sequence, from head to tail. |
1060 |
|
return new Itr(); |
1061 |
|
} |
1062 |
|
|
671 |
– |
/** |
672 |
– |
* Iterators. Basic strategy is to traverse list, treating |
673 |
– |
* non-data (i.e., request) nodes as terminating list. |
674 |
– |
* Once a valid data node is found, the item is cached |
675 |
– |
* so that the next call to next() will return it even |
676 |
– |
* if subsequently removed. |
677 |
– |
*/ |
678 |
– |
class Itr implements Iterator<E> { |
679 |
– |
Node<E> next; // node to return next |
680 |
– |
Node<E> pnext; // predecessor of next |
681 |
– |
Node<E> curr; // last returned node, for remove() |
682 |
– |
Node<E> pcurr; // predecessor of curr, for remove() |
683 |
– |
E nextItem; // Cache of next item, once committed to in next |
684 |
– |
|
685 |
– |
Itr() { |
686 |
– |
advance(); |
687 |
– |
} |
688 |
– |
|
689 |
– |
/** |
690 |
– |
* Moves to next valid node and returns item to return for |
691 |
– |
* next(), or null if no such. |
692 |
– |
*/ |
693 |
– |
private E advance() { |
694 |
– |
pcurr = pnext; |
695 |
– |
curr = next; |
696 |
– |
E item = nextItem; |
697 |
– |
|
698 |
– |
for (;;) { |
699 |
– |
pnext = (next == null) ? traversalHead() : next; |
700 |
– |
next = pnext.next; |
701 |
– |
if (next == pnext) { |
702 |
– |
next = null; |
703 |
– |
continue; // restart |
704 |
– |
} |
705 |
– |
if (next == null) |
706 |
– |
break; |
707 |
– |
Object x = next.get(); |
708 |
– |
if (x != null && x != next) { |
709 |
– |
nextItem = (E) x; |
710 |
– |
break; |
711 |
– |
} |
712 |
– |
} |
713 |
– |
return item; |
714 |
– |
} |
715 |
– |
|
716 |
– |
public boolean hasNext() { |
717 |
– |
return next != null; |
718 |
– |
} |
719 |
– |
|
720 |
– |
public E next() { |
721 |
– |
if (next == null) |
722 |
– |
throw new NoSuchElementException(); |
723 |
– |
return advance(); |
724 |
– |
} |
725 |
– |
|
726 |
– |
public void remove() { |
727 |
– |
Node<E> p = curr; |
728 |
– |
if (p == null) |
729 |
– |
throw new IllegalStateException(); |
730 |
– |
Object x = p.get(); |
731 |
– |
if (x != null && x != p && p.compareAndSet(x, p)) |
732 |
– |
clean(pcurr, p); |
733 |
– |
} |
734 |
– |
} |
735 |
– |
|
1063 |
|
public E peek() { |
1064 |
< |
for (;;) { |
738 |
< |
Node<E> h = traversalHead(); |
739 |
< |
Node<E> p = h.next; |
740 |
< |
if (p == null) |
741 |
< |
return null; |
742 |
< |
Object x = p.get(); |
743 |
< |
if (p != x) { |
744 |
< |
if (!p.isData) |
745 |
< |
return null; |
746 |
< |
if (x != null) |
747 |
< |
return (E) x; |
748 |
< |
} |
749 |
< |
} |
1064 |
> |
return (E) firstDataItem(); |
1065 |
|
} |
1066 |
|
|
1067 |
+ |
/** |
1068 |
+ |
* Returns {@code true} if this queue contains no elements. |
1069 |
+ |
* |
1070 |
+ |
* @return {@code true} if this queue contains no elements |
1071 |
+ |
*/ |
1072 |
|
public boolean isEmpty() { |
1073 |
< |
for (;;) { |
754 |
< |
Node<E> h = traversalHead(); |
755 |
< |
Node<E> p = h.next; |
756 |
< |
if (p == null) |
757 |
< |
return true; |
758 |
< |
Object x = p.get(); |
759 |
< |
if (p != x) { |
760 |
< |
if (!p.isData) |
761 |
< |
return true; |
762 |
< |
if (x != null) |
763 |
< |
return false; |
764 |
< |
} |
765 |
< |
} |
1073 |
> |
return firstOfMode(true) == null; |
1074 |
|
} |
1075 |
|
|
1076 |
|
public boolean hasWaitingConsumer() { |
1077 |
< |
for (;;) { |
770 |
< |
Node<E> h = traversalHead(); |
771 |
< |
Node<E> p = h.next; |
772 |
< |
if (p == null) |
773 |
< |
return false; |
774 |
< |
Object x = p.get(); |
775 |
< |
if (p != x) |
776 |
< |
return !p.isData; |
777 |
< |
} |
1077 |
> |
return firstOfMode(false) != null; |
1078 |
|
} |
1079 |
|
|
1080 |
|
/** |
1090 |
|
* @return the number of elements in this queue |
1091 |
|
*/ |
1092 |
|
public int size() { |
1093 |
< |
for (;;) { |
794 |
< |
int count = 0; |
795 |
< |
Node<E> pred = traversalHead(); |
796 |
< |
for (;;) { |
797 |
< |
Node<E> q = pred.next; |
798 |
< |
if (q == pred) // restart |
799 |
< |
break; |
800 |
< |
