1 /*
2 * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 # include "incls/_precompiled.incl"
26 # include "incls/_referenceProcessor.cpp.incl"
27
28 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
29 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
30 oop ReferenceProcessor::_sentinelRef = NULL;
31 const int subclasses_of_ref = REF_PHANTOM - REF_OTHER;
32
33 // List of discovered references.
34 class DiscoveredList {
35 public:
36 DiscoveredList() : _len(0), _compressed_head(0), _oop_head(NULL) { }
37 oop head() const {
38 return UseCompressedOops ? oopDesc::decode_heap_oop_not_null(_compressed_head) :
39 _oop_head;
40 }
41 HeapWord* adr_head() {
42 return UseCompressedOops ? (HeapWord*)&_compressed_head :
43 (HeapWord*)&_oop_head;
44 }
45 void set_head(oop o) {
46 if (UseCompressedOops) {
47 // Must compress the head ptr.
48 _compressed_head = oopDesc::encode_heap_oop_not_null(o);
49 } else {
50 _oop_head = o;
51 }
52 }
53 bool empty() const { return head() == ReferenceProcessor::sentinel_ref(); }
54 size_t length() { return _len; }
55 void set_length(size_t len) { _len = len; }
56 void inc_length(size_t inc) { _len += inc; assert(_len > 0, "Error"); }
57 void dec_length(size_t dec) { _len -= dec; }
58 private:
59 // Set value depending on UseCompressedOops. This could be a template class
60 // but then we have to fix all the instantiations and declarations that use this class.
61 oop _oop_head;
62 narrowOop _compressed_head;
63 size_t _len;
64 };
65
66 void referenceProcessor_init() {
67 ReferenceProcessor::init_statics();
68 }
69
70 void ReferenceProcessor::init_statics() {
71 assert(_sentinelRef == NULL, "should be initialized precisely once");
72 EXCEPTION_MARK;
73 _sentinelRef = instanceKlass::cast(
74 SystemDictionary::Reference_klass())->
75 allocate_permanent_instance(THREAD);
76
77 // Initialize the master soft ref clock.
78 java_lang_ref_SoftReference::set_clock(os::javaTimeMillis());
79
80 if (HAS_PENDING_EXCEPTION) {
81 Handle ex(THREAD, PENDING_EXCEPTION);
82 vm_exit_during_initialization(ex);
83 }
84 assert(_sentinelRef != NULL && _sentinelRef->is_oop(),
85 "Just constructed it!");
86 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
87 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
88 NOT_COMPILER2(LRUCurrentHeapPolicy());
89 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
90 vm_exit_during_initialization("Could not allocate reference policy object");
91 }
92 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
93 RefDiscoveryPolicy == ReferentBasedDiscovery,
94 "Unrecongnized RefDiscoveryPolicy");
95 }
96
97 ReferenceProcessor*
98 ReferenceProcessor::create_ref_processor(MemRegion span,
99 bool atomic_discovery,
100 bool mt_discovery,
101 BoolObjectClosure* is_alive_non_header,
102 int parallel_gc_threads,
103 bool mt_processing,
104 bool dl_needs_barrier) {
105 int mt_degree = 1;
106 if (parallel_gc_threads > 1) {
107 mt_degree = parallel_gc_threads;
108 }
109 ReferenceProcessor* rp =
110 new ReferenceProcessor(span, atomic_discovery,
111 mt_discovery, mt_degree,
112 mt_processing && (parallel_gc_threads > 0),
113 dl_needs_barrier);
114 if (rp == NULL) {
115 vm_exit_during_initialization("Could not allocate ReferenceProcessor object");
116 }
117 rp->set_is_alive_non_header(is_alive_non_header);
118 rp->setup_policy(false /* default soft ref policy */);
119 return rp;
120 }
121
122 ReferenceProcessor::ReferenceProcessor(MemRegion span,
123 bool atomic_discovery,
124 bool mt_discovery,
125 int mt_degree,
126 bool mt_processing,
127 bool discovered_list_needs_barrier) :
128 _discovering_refs(false),
129 _enqueuing_is_done(false),
130 _is_alive_non_header(NULL),
131 _discovered_list_needs_barrier(discovered_list_needs_barrier),
132 _bs(NULL),
133 _processing_is_mt(mt_processing),
134 _next_id(0)
135 {
136 _span = span;
137 _discovery_is_atomic = atomic_discovery;
138 _discovery_is_mt = mt_discovery;
139 _num_q = mt_degree;
140 _max_num_q = mt_degree;
141 _discoveredSoftRefs = NEW_C_HEAP_ARRAY(DiscoveredList, _max_num_q * subclasses_of_ref);
142 if (_discoveredSoftRefs == NULL) {
143 vm_exit_during_initialization("Could not allocated RefProc Array");
144 }
145 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
146 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
147 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
148 assert(sentinel_ref() != NULL, "_sentinelRef is NULL");
149 // Initialized all entries to _sentinelRef
150 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
151 _discoveredSoftRefs[i].set_head(sentinel_ref());
152 _discoveredSoftRefs[i].set_length(0);
153 }
154 // If we do barreirs, cache a copy of the barrier set.
155 if (discovered_list_needs_barrier) {
156 _bs = Universe::heap()->barrier_set();
157 }
158 }
159
160 #ifndef PRODUCT
161 void ReferenceProcessor::verify_no_references_recorded() {
162 guarantee(!_discovering_refs, "Discovering refs?");
163 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
164 guarantee(_discoveredSoftRefs[i].empty(),
165 "Found non-empty discovered list");
166 }
167 }
168 #endif
169
170 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
171 // Should this instead be
172 // for (int i = 0; i < subclasses_of_ref; i++_ {
173 // for (int j = 0; j < _num_q; j++) {
174 // int index = i * _max_num_q + j;
175 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
176 if (UseCompressedOops) {
177 f->do_oop((narrowOop*)_discoveredSoftRefs[i].adr_head());
178 } else {
179 f->do_oop((oop*)_discoveredSoftRefs[i].adr_head());
180 }
181 }
182 }
183
184 void ReferenceProcessor::oops_do(OopClosure* f) {
185 f->do_oop(adr_sentinel_ref());
186 }
187
188 void ReferenceProcessor::update_soft_ref_master_clock() {
189 // Update (advance) the soft ref master clock field. This must be done
190 // after processing the soft ref list.
