1 /*
2 * Copyright (c) 2001, 2011, 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
40 bool DiscoveredList::empty() const {
41 return head() == ReferenceProcessor::sentinel_ref();
42 }
43
44 void referenceProcessor_init() {
45 ReferenceProcessor::init_statics();
46 }
47
48 void ReferenceProcessor::init_statics() {
49 assert(_sentinelRef == NULL, "should be initialized precisely once");
50 EXCEPTION_MARK;
51 _sentinelRef = instanceKlass::cast(
52 SystemDictionary::Reference_klass())->
53 allocate_permanent_instance(THREAD);
54
55 // Initialize the master soft ref clock.
56 java_lang_ref_SoftReference::set_clock(os::javaTimeMillis());
57
58 if (HAS_PENDING_EXCEPTION) {
59 Handle ex(THREAD, PENDING_EXCEPTION);
60 vm_exit_during_initialization(ex);
61 }
62 assert(_sentinelRef != NULL && _sentinelRef->is_oop(),
63 "Just constructed it!");
64 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
65 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
66 NOT_COMPILER2(LRUCurrentHeapPolicy());
67 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
68 vm_exit_during_initialization("Could not allocate reference policy object");
69 }
70 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
71 RefDiscoveryPolicy == ReferentBasedDiscovery,
72 "Unrecongnized RefDiscoveryPolicy");
73 }
74
75 ReferenceProcessor::ReferenceProcessor(MemRegion span,
76 bool mt_processing,
77 int mt_processing_degree,
78 bool mt_discovery,
79 int mt_discovery_degree,
80 bool atomic_discovery,
81 BoolObjectClosure* is_alive_non_header,
82 bool discovered_list_needs_barrier) :
83 _discovering_refs(false),
84 _enqueuing_is_done(false),
85 _is_alive_non_header(is_alive_non_header),
86 _discovered_list_needs_barrier(discovered_list_needs_barrier),
87 _bs(NULL),
88 _processing_is_mt(mt_processing),
89 _next_id(0)
90 {
91 _span = span;
92 _discovery_is_atomic = atomic_discovery;
93 _discovery_is_mt = mt_discovery;
94 _num_q = MAX2(1, mt_processing_degree);
95 _max_num_q = MAX2(_num_q, mt_discovery_degree);
96 _discoveredSoftRefs = NEW_C_HEAP_ARRAY(DiscoveredList, _max_num_q * subclasses_of_ref());
97 if (_discoveredSoftRefs == NULL) {
98 vm_exit_during_initialization("Could not allocated RefProc Array");
99 }
100 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
101 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
102 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
103 assert(sentinel_ref() != NULL, "_sentinelRef is NULL");
104 // Initialized all entries to _sentinelRef
105 for (int i = 0; i < _max_num_q * subclasses_of_ref(); i++) {
106 _discoveredSoftRefs[i].set_head(sentinel_ref());
107 _discoveredSoftRefs[i].set_length(0);
108 }
109 // If we do barriers, cache a copy of the barrier set.
110 if (discovered_list_needs_barrier) {
111 _bs = Universe::heap()->barrier_set();
112 }
113 setup_policy(false /* default soft ref policy */);
114 }
115
116 #ifndef PRODUCT
117 void ReferenceProcessor::verify_no_references_recorded() {
118 guarantee(!_discovering_refs, "Discovering refs?");
119 for (int i = 0; i < _max_num_q * subclasses_of_ref(); i++) {
120 guarantee(_discoveredSoftRefs[i].empty(),
121 "Found non-empty discovered list");
122 }
123 }
124 #endif
125
126 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
127 // Should this instead be
128 // for (int i = 0; i < subclasses_of_ref(); i++_ {
129 // for (int j = 0; j < _num_q; j++) {
130 // int index = i * _max_num_q + j;
131 for (int i = 0; i < _max_num_q * subclasses_of_ref(); i++) {
132 if (UseCompressedOops) {
133 f->do_oop((narrowOop*)_discoveredSoftRefs[i].adr_head());
134 } else {
135 f->do_oop((oop*)_discoveredSoftRefs[i].adr_head());
136 }
137 }
138 }
139
140 void ReferenceProcessor::oops_do(OopClosure* f) {
141 f->do_oop(adr_sentinel_ref());
142 }
143
144 void ReferenceProcessor::update_soft_ref_master_clock() {
145 // Update (advance) the soft ref master clock field. This must be done
146 // after processing the soft ref list.
