1 |
jsr166 |
1.1 |
/* |
2 |
|
|
* Written by Doug Lea with assistance from members of JCP JSR-166 |
3 |
|
|
* Expert Group and released to the public domain, as explained at |
4 |
|
|
* http://creativecommons.org/licenses/publicdomain |
5 |
|
|
*/ |
6 |
|
|
|
7 |
|
|
package java.util.concurrent; |
8 |
|
|
|
9 |
|
|
import java.util.AbstractQueue; |
10 |
|
|
import java.util.Collection; |
11 |
jsr166 |
1.5 |
import java.util.ConcurrentModificationException; |
12 |
jsr166 |
1.1 |
import java.util.Iterator; |
13 |
|
|
import java.util.NoSuchElementException; |
14 |
jsr166 |
1.5 |
import java.util.Queue; |
15 |
jsr166 |
1.1 |
import java.util.concurrent.locks.LockSupport; |
16 |
|
|
/** |
17 |
jsr166 |
1.6 |
* An unbounded {@link TransferQueue} based on linked nodes. |
18 |
jsr166 |
1.1 |
* This queue orders elements FIFO (first-in-first-out) with respect |
19 |
|
|
* to any given producer. The <em>head</em> of the queue is that |
20 |
|
|
* element that has been on the queue the longest time for some |
21 |
|
|
* producer. The <em>tail</em> of the queue is that element that has |
22 |
|
|
* been on the queue the shortest time for some producer. |
23 |
|
|
* |
24 |
|
|
* <p>Beware that, unlike in most collections, the {@code size} |
25 |
|
|
* method is <em>NOT</em> a constant-time operation. Because of the |
26 |
|
|
* asynchronous nature of these queues, determining the current number |
27 |
|
|
* of elements requires a traversal of the elements. |
28 |
|
|
* |
29 |
|
|
* <p>This class and its iterator implement all of the |
30 |
|
|
* <em>optional</em> methods of the {@link Collection} and {@link |
31 |
|
|
* Iterator} interfaces. |
32 |
|
|
* |
33 |
|
|
* <p>Memory consistency effects: As with other concurrent |
34 |
|
|
* collections, actions in a thread prior to placing an object into a |
35 |
|
|
* {@code LinkedTransferQueue} |
36 |
|
|
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
37 |
|
|
* actions subsequent to the access or removal of that element from |
38 |
|
|
* the {@code LinkedTransferQueue} in another thread. |
39 |
|
|
* |
40 |
|
|
* <p>This class is a member of the |
41 |
|
|
* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
42 |
|
|
* Java Collections Framework</a>. |
43 |
|
|
* |
44 |
|
|
* @since 1.7 |
45 |
|
|
* @author Doug Lea |
46 |
|
|
* @param <E> the type of elements held in this collection |
47 |
|
|
*/ |
48 |
|
|
public class LinkedTransferQueue<E> extends AbstractQueue<E> |
49 |
|
|
implements TransferQueue<E>, java.io.Serializable { |
50 |
|
|
private static final long serialVersionUID = -3223113410248163686L; |
51 |
|
|
|
52 |
|
|
/* |
53 |
jsr166 |
1.8 |
* *** Overview of Dual Queues with Slack *** |
54 |
jsr166 |
1.1 |
* |
55 |
jsr166 |
1.8 |
* Dual Queues, introduced by Scherer and Scott |
56 |
|
|
* (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are |
57 |
|
|
* (linked) queues in which nodes may represent either data or |
58 |
|
|
* requests. When a thread tries to enqueue a data node, but |
59 |
|
|
* encounters a request node, it instead "matches" and removes it; |
60 |
|
|
* and vice versa for enqueuing requests. Blocking Dual Queues |
61 |
|
|
* arrange that threads enqueuing unmatched requests block until |
62 |
|
|
* other threads provide the match. Dual Synchronous Queues (see |
63 |
|
|
* Scherer, Lea, & Scott |
64 |
|
|
* http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf) |
65 |
|
|
* additionally arrange that threads enqueuing unmatched data also |
66 |
|
|
* block. Dual Transfer Queues support all of these modes, as |
67 |
|
|
* dictated by callers. |
68 |
|
|
* |
69 |
|
|
* A FIFO dual queue may be implemented using a variation of the |
70 |
|
|
* Michael & Scott (M&S) lock-free queue algorithm |
71 |
|
|
* (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf). |
72 |
|
|
* It maintains two pointer fields, "head", pointing to a |
73 |
|
|
* (matched) node that in turn points to the first actual |
74 |
|
|
* (unmatched) queue node (or null if empty); and "tail" that |
75 |
|
|
* points to the last node on the queue (or again null if |
76 |
|
|
* empty). For example, here is a possible queue with four data |
77 |
|
|
* elements: |
78 |
|
|
* |
79 |
|
|
* head tail |
80 |
|
|
* | | |
81 |
|
|
* v v |
82 |
|
|
* M -> U -> U -> U -> U |
83 |
|
|
* |
84 |
|
|
* The M&S queue algorithm is known to be prone to scalability and |
85 |
|
|
* overhead limitations when maintaining (via CAS) these head and |
86 |
|
|
* tail pointers. This has led to the development of |
87 |
|
|
* contention-reducing variants such as elimination arrays (see |
88 |
|
|
* Moir et al http://portal.acm.org/citation.cfm?id=1074013) and |
89 |
|
|
* optimistic back pointers (see Ladan-Mozes & Shavit |
90 |
|
|
* http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf). |
91 |
|
|
* However, the nature of dual queues enables a simpler tactic for |
92 |
|
|
* improving M&S-style implementations when dual-ness is needed. |
93 |
|
|
* |
94 |
|
|
* In a dual queue, each node must atomically maintain its match |
95 |
|
|
* status. While there are other possible variants, we implement |
96 |
|
|
* this here as: for a data-mode node, matching entails CASing an |
97 |
|
|
* "item" field from a non-null data value to null upon match, and |
98 |
|
|
* vice-versa for request nodes, CASing from null to a data |
99 |
|
|
* value. (Note that the linearization properties of this style of |
100 |
|
|
* queue are easy to verify -- elements are made available by |
101 |
|
|
* linking, and unavailable by matching.) Compared to plain M&S |
102 |
|
|
* queues, this property of dual queues requires one additional |
103 |
|
|
* successful atomic operation per enq/deq pair. But it also |
104 |
|
|
* enables lower cost variants of queue maintenance mechanics. (A |
105 |
|
|
* variation of this idea applies even for non-dual queues that |
106 |
|
|
* support deletion of interior elements, such as |
107 |
|
|
* j.u.c.ConcurrentLinkedQueue.) |
108 |
|
|
* |
109 |
|
|
* Once a node is matched, its match status can never again |
110 |
|
|
* change. We may thus arrange that the linked list of them |
111 |
|
|
* contain a prefix of zero or more matched nodes, followed by a |
112 |
|
|
* suffix of zero or more unmatched nodes. (Note that we allow |
113 |
|
|
* both the prefix and suffix to be zero length, which in turn |
114 |
|
|
* means that we do not use a dummy header.) If we were not |
115 |
|
|
* concerned with either time or space efficiency, we could |
116 |
|
|
* correctly perform enqueue and dequeue operations by traversing |
117 |
|
|
* from a pointer to the initial node; CASing the item of the |
118 |
|
|
* first unmatched node on match and CASing the next field of the |
119 |
|
|
* trailing node on appends. (Plus some special-casing when |
120 |
|
|
* initially empty). While this would be a terrible idea in |
121 |
|
|
* itself, it does have the benefit of not requiring ANY atomic |
122 |
|
|
* updates on head/tail fields. |
123 |
|
|
* |
124 |
|
|
* We introduce here an approach that lies between the extremes of |
125 |
|
|
* never versus always updating queue (head and tail) pointers. |
126 |
|
|
* This offers a tradeoff between sometimes requiring extra |
127 |
|
|
* traversal steps to locate the first and/or last unmatched |
128 |
|
|
* nodes, versus the reduced overhead and contention of fewer |
129 |
|
|