191 jlong now = os::javaTimeMillis();
192 jlong clock = java_lang_ref_SoftReference::clock();
193 NOT_PRODUCT(
194 if (now < clock) {
195 warning("time warp: %d to %d", clock, now);
196 }
197 )
198 // In product mode, protect ourselves from system time being adjusted
199 // externally and going backward; see note in the implementation of
200 // GenCollectedHeap::time_since_last_gc() for the right way to fix
201 // this uniformly throughout the VM; see bug-id 4741166. XXX
202 if (now > clock) {
203 java_lang_ref_SoftReference::set_clock(now);
204 }
205 // Else leave clock stalled at its old value until time progresses
206 // past clock value.
207 }
208
209 void ReferenceProcessor::process_discovered_references(
210 BoolObjectClosure* is_alive,
211 OopClosure* keep_alive,
212 VoidClosure* complete_gc,
213 AbstractRefProcTaskExecutor* task_executor) {
214 NOT_PRODUCT(verify_ok_to_handle_reflists());
215
216 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
217 // Stop treating discovered references specially.
218 disable_discovery();
219
220 bool trace_time = PrintGCDetails && PrintReferenceGC;
221 // Soft references
222 {
223 TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);
224 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
225 is_alive, keep_alive, complete_gc, task_executor);
226 }
227
228 update_soft_ref_master_clock();
229
230 // Weak references
231 {
232 TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);
233 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
234 is_alive, keep_alive, complete_gc, task_executor);
235 }
236
237 // Final references
238 {
239 TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);
240 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
241 is_alive, keep_alive, complete_gc, task_executor);
242 }
243
244 // Phantom references
245 {
246 TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);
247 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
248 is_alive, keep_alive, complete_gc, task_executor);
249 }
250
251 // Weak global JNI references. It would make more sense (semantically) to
252 // traverse these simultaneously with the regular weak references above, but
253 // that is not how the JDK1.2 specification is. See #4126360. Native code can
254 // thus use JNI weak references to circumvent the phantom references and
255 // resurrect a "post-mortem" object.
256 {
257 TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);
258 if (task_executor != NULL) {
259 task_executor->set_single_threaded_mode();
260 }
261 process_phaseJNI(is_alive, keep_alive, complete_gc);
262 }
263 }
264
265 #ifndef PRODUCT
266 // Calculate the number of jni handles.
267 uint ReferenceProcessor::count_jni_refs() {
268 class AlwaysAliveClosure: public BoolObjectClosure {
269 public:
270 virtual bool do_object_b(oop obj) { return true; }
271 virtual void do_object(oop obj) { assert(false, "Don't call"); }
272 };
273
274 class CountHandleClosure: public OopClosure {
275 private:
276 int _count;
277 public:
278 CountHandleClosure(): _count(0) {}
279 void do_oop(oop* unused) { _count++; }
280 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
281 int count() { return _count; }
282 };
283 CountHandleClosure global_handle_count;
284 AlwaysAliveClosure always_alive;
285 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
286 return global_handle_count.count();
287 }
288 #endif
289
290 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
291 OopClosure* keep_alive,
292 VoidClosure* complete_gc) {
293 #ifndef PRODUCT
294 if (PrintGCDetails && PrintReferenceGC) {
295 unsigned int count = count_jni_refs();
296 gclog_or_tty->print(", %u refs", count);
297 }
298 #endif
299 JNIHandles::weak_oops_do(is_alive, keep_alive);
300 // Finally remember to keep sentinel around
301 keep_alive->do_oop(adr_sentinel_ref());
302 complete_gc->do_void();
303 }
304
305
306 template <class T>
307 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
308 AbstractRefProcTaskExecutor* task_executor) {
309
310 // Remember old value of pending references list
311 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
312 T old_pending_list_value = *pending_list_addr;
313
314 // Enqueue references that are not made active again, and
315 // clear the decks for the next collection (cycle).
316 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
317 // Do the oop-check on pending_list_addr missed in
318 // enqueue_discovered_reflist. We should probably
319 // do a raw oop_check so that future such idempotent
320 // oop_stores relying on the oop-check side-effect
321 // may be elided automatically and safely without
322 // affecting correctness.
323 oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
324
325 // Stop treating discovered references specially.
326 ref->disable_discovery();
327
328 // Return true if new pending references were added
329 return old_pending_list_value != *pending_list_addr;
330 }
331
332 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
333 NOT_PRODUCT(verify_ok_to_handle_reflists());
334 if (UseCompressedOops) {
335 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
336 } else {
337 return enqueue_discovered_ref_helper<oop>(this, task_executor);
338 }
339 }
340
341 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
342 HeapWord* pending_list_addr) {
343 // Given a list of refs linked through the "discovered" field
344 // (java.lang.ref.Reference.discovered) chain them through the
345 // "next" field (java.lang.ref.Reference.next) and prepend
346 // to the pending list.
347 if (TraceReferenceGC && PrintGCDetails) {
348 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
349 INTPTR_FORMAT, (address)refs_list.head());
350 }
351 oop obj = refs_list.head();
352 // Walk down the list, copying the discovered field into
353 // the next field and clearing it (except for the last
354 // non-sentinel object which is treated specially to avoid
355 // confusion with an active reference).
356 while (obj != sentinel_ref()) {
357 assert(obj->is_instanceRef(), "should be reference object");
358 oop next = java_lang_ref_Reference::discovered(obj);
359 if (TraceReferenceGC && PrintGCDetails) {
360 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next " INTPTR_FORMAT,
361 obj, next);
362 }
363 assert(java_lang_ref_Reference::next(obj) == NULL,
364 "The reference should not be enqueued");
365 if (next == sentinel_ref()) { // obj is last
366 // Swap refs_list into pendling_list_addr and
367 // set obj's next to what we read from pending_list_addr.
368 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
369 // Need oop_check on pending_list_addr above;
370 // see special oop-check code at the end of
371 // enqueue_discovered_reflists() further below.
372 if (old == NULL) {
373 // obj should be made to point to itself, since
374 // pending list was empty.