147 jlong now = os::javaTimeMillis();
148 jlong clock = java_lang_ref_SoftReference::clock();
149 NOT_PRODUCT(
150 if (now < clock) {
151 warning("time warp: %d to %d", clock, now);
152 }
153 )
154 // In product mode, protect ourselves from system time being adjusted
155 // externally and going backward; see note in the implementation of
156 // GenCollectedHeap::time_since_last_gc() for the right way to fix
157 // this uniformly throughout the VM; see bug-id 4741166. XXX
158 if (now > clock) {
159 java_lang_ref_SoftReference::set_clock(now);
160 }
161 // Else leave clock stalled at its old value until time progresses
162 // past clock value.
163 }
164
165 void ReferenceProcessor::process_discovered_references(
166 BoolObjectClosure* is_alive,
167 OopClosure* keep_alive,
168 VoidClosure* complete_gc,
169 AbstractRefProcTaskExecutor* task_executor) {
170 NOT_PRODUCT(verify_ok_to_handle_reflists());
171
172 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
173 // Stop treating discovered references specially.
174 disable_discovery();
175
176 bool trace_time = PrintGCDetails && PrintReferenceGC;
177 // Soft references
178 {
179 TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);
180 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
181 is_alive, keep_alive, complete_gc, task_executor);
182 }
183
184 update_soft_ref_master_clock();
185
186 // Weak references
187 {
188 TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);
189 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
190 is_alive, keep_alive, complete_gc, task_executor);
191 }
192
193 // Final references
194 {
195 TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);
196 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
197 is_alive, keep_alive, complete_gc, task_executor);
198 }
199
200 // Phantom references
201 {
202 TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);
203 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
204 is_alive, keep_alive, complete_gc, task_executor);
205 }
206
207 // Weak global JNI references. It would make more sense (semantically) to
208 // traverse these simultaneously with the regular weak references above, but
209 // that is not how the JDK1.2 specification is. See #4126360. Native code can
210 // thus use JNI weak references to circumvent the phantom references and
211 // resurrect a "post-mortem" object.
212 {
213 TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);
214 if (task_executor != NULL) {
215 task_executor->set_single_threaded_mode();
216 }
217 process_phaseJNI(is_alive, keep_alive, complete_gc);
218 }
219 }
220
221 #ifndef PRODUCT
222 // Calculate the number of jni handles.
223 uint ReferenceProcessor::count_jni_refs() {
224 class AlwaysAliveClosure: public BoolObjectClosure {
225 public:
226 virtual bool do_object_b(oop obj) { return true; }
227 virtual void do_object(oop obj) { assert(false, "Don't call"); }
228 };
229
230 class CountHandleClosure: public OopClosure {
231 private:
232 int _count;
233 public:
234 CountHandleClosure(): _count(0) {}
235 void do_oop(oop* unused) { _count++; }
236 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
237 int count() { return _count; }
238 };
239 CountHandleClosure global_handle_count;
240 AlwaysAliveClosure always_alive;
241 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
242 return global_handle_count.count();
243 }
244 #endif
245
246 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
247 OopClosure* keep_alive,
248 VoidClosure* complete_gc) {
249 #ifndef PRODUCT
250 if (PrintGCDetails && PrintReferenceGC) {
251 unsigned int count = count_jni_refs();
252 gclog_or_tty->print(", %u refs", count);
253 }
254 #endif
255 JNIHandles::weak_oops_do(is_alive, keep_alive);
256 // Finally remember to keep sentinel around
257 keep_alive->do_oop(adr_sentinel_ref());
258 complete_gc->do_void();
259 }
260
261
262 template <class T>
263 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
264 AbstractRefProcTaskExecutor* task_executor) {
265
266 // Remember old value of pending references list
267 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
268 T old_pending_list_value = *pending_list_addr;
269
270 // Enqueue references that are not made active again, and
271 // clear the decks for the next collection (cycle).