* updates to queue pointers. For example, a possible snapshot of |
130 |
|
|
* a queue is: |
131 |
|
|
* |
132 |
|
|
* head tail |
133 |
|
|
* | | |
134 |
|
|
* v v |
135 |
|
|
* M -> M -> U -> U -> U -> U |
136 |
|
|
* |
137 |
|
|
* The best value for this "slack" (the targeted maximum distance |
138 |
|
|
* between the value of "head" and the first unmatched node, and |
139 |
|
|
* similarly for "tail") is an empirical matter. We have found |
140 |
|
|
* that using very small constants in the range of 1-3 work best |
141 |
|
|
* over a range of platforms. Larger values introduce increasing |
142 |
|
|
* costs of cache misses and risks of long traversal chains, while |
143 |
|
|
* smaller values increase CAS contention and overhead. |
144 |
|
|
* |
145 |
|
|
* Dual queues with slack differ from plain M&S dual queues by |
146 |
|
|
* virtue of only sometimes updating head or tail pointers when |
147 |
|
|
* matching, appending, or even traversing nodes; in order to |
148 |
|
|
* maintain a targeted slack. The idea of "sometimes" may be |
149 |
|
|
* operationalized in several ways. The simplest is to use a |
150 |
|
|
* per-operation counter incremented on each traversal step, and |
151 |
|
|
* to try (via CAS) to update the associated queue pointer |
152 |
|
|
* whenever the count exceeds a threshold. Another, that requires |
153 |
|
|
* more overhead, is to use random number generators to update |
154 |
|
|
* with a given probability per traversal step. |
155 |
|
|
* |
156 |
|
|
* In any strategy along these lines, because CASes updating |
157 |
|
|
* fields may fail, the actual slack may exceed targeted |
158 |
|
|
* slack. However, they may be retried at any time to maintain |
159 |
|
|
* targets. Even when using very small slack values, this |
160 |
|
|
* approach works well for dual queues because it allows all |
161 |
|
|
* operations up to the point of matching or appending an item |
162 |
|
|
* (hence potentially allowing progress by another thread) to be |
163 |
|
|
* read-only, thus not introducing any further contention. As |
164 |
|
|
* described below, we implement this by performing slack |
165 |
|
|
* maintenance retries only after these points. |
166 |
|
|
* |
167 |
|
|
* As an accompaniment to such techniques, traversal overhead can |
168 |
|
|
* be further reduced without increasing contention of head |
169 |
|
|
* pointer updates: Threads may sometimes shortcut the "next" link |
170 |
|
|
* path from the current "head" node to be closer to the currently |
171 |
|
|
* known first unmatched node, and similarly for tail. Again, this |
172 |
|
|
* may be triggered with using thresholds or randomization. |
173 |
|
|
* |
174 |
|
|
* These ideas must be further extended to avoid unbounded amounts |
175 |
|
|
* of costly-to-reclaim garbage caused by the sequential "next" |
176 |
|
|
* links of nodes starting at old forgotten head nodes: As first |
177 |
|
|
* described in detail by Boehm |
178 |
|
|
* (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC |
179 |
|
|
* delays noticing that any arbitrarily old node has become |
180 |
|
|
* garbage, all newer dead nodes will also be unreclaimed. |
181 |
|
|
* (Similar issues arise in non-GC environments.) To cope with |
182 |
|
|
* this in our implementation, upon CASing to advance the head |
183 |
|
|
* pointer, we set the "next" link of the previous head to point |
184 |
|
|
* only to itself; thus limiting the length of connected dead lists. |
185 |
|
|
* (We also take similar care to wipe out possibly garbage |
186 |
|
|
* retaining values held in other Node fields.) However, doing so |
187 |
|
|
* adds some further complexity to traversal: If any "next" |
188 |
|
|
* pointer links to itself, it indicates that the current thread |
189 |
|
|
* has lagged behind a head-update, and so the traversal must |
190 |
|
|
* continue from the "head". Traversals trying to find the |
191 |
|
|
* current tail starting from "tail" may also encounter |
192 |
|
|
* self-links, in which case they also continue at "head". |
193 |
|
|
* |
194 |
|
|
* It is tempting in slack-based scheme to not even use CAS for |
195 |
|
|
* updates (similarly to Ladan-Mozes & Shavit). However, this |
196 |
|
|
* cannot be done for head updates under the above link-forgetting |
197 |
|
|
* mechanics because an update may leave head at a detached node. |
198 |
|
|
* And while direct writes are possible for tail updates, they |
199 |
|
|
* increase the risk of long retraversals, and hence long garbage |
200 |
|
|
* chains, which can be much more costly than is worthwhile |
201 |
|
|
* considering that the cost difference of performing a CAS vs |
202 |
|
|
* write is smaller when they are not triggered on each operation |
203 |
|
|
* (especially considering that writes and CASes equally require |
204 |
|
|
* additional GC bookkeeping ("write barriers") that are sometimes |
205 |
|
|
* more costly than the writes themselves because of contention). |
206 |
|
|
* |
207 |
|
|
* *** Overview of implementation *** |
208 |
|
|
* |
209 |
|
|
* We use a threshold-based approach to updates, with a slack |
210 |
|
|
* threshold of two -- that is, we update head/tail when the |
211 |
|
|
* current pointer appears to be two or more steps away from the |
212 |
|
|
* first/last node. The slack value is hard-wired: a path greater |
213 |
|
|
* than one is naturally implemented by checking equality of |
214 |
|
|
* traversal pointers except when the list has only one element, |
215 |
|
|
* in which case we keep slack threshold at one. Avoiding tracking |
216 |
|
|
* explicit counts across method calls slightly simplifies an |
217 |
|
|
* already-messy implementation. Using randomization would |
218 |
|
|
* probably work better if there were a low-quality dirt-cheap |
219 |
|
|
* per-thread one available, but even ThreadLocalRandom is too |
220 |
|
|
* heavy for these purposes. |
221 |
|
|
* |
222 |
dl |
1.16 |
* With such a small slack threshold value, it is not worthwhile |
223 |
|
|
* to augment this with path short-circuiting (i.e., unsplicing |
224 |
|
|
* interior nodes) except in the case of cancellation/removal (see |
225 |
|
|
* below). |
226 |
jsr166 |
1.8 |
* |
227 |
|
|
* We allow both the head and tail fields to be null before any |
228 |
|
|
* nodes are enqueued; initializing upon first append. This |
229 |
|
|
* simplifies some other logic, as well as providing more |
230 |
|
|
* efficient explicit control paths instead of letting JVMs insert |
231 |
|
|
* implicit NullPointerExceptions when they are null. While not |
232 |
|
|
* currently fully implemented, we also leave open the possibility |
233 |
|
|
* of re-nulling these fields when empty (which is complicated to |
234 |
|
|
* arrange, for little benefit.) |
235 |
|
|
* |
236 |
|
|
* All enqueue/dequeue operations are handled by the single method |
237 |
|
|
* "xfer" with parameters indicating whether to act as some form |
238 |
|
|
* of offer, put, poll, take, or transfer (each possibly with |
239 |
|
|
* timeout). The relative complexity of using one monolithic |
240 |
|
|
* method outweighs the code bulk and maintenance problems of |
241 |
|
|
* using separate methods for each case. |
242 |
|
|
* |
243 |
|
|
* Operation consists of up to three phases. The first is |
244 |
|
|
* implemented within method xfer, the second in tryAppend, and |
245 |
|
|
* the third in method awaitMatch. |