if (q == null || !q.isData) |
801 |
< |
return count; |
802 |
< |
Object x = q.get(); |
803 |
< |
if (x != null && x != q) { |
804 |
< |
if (++count == Integer.MAX_VALUE) // saturated |
805 |
< |
return count; |
806 |
< |
} |
807 |
< |
pred = q; |
808 |
< |
} |
809 |
< |
} |
1093 |
> |
return countOfMode(true); |
1094 |
|
} |
1095 |
|
|
1096 |
|
public int getWaitingConsumerCount() { |
1097 |
< |
// converse of size -- count valid non-data nodes |
814 |
< |
for (;;) { |
815 |
< |
int count = 0; |
816 |
< |
Node<E> pred = traversalHead(); |
817 |
< |
for (;;) { |
818 |
< |
Node<E> q = pred.next; |
819 |
< |
if (q == pred) // restart |
820 |
< |
break; |
821 |
< |
if (q == null || q.isData) |
822 |
< |
return count; |
823 |
< |
Object x = q.get(); |
824 |
< |
if (x == null) { |
825 |
< |
if (++count == Integer.MAX_VALUE) // saturated |
826 |
< |
return count; |
827 |
< |
} |
828 |
< |
pred = q; |
829 |
< |
} |
830 |
< |
} |
1097 |
> |
return countOfMode(false); |
1098 |
|
} |
1099 |
|
|
1100 |
+ |
/** |
1101 |
+ |
* Removes a single instance of the specified element from this queue, |
1102 |
+ |
* if it is present. More formally, removes an element {@code e} such |
1103 |
+ |
* that {@code o.equals(e)}, if this queue contains one or more such |
1104 |
+ |
* elements. |
1105 |
+ |
* Returns {@code true} if this queue contained the specified element |
1106 |
+ |
* (or equivalently, if this queue changed as a result of the call). |
1107 |
+ |
* |
1108 |
+ |
* @param o element to be removed from this queue, if present |
1109 |
+ |
* @return {@code true} if this queue changed as a result of the call |
1110 |
+ |
*/ |
1111 |
|
public boolean remove(Object o) { |
1112 |
< |
if (o == null) |
835 |
< |
return false; |
836 |
< |
for (;;) { |
837 |
< |
Node<E> pred = traversalHead(); |
838 |
< |
for (;;) { |
839 |
< |
Node<E> q = pred.next; |
840 |
< |
if (q == pred) // restart |
841 |
< |
break; |
842 |
< |
if (q == null || !q.isData) |
843 |
< |
return false; |
844 |
< |
Object x = q.get(); |
845 |
< |
if (x != null && x != q && o.equals(x) && |
846 |
< |
q.compareAndSet(x, q)) { |
847 |
< |
clean(pred, q); |
848 |
< |
return true; |
849 |
< |
} |
850 |
< |
pred = q; |
851 |
< |
} |
852 |
< |
} |
1112 |
> |
return findAndRemove(o); |
1113 |
|
} |
1114 |
|
|
1115 |
|
/** |
1148 |
|
private void readObject(java.io.ObjectInputStream s) |
1149 |
|
throws java.io.IOException, ClassNotFoundException { |
1150 |
|
s.defaultReadObject(); |
891 |
– |
resetHeadAndTail(); |
1151 |
|
for (;;) { |
1152 |
|
@SuppressWarnings("unchecked") E item = (E) s.readObject(); |
1153 |
|
if (item == null) |
1157 |
|
} |
1158 |
|
} |
1159 |
|
|
901 |
– |
// Support for resetting head/tail while deserializing |
902 |
– |
private void resetHeadAndTail() { |
903 |
– |
Node<E> dummy = new Node<E>(null, false); |
904 |
– |
UNSAFE.putObjectVolatile(this, headOffset, |
905 |
– |
new PaddedAtomicReference<Node<E>>(dummy)); |
906 |
– |
UNSAFE.putObjectVolatile(this, tailOffset, |
907 |
– |
new PaddedAtomicReference<Node<E>>(dummy)); |
908 |
– |
UNSAFE.putObjectVolatile(this, cleanMeOffset, |
909 |
– |
new PaddedAtomicReference<Node<E>>(null)); |
910 |
– |
} |
1160 |
|
|
1161 |
|
// Unsafe mechanics |
1162 |
|
|
1168 |
|
private static final long cleanMeOffset = |
1169 |
|
objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class); |
1170 |
|
|
922 |
– |
|
1171 |
|
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
1172 |
|
String field, Class<?> klazz) { |
1173 |
|
try { |
1180 |
|
} |
1181 |
|
} |
1182 |
|
|
935 |
– |
/** |
936 |
– |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
937 |
– |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
938 |
– |
* into a jdk. |
939 |
– |
* |
940 |
– |
* @return a sun.misc.Unsafe |
941 |
– |
*/ |
1183 |
|
private static sun.misc.Unsafe getUnsafe() { |
1184 |
|
try { |
1185 |
|
return sun.misc.Unsafe.getUnsafe(); |
1200 |
|
} |
1201 |
|
} |
1202 |
|
} |
1203 |
+ |
|
1204 |
|
} |