375 java_lang_ref_Reference::set_next(obj, obj);
376 } else {
377 java_lang_ref_Reference::set_next(obj, old);
378 }
379 } else {
380 java_lang_ref_Reference::set_next(obj, next);
381 }
382 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
383 obj = next;
384 }
385 }
386
387 // Parallel enqueue task
388 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
389 public:
390 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
391 DiscoveredList discovered_refs[],
392 HeapWord* pending_list_addr,
393 oop sentinel_ref,
394 int n_queues)
395 : EnqueueTask(ref_processor, discovered_refs,
396 pending_list_addr, sentinel_ref, n_queues)
397 { }
398
399 virtual void work(unsigned int work_id) {
400 assert(work_id < (unsigned int)_ref_processor.num_q(), "Index out-of-bounds");
401 // Simplest first cut: static partitioning.
402 int index = work_id;
403 // The increment on "index" must correspond to the maximum number of queues
404 // (n_queues) with which that ReferenceProcessor was created. That
405 // is because of the "clever" way the discovered references lists were
406 // allocated and are indexed into. That number is ParallelGCThreads
407 // currently. Assert that.
408 assert(_n_queues == (int) ParallelGCThreads, "Different number not expected");
409 for (int j = 0;
410 j < subclasses_of_ref;
411 j++, index += _n_queues) {
412 _ref_processor.enqueue_discovered_reflist(
413 _refs_lists[index], _pending_list_addr);
414 _refs_lists[index].set_head(_sentinel_ref);
415 _refs_lists[index].set_length(0);
416 }
417 }
418 };
419
420 // Enqueue references that are not made active again
421 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
422 AbstractRefProcTaskExecutor* task_executor) {
423 if (_processing_is_mt && task_executor != NULL) {
424 // Parallel code
425 RefProcEnqueueTask tsk(*this, _discoveredSoftRefs,
426 pending_list_addr, sentinel_ref(), _max_num_q);
427 task_executor->execute(tsk);
428 } else {
429 // Serial code: call the parent class's implementation
430 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
431 enqueue_discovered_reflist(_discoveredSoftRefs[i], pending_list_addr);
432 _discoveredSoftRefs[i].set_head(sentinel_ref());
433 _discoveredSoftRefs[i].set_length(0);
434 }
435 }
436 }
437
438 // Iterator for the list of discovered references.
439 class DiscoveredListIterator {
440 public:
441 inline DiscoveredListIterator(DiscoveredList& refs_list,
442 OopClosure* keep_alive,
443 BoolObjectClosure* is_alive);
444
445 // End Of List.
446 inline bool has_next() const { return _next != ReferenceProcessor::sentinel_ref(); }
447
448 // Get oop to the Reference object.
449 inline oop obj() const { return _ref; }
450
451 // Get oop to the referent object.
452 inline oop referent() const { return _referent; }
453
454 // Returns true if referent is alive.
455 inline bool is_referent_alive() const;
456
457 // Loads data for the current reference.
458 // The "allow_null_referent" argument tells us to allow for the possibility
459 // of a NULL referent in the discovered Reference object. This typically
460 // happens in the case of concurrent collectors that may have done the
461 // discovery concurrently, or interleaved, with mutator execution.
462 inline void load_ptrs(DEBUG_ONLY(bool allow_null_referent));
463
464 // Move to the next discovered reference.
465 inline void next();
466
467 // Remove the current reference from the list
468 inline void remove();
469
470 // Make the Reference object active again.
471 inline void make_active() { java_lang_ref_Reference::set_next(_ref, NULL); }
472
473 // Make the referent alive.
474 inline void make_referent_alive() {
475 if (UseCompressedOops) {
476 _keep_alive->do_oop((narrowOop*)_referent_addr);
477 } else {
478 _keep_alive->do_oop((oop*)_referent_addr);
479 }
480 }
481
482 // Update the discovered field.
483 inline void update_discovered() {
484 // First _prev_next ref actually points into DiscoveredList (gross).
485 if (UseCompressedOops) {
486 _keep_alive->do_oop((narrowOop*)_prev_next);
487 } else {
488 _keep_alive->do_oop((oop*)_prev_next);
489 }
490 }
491
492 // NULL out referent pointer.
493 inline void clear_referent() { oop_store_raw(_referent_addr, NULL); }
494
495 // Statistics
496 NOT_PRODUCT(
497 inline size_t processed() const { return _processed; }
498 inline size_t removed() const { return _removed; }
499 )
500
501 inline void move_to_next();
502
503 private:
504 DiscoveredList& _refs_list;
505 HeapWord* _prev_next;
506 oop _ref;
507 HeapWord* _discovered_addr;
508 oop _next;
509 HeapWord* _referent_addr;
510 oop _referent;
511 OopClosure* _keep_alive;
512 BoolObjectClosure* _is_alive;
513 DEBUG_ONLY(
514 oop _first_seen; // cyclic linked list check
515 )
516 NOT_PRODUCT(
517 size_t _processed;
518 size_t _removed;
519 )
520 };
521
522 inline DiscoveredListIterator::DiscoveredListIterator(DiscoveredList& refs_list,
523 OopClosure* keep_alive,
524 BoolObjectClosure* is_alive)
525 : _refs_list(refs_list),
526 _prev_next(refs_list.adr_head()),
527 _ref(refs_list.head()),
528 #ifdef ASSERT
529 _first_seen(refs_list.head()),
530 #endif
531 #ifndef PRODUCT
532 _processed(0),
533 _removed(0),
534 #endif
535 _next(refs_list.head()),
536 _keep_alive(keep_alive),
537 _is_alive(is_alive)
538 { }
539
540 inline bool DiscoveredListIterator::is_referent_alive() const {
541 return _is_alive->do_object_b(_referent);
542 }
543
544 inline void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
545 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
546 oop discovered = java_lang_ref_Reference::discovered(_ref);
547 assert(_discovered_addr && discovered->is_oop_or_null(),
548 "discovered field is bad");
549 _next = discovered;
550 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
551 _referent = java_lang_ref_Reference::referent(_ref);
552 assert(Universe::heap()->is_in_reserved_or_null(_referent),
553 "Wrong oop found in java.lang.Reference object");
554 assert(allow_null_referent ?
555 _referent->is_oop_or_null()
556 : _referent->is_oop(),
557 "bad referent");
558 }
559
560 inline void DiscoveredListIterator::next() {
561 _prev_next = _discovered_addr;
562 move_to_next();
563 }
564
565 inline void DiscoveredListIterator::remove() {
566 assert(_ref->is_oop(), "Dropping a bad reference");
567 oop_store_raw(_discovered_addr, NULL);
568 // First _prev_next ref actually points into DiscoveredList (gross).