246 |
|
|
* |
247 |
|
|
* 1. Try to match an existing node |
248 |
|
|
* |
249 |
|
|
* Starting at head, skip already-matched nodes until finding |
250 |
|
|
* an unmatched node of opposite mode, if one exists, in which |
251 |
|
|
* case matching it and returning, also if necessary updating |
252 |
|
|
* head to one past the matched node (or the node itself if the |
253 |
|
|
* list has no other unmatched nodes). If the CAS misses, then |
254 |
|
|
* a loop retries advancing head by two steps until either |
255 |
|
|
* success or the slack is at most two. By requiring that each |
256 |
|
|
* attempt advances head by two (if applicable), we ensure that |
257 |
|
|
* the slack does not grow without bound. Traversals also check |
258 |
|
|
* if the initial head is now off-list, in which case they |
259 |
|
|
* start at the new head. |
260 |
|
|
* |
261 |
|
|
* If no candidates are found and the call was untimed |
262 |
|
|
* poll/offer, (argument "how" is NOW) return. |
263 |
|
|
* |
264 |
|
|
* 2. Try to append a new node (method tryAppend) |
265 |
|
|
* |
266 |
|
|
* Starting at current tail pointer, find the actual last node |
267 |
|
|
* and try to append a new node (or if head was null, establish |
268 |
|
|
* the first node). Nodes can be appended only if their |
269 |
|
|
* predecessors are either already matched or are of the same |
270 |
|
|
* mode. If we detect otherwise, then a new node with opposite |
271 |
|
|
* mode must have been appended during traversal, so we must |
272 |
|
|
* restart at phase 1. The traversal and update steps are |
273 |
|
|
* otherwise similar to phase 1: Retrying upon CAS misses and |
274 |
|
|
* checking for staleness. In particular, if a self-link is |
275 |
|
|
* encountered, then we can safely jump to a node on the list |
276 |
|
|
* by continuing the traversal at current head. |
277 |
|
|
* |
278 |
|
|
* On successful append, if the call was ASYNC, return. |
279 |
|
|
* |
280 |
|
|
* 3. Await match or cancellation (method awaitMatch) |
281 |
|
|
* |
282 |
|
|
* Wait for another thread to match node; instead cancelling if |
283 |
|
|
* the current thread was interrupted or the wait timed out. On |
284 |
|
|
* multiprocessors, we use front-of-queue spinning: If a node |
285 |
|
|
* appears to be the first unmatched node in the queue, it |
286 |
|
|
* spins a bit before blocking. In either case, before blocking |
287 |
|
|
* it tries to unsplice any nodes between the current "head" |
288 |
|
|
* and the first unmatched node. |
289 |
|
|
* |
290 |
|
|
* Front-of-queue spinning vastly improves performance of |
291 |
|
|
* heavily contended queues. And so long as it is relatively |
292 |
|
|
* brief and "quiet", spinning does not much impact performance |
293 |
|
|
* of less-contended queues. During spins threads check their |
294 |
|
|
* interrupt status and generate a thread-local random number |
295 |
|
|
* to decide to occasionally perform a Thread.yield. While |
296 |
|
|
* yield has underdefined specs, we assume that might it help, |
297 |
|
|
* and will not hurt in limiting impact of spinning on busy |
298 |
|
|
* systems. We also use smaller (1/2) spins for nodes that are |
299 |
|
|
* not known to be front but whose predecessors have not |
300 |
|
|
* blocked -- these "chained" spins avoid artifacts of |
301 |
|
|
* front-of-queue rules which otherwise lead to alternating |
302 |
|
|
* nodes spinning vs blocking. Further, front threads that |
303 |
|
|
* represent phase changes (from data to request node or vice |
304 |
|
|
* versa) compared to their predecessors receive additional |
305 |
|
|
* chained spins, reflecting longer paths typically required to |
306 |
|
|
* unblock threads during phase changes. |
307 |
dl |
1.16 |
* |
308 |
|
|
* |
309 |
|
|
* ** Unlinking removed interior nodes ** |
310 |
|
|
* |
311 |
|
|
* In addition to minimizing garbage retention via self-linking |
312 |
|
|
* described above, we also unlink removed interior nodes. These |
313 |
|
|
* may arise due to timed out or interrupted waits, or calls to |
314 |
|
|
* remove(x) or Iterator.remove. Normally, given a node that was |
315 |
|
|
* at one time known to be the predecessor of some node s that is |
316 |
|
|
* to be removed, we can unsplice s by CASing the next field of |
317 |
|
|
* its predecessor if it still points to s (otherwise s must |
318 |
|
|
* already have been removed or is now offlist). But there are two |
319 |
|
|
* situations in which we cannot guarantee to make node s |
320 |
|
|
* unreachable in this way: (1) If s is the trailing node of list |
321 |
|
|
* (i.e., with null next), then it is pinned as the target node |
322 |
|
|
* for appends, so can only be removed later when other nodes are |
323 |
|
|
* appended. (2) We cannot necessarily unlink s given a |
324 |
|
|
* predecessor node that is matched (including the case of being |
325 |
|
|
* cancelled): the predecessor may already be already unspliced, |
326 |
|
|
* in which case some previous reachable node may still point to |
327 |
|
|
* s. (For further explanation see Herlihy & Shavit "The Art of |
328 |
|
|
* Multiprocessor Programming" chapter 9). Although, in both |
329 |
|
|
* cases, we can rule out the need for further action if either s |
330 |
|
|
* or its predecessor are (or can be made to be) at, or fall off |
331 |
|
|
* from, the head of list. |
332 |
|
|
* |
333 |
|
|
* Without taking these into account, it would be possible for an |
334 |
|
|
* unbounded number of supposedly removed nodes to remain |
335 |
|
|
* reachable. Situations leading to such buildup are uncommon but |
336 |
|
|
* can occur in practice; for example when a series of short timed |
337 |
|
|
* calls to poll repeatedly time out but never otherwise fall off |
338 |
|
|
* the list because of an untimed call to take at the front of the |
339 |
|
|
* queue. |
340 |
|
|
* |
341 |
|
|
* When these cases arise, rather than always retraversing the |
342 |
|
|
* entire list to find an actual predecessor to unlink (which |
343 |
|
|
* won't help for case (1) anyway), we record a conservative |
344 |
|
|
* estimate of possible unsplice failures (in "sweepVotes). We |
345 |
|
|
* trigger a full sweep when the estimate exceeds a threshold |
346 |
|
|
* indicating the maximum number of estimated removal failures to |
347 |
|
|
* tolerate before sweeping through, unlinking cancelled nodes |
348 |
|
|
* that were not unlinked upon initial removal. We perform sweeps |
349 |
|
|
* by the thread hitting threshold (rather than background threads |
350 |
|
|
* or by spreading work to other threads) because in the main |
351 |
|
|
* contexts in which removal occurs, the caller is already |
352 |
|
|
* timed-out, cancelled, or performing a potentially O(n) |
353 |
|
|
* operation (i.e., remove(x)), none of which are time-critical |
354 |
|
|
* enough to warrant the overhead that alternatives would impose |
355 |
|
|
* on other threads. |
356 |
|
|
* |
357 |
|
|
* Because the sweepVotes estimate is conservative, and because |
358 |
|
|
* nodes become unlinked "naturally" as they fall off the head of |
359 |
|
|
* the queue, and because we allow votes to accumulate even while |
360 |
|
|
* sweeps are in progress, there are typically signficantly fewer |
361 |
|
|
* such nodes than estimated. Choice of a threshold value |
362 |
|
|
* balances the likelihood of wasted effort and contention, versus |