272 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
273 // Do the oop-check on pending_list_addr missed in
274 // enqueue_discovered_reflist. We should probably
275 // do a raw oop_check so that future such idempotent
276 // oop_stores relying on the oop-check side-effect
277 // may be elided automatically and safely without
278 // affecting correctness.
279 oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
280
281 // Stop treating discovered references specially.
282 ref->disable_discovery();
283
284 // Return true if new pending references were added
285 return old_pending_list_value != *pending_list_addr;
286 }
287
288 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
289 NOT_PRODUCT(verify_ok_to_handle_reflists());
290 if (UseCompressedOops) {
291 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
292 } else {
293 return enqueue_discovered_ref_helper<oop>(this, task_executor);
294 }
295 }
296
297 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
298 HeapWord* pending_list_addr) {
299 // Given a list of refs linked through the "discovered" field
300 // (java.lang.ref.Reference.discovered) chain them through the
301 // "next" field (java.lang.ref.Reference.next) and prepend
302 // to the pending list.
303 if (TraceReferenceGC && PrintGCDetails) {
304 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
305 INTPTR_FORMAT, (address)refs_list.head());
306 }
307 oop obj = refs_list.head();
308 // Walk down the list, copying the discovered field into
309 // the next field and clearing it (except for the last
310 // non-sentinel object which is treated specially to avoid
311 // confusion with an active reference).
312 while (obj != sentinel_ref()) {
313 assert(obj->is_instanceRef(), "should be reference object");
314 oop next = java_lang_ref_Reference::discovered(obj);
315 if (TraceReferenceGC && PrintGCDetails) {
316 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next " INTPTR_FORMAT,
317 obj, next);
318 }
319 assert(java_lang_ref_Reference::next(obj) == NULL,
320 "The reference should not be enqueued");
321 if (next == sentinel_ref()) { // obj is last
322 // Swap refs_list into pendling_list_addr and
323 // set obj's next to what we read from pending_list_addr.
324 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
325 // Need oop_check on pending_list_addr above;
326 // see special oop-check code at the end of
327 // enqueue_discovered_reflists() further below.
328 if (old == NULL) {
329 // obj should be made to point to itself, since
330 // pending list was empty.
331 java_lang_ref_Reference::set_next(obj, obj);
332 } else {
333 java_lang_ref_Reference::set_next(obj, old);
334 }
335 } else {
336 java_lang_ref_Reference::set_next(obj, next);
337 }
338 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
339 obj = next;
340 }
341 }
342
343 // Parallel enqueue task
344 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
345 public:
346 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
347 DiscoveredList discovered_refs[],
348 HeapWord* pending_list_addr,
349 oop sentinel_ref,
350 int n_queues)
351 : EnqueueTask(ref_processor, discovered_refs,
352 pending_list_addr, sentinel_ref, n_queues)
353 { }
354
355 virtual void work(unsigned int work_id) {
356 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
357 // Simplest first cut: static partitioning.
358 int index = work_id;
359 // The increment on "index" must correspond to the maximum number of queues
360 // (n_queues) with which that ReferenceProcessor was created. That
361 // is because of the "clever" way the discovered references lists were
362 // allocated and are indexed into.