569 if (UseCompressedOops) {
570 // Remove Reference object from list.
571 oopDesc::encode_store_heap_oop_not_null((narrowOop*)_prev_next, _next);
572 } else {
573 // Remove Reference object from list.
574 oopDesc::store_heap_oop((oop*)_prev_next, _next);
575 }
576 NOT_PRODUCT(_removed++);
577 _refs_list.dec_length(1);
578 }
579
580 inline void DiscoveredListIterator::move_to_next() {
581 _ref = _next;
582 assert(_ref != _first_seen, "cyclic ref_list found");
583 NOT_PRODUCT(_processed++);
584 }
585
586 // NOTE: process_phase*() are largely similar, and at a high level
587 // merely iterate over the extant list applying a predicate to
588 // each of its elements and possibly removing that element from the
589 // list and applying some further closures to that element.
590 // We should consider the possibility of replacing these
591 // process_phase*() methods by abstracting them into
592 // a single general iterator invocation that receives appropriate
593 // closures that accomplish this work.
594
595 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
596 // referents are not alive, but that should be kept alive for policy reasons.
597 // Keep alive the transitive closure of all such referents.
598 void
599 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
600 ReferencePolicy* policy,
601 BoolObjectClosure* is_alive,
602 OopClosure* keep_alive,
603 VoidClosure* complete_gc) {
604 assert(policy != NULL, "Must have a non-NULL policy");
605 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
606 // Decide which softly reachable refs should be kept alive.
607 while (iter.has_next()) {
608 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
609 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
610 if (referent_is_dead && !policy->should_clear_reference(iter.obj())) {
611 if (TraceReferenceGC) {
612 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
613 iter.obj(), iter.obj()->blueprint()->internal_name());
614 }
615 // Remove Reference object from list
616 iter.remove();
617 // Make the Reference object active again
618 iter.make_active();
619 // keep the referent around
620 iter.make_referent_alive();
621 iter.move_to_next();
622 } else {
623 iter.next();
624 }
625 }
626 // Close the reachable set
627 complete_gc->do_void();
628 NOT_PRODUCT(
629 if (PrintGCDetails && TraceReferenceGC) {
630 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
631 "discovered Refs by policy list " INTPTR_FORMAT,
632 iter.removed(), iter.processed(), (address)refs_list.head());
633 }
634 )
635 }
636
637 // Traverse the list and remove any Refs that are not active, or
638 // whose referents are either alive or NULL.
639 void
640 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
641 BoolObjectClosure* is_alive,
642 OopClosure* keep_alive) {
643 assert(discovery_is_atomic(), "Error");
644 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
645 while (iter.has_next()) {
646 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
647 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
648 assert(next == NULL, "Should not discover inactive Reference");
649 if (iter.is_referent_alive()) {
650 if (TraceReferenceGC) {
651 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
652 iter.obj(), iter.obj()->blueprint()->internal_name());
653 }
654 // The referent is reachable after all.
655 // Remove Reference object from list.
656 iter.remove();
657 // Update the referent pointer as necessary: Note that this
658 // should not entail any recursive marking because the
659 // referent must already have been traversed.
660 iter.make_referent_alive();
661 iter.move_to_next();
662 } else {
663 iter.next();
664 }
665 }
666 NOT_PRODUCT(
667 if (PrintGCDetails && TraceReferenceGC) {
668 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
669 "Refs in discovered list " INTPTR_FORMAT,
670 iter.removed(), iter.processed(), (address)refs_list.head());
671 }
672 )
673 }
674
675 void
676 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
677 BoolObjectClosure* is_alive,
678 OopClosure* keep_alive,
679 VoidClosure* complete_gc) {
680 assert(!discovery_is_atomic(), "Error");
681 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
682 while (iter.has_next()) {
683 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
684 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
685 oop next = java_lang_ref_Reference::next(iter.obj());
686 if ((iter.referent() == NULL || iter.is_referent_alive() ||
687 next != NULL)) {
688 assert(next->is_oop_or_null(), "bad next field");
689 // Remove Reference object from list
690 iter.remove();
691 // Trace the cohorts
692 iter.make_referent_alive();
693 if (UseCompressedOops) {
694 keep_alive->do_oop((narrowOop*)next_addr);
695 } else {
696 keep_alive->do_oop((oop*)next_addr);
697 }
698 iter.move_to_next();
699 } else {
700 iter.next();
701 }
702 }
703 // Now close the newly reachable set
704 complete_gc->do_void();
705 NOT_PRODUCT(
706 if (PrintGCDetails && TraceReferenceGC) {
707 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
708 "Refs in discovered list " INTPTR_FORMAT,
709 iter.removed(), iter.processed(), (address)refs_list.head());
710 }
711 )
712 }
713
714 // Traverse the list and process the referents, by either
715 // clearing them or keeping them (and their reachable
716 // closure) alive.