363 |
|
|
* providing a worst-case bound on retention of interior nodes in |
364 |
|
|
* quiescent queues. The value defined below was chosen |
365 |
|
|
* empirically to balance these under various timeout scenarios. |
366 |
|
|
* |
367 |
|
|
* Note that we cannot self-link unlinked interior nodes during |
368 |
|
|
* sweeps. However, the associated garbage chains terminate when |
369 |
|
|
* some successor ultimately falls off the head of the list and is |
370 |
|
|
* self-linked. |
371 |
jsr166 |
1.8 |
*/ |
372 |
|
|
|
373 |
|
|
/** True if on multiprocessor */ |
374 |
|
|
private static final boolean MP = |
375 |
|
|
Runtime.getRuntime().availableProcessors() > 1; |
376 |
|
|
|
377 |
|
|
/** |
378 |
|
|
* The number of times to spin (with randomly interspersed calls |
379 |
|
|
* to Thread.yield) on multiprocessor before blocking when a node |
380 |
|
|
* is apparently the first waiter in the queue. See above for |
381 |
|
|
* explanation. Must be a power of two. The value is empirically |
382 |
|
|
* derived -- it works pretty well across a variety of processors, |
383 |
|
|
* numbers of CPUs, and OSes. |
384 |
|
|
*/ |
385 |
|
|
private static final int FRONT_SPINS = 1 << 7; |
386 |
|
|
|
387 |
|
|
/** |
388 |
|
|
* The number of times to spin before blocking when a node is |
389 |
|
|
* preceded by another node that is apparently spinning. Also |
390 |
|
|
* serves as an increment to FRONT_SPINS on phase changes, and as |
391 |
|
|
* base average frequency for yielding during spins. Must be a |
392 |
|
|
* power of two. |
393 |
|
|
*/ |
394 |
|
|
private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; |
395 |
|
|
|
396 |
|
|
/** |
397 |
dl |
1.16 |
* The maximum number of estimated removal failures (sweepVotes) |
398 |
|
|
* to tolerate before sweeping through the queue unlinking |
399 |
|
|
* cancelled nodes that were not unlinked upon initial |
400 |
|
|
* removal. See above for explanation. The value must be at least |
401 |
|
|
* two to avoid useless sweeps when removing trailing nodes. |
402 |
|
|
*/ |
403 |
|
|
static final int SWEEP_THRESHOLD = 32; |
404 |
|
|
|
405 |
|
|
/** |
406 |
jsr166 |
1.8 |
* Queue nodes. Uses Object, not E, for items to allow forgetting |
407 |
|
|
* them after use. Relies heavily on Unsafe mechanics to minimize |
408 |
dl |
1.16 |
* unnecessary ordering constraints: Writes that are intrinsically |
409 |
|
|
* ordered wrt other accesses or CASes use simple relaxed forms. |
410 |
jsr166 |
1.8 |
*/ |
411 |
jsr166 |
1.14 |
static final class Node { |
412 |
jsr166 |
1.8 |
final boolean isData; // false if this is a request node |
413 |
|
|
volatile Object item; // initially non-null if isData; CASed to match |
414 |
jsr166 |
1.14 |
volatile Node next; |
415 |
jsr166 |
1.8 |
volatile Thread waiter; // null until waiting |
416 |
|
|
|
417 |
|
|
// CAS methods for fields |
418 |
jsr166 |
1.14 |
final boolean casNext(Node cmp, Node val) { |
419 |
jsr166 |
1.8 |
return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
420 |
|
|
} |
421 |
jsr166 |
1.1 |
|
422 |
jsr166 |
1.8 |
final boolean casItem(Object cmp, Object val) { |
423 |
jsr166 |
1.9 |
assert cmp == null || cmp.getClass() != Node.class; |
424 |
jsr166 |
1.8 |
return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
425 |
|
|
} |
426 |
jsr166 |
1.1 |
|
427 |
jsr166 |
1.8 |
/** |
428 |
|
|
* Creates a new node. Uses relaxed write because item can only |
429 |
|
|
* be seen if followed by CAS. |
430 |
|
|
*/ |
431 |
jsr166 |
1.14 |
Node(Object item, boolean isData) { |
432 |
jsr166 |
1.8 |
UNSAFE.putObject(this, itemOffset, item); // relaxed write |
433 |
|
|
this.isData = isData; |
434 |
|
|
} |
435 |
jsr166 |
1.1 |
|
436 |
jsr166 |
1.8 |
/** |
437 |
|
|
* Links node to itself to avoid garbage retention. Called |
438 |
|
|
* only after CASing head field, so uses relaxed write. |
439 |
|
|
*/ |
440 |
|
|
final void forgetNext() { |
441 |
|
|
UNSAFE.putObject(this, nextOffset, this); |
442 |
|
|
} |
443 |
jsr166 |
1.1 |
|
444 |
jsr166 |
1.8 |
/** |
445 |
dl |
1.16 |
* Sets item to self and waiter to null, to avoid garbage |
446 |
|
|
* retention after matching or cancelling. Uses relaxed writes |
447 |
|
|
* bacause order is already constrained in the only calling |
448 |
|
|
* contexts: item is forgotten only after volatile/atomic |
449 |
|
|
* mechanics that extract items. Similarly, clearing waiter |
450 |
|
|
* follows either CAS or return from park (if ever parked; |
451 |
|
|
* else we don't care). |
452 |
jsr166 |
1.8 |
*/ |
453 |
|
|
final void forgetContents() { |
454 |
dl |
1.16 |
UNSAFE.putObject(this, itemOffset, this); |
455 |
|
|
UNSAFE.putObject(this, waiterOffset, null); |
456 |
jsr166 |
1.8 |
} |
457 |
jsr166 |
1.1 |
|
458 |
jsr166 |
1.8 |
/** |
459 |
|
|
* Returns true if this node has been matched, including the |
460 |
|
|
* case of artificial matches due to cancellation. |
461 |
|
|
*/ |
462 |
|
|
final boolean isMatched() { |
463 |
|
|
Object x = item; |
464 |
jsr166 |
1.11 |
return (x == this) || ((x == null) == isData); |
465 |
|
|
} |
466 |
|
|
|
467 |
|
|
/** |
468 |
|
|
* Returns true if this is an unmatched request node. |
469 |
|
|
*/ |
470 |
|
|
final boolean isUnmatchedRequest() { |
471 |
|
|
return !isData && item == null; |
472 |
jsr166 |
1.8 |
} |
473 |
jsr166 |
1.1 |
|
474 |
jsr166 |
1.8 |
/** |
475 |
|
|
* Returns true if a node with the given mode cannot be |
476 |
|
|
* appended to this node because this node is unmatched and |
477 |
|
|
* has opposite data mode. |
478 |
|
|
*/ |
479 |
|
|
final boolean cannotPrecede(boolean haveData) { |
480 |
|
|
boolean d = isData; |
481 |
|
|
Object x; |
482 |
|
|
return d != haveData && (x = item) != this && (x != null) == d; |
483 |
|
|
} |
484 |
jsr166 |
1.1 |
|
485 |
jsr166 |
1.8 |
/** |
486 |
|
|
* Tries to artificially match a data node -- used by remove. |
487 |
|
|
*/ |
488 |
|
|
final boolean tryMatchData() { |
489 |
jsr166 |
1.11 |
assert isData; |
490 |
jsr166 |
1.8 |
Object x = item; |
491 |
|
|
if (x != null && x != this && casItem(x, null)) { |
492 |
|
|
LockSupport.unpark(waiter); |
493 |
|
|
return true; |
494 |
|
|
} |
495 |
|
|
return false; |
496 |
jsr166 |
1.1 |
} |
497 |
|
|
|
498 |
jsr166 |
1.4 |
// Unsafe mechanics |
499 |
|
|
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe(); |
500 |
|
|
private static final long nextOffset = |
501 |
|
|
objectFieldOffset(UNSAFE, "next", Node.class); |
502 |
jsr166 |
1.8 |
private static final long itemOffset = |
503 |
|
|
objectFieldOffset(UNSAFE, "item", Node.class); |
504 |
|
|
private static final long waiterOffset = |
505 |
|
|
objectFieldOffset(UNSAFE, "waiter", Node.class); |
506 |
jsr166 |
1.1 |
|
507 |
|
|
private static final long serialVersionUID = -3375979862319811754L; |
508 |
|
|
} |
509 |
|
|
|
510 |
jsr166 |
1.8 |
/** head of the queue; null until first enqueue */ |
511 |
jsr166 |
1.14 |
transient volatile Node head; |
512 |
jsr166 |
1.8 |
|
513 |
|
|
/** tail of the queue; null until first append */ |
514 |
jsr166 |
1.14 |
private transient volatile Node tail; |
515 |
jsr166 |
1.1 |
|
516 |
dl |
1.16 |
/** The number of apparent failures to unsplice removed nodes */ |
517 |
|
|
private transient volatile int sweepVotes; |
518 |
|
|
|
519 |
jsr166 |
1.8 |
// CAS methods for fields |
520 |
jsr166 |
1.14 |
private boolean casTail(Node cmp, Node val) { |
521 |
jsr166 |
1.8 |