363 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
364 for (int j = 0;
365 j < ReferenceProcessor::subclasses_of_ref();
366 j++, index += _n_queues) {
367 _ref_processor.enqueue_discovered_reflist(
368 _refs_lists[index], _pending_list_addr);
369 _refs_lists[index].set_head(_sentinel_ref);
370 _refs_lists[index].set_length(0);
371 }
372 }
373 };
374
375 // Enqueue references that are not made active again
376 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
377 AbstractRefProcTaskExecutor* task_executor) {
378 if (_processing_is_mt && task_executor != NULL) {
379 // Parallel code
380 RefProcEnqueueTask tsk(*this, _discoveredSoftRefs,
381 pending_list_addr, sentinel_ref(), _max_num_q);
382 task_executor->execute(tsk);
383 } else {
384 // Serial code: call the parent class's implementation
385 for (int i = 0; i < _max_num_q * subclasses_of_ref(); i++) {
386 enqueue_discovered_reflist(_discoveredSoftRefs[i], pending_list_addr);
387 _discoveredSoftRefs[i].set_head(sentinel_ref());
388 _discoveredSoftRefs[i].set_length(0);
389 }
390 }
391 }
392
393 // Iterator for the list of discovered references.
394 class DiscoveredListIterator {
395 public:
396 inline DiscoveredListIterator(DiscoveredList& refs_list,
397 OopClosure* keep_alive,
398 BoolObjectClosure* is_alive);
399
400 // End Of List.
401 inline bool has_next() const { return _next != ReferenceProcessor::sentinel_ref(); }
402
403 // Get oop to the Reference object.
404 inline oop obj() const { return _ref; }
405
406 // Get oop to the referent object.
407 inline oop referent() const { return _referent; }
408
409 // Returns true if referent is alive.
410 inline bool is_referent_alive() const;
411
412 // Loads data for the current reference.
413 // The "allow_null_referent" argument tells us to allow for the possibility
414 // of a NULL referent in the discovered Reference object. This typically
415 // happens in the case of concurrent collectors that may have done the
416 // discovery concurrently, or interleaved, with mutator execution.
417 inline void load_ptrs(DEBUG_ONLY(bool allow_null_referent));
418
419 // Move to the next discovered reference.
420 inline void next();
421
422 // Remove the current reference from the list
423 inline void remove();
424
425 // Make the Reference object active again.
426 inline void make_active() {
427 // For G1 we don't want to use set_next - it
428 // will dirty the card for the next field of
429 // the reference object and will fail
430 // CT verification.
431 if (UseG1GC) {
432 BarrierSet* bs = oopDesc::bs();
433 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
434
435 if (UseCompressedOops) {
436 bs->write_ref_field_pre((narrowOop*)next_addr, NULL);
437 } else {
438 bs->write_ref_field_pre((oop*)next_addr, NULL);
439 }
440 java_lang_ref_Reference::set_next_raw(_ref, NULL);
441 } else {
442 java_lang_ref_Reference::set_next(_ref, NULL);
443 }
444 }
445
446 // Make the referent alive.
447 inline void make_referent_alive() {
448 if (UseCompressedOops) {
449 _keep_alive->do_oop((narrowOop*)_referent_addr);
450 } else {
451 _keep_alive->do_oop((oop*)_referent_addr);
452 }
453 }
454
455 // Update the discovered field.
456 inline void update_discovered() {
457 // First _prev_next ref actually points into DiscoveredList (gross).
458 if (UseCompressedOops) {
459 _keep_alive->do_oop((narrowOop*)_prev_next);
460 } else {
461 _keep_alive->do_oop((oop*)_prev_next);
462 }
463 }
464
465 // NULL out referent pointer.