717 void
718 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
719 bool clear_referent,
720 BoolObjectClosure* is_alive,
721 OopClosure* keep_alive,
722 VoidClosure* complete_gc) {
723 ResourceMark rm;
724 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
725 while (iter.has_next()) {
726 iter.update_discovered();
727 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
728 if (clear_referent) {
729 // NULL out referent pointer
730 iter.clear_referent();
731 } else {
732 // keep the referent around
733 iter.make_referent_alive();
734 }
735 if (TraceReferenceGC) {
736 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
737 clear_referent ? "cleared " : "",
738 iter.obj(), iter.obj()->blueprint()->internal_name());
739 }
740 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
741 iter.next();
742 }
743 // Remember to keep sentinel pointer around
744 iter.update_discovered();
745 // Close the reachable set
746 complete_gc->do_void();
747 }
748
749 void
750 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
751 oop obj = refs_list.head();
752 while (obj != sentinel_ref()) {
753 oop discovered = java_lang_ref_Reference::discovered(obj);
754 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
755 obj = discovered;
756 }
757 refs_list.set_head(sentinel_ref());
758 refs_list.set_length(0);
759 }
760
761 void ReferenceProcessor::abandon_partial_discovery() {
762 // loop over the lists
763 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
764 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
765 gclog_or_tty->print_cr(
766 "\nAbandoning %s discovered list",
767 list_name(i));
768 }
769 abandon_partial_discovered_list(_discoveredSoftRefs[i]);
770 }
771 }
772
773 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
774 public:
775 RefProcPhase1Task(ReferenceProcessor& ref_processor,
776 DiscoveredList refs_lists[],
777 ReferencePolicy* policy,
778 bool marks_oops_alive)
779 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
780 _policy(policy)
781 { }
782 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
783 OopClosure& keep_alive,
784 VoidClosure& complete_gc)
785 {
786 Thread* thr = Thread::current();
787 int refs_list_index = ((WorkerThread*)thr)->id();
788 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
789 &is_alive, &keep_alive, &complete_gc);
790 }
791 private:
792 ReferencePolicy* _policy;
793 };
794
795 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
796 public:
797 RefProcPhase2Task(ReferenceProcessor& ref_processor,
798 DiscoveredList refs_lists[],
799 bool marks_oops_alive)
800 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
801 { }
802 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
803 OopClosure& keep_alive,
804 VoidClosure& complete_gc)
805 {
806 _ref_processor.process_phase2(_refs_lists[i],
807 &is_alive, &keep_alive, &complete_gc);
808 }
809 };
810
811 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
812 public:
813 RefProcPhase3Task(ReferenceProcessor& ref_processor,
814 DiscoveredList refs_lists[],
815 bool clear_referent,
816 bool marks_oops_alive)
817 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
818 _clear_referent(clear_referent)
819 { }
820 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
821 OopClosure& keep_alive,
822 VoidClosure& complete_gc)
823 {
824 // Don't use "refs_list_index" calculated in this way because
825 // balance_queues() has moved the Ref's into the first n queues.
826 // Thread* thr = Thread::current();
827 // int refs_list_index = ((WorkerThread*)thr)->id();
828 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
829 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
830 &is_alive, &keep_alive, &complete_gc);
831 }
832 private:
833 bool _clear_referent;
834 };
835
836 // Balances reference queues.
837 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
838 // queues[0, 1, ..., _num_q-1] because only the first _num_q
839 // corresponding to the active workers will be processed.
840 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
841 {
842 // calculate total length
843 size_t total_refs = 0;
844 if (TraceReferenceGC && PrintGCDetails) {
845 gclog_or_tty->print_cr("\nBalance ref_lists ");
846 }
847
848 for (int i = 0; i < _max_num_q; ++i) {
849 total_refs += ref_lists[i].length();
850 if (TraceReferenceGC && PrintGCDetails) {
851 gclog_or_tty->print("%d ", ref_lists[i].length());
852 }
853 }
854 if (TraceReferenceGC && PrintGCDetails) {
855 gclog_or_tty->print_cr(" = %d", total_refs);
856 }
857 size_t avg_refs = total_refs / _num_q + 1;
858 int to_idx = 0;
859 for (int from_idx = 0; from_idx < _max_num_q; from_idx++) {
860 bool move_all = false;
861 if (from_idx >= _num_q) {
862 move_all = ref_lists[from_idx].length() > 0;
863 }
864 while ((ref_lists[from_idx].length() > avg_refs) ||
865 move_all) {
866 assert(to_idx < _num_q, "Sanity Check!");
867 if (ref_lists[to_idx].length() < avg_refs) {
868 // move superfluous refs
869 size_t refs_to_move;
870 // Move all the Ref's if the from queue will not be processed.
871 if (move_all) {
872 refs_to_move = MIN2(ref_lists[from_idx].length(),
873 avg_refs - ref_lists[to_idx].length());
874 } else {
875 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
876 avg_refs - ref_lists[to_idx].length());
877 }
878 oop move_head = ref_lists[from_idx].head();
879 oop move_tail = move_head;
880 oop new_head = move_head;
881 // find an element to split the list on
882 for (size_t j = 0; j < refs_to_move; ++j) {
883 move_tail = new_head;
884 new_head = java_lang_ref_Reference::discovered(new_head);
885 }
886 java_lang_ref_Reference::set_discovered(move_tail, ref_lists[to_idx].head());
887 ref_lists[to_idx].set_head(move_head);
888 ref_lists[to_idx].inc_length(refs_to_move);
889 ref_lists[from_idx].set_head(new_head);
890 ref_lists[from_idx].dec_length(refs_to_move);
891 if (ref_lists[from_idx].length() == 0) {
892 break;
893 }
894 } else {
895 to_idx = (to_idx + 1) % _num_q;
896 }
897 }
898 }
899 #ifdef ASSERT
900 size_t balanced_total_refs = 0;
901 for (int i = 0; i < _max_num_q; ++i) {
902 balanced_total_refs += ref_lists[i].length();
903 if (TraceReferenceGC && PrintGCDetails) {
904 gclog_or_tty->print("%d ", ref_lists[i].length());
905 }
906 }
907 if (TraceReferenceGC && PrintGCDetails) {
908 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
909 gclog_or_tty->flush();
910 }
911 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
912 #endif
913 }
914
915 void ReferenceProcessor::balance_all_queues() {
916 balance_queues(_discoveredSoftRefs);
917 balance_queues(_discoveredWeakRefs);
918 balance_queues(_discoveredFinalRefs);
919 balance_queues(_discoveredPhantomRefs);
920 }
921
922 void
923 ReferenceProcessor::process_discovered_reflist(
924 DiscoveredList refs_lists[],
925 ReferencePolicy* policy,
926 bool clear_referent,
927 BoolObjectClosure* is_alive,
928 OopClosure* keep_alive,
929 VoidClosure* complete_gc,
930 AbstractRefProcTaskExecutor* task_executor)
931 {
932 bool mt_processing = task_executor != NULL && _processing_is_mt;
933 // If discovery used MT and a dynamic number of GC threads, then
934 // the queues must be balanced for correctness if fewer than the
935 // maximum number of queues were used. The number of queue used
936 // during discovery may be different than the number to be used
937 // for processing so don't depend of _num_q < _max_num_q as part
938 // of the test.
939 bool must_balance = _discovery_is_mt;
940
941 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
942 must_balance) {
943 balance_queues(refs_lists);
944 }
945 if (PrintReferenceGC && PrintGCDetails) {
946 size_t total = 0;
947 for (int i = 0; i < _num_q; ++i) {
948 total += refs_lists[i].length();
949 }
950 gclog_or_tty->print(", %u refs", total);
951 }
952
953 // Phase 1 (soft refs only):
954 // . Traverse the list and remove any SoftReferences whose
955 // referents are not alive, but that should be kept alive for
956 // policy reasons. Keep alive the transitive closure of all
957 // such referents.