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
522 |
|
|
} |
523 |
jsr166 |
1.1 |
|
524 |
jsr166 |
1.14 |
private boolean casHead(Node cmp, Node val) { |
525 |
jsr166 |
1.8 |
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
526 |
|
|
} |
527 |
jsr166 |
1.1 |
|
528 |
dl |
1.16 |
private boolean casSweepVotes(int cmp, int val) { |
529 |
|
|
return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val); |
530 |
jsr166 |
1.8 |
} |
531 |
jsr166 |
1.1 |
|
532 |
jsr166 |
1.8 |
/* |
533 |
jsr166 |
1.14 |
* Possible values for "how" argument in xfer method. |
534 |
jsr166 |
1.1 |
*/ |
535 |
jsr166 |
1.14 |
private static final int NOW = 0; // for untimed poll, tryTransfer |
536 |
|
|
private static final int ASYNC = 1; // for offer, put, add |
537 |
|
|
private static final int SYNC = 2; // for transfer, take |
538 |
|
|
private static final int TIMED = 3; // for timed poll, tryTransfer |
539 |
jsr166 |
1.1 |
|
540 |
jsr166 |
1.10 |
@SuppressWarnings("unchecked") |
541 |
|
|
static <E> E cast(Object item) { |
542 |
|
|
assert item == null || item.getClass() != Node.class; |
543 |
|
|
return (E) item; |
544 |
|
|
} |
545 |
|
|
|
546 |
jsr166 |
1.1 |
/** |
547 |
jsr166 |
1.8 |
* Implements all queuing methods. See above for explanation. |
548 |
jsr166 |
1.1 |
* |
549 |
jsr166 |
1.8 |
* @param e the item or null for take |
550 |
|
|
* @param haveData true if this is a put, else a take |
551 |
jsr166 |
1.14 |
* @param how NOW, ASYNC, SYNC, or TIMED |
552 |
|
|
* @param nanos timeout in nanosecs, used only if mode is TIMED |
553 |
jsr166 |
1.8 |
* @return an item if matched, else e |
554 |
|
|
* @throws NullPointerException if haveData mode but e is null |
555 |
jsr166 |
1.1 |
*/ |
556 |
jsr166 |
1.8 |
private E xfer(E e, boolean haveData, int how, long nanos) { |
557 |
|
|
if (haveData && (e == null)) |
558 |
|
|
throw new NullPointerException(); |
559 |
jsr166 |
1.14 |
Node s = null; // the node to append, if needed |
560 |
jsr166 |
1.1 |
|
561 |
jsr166 |
1.8 |
retry: for (;;) { // restart on append race |
562 |
jsr166 |
1.1 |
|
563 |
jsr166 |
1.14 |
for (Node h = head, p = h; p != null;) { // find & match first node |
564 |
jsr166 |
1.8 |
boolean isData = p.isData; |
565 |
|
|
Object item = p.item; |
566 |
|
|
if (item != p && (item != null) == isData) { // unmatched |
567 |
|
|
if (isData == haveData) // can't match |
568 |
|
|
break; |
569 |
|
|
if (p.casItem(item, e)) { // match |
570 |
jsr166 |
1.14 |
for (Node q = p; q != h;) { |
571 |
dl |
1.16 |
Node n = q.next; // update by 2 unless singleton |
572 |
|
|
if (head == h && casHead(h, n == null? q : n)) { |
573 |
jsr166 |
1.8 |
h.forgetNext(); |
574 |
|
|
break; |
575 |
|
|
} // advance and retry |
576 |
|
|
if ((h = head) == null || |
577 |
|
|
(q = h.next) == null || !q.isMatched()) |
578 |
|
|
break; // unless slack < 2 |
579 |
|
|
} |
580 |
|
|
LockSupport.unpark(p.waiter); |
581 |
|
|
return this.<E>cast(item); |
582 |
jsr166 |
1.1 |
} |
583 |
|
|
} |
584 |
jsr166 |
1.14 |
Node n = p.next; |
585 |
jsr166 |
1.8 |
p = (p != n) ? n : (h = head); // Use head if p offlist |
586 |
|
|
} |
587 |
|
|
|
588 |
jsr166 |
1.14 |
if (how != NOW) { // No matches available |
589 |
jsr166 |
1.8 |
if (s == null) |
590 |
jsr166 |
1.14 |
s = new Node(e, haveData); |
591 |
|
|
Node pred = tryAppend(s, haveData); |
592 |
jsr166 |
1.8 |
if (pred == null) |
593 |
|
|
continue retry; // lost race vs opposite mode |
594 |
jsr166 |
1.14 |
if (how != ASYNC) |
595 |
|
|
return awaitMatch(s, pred, e, (how == TIMED), nanos); |
596 |
jsr166 |
1.1 |
} |
597 |
jsr166 |
1.8 |
return e; // not waiting |
598 |
jsr166 |
1.1 |
} |
599 |
|
|
} |
600 |
|
|
|
601 |
|
|
/** |
602 |
jsr166 |
1.8 |
* Tries to append node s as tail. |
603 |
|
|
* |
604 |
|
|
* @param s the node to append |
605 |
|
|
* @param haveData true if appending in data mode |
606 |
|
|
* @return null on failure due to losing race with append in |
607 |
|
|
* different mode, else s's predecessor, or s itself if no |
608 |
|
|
* predecessor |
609 |
jsr166 |
1.1 |
*/ |
610 |
jsr166 |
1.14 |
private Node tryAppend(Node s, boolean haveData) { |
611 |
|
|
for (Node t = tail, p = t;;) { // move p to last node and append |
612 |
|
|
Node n, u; // temps for reads of next & tail |
613 |
jsr166 |
1.8 |
if (p == null && (p = head) == null) { |
614 |
|
|
if (casHead(null, s)) |
615 |
|
|
return s; // initialize |
616 |
|
|
} |
617 |
|
|
else if (p.cannotPrecede(haveData)) |
618 |
|
|
return null; // lost race vs opposite mode |
619 |
|
|
else if ((n = p.next) != null) // not last; keep traversing |
620 |
|
|
p = p != t && t != (u = tail) ? (t = u) : // stale tail |
621 |
|
|
(p != n) ? n : null; // restart if off list |
622 |
|
|
else if (!p.casNext(null, s)) |
623 |
|
|
p = p.next; // re-read on CAS failure |
624 |
|
|
else { |
625 |
|
|
if (p != t) { // update if slack now >= 2 |
626 |
|
|
while ((tail != t || !casTail(t, s)) && |
627 |
|
|
(t = tail) != null && |
628 |
|
|
(s = t.next) != null && // advance and retry |
629 |
|
|
(s = s.next) != null && s != t); |
630 |
jsr166 |
1.1 |
} |
631 |
jsr166 |
1.8 |
return p; |
632 |
jsr166 |
1.1 |
} |
633 |
|
|
} |
634 |
|
|
} |
635 |
|
|
|
636 |
|
|
/** |
637 |
jsr166 |
1.8 |
* Spins/yields/blocks until node s is matched or caller gives up. |
638 |
jsr166 |
1.1 |
* |
639 |
|
|
* @param s the waiting node |
640 |
jsr166 |
1.8 |
* @param pred the predecessor of s, or s itself if it has no |
641 |
|
|
* predecessor, or null if unknown (the null case does not occur |
642 |
|
|
* in any current calls but may in possible future extensions) |
643 |
jsr166 |
1.1 |
* @param e the comparison value for checking match |
644 |
jsr166 |
1.14 |
* @param timed if true, wait only until timeout elapses |
645 |
|
|
* @param nanos timeout in nanosecs, used only if timed is true |
646 |
jsr166 |
1.8 |
* @return matched item, or e if unmatched on interrupt or timeout |
647 |
jsr166 |
1.1 |
*/ |
648 |
jsr166 |
1.14 |
private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) { |
649 |
|
|
long lastTime = timed ? System.nanoTime() : 0L; |
650 |
jsr166 |
1.8 |
Thread w = Thread.currentThread(); |
651 |
|
|
int spins = -1; // initialized after first item and cancel checks |
652 |
|
|
ThreadLocalRandom randomYields = null; // bound if needed |
653 |
jsr166 |
1.1 |
|
654 |
|
|
for (;;) { |
655 |
jsr166 |
1.8 |
Object item = s.item; |
656 |
|
|
if (item != e) { // matched |
657 |
|
|
assert item != s; |
658 |
|
|
s.forgetContents(); // avoid garbage |
659 |
|
|
return this.<E>cast(item); |
660 |
|
|
} |
661 |
jsr166 |
1.14 |
if ((w.isInterrupted() || (timed && nanos <= 0)) && |
662 |
dl |
1.16 |
s.casItem(e, s)) { // cancel |
663 |
jsr166 |
1.8 |
unsplice(pred, s); |
664 |
|
|
return e; |
665 |
|
|
} |
666 |
|
|
|
667 |
|
|
if (spins < 0) { // establish spins at/near front |
668 |
|
|
if ((spins = spinsFor(pred, s.isData)) > 0) |
669 |
|
|
randomYields = ThreadLocalRandom.current(); |
670 |
|
|
} |
671 |
|
|
else if (spins > 0) { // spin |
672 |
dl |
1.16 |
--spins; |
673 |
|
|
if (randomYields.nextInt(CHAINED_SPINS) == 0) |
674 |
jsr166 |
1.8 |
Thread.yield(); // occasionally yield |
675 |
|
|
} |
676 |
|
|
else if (s.waiter == null) { |
677 |
|
|
s.waiter = w; // request unpark then recheck |
678 |
jsr166 |
1.1 |
} |
679 |
jsr166 |
1.14 |
else if (timed) { |
680 |
jsr166 |
1.1 |
long now = System.nanoTime(); |
681 |
jsr166 |
1.8 |
if ((nanos -= now - lastTime) > 0) |