466 inline void clear_referent() { oop_store_raw(_referent_addr, NULL); }
467
468 // Statistics
469 NOT_PRODUCT(
470 inline size_t processed() const { return _processed; }
471 inline size_t removed() const { return _removed; }
472 )
473
474 inline void move_to_next();
475
476 private:
477 DiscoveredList& _refs_list;
478 HeapWord* _prev_next;
479 oop _ref;
480 HeapWord* _discovered_addr;
481 oop _next;
482 HeapWord* _referent_addr;
483 oop _referent;
484 OopClosure* _keep_alive;
485 BoolObjectClosure* _is_alive;
486
487 DEBUG_ONLY(
488 oop _first_seen; // cyclic linked list check
489 )
490
491 NOT_PRODUCT(
492 size_t _processed;
493 size_t _removed;
494 )
495 };
496
497 inline DiscoveredListIterator::DiscoveredListIterator(DiscoveredList& refs_list,
498 OopClosure* keep_alive,
499 BoolObjectClosure* is_alive) :
500 _refs_list(refs_list),
501 _prev_next(refs_list.adr_head()),
502 _ref(refs_list.head()),
503 #ifdef ASSERT
504 _first_seen(refs_list.head()),
505 #endif
506 #ifndef PRODUCT
507 _processed(0),
508 _removed(0),
509 #endif
510 _next(refs_list.head()),
511 _keep_alive(keep_alive),
512 _is_alive(is_alive)
513 { }
514
515 inline bool DiscoveredListIterator::is_referent_alive() const {
516 return _is_alive->do_object_b(_referent);
517 }
518
519 inline void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
520 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
521 oop discovered = java_lang_ref_Reference::discovered(_ref);
522 assert(_discovered_addr && discovered->is_oop_or_null(),
523 "discovered field is bad");
524 _next = discovered;
525 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
526 _referent = java_lang_ref_Reference::referent(_ref);
527 assert(Universe::heap()->is_in_reserved_or_null(_referent),
528 "Wrong oop found in java.lang.Reference object");
529 assert(allow_null_referent ?
530 _referent->is_oop_or_null()
531 : _referent->is_oop(),
532 "bad referent");
533 }
534
535 inline void DiscoveredListIterator::next() {
536 _prev_next = _discovered_addr;
537 move_to_next();
538 }
539
540 inline void DiscoveredListIterator::remove() {
541 assert(_ref->is_oop(), "Dropping a bad reference");
542 oop_store_raw(_discovered_addr, NULL);
543 // First _prev_next ref actually points into DiscoveredList (gross).
544 if (UseCompressedOops) {
545 // Remove Reference object from list.
546 oopDesc::encode_store_heap_oop_not_null((narrowOop*)_prev_next, _next);
547 } else {
548 // Remove Reference object from list.
549 oopDesc::store_heap_oop((oop*)_prev_next, _next);
550 }
551 NOT_PRODUCT(_removed++);
552 _refs_list.dec_length(1);
553 }
554
555 inline void DiscoveredListIterator::move_to_next() {
556 _ref = _next;
557 assert(_ref != _first_seen, "cyclic ref_list found");
558 NOT_PRODUCT(_processed++);
559 }
560
561 // NOTE: process_phase*() are largely similar, and at a high level
562 // merely iterate over the extant list applying a predicate to
563 // each of its elements and possibly removing that element from the
564 // list and applying some further closures to that element.
565 // We should consider the possibility of replacing these
566 // process_phase*() methods by abstracting them into
567 // a single general iterator invocation that receives appropriate
568 // closures that accomplish this work.
569
570 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
571 // referents are not alive, but that should be kept alive for policy reasons.
572 // Keep alive the transitive closure of all such referents.
573 void
574 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
575 ReferencePolicy* policy,
576 BoolObjectClosure* is_alive,
577 OopClosure* keep_alive,
578 VoidClosure* complete_gc) {
579 assert(policy != NULL, "Must have a non-NULL policy");
580 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
581 // Decide which softly reachable refs should be kept alive.