958 if (policy != NULL) {
959 if (mt_processing) {
960 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
961 task_executor->execute(phase1);
962 } else {
963 for (int i = 0; i < _num_q; i++) {
964 process_phase1(refs_lists[i], policy,
965 is_alive, keep_alive, complete_gc);
966 }
967 }
968 } else { // policy == NULL
969 assert(refs_lists != _discoveredSoftRefs,
970 "Policy must be specified for soft references.");
971 }
972
973 // Phase 2:
974 // . Traverse the list and remove any refs whose referents are alive.
975 if (mt_processing) {
976 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
977 task_executor->execute(phase2);
978 } else {
979 for (int i = 0; i < _num_q; i++) {
980 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
981 }
982 }
983
984 // Phase 3:
985 // . Traverse the list and process referents as appropriate.
986 if (mt_processing) {
987 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
988 task_executor->execute(phase3);
989 } else {
990 for (int i = 0; i < _num_q; i++) {
991 process_phase3(refs_lists[i], clear_referent,
992 is_alive, keep_alive, complete_gc);
993 }
994 }
995 }
996
997 void ReferenceProcessor::clean_up_discovered_references() {
998 // loop over the lists
999 // Should this instead be
1000 // for (int i = 0; i < subclasses_of_ref; i++_ {
1001 // for (int j = 0; j < _num_q; j++) {
1002 // int index = i * _max_num_q + j;
1003 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
1004 if (TraceReferenceGC && PrintGCDetails && ((i % _num_q) == 0)) {
1005 gclog_or_tty->print_cr(
1006 "\nScrubbing %s discovered list of Null referents",
1007 list_name(i));
1008 }
1009 clean_up_discovered_reflist(_discoveredSoftRefs[i]);
1010 }
1011 }
1012
1013 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
1014 assert(!discovery_is_atomic(), "Else why call this method?");
1015 DiscoveredListIterator iter(refs_list, NULL, NULL);
1016 while (iter.has_next()) {
1017 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1018 oop next = java_lang_ref_Reference::next(iter.obj());
1019 assert(next->is_oop_or_null(), "bad next field");
1020 // If referent has been cleared or Reference is not active,
1021 // drop it.
1022 if (iter.referent() == NULL || next != NULL) {
1023 debug_only(
1024 if (PrintGCDetails && TraceReferenceGC) {
1025 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
1026 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
1027 " and referent: " INTPTR_FORMAT,
1028 iter.obj(), next, iter.referent());
1029 }
1030 )
1031 // Remove Reference object from list
1032 iter.remove();
1033 iter.move_to_next();
1034 } else {
1035 iter.next();
1036 }
1037 }
1038 NOT_PRODUCT(
1039 if (PrintGCDetails && TraceReferenceGC) {
1040 gclog_or_tty->print(
1041 " Removed %d Refs with NULL referents out of %d discovered Refs",
1042 iter.removed(), iter.processed());
1043 }
1044 )
1045 }
1046
1047 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1048 int id = 0;
1049 // Determine the queue index to use for this object.
1050 if (_discovery_is_mt) {
1051 // During a multi-threaded discovery phase,
1052 // each thread saves to its "own" list.
1053 Thread* thr = Thread::current();
1054 assert(thr->is_GC_task_thread(),
1055 "Dubious cast from Thread* to WorkerThread*?");
1056 id = ((WorkerThread*)thr)->id();
1057 } else {
1058 // single-threaded discovery, we save in round-robin
1059 // fashion to each of the lists.
1060 if (_processing_is_mt) {
1061 id = next_id();
1062 }
1063 }
1064 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1065
1066 // Get the discovered queue to which we will add
1067 DiscoveredList* list = NULL;
1068 switch (rt) {
1069 case REF_OTHER:
1070 // Unknown reference type, no special treatment
1071 break;
1072 case REF_SOFT:
1073 list = &_discoveredSoftRefs[id];
1074 break;
1075 case REF_WEAK:
1076 list = &_discoveredWeakRefs[id];
1077 break;
1078 case REF_FINAL:
1079 list = &_discoveredFinalRefs[id];
1080 break;
1081 case REF_PHANTOM:
1082 list = &_discoveredPhantomRefs[id];
1083 break;
1084 case REF_NONE:
1085 // we should not reach here if we are an instanceRefKlass
1086 default:
1087 ShouldNotReachHere();
1088 }
1089 if (TraceReferenceGC && PrintGCDetails) {
1090 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT,
1091 id, list);
1092 }
1093 return list;
1094 }
1095
1096 inline void
1097 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1098 oop obj,
1099 HeapWord* discovered_addr) {
1100 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1101 // First we must make sure this object is only enqueued once. CAS in a non null
1102 // discovered_addr.
1103 oop current_head = refs_list.head();
1104
1105 // Note: In the case of G1, this specific pre-barrier is strictly
1106 // not necessary because the only case we are interested in
1107 // here is when *discovered_addr is NULL (see the CAS further below),
1108 // so this will expand to nothing. As a result, we have manually
1109 // elided this out for G1, but left in the test for some future
1110 // collector that might have need for a pre-barrier here.
1111 if (_discovered_list_needs_barrier && !UseG1GC) {
1112 if (UseCompressedOops) {
1113 _bs->write_ref_field_pre((narrowOop*)discovered_addr, current_head);
1114 } else {
1115 _bs->write_ref_field_pre((oop*)discovered_addr, current_head);
1116 }
1117 guarantee(false, "Need to check non-G1 collector");
1118 }
1119 oop retest = oopDesc::atomic_compare_exchange_oop(current_head, discovered_addr,
1120 NULL);
1121 if (retest == NULL) {
1122 // This thread just won the right to enqueue the object.
1123 // We have separate lists for enqueueing so no synchronization
1124 // is necessary.
1125 refs_list.set_head(obj);
1126 refs_list.inc_length(1);
1127 if (_discovered_list_needs_barrier) {
1128 _bs->write_ref_field((void*)discovered_addr, current_head);
1129 }
1130 } else {
1131 // If retest was non NULL, another thread beat us to it:
1132 // The reference has already been discovered...