682 |
|
|
LockSupport.parkNanos(this, nanos); |
683 |
jsr166 |
1.1 |
lastTime = now; |
684 |
|
|
} |
685 |
jsr166 |
1.8 |
else { |
686 |
jsr166 |
1.1 |
LockSupport.park(this); |
687 |
|
|
} |
688 |
jsr166 |
1.8 |
} |
689 |
|
|
} |
690 |
|
|
|
691 |
|
|
/** |
692 |
|
|
* Returns spin/yield value for a node with given predecessor and |
693 |
|
|
* data mode. See above for explanation. |
694 |
|
|
*/ |
695 |
jsr166 |
1.14 |
private static int spinsFor(Node pred, boolean haveData) { |
696 |
jsr166 |
1.8 |
if (MP && pred != null) { |
697 |
|
|
if (pred.isData != haveData) // phase change |
698 |
|
|
return FRONT_SPINS + CHAINED_SPINS; |
699 |
|
|
if (pred.isMatched()) // probably at front |
700 |
|
|
return FRONT_SPINS; |
701 |
|
|
if (pred.waiter == null) // pred apparently spinning |
702 |
|
|
return CHAINED_SPINS; |
703 |
|
|
} |
704 |
|
|
return 0; |
705 |
|
|
} |
706 |
|
|
|
707 |
|
|
/* -------------- Traversal methods -------------- */ |
708 |
|
|
|
709 |
|
|
/** |
710 |
jsr166 |
1.14 |
* Returns the successor of p, or the head node if p.next has been |
711 |
|
|
* linked to self, which will only be true if traversing with a |
712 |
|
|
* stale pointer that is now off the list. |
713 |
|
|
*/ |
714 |
|
|
final Node succ(Node p) { |
715 |
|
|
Node next = p.next; |
716 |
|
|
return (p == next) ? head : next; |
717 |
|
|
} |
718 |
|
|
|
719 |
|
|
/** |
720 |
jsr166 |
1.8 |
* Returns the first unmatched node of the given mode, or null if |
721 |
|
|
* none. Used by methods isEmpty, hasWaitingConsumer. |
722 |
|
|
*/ |
723 |
jsr166 |
1.14 |
private Node firstOfMode(boolean isData) { |
724 |
|
|
for (Node p = head; p != null; p = succ(p)) { |
725 |
jsr166 |
1.8 |
if (!p.isMatched()) |
726 |
jsr166 |
1.14 |
return (p.isData == isData) ? p : null; |
727 |
jsr166 |
1.8 |
} |
728 |
|
|
return null; |
729 |
|
|
} |
730 |
|
|
|
731 |
|
|
/** |
732 |
|
|
* Returns the item in the first unmatched node with isData; or |
733 |
|
|
* null if none. Used by peek. |
734 |
|
|
*/ |
735 |
|
|
private E firstDataItem() { |
736 |
jsr166 |
1.14 |
for (Node p = head; p != null; p = succ(p)) { |
737 |
jsr166 |
1.8 |
Object item = p.item; |
738 |
jsr166 |
1.14 |
if (p.isData) { |
739 |
|
|
if (item != null && item != p) |
740 |
|
|
return this.<E>cast(item); |
741 |
|
|
} |
742 |
|
|
else if (item == null) |
743 |
|
|
return null; |
744 |
jsr166 |
1.8 |
} |
745 |
|
|
return null; |
746 |
|
|
} |
747 |
|
|
|
748 |
jsr166 |
1.1 |
/** |
749 |
jsr166 |
1.8 |
* Traverses and counts unmatched nodes of the given mode. |
750 |
|
|
* Used by methods size and getWaitingConsumerCount. |
751 |
jsr166 |
1.1 |
*/ |
752 |
jsr166 |
1.8 |
private int countOfMode(boolean data) { |
753 |
|
|
int count = 0; |
754 |
jsr166 |
1.14 |
for (Node p = head; p != null; ) { |
755 |
jsr166 |
1.8 |
if (!p.isMatched()) { |
756 |
|
|
if (p.isData != data) |
757 |
|
|
return 0; |
758 |
|
|
if (++count == Integer.MAX_VALUE) // saturated |
759 |
|
|
break; |
760 |
|
|
} |
761 |
jsr166 |
1.14 |
Node n = p.next; |
762 |
jsr166 |
1.8 |
if (n != p) |
763 |
|
|
p = n; |
764 |
|
|
else { |
765 |
|
|
count = 0; |
766 |
|
|
p = head; |
767 |
jsr166 |
1.1 |
} |
768 |
jsr166 |
1.8 |
} |
769 |
|
|
return count; |
770 |
|
|
} |
771 |
|
|
|
772 |
|
|
final class Itr implements Iterator<E> { |
773 |
jsr166 |
1.14 |
private Node nextNode; // next node to return item for |
774 |
|
|
private E nextItem; // the corresponding item |
775 |
|
|
private Node lastRet; // last returned node, to support remove |
776 |
|
|
private Node lastPred; // predecessor to unlink lastRet |
777 |
jsr166 |
1.8 |
|
778 |
|
|
/** |
779 |
|
|
* Moves to next node after prev, or first node if prev null. |
780 |
|
|
*/ |
781 |
jsr166 |
1.14 |
private void advance(Node prev) { |
782 |
jsr166 |
1.13 |
lastPred = lastRet; |
783 |
jsr166 |
1.8 |
lastRet = prev; |
784 |
jsr166 |
1.14 |
for (Node p = (prev == null) ? head : succ(prev); |
785 |
|
|
p != null; p = succ(p)) { |
786 |
jsr166 |
1.8 |
Object item = p.item; |
787 |
|
|
if (p.isData) { |
788 |
|
|
if (item != null && item != p) { |
789 |
|
|
nextItem = LinkedTransferQueue.this.<E>cast(item); |
790 |
|
|
nextNode = p; |
791 |
|
|
return; |
792 |
|
|
} |
793 |
|
|
} |
794 |
|
|
else if (item == null) |
795 |
|
|
break; |
796 |
jsr166 |
1.1 |
} |
797 |
jsr166 |
1.8 |
nextNode = null; |
798 |
|
|
} |
799 |
|
|
|
800 |
|
|
Itr() { |
801 |
|
|
advance(null); |
802 |
|
|
} |
803 |
|
|
|
804 |
|
|
public final boolean hasNext() { |
805 |
|
|
return nextNode != null; |
806 |
|
|
} |
807 |
|
|
|
808 |
|
|
public final E next() { |
809 |
jsr166 |
1.14 |
Node p = nextNode; |
810 |
jsr166 |
1.8 |
if (p == null) throw new NoSuchElementException(); |
811 |
|
|
E e = nextItem; |
812 |
|
|
advance(p); |
813 |
|
|
return e; |
814 |
|
|
} |
815 |
|
|
|
816 |
|
|
public final void remove() { |
817 |
jsr166 |
1.14 |
Node p = lastRet; |
818 |
jsr166 |
1.8 |
if (p == null) throw new IllegalStateException(); |
819 |
dl |
1.16 |
if (p.tryMatchData()) |
820 |
|
|
unsplice(lastPred, p); |
821 |
jsr166 |
1.1 |
} |
822 |
|
|
} |
823 |
|
|
|
824 |
jsr166 |
1.8 |
/* -------------- Removal methods -------------- */ |
825 |
|
|
|
826 |
jsr166 |
1.1 |
/** |
827 |
jsr166 |
1.8 |
* Unsplices (now or later) the given deleted/cancelled node with |
828 |
|
|
* the given predecessor. |
829 |
jsr166 |
1.1 |
* |
830 |
dl |
1.16 |
* @param pred a node that was at one time known to be the |
831 |
|
|
* predecessor of s, or null or s itself if s is/was at head |
832 |
jsr166 |
1.8 |
* @param s the node to be unspliced |
833 |
jsr166 |
1.1 |
*/ |
834 |
dl |
1.16 |
final void unsplice(Node pred, Node s) { |
835 |
|
|
s.forgetContents(); // forget unneeded fields |
836 |
jsr166 |
1.1 |
/* |
837 |
dl |
1.16 |
* See above for rationale. Briefly: if pred still points to |
838 |
|
|
* s, try to unlink s. If s cannot be unlinked, because it is |
839 |
|
|
* trailing node or pred might be unlinked, and neither pred |
840 |
|
|
* nor s are head or offlist, add to sweepVotes, and if enough |
841 |
|
|
* votes have accumulated, sweep. |
842 |
jsr166 |
1.1 |
*/ |
843 |
dl |
1.16 |
if (pred != null && pred != s && pred.next == s) { |
844 |
|
|
Node n = s.next; |
845 |
|
|
if (n == null || |
846 |
|
|
(n != s && pred.casNext(s, n) && pred.isMatched())) { |
847 |
|
|
for (;;) { // check if at, or could be, head |
848 |
|
|
Node h = head; |
849 |
|
|
if (h == pred || h == s || h == null) |
850 |
|
|
return; // at head or list empty |
851 |
|
|
if (!h.isMatched()) |
852 |
|
|
break; |
853 |
|
|
Node hn = h.next; |
854 |
|
|
if (hn == null) |
855 |
|
|
return; // now empty |
856 |
|
|
if (hn != h && casHead(h, hn)) |
857 |
|
|
h.forgetNext(); // advance head |
858 |
jsr166 |
1.8 |
} |
859 |
dl |
1.16 |
if (pred.next != pred && s.next != s) { // recheck if offlist |
860 |
|
|
for (;;) { // sweep now if enough votes |
861 |
|
|
int v = sweepVotes; |
862 |
|
|
if (v < SWEEP_THRESHOLD) { |
863 |
|
|
if (casSweepVotes(v, v + 1)) |
864 |
|
|
break; |
865 |
|
|
} |
866 |
|
|
else if (casSweepVotes(v, 0)) { |
867 |
|
|
sweep(); |
868 |
|
|
break; |
869 |
|
|
} |
870 |
|
|
} |
871 |
jsr166 |
1.12 |
} |
872 |
jsr166 |
1.1 |
} |
873 |
|
|
} |
874 |
|
|
} |
875 |
|
|
|
876 |
|
|
/** |
877 |
dl |
1.16 |
* Unlink matched nodes encountered in a traversal from head |
878 |
jsr166 |
1.1 |
*/ |
879 |
dl |
1.16 |
private void sweep() { |
880 |
|
|
Node p = head, s, n; |
881 |
|
|