582 while (iter.has_next()) {
583 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
584 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
585 if (referent_is_dead && !policy->should_clear_reference(iter.obj())) {
586 if (TraceReferenceGC) {
587 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
588 iter.obj(), iter.obj()->blueprint()->internal_name());
589 }
590 // Remove Reference object from list
591 iter.remove();
592 // Make the Reference object active again
593 iter.make_active();
594 // keep the referent around
595 iter.make_referent_alive();
596 iter.move_to_next();
597 } else {
598 iter.next();
599 }
600 }
601 // Close the reachable set
602 complete_gc->do_void();
603 NOT_PRODUCT(
604 if (PrintGCDetails && TraceReferenceGC) {
605 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
606 "discovered Refs by policy list " INTPTR_FORMAT,
607 iter.removed(), iter.processed(), (address)refs_list.head());
608 }
609 )
610 }
611
612 // Traverse the list and remove any Refs that are not active, or
613 // whose referents are either alive or NULL.
614 void
615 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
616 BoolObjectClosure* is_alive,
617 OopClosure* keep_alive) {
618 assert(discovery_is_atomic(), "Error");
619 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
620 while (iter.has_next()) {
621 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
622 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
623 assert(next == NULL, "Should not discover inactive Reference");
624 if (iter.is_referent_alive()) {
625 if (TraceReferenceGC) {
626 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
627 iter.obj(), iter.obj()->blueprint()->internal_name());
628 }
629 // The referent is reachable after all.
630 // Remove Reference object from list.
631 iter.remove();
632 // Update the referent pointer as necessary: Note that this
633 // should not entail any recursive marking because the
634 // referent must already have been traversed.
635 iter.make_referent_alive();
636 iter.move_to_next();
637 } else {
638 iter.next();
639 }
640 }
641 NOT_PRODUCT(
642 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
643 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
644 "Refs in discovered list " INTPTR_FORMAT,
645 iter.removed(), iter.processed(), (address)refs_list.head());
646 }
647 )
648 }
649
650 void
651 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
652 BoolObjectClosure* is_alive,
653 OopClosure* keep_alive,
654 VoidClosure* complete_gc) {
655 assert(!discovery_is_atomic(), "Error");
656 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
657 while (iter.has_next()) {
658 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
659 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
660 oop next = java_lang_ref_Reference::next(iter.obj());
661 if ((iter.referent() == NULL || iter.is_referent_alive() ||
662 next != NULL)) {
663 assert(next->is_oop_or_null(), "bad next field");
664 // Remove Reference object from list
665 iter.remove();
666 // Trace the cohorts
667 iter.make_referent_alive();
668 if (UseCompressedOops) {
669 keep_alive->do_oop((narrowOop*)next_addr);
670 } else {
671 keep_alive->do_oop((oop*)next_addr);
672 }
673 iter.move_to_next();
674 } else {
675 iter.next();
676 }
677 }
678 // Now close the newly reachable set
679 complete_gc->do_void();
680 NOT_PRODUCT(
681 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
682 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
683 "Refs in discovered list " INTPTR_FORMAT,
684 iter.removed(), iter.processed(), (address)refs_list.head());
685 }
686 )
687 }
688
689 // Traverse the list and process the referents, by either
690 // clearing them or keeping them (and their reachable
691 // closure) alive.