1133 if (TraceReferenceGC) {
1134 gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",
1135 obj, obj->blueprint()->internal_name());
1136 }
1137 }
1138 }
1139
1140 // We mention two of several possible choices here:
1141 // #0: if the reference object is not in the "originating generation"
1142 // (or part of the heap being collected, indicated by our "span"
1143 // we don't treat it specially (i.e. we scan it as we would
1144 // a normal oop, treating its references as strong references).
1145 // This means that references can't be enqueued unless their
1146 // referent is also in the same span. This is the simplest,
1147 // most "local" and most conservative approach, albeit one
1148 // that may cause weak references to be enqueued least promptly.
1149 // We call this choice the "ReferenceBasedDiscovery" policy.
1150 // #1: the reference object may be in any generation (span), but if
1151 // the referent is in the generation (span) being currently collected
1152 // then we can discover the reference object, provided
1153 // the object has not already been discovered by
1154 // a different concurrently running collector (as may be the
1155 // case, for instance, if the reference object is in CMS and
1156 // the referent in DefNewGeneration), and provided the processing
1157 // of this reference object by the current collector will
1158 // appear atomic to every other collector in the system.
1159 // (Thus, for instance, a concurrent collector may not
1160 // discover references in other generations even if the
1161 // referent is in its own generation). This policy may,
1162 // in certain cases, enqueue references somewhat sooner than
1163 // might Policy #0 above, but at marginally increased cost
1164 // and complexity in processing these references.
1165 // We call this choice the "RefeferentBasedDiscovery" policy.
1166 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1167 // We enqueue references only if we are discovering refs
1168 // (rather than processing discovered refs).
1169 if (!_discovering_refs || !RegisterReferences) {
1170 return false;
1171 }
1172 // We only enqueue active references.
1173 oop next = java_lang_ref_Reference::next(obj);
1174 if (next != NULL) {
1175 return false;
1176 }
1177
1178 HeapWord* obj_addr = (HeapWord*)obj;
1179 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1180 !_span.contains(obj_addr)) {
1181 // Reference is not in the originating generation;
1182 // don't treat it specially (i.e. we want to scan it as a normal
1183 // object with strong references).
1184 return false;
1185 }
1186
1187 // We only enqueue references whose referents are not (yet) strongly
1188 // reachable.
1189 if (is_alive_non_header() != NULL) {
1190 oop referent = java_lang_ref_Reference::referent(obj);
1191 // In the case of non-concurrent discovery, the last
1192 // disjunct below should hold. It may not hold in the
1193 // case of concurrent discovery because mutators may
1194 // concurrently clear() a Reference.
1195 assert(UseConcMarkSweepGC || UseG1GC || referent != NULL,
1196 "Refs with null referents already filtered");
1197 if (is_alive_non_header()->do_object_b(referent)) {
1198 return false; // referent is reachable
1199 }
1200 }
1201 if (rt == REF_SOFT) {
1202 // For soft refs we can decide now if these are not
1203 // current candidates for clearing, in which case we
1204 // can mark through them now, rather than delaying that
1205 // to the reference-processing phase. Since all current
1206 // time-stamp policies advance the soft-ref clock only
1207 // at a major collection cycle, this is always currently
1208 // accurate.
1209 if (!_current_soft_ref_policy->should_clear_reference(obj)) {
1210 return false;
1211 }
1212 }
1213
1214 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1215 const oop discovered = java_lang_ref_Reference::discovered(obj);
1216 assert(discovered->is_oop_or_null(), "bad discovered field");
1217 if (discovered != NULL) {
1218 // The reference has already been discovered...
1219 if (TraceReferenceGC) {
1220 gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",
1221 obj, obj->blueprint()->internal_name());
1222 }
1223 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1224 // assumes that an object is not processed twice;
1225 // if it's been already discovered it must be on another
1226 // generation's discovered list; so we won't discover it.
1227 return false;
1228 } else {
1229 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1230 "Unrecognized policy");
1231 // Check assumption that an object is not potentially
1232 // discovered twice except by concurrent collectors that potentially
1233 // trace the same Reference object twice.
1234 assert(UseConcMarkSweepGC,
1235 "Only possible with an incremental-update concurrent collector");
1236 return true;
1237 }
1238 }
1239
1240 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1241 oop referent = java_lang_ref_Reference::referent(obj);
1242 assert(referent->is_oop(), "bad referent");
1243 // enqueue if and only if either:
1244 // reference is in our span or
1245 // we are an atomic collector and referent is in our span
1246 if (_span.contains(obj_addr) ||
1247 (discovery_is_atomic() && _span.contains(referent))) {
1248 // should_enqueue = true;
1249 } else {
1250 return false;
1251 }
1252 } else {
1253 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1254 _span.contains(obj_addr), "code inconsistency");
1255 }
1256
1257 // Get the right type of discovered queue head.
1258 DiscoveredList* list = get_discovered_list(rt);
1259 if (list == NULL) {
1260 return false; // nothing special needs to be done
1261 }
1262
1263 if (_discovery_is_mt) {
1264 add_to_discovered_list_mt(*list, obj, discovered_addr);
1265 } else {
1266 // If "_discovered_list_needs_barrier", we do write barriers when
1267 // updating the discovered reference list. Otherwise, we do a raw store
1268 // here: the field will be visited later when processing the discovered
1269 // references.
1270 oop current_head = list->head();
1271 // As in the case further above, since we are over-writing a NULL
1272 // pre-value, we can safely elide the pre-barrier here for the case of G1.
1273 assert(discovered == NULL, "control point invariant");
1274 if (_discovered_list_needs_barrier && !UseG1GC) { // safe to elide for G1
1275 if (UseCompressedOops) {
1276 _bs->write_ref_field_pre((narrowOop*)discovered_addr, current_head);
1277 } else {
1278 _bs->write_ref_field_pre((oop*)discovered_addr, current_head);
1279 }
1280 guarantee(false, "Need to check non-G1 collector");
1281 }
1282 oop_store_raw(discovered_addr, current_head);
1283 if (_discovered_list_needs_barrier) {
1284 _bs->write_ref_field((void*)discovered_addr, current_head);
1285 }
1286 list->set_head(obj);
1287 list->inc_length(1);
1288 }
1289
1290 // In the MT discovery case, it is currently possible to see
1291 // the following message multiple times if several threads
1292 // discover a reference about the same time. Only one will
1293 // however have actually added it to the disocvered queue.