while (p != null && (s = p.next) != null && (n = s.next) != null) { |
882 |
|
|
if (p == s || s == n) |
883 |
|
|
p = head; // stale |
884 |
|
|
else if (s.isMatched()) |
885 |
|
|
p.casNext(s, n); |
886 |
jsr166 |
1.8 |
else |
887 |
dl |
1.16 |
p = s; |
888 |
jsr166 |
1.8 |
} |
889 |
|
|
} |
890 |
|
|
|
891 |
|
|
/** |
892 |
|
|
* Main implementation of remove(Object) |
893 |
|
|
*/ |
894 |
|
|
private boolean findAndRemove(Object e) { |
895 |
|
|
if (e != null) { |
896 |
jsr166 |
1.14 |
for (Node pred = null, p = head; p != null; ) { |
897 |
jsr166 |
1.8 |
Object item = p.item; |
898 |
|
|
if (p.isData) { |
899 |
|
|
if (item != null && item != p && e.equals(item) && |
900 |
|
|
p.tryMatchData()) { |
901 |
|
|
unsplice(pred, p); |
902 |
|
|
return true; |
903 |
|
|
} |
904 |
|
|
} |
905 |
|
|
else if (item == null) |
906 |
|
|
break; |
907 |
|
|
pred = p; |
908 |
jsr166 |
1.11 |
if ((p = p.next) == pred) { // stale |
909 |
jsr166 |
1.8 |
pred = null; |
910 |
|
|
p = head; |
911 |
|
|
} |
912 |
|
|
} |
913 |
|
|
} |
914 |
|
|
return false; |
915 |
|
|
} |
916 |
|
|
|
917 |
|
|
|
918 |
|
|
/** |
919 |
jsr166 |
1.1 |
* Creates an initially empty {@code LinkedTransferQueue}. |
920 |
|
|
*/ |
921 |
|
|
public LinkedTransferQueue() { |
922 |
|
|
} |
923 |
|
|
|
924 |
|
|
/** |
925 |
|
|
* Creates a {@code LinkedTransferQueue} |
926 |
|
|
* initially containing the elements of the given collection, |
927 |
|
|
* added in traversal order of the collection's iterator. |
928 |
|
|
* |
929 |
|
|
* @param c the collection of elements to initially contain |
930 |
|
|
* @throws NullPointerException if the specified collection or any |
931 |
|
|
* of its elements are null |
932 |
|
|
*/ |
933 |
|
|
public LinkedTransferQueue(Collection<? extends E> c) { |
934 |
|
|
this(); |
935 |
|
|
addAll(c); |
936 |
|
|
} |
937 |
|
|
|
938 |
jsr166 |
1.4 |
/** |
939 |
jsr166 |
1.5 |
* Inserts the specified element at the tail of this queue. |
940 |
|
|
* As the queue is unbounded, this method will never block. |
941 |
|
|
* |
942 |
|
|
* @throws NullPointerException if the specified element is null |
943 |
jsr166 |
1.4 |
*/ |
944 |
jsr166 |
1.5 |
public void put(E e) { |
945 |
jsr166 |
1.8 |
xfer(e, true, ASYNC, 0); |
946 |
jsr166 |
1.1 |
} |
947 |
|
|
|
948 |
jsr166 |
1.4 |
/** |
949 |
jsr166 |
1.5 |
* Inserts the specified element at the tail of this queue. |
950 |
|
|
* As the queue is unbounded, this method will never block or |
951 |
|
|
* return {@code false}. |
952 |
|
|
* |
953 |
|
|
* @return {@code true} (as specified by |
954 |
|
|
* {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer}) |
955 |
|
|
* @throws NullPointerException if the specified element is null |
956 |
jsr166 |
1.4 |
*/ |
957 |
jsr166 |
1.5 |
public boolean offer(E e, long timeout, TimeUnit unit) { |
958 |
jsr166 |
1.8 |
xfer(e, true, ASYNC, 0); |
959 |
|
|
return true; |
960 |
jsr166 |
1.1 |
} |
961 |
|
|
|
962 |
jsr166 |
1.4 |
/** |
963 |
jsr166 |
1.5 |
* Inserts the specified element at the tail of this queue. |
964 |
|
|
* As the queue is unbounded, this method will never return {@code false}. |
965 |
|
|
* |
966 |
|
|
* @return {@code true} (as specified by |
967 |
|
|
* {@link BlockingQueue#offer(Object) BlockingQueue.offer}) |
968 |
|
|
* @throws NullPointerException if the specified element is null |
969 |
jsr166 |
1.4 |
*/ |
970 |
jsr166 |
1.1 |
public boolean offer(E e) { |
971 |
jsr166 |
1.8 |
xfer(e, true, ASYNC, 0); |
972 |
jsr166 |
1.1 |
return true; |
973 |
|
|
} |
974 |
|
|
|
975 |
jsr166 |
1.4 |
/** |
976 |
jsr166 |
1.5 |
* Inserts the specified element at the tail of this queue. |
977 |
|
|
* As the queue is unbounded, this method will never throw |
978 |
|
|
* {@link IllegalStateException} or return {@code false}. |
979 |
|
|
* |
980 |
|
|
* @return {@code true} (as specified by {@link Collection#add}) |
981 |
|
|
* @throws NullPointerException if the specified element is null |
982 |
jsr166 |
1.4 |
*/ |
983 |
jsr166 |
1.1 |
public boolean add(E e) { |
984 |
jsr166 |
1.8 |
xfer(e, true, ASYNC, 0); |
985 |
|
|
return true; |
986 |
jsr166 |
1.5 |
} |
987 |
|
|
|
988 |
|
|
/** |
989 |
jsr166 |
1.6 |
* Transfers the element to a waiting consumer immediately, if possible. |
990 |
|
|
* |
991 |
|
|
* <p>More precisely, transfers the specified element immediately |
992 |
|
|
* if there exists a consumer already waiting to receive it (in |
993 |
|
|
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
994 |
|
|
* otherwise returning {@code false} without enqueuing the element. |
995 |
jsr166 |
1.5 |
* |
996 |
|
|
* @throws NullPointerException if the specified element is null |
997 |
|
|
*/ |
998 |
|
|
public boolean tryTransfer(E e) { |
999 |
jsr166 |
1.8 |
return xfer(e, true, NOW, 0) == null; |
1000 |
jsr166 |
1.1 |
} |
1001 |
|
|
|
1002 |
jsr166 |
1.4 |
/** |
1003 |
jsr166 |
1.6 |
* Transfers the element to a consumer, waiting if necessary to do so. |
1004 |
|
|
* |
1005 |
|
|
* <p>More precisely, transfers the specified element immediately |
1006 |
|
|
* if there exists a consumer already waiting to receive it (in |
1007 |
|
|
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
1008 |
|
|
* else inserts the specified element at the tail of this queue |
1009 |
|
|
* and waits until the element is received by a consumer. |
1010 |
jsr166 |
1.5 |
* |
1011 |
|
|
* @throws NullPointerException if the specified element is null |
1012 |
jsr166 |
1.4 |
*/ |
1013 |
jsr166 |
1.1 |
public void transfer(E e) throws InterruptedException { |
1014 |
jsr166 |
1.8 |
if (xfer(e, true, SYNC, 0) != null) { |
1015 |
|
|
Thread.interrupted(); // failure possible only due to interrupt |
1016 |
jsr166 |
1.1 |
throw new InterruptedException(); |
1017 |
|
|
} |
1018 |
|
|
} |
1019 |
|
|
|
1020 |
jsr166 |
1.4 |
/** |
1021 |
jsr166 |
1.6 |
* Transfers the element to a consumer if it is possible to do so |
1022 |
|
|
* before the timeout elapses. |
1023 |
|
|
* |
1024 |
|
|
* <p>More precisely, transfers the specified element immediately |
1025 |
|
|
* if there exists a consumer already waiting to receive it (in |
1026 |
|
|
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
1027 |
|
|
* else inserts the specified element at the tail of this queue |
1028 |
|
|
* and waits until the element is received by a consumer, |
1029 |
|
|
* returning {@code false} if the specified wait time elapses |
1030 |
|
|
* before the element can be transferred. |
1031 |
jsr166 |
1.5 |
* |
1032 |
|
|
* @throws NullPointerException if the specified element is null |
1033 |
jsr166 |
1.4 |
*/ |
1034 |
jsr166 |
1.1 |
public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
1035 |
|
|
throws InterruptedException { |
1036 |
jsr166 |
1.14 |
if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) |
1037 |
jsr166 |
1.1 |
return true; |
1038 |
|
|
if (!Thread.interrupted()) |
1039 |
|
|
return false; |
1040 |
|
|
throw new InterruptedException(); |
1041 |
|
|
} |
1042 |
|
|
|
1043 |
|
|
public E take() throws InterruptedException { |
1044 |
jsr166 |
1.8 |
E e = xfer(null, false, SYNC, 0); |
1045 |
jsr166 |
1.1 |
if (e != null) |
1046 |
jsr166 |
1.5 |
return e; |
1047 |
jsr166 |
1.1 |
Thread.interrupted(); |
1048 |
|
|
throw new InterruptedException(); |
1049 |
|
|
} |
1050 |
|
|
|
1051 |
|
|
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
1052 |
jsr166 |
1.14 |
E e = xfer(null, false, TIMED, unit.toNanos(timeout)); |
1053 |