692 void
693 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
694 bool clear_referent,
695 BoolObjectClosure* is_alive,
696 OopClosure* keep_alive,
697 VoidClosure* complete_gc) {
698 ResourceMark rm;
699 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
700 while (iter.has_next()) {
701 iter.update_discovered();
702 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
703 if (clear_referent) {
704 // NULL out referent pointer
705 iter.clear_referent();
706 } else {
707 // keep the referent around
708 iter.make_referent_alive();
709 }
710 if (TraceReferenceGC) {
711 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
712 clear_referent ? "cleared " : "",
713 iter.obj(), iter.obj()->blueprint()->internal_name());
714 }
715 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
716 iter.next();
717 }
718 // Remember to keep sentinel pointer around
719 iter.update_discovered();
720 // Close the reachable set
721 complete_gc->do_void();
722 }
723
724 void
725 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
726 oop obj = refs_list.head();
727 while (obj != sentinel_ref()) {
728 oop discovered = java_lang_ref_Reference::discovered(obj);
729 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
730 obj = discovered;
731 }
732 refs_list.set_head(sentinel_ref());
733 refs_list.set_length(0);
734 }
735
736 void ReferenceProcessor::abandon_partial_discovery() {
737 // loop over the lists
738 for (int i = 0; i < _max_num_q * subclasses_of_ref(); i++) {
739 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
740 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
741 }
742 abandon_partial_discovered_list(_discoveredSoftRefs[i]);
743 }
744 }
745
746 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
747 public:
748 RefProcPhase1Task(ReferenceProcessor& ref_processor,
749 DiscoveredList refs_lists[],
750 ReferencePolicy* policy,
751 bool marks_oops_alive)
752 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
753 _policy(policy)
754 { }
755 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
756 OopClosure& keep_alive,
757 VoidClosure& complete_gc)
758 {
759 Thread* thr = Thread::current();
760 int refs_list_index = ((WorkerThread*)thr)->id();
761 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
762 &is_alive, &keep_alive, &complete_gc);
763 }
764 private:
765 ReferencePolicy* _policy;
766 };
767
768 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
769 public:
770 RefProcPhase2Task(ReferenceProcessor& ref_processor,
771 DiscoveredList refs_lists[],
772 bool marks_oops_alive)
773 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
774 { }
775 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
776 OopClosure& keep_alive,
777 VoidClosure& complete_gc)
778 {
779 _ref_processor.process_phase2(_refs_lists[i],
780 &is_alive, &keep_alive, &complete_gc);
781 }
782 };
783
784 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
785 public:
786 RefProcPhase3Task(ReferenceProcessor& ref_processor,
787 DiscoveredList refs_lists[],
788 bool clear_referent,
789 bool marks_oops_alive)
790 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
791 _clear_referent(clear_referent)
792 { }
793 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
794 OopClosure& keep_alive,
795 VoidClosure& complete_gc)
796 {
797 // Don't use "refs_list_index" calculated in this way because
798 // balance_queues() has moved the Ref's into the first n queues.
799 // Thread* thr = Thread::current();
800 // int refs_list_index = ((WorkerThread*)thr)->id();
801 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
802 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
803 &is_alive, &keep_alive, &complete_gc);
804 }
805 private:
806 bool _clear_referent;
807 };
808
809 // Balances reference queues.
810 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
811 // queues[0, 1, ..., _num_q-1] because only the first _num_q
812 // corresponding to the active workers will be processed.
813 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
814 {
815 // calculate total length
816 size_t total_refs = 0;
817 if (TraceReferenceGC && PrintGCDetails) {
818 gclog_or_tty->print_cr("\nBalance ref_lists ");
819 }
820
821 for (int i = 0; i < _max_num_q; ++i) {
822 total_refs += ref_lists[i].length();
823 if (TraceReferenceGC && PrintGCDetails) {
824 gclog_or_tty->print("%d ", ref_lists[i].length());
825 }
826 }
827 if (TraceReferenceGC && PrintGCDetails) {
828 gclog_or_tty->print_cr(" = %d", total_refs);
829 }
830 size_t avg_refs = total_refs / _num_q + 1;
831 int to_idx = 0;
832 for (int from_idx = 0; from_idx < _max_num_q; from_idx++) {
833 bool move_all = false;
834 if (from_idx >= _num_q) {
835 move_all = ref_lists[from_idx].length() > 0;
836 }
837 while ((ref_lists[from_idx].length() > avg_refs) ||
838 move_all) {
839 assert(to_idx < _num_q, "Sanity Check!");
840 if (ref_lists[to_idx].length() < avg_refs) {
841 // move superfluous refs
842 size_t refs_to_move;
843 // Move all the Ref's if the from queue will not be processed.