1294 // One could let add_to_discovered_list_mt() return an
1295 // indication for success in queueing (by 1 thread) or
1296 // failure (by all other threads), but I decided the extra
1297 // code was not worth the effort for something that is
1298 // only used for debugging support.
1299 if (TraceReferenceGC) {
1300 oop referent = java_lang_ref_Reference::referent(obj);
1301 if (PrintGCDetails) {
1302 gclog_or_tty->print_cr("Enqueued reference (" INTPTR_FORMAT ": %s)",
1303 obj, obj->blueprint()->internal_name());
1304 }
1305 assert(referent->is_oop(), "Enqueued a bad referent");
1306 }
1307 assert(obj->is_oop(), "Enqueued a bad reference");
1308 return true;
1309 }
1310
1311 // Preclean the discovered references by removing those
1312 // whose referents are alive, and by marking from those that
1313 // are not active. These lists can be handled here
1314 // in any order and, indeed, concurrently.
1315 void ReferenceProcessor::preclean_discovered_references(
1316 BoolObjectClosure* is_alive,
1317 OopClosure* keep_alive,
1318 VoidClosure* complete_gc,
1319 YieldClosure* yield,
1320 bool should_unload_classes) {
1321
1322 NOT_PRODUCT(verify_ok_to_handle_reflists());
1323
1324 #ifdef ASSERT
1325 bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC ||
1326 CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
1327 ExplicitGCInvokesConcurrentAndUnloadsClasses &&
1328 UseConcMarkSweepGC && should_unload_classes;
1329 RememberKlassesChecker mx(must_remember_klasses);
1330 #endif
1331 // Soft references
1332 {
1333 TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1334 false, gclog_or_tty);
1335 for (int i = 0; i < _max_num_q; i++) {
1336 if (yield->should_return()) {
1337 return;
1338 }
1339 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1340 keep_alive, complete_gc, yield);
1341 }
1342 }
1343
1344 // Weak references
1345 {
1346 TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1347 false, gclog_or_tty);
1348 for (int i = 0; i < _num_q; i++) {
1349 if (yield->should_return()) {
1350 return;
1351 }
1352 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1353 keep_alive, complete_gc, yield);
1354 }
1355 }
1356
1357 // Final references
1358 {
1359 TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1360 false, gclog_or_tty);
1361 for (int i = 0; i < _num_q; i++) {
1362 if (yield->should_return()) {
1363 return;
1364 }
1365 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1366 keep_alive, complete_gc, yield);
1367 }
1368 }
1369
1370 // Phantom references
1371 {
1372 TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1373 false, gclog_or_tty);
1374 for (int i = 0; i < _num_q; i++) {
1375 if (yield->should_return()) {
1376 return;
1377 }
1378 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1379 keep_alive, complete_gc, yield);
1380 }
1381 }
1382 }
1383
1384 // Walk the given discovered ref list, and remove all reference objects
1385 // whose referents are still alive, whose referents are NULL or which
1386 // are not active (have a non-NULL next field). NOTE: When we are
1387 // thus precleaning the ref lists (which happens single-threaded today),
1388 // we do not disable refs discovery to honour the correct semantics of
1389 // java.lang.Reference. As a result, we need to be careful below
1390 // that ref removal steps interleave safely with ref discovery steps
1391 // (in this thread).
1392 void
1393 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1394 BoolObjectClosure* is_alive,
1395 OopClosure* keep_alive,
1396 VoidClosure* complete_gc,
1397 YieldClosure* yield) {
1398 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1399 while (iter.has_next()) {
1400 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1401 oop obj = iter.obj();
1402 oop next = java_lang_ref_Reference::next(obj);
1403 if (iter.referent() == NULL || iter.is_referent_alive() ||
1404 next != NULL) {
1405 // The referent has been cleared, or is alive, or the Reference is not
1406 // active; we need to trace and mark its cohort.
1407 if (TraceReferenceGC) {
1408 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1409 iter.obj(), iter.obj()->blueprint()->internal_name());
1410 }
1411 // Remove Reference object from list
1412 iter.remove();
1413 // Keep alive its cohort.
1414 iter.make_referent_alive();
1415 if (UseCompressedOops) {
1416 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1417 keep_alive->do_oop(next_addr);
1418 } else {
1419 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1420 keep_alive->do_oop(next_addr);
1421 }
1422 iter.move_to_next();
1423 } else {
1424 iter.next();
1425 }
1426 }
1427 // Close the reachable set
1428 complete_gc->do_void();
1429
1430 NOT_PRODUCT(
1431 if (PrintGCDetails && PrintReferenceGC) {
1432 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1433 "Refs in discovered list " INTPTR_FORMAT,
1434 iter.removed(), iter.processed(), (address)refs_list.head());
1435 }
1436 )
1437 }
1438
1439 const char* ReferenceProcessor::list_name(int i) {
1440 assert(i >= 0 && i <= _max_num_q * subclasses_of_ref, "Out of bounds index");
1441 int j = i / _max_num_q;
1442 switch (j) {
1443 case 0: return "SoftRef";
1444 case 1: return "WeakRef";
1445 case 2: return "FinalRef";
1446 case 3: return "PhantomRef";
1447 }
1448 ShouldNotReachHere();
1449 return NULL;
1450 }
1451
1452 #ifndef PRODUCT
1453 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1454 // empty for now
1455 }
1456 #endif
1457
1458 void ReferenceProcessor::verify() {
1459 guarantee(sentinel_ref() != NULL && sentinel_ref()->is_oop(), "Lost _sentinelRef");
1460 }
1461
1462 #ifndef PRODUCT
1463 void ReferenceProcessor::clear_discovered_references() {
1464 guarantee(!_discovering_refs, "Discovering refs?");
1465 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
1466 oop obj = _discoveredSoftRefs[i].head();
1467 while (obj != sentinel_ref()) {
1468 oop next = java_lang_ref_Reference::discovered(obj);
1469 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
1470 obj = next;
1471 }
1472 _discoveredSoftRefs[i].set_head(sentinel_ref());
1473 _discoveredSoftRefs[i].set_length(0);
1474 }
1475 }
1476 #endif // PRODUCT