jsr166 |
1.1 |
if (e != null || !Thread.interrupted()) |
1054 |
jsr166 |
1.5 |
return e; |
1055 |
jsr166 |
1.1 |
throw new InterruptedException(); |
1056 |
|
|
} |
1057 |
|
|
|
1058 |
|
|
public E poll() { |
1059 |
jsr166 |
1.8 |
return xfer(null, false, NOW, 0); |
1060 |
jsr166 |
1.1 |
} |
1061 |
|
|
|
1062 |
jsr166 |
1.4 |
/** |
1063 |
|
|
* @throws NullPointerException {@inheritDoc} |
1064 |
|
|
* @throws IllegalArgumentException {@inheritDoc} |
1065 |
|
|
*/ |
1066 |
jsr166 |
1.1 |
public int drainTo(Collection<? super E> c) { |
1067 |
|
|
if (c == null) |
1068 |
|
|
throw new NullPointerException(); |
1069 |
|
|
if (c == this) |
1070 |
|
|
throw new IllegalArgumentException(); |
1071 |
|
|
int n = 0; |
1072 |
|
|
E e; |
1073 |
|
|
while ( (e = poll()) != null) { |
1074 |
|
|
c.add(e); |
1075 |
|
|
++n; |
1076 |
|
|
} |
1077 |
|
|
return n; |
1078 |
|
|
} |
1079 |
|
|
|
1080 |
jsr166 |
1.4 |
/** |
1081 |
|
|
* @throws NullPointerException {@inheritDoc} |
1082 |
|
|
* @throws IllegalArgumentException {@inheritDoc} |
1083 |
|
|
*/ |
1084 |
jsr166 |
1.1 |
public int drainTo(Collection<? super E> c, int maxElements) { |
1085 |
|
|
if (c == null) |
1086 |
|
|
throw new NullPointerException(); |
1087 |
|
|
if (c == this) |
1088 |
|
|
throw new IllegalArgumentException(); |
1089 |
|
|
int n = 0; |
1090 |
|
|
E e; |
1091 |
|
|
while (n < maxElements && (e = poll()) != null) { |
1092 |
|
|
c.add(e); |
1093 |
|
|
++n; |
1094 |
|
|
} |
1095 |
|
|
return n; |
1096 |
|
|
} |
1097 |
|
|
|
1098 |
jsr166 |
1.5 |
/** |
1099 |
|
|
* Returns an iterator over the elements in this queue in proper |
1100 |
|
|
* sequence, from head to tail. |
1101 |
|
|
* |
1102 |
|
|
* <p>The returned iterator is a "weakly consistent" iterator that |
1103 |
|
|
* will never throw |
1104 |
|
|
* {@link ConcurrentModificationException ConcurrentModificationException}, |
1105 |
|
|
* and guarantees to traverse elements as they existed upon |
1106 |
|
|
* construction of the iterator, and may (but is not guaranteed |
1107 |
|
|
* to) reflect any modifications subsequent to construction. |
1108 |
|
|
* |
1109 |
|
|
* @return an iterator over the elements in this queue in proper sequence |
1110 |
|
|
*/ |
1111 |
jsr166 |
1.1 |
public Iterator<E> iterator() { |
1112 |
|
|
return new Itr(); |
1113 |
|
|
} |
1114 |
|
|
|
1115 |
|
|
public E peek() { |
1116 |
jsr166 |
1.8 |
return firstDataItem(); |
1117 |
jsr166 |
1.1 |
} |
1118 |
|
|
|
1119 |
jsr166 |
1.6 |
/** |
1120 |
|
|
* Returns {@code true} if this queue contains no elements. |
1121 |
|
|
* |
1122 |
|
|
* @return {@code true} if this queue contains no elements |
1123 |
|
|
*/ |
1124 |
jsr166 |
1.1 |
public boolean isEmpty() { |
1125 |
jsr166 |
1.8 |
return firstOfMode(true) == null; |
1126 |
jsr166 |
1.1 |
} |
1127 |
|
|
|
1128 |
|
|
public boolean hasWaitingConsumer() { |
1129 |
jsr166 |
1.8 |
return firstOfMode(false) != null; |
1130 |
jsr166 |
1.1 |
} |
1131 |
|
|
|
1132 |
|
|
/** |
1133 |
|
|
* Returns the number of elements in this queue. If this queue |
1134 |
|
|
* contains more than {@code Integer.MAX_VALUE} elements, returns |
1135 |
|
|
* {@code Integer.MAX_VALUE}. |
1136 |
|
|
* |
1137 |
|
|
* <p>Beware that, unlike in most collections, this method is |
1138 |
|
|
* <em>NOT</em> a constant-time operation. Because of the |
1139 |
|
|
* asynchronous nature of these queues, determining the current |
1140 |
|
|
* number of elements requires an O(n) traversal. |
1141 |
|
|
* |
1142 |
|
|
* @return the number of elements in this queue |
1143 |
|
|
*/ |
1144 |
|
|
public int size() { |
1145 |
jsr166 |
1.8 |
return countOfMode(true); |
1146 |
jsr166 |
1.1 |
} |
1147 |
|
|
|
1148 |
|
|
public int getWaitingConsumerCount() { |
1149 |
jsr166 |
1.8 |
return countOfMode(false); |
1150 |
jsr166 |
1.1 |
} |
1151 |
|
|
|
1152 |
jsr166 |
1.6 |
/** |
1153 |
|
|
* Removes a single instance of the specified element from this queue, |
1154 |
|
|
* if it is present. More formally, removes an element {@code e} such |
1155 |
|
|
* that {@code o.equals(e)}, if this queue contains one or more such |
1156 |
|
|
* elements. |
1157 |
|
|
* Returns {@code true} if this queue contained the specified element |
1158 |
|
|
* (or equivalently, if this queue changed as a result of the call). |
1159 |
|
|
* |
1160 |
|
|
* @param o element to be removed from this queue, if present |
1161 |
|
|
* @return {@code true} if this queue changed as a result of the call |
1162 |
|
|
*/ |
1163 |
jsr166 |
1.1 |
public boolean remove(Object o) { |
1164 |
jsr166 |
1.8 |
return findAndRemove(o); |
1165 |
jsr166 |
1.1 |
} |
1166 |
|
|
|
1167 |
|
|
/** |
1168 |
jsr166 |
1.5 |
* Always returns {@code Integer.MAX_VALUE} because a |
1169 |
|
|
* {@code LinkedTransferQueue} is not capacity constrained. |
1170 |
|
|
* |
1171 |
|
|
* @return {@code Integer.MAX_VALUE} (as specified by |
1172 |
|
|
* {@link BlockingQueue#remainingCapacity()}) |
1173 |
|
|
*/ |
1174 |
|
|
public int remainingCapacity() { |
1175 |
|
|
return Integer.MAX_VALUE; |
1176 |
|
|
} |
1177 |
|
|
|
1178 |
|
|
/** |
1179 |
jsr166 |
1.8 |
* Saves the state to a stream (that is, serializes it). |
1180 |
jsr166 |
1.1 |
* |
1181 |
|
|
* @serialData All of the elements (each an {@code E}) in |
1182 |
|
|
* the proper order, followed by a null |
1183 |
|
|
* @param s the stream |
1184 |
|
|
*/ |
1185 |
|
|
private void writeObject(java.io.ObjectOutputStream s) |
1186 |
|
|
throws java.io.IOException { |
1187 |
|
|
s.defaultWriteObject(); |
1188 |
|
|
for (E e : this) |
1189 |
|
|
s.writeObject(e); |
1190 |
|
|
// Use trailing null as sentinel |
1191 |
|
|
s.writeObject(null); |
1192 |
|
|
} |
1193 |
|
|
|
1194 |
|
|
/** |
1195 |
jsr166 |
1.8 |
* Reconstitutes the Queue instance from a stream (that is, |
1196 |
|
|
* deserializes it). |
1197 |
jsr166 |
1.1 |
* |
1198 |
|
|
* @param s the stream |
1199 |
|
|
*/ |
1200 |
|
|
private void readObject(java.io.ObjectInputStream s) |
1201 |
|
|
throws java.io.IOException, ClassNotFoundException { |
1202 |
|
|
s.defaultReadObject(); |
1203 |
|
|
for (;;) { |
1204 |
|
|
@SuppressWarnings("unchecked") E item = (E) s.readObject(); |
1205 |
|
|
if (item == null) |
1206 |
|
|
break; |
1207 |
|
|
else |
1208 |
|
|
offer(item); |
1209 |
|
|
} |
1210 |
|
|
} |
1211 |
|
|
|
1212 |
jsr166 |
1.3 |
// Unsafe mechanics |
1213 |
jsr166 |
1.1 |
|
1214 |
jsr166 |
1.3 |
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe(); |
1215 |
|
|
private static final long headOffset = |
1216 |
jsr166 |
1.4 |
objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class); |
1217 |
jsr166 |
1.3 |
private static final long tailOffset = |
1218 |
jsr166 |
1.4 |
objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class); |
1219 |
dl |
1.16 |
private static final long sweepVotesOffset = |
1220 |
|
|
objectFieldOffset(UNSAFE, "sweepVotes", LinkedTransferQueue.class); |
1221 |
jsr166 |
1.4 |
|
1222 |
|
|
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
1223 |
|
|
String field, Class<?> klazz) { |
1224 |
jsr166 |
1.1 |
try { |
1225 |
jsr166 |
1.3 |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
1226 |
jsr166 |
1.1 |
} catch (NoSuchFieldException e) { |
1227 |
jsr166 |
1.3 |
// Convert Exception to corresponding Error |
1228 |
|
|
NoSuchFieldError error = new NoSuchFieldError(field); |
1229 |
jsr166 |
1.1 |
error.initCause(e); |
1230 |
|
|
throw error; |
1231 |
|
|
} |
1232 |
|
|
} |
1233 |
|
|
} |