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