844 if (move_all) {
845 refs_to_move = MIN2(ref_lists[from_idx].length(),
846 avg_refs - ref_lists[to_idx].length());
847 } else {
848 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
849 avg_refs - ref_lists[to_idx].length());
850 }
851 oop move_head = ref_lists[from_idx].head();
852 oop move_tail = move_head;
853 oop new_head = move_head;
854 // find an element to split the list on
855 for (size_t j = 0; j < refs_to_move; ++j) {
856 move_tail = new_head;
857 new_head = java_lang_ref_Reference::discovered(new_head);
858 }
859
860 if (_discovered_list_needs_barrier) {
861 java_lang_ref_Reference::set_discovered(move_tail, ref_lists[to_idx].head());
862 } else {
863 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(move_tail);
864 oop_store_raw(discovered_addr, ref_lists[to_idx].head());
865 }
866
867 ref_lists[to_idx].set_head(move_head);
868 ref_lists[to_idx].inc_length(refs_to_move);
869 ref_lists[from_idx].set_head(new_head);
870 ref_lists[from_idx].dec_length(refs_to_move);
871 if (ref_lists[from_idx].length() == 0) {
872 break;
873 }
874 } else {
875 to_idx = (to_idx + 1) % _num_q;
876 }
877 }
878 }
879 #ifdef ASSERT
880 size_t balanced_total_refs = 0;
881 for (int i = 0; i < _max_num_q; ++i) {
882 balanced_total_refs += ref_lists[i].length();
883 if (TraceReferenceGC && PrintGCDetails) {
884 gclog_or_tty->print("%d ", ref_lists[i].length());
885 }
886 }
887 if (TraceReferenceGC && PrintGCDetails) {
888 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
889 gclog_or_tty->flush();
890 }
891 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
892 #endif
893 }
894
895 void ReferenceProcessor::balance_all_queues() {
896 balance_queues(_discoveredSoftRefs);
897 balance_queues(_discoveredWeakRefs);
898 balance_queues(_discoveredFinalRefs);
899 balance_queues(_discoveredPhantomRefs);
900 }
901
902 void
903 ReferenceProcessor::process_discovered_reflist(
904 DiscoveredList refs_lists[],
905 ReferencePolicy* policy,
906 bool clear_referent,
907 BoolObjectClosure* is_alive,
908 OopClosure* keep_alive,
909 VoidClosure* complete_gc,
910 AbstractRefProcTaskExecutor* task_executor)
911 {
912 bool mt_processing = task_executor != NULL && _processing_is_mt;
913 // If discovery used MT and a dynamic number of GC threads, then
914 // the queues must be balanced for correctness if fewer than the
915 // maximum number of queues were used. The number of queue used
916 // during discovery may be different than the number to be used
917 // for processing so don't depend of _num_q < _max_num_q as part
918 // of the test.
919 bool must_balance = _discovery_is_mt;
920
921 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
922 must_balance) {
923 balance_queues(refs_lists);
924 }
925 if (PrintReferenceGC && PrintGCDetails) {
926 size_t total = 0;
927 for (int i = 0; i < _max_num_q; ++i) {
928 total += refs_lists[i].length();
929 }
930 gclog_or_tty->print(", %u refs", total);
931 }
932
933 // Phase 1 (soft refs only):
934 // . Traverse the list and remove any SoftReferences whose
935 // referents are not alive, but that should be kept alive for
936 // policy reasons. Keep alive the transitive closure of all
937 // such referents.
938 if (policy != NULL) {
939 if (mt_processing) {
940 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
941 task_executor->execute(phase1);
942 } else {
943 for (int i = 0; i < _max_num_q; i++) {
944 process_phase1(refs_lists[i], policy,
945 is_alive, keep_alive, complete_gc);
946 }
947 }
948 } else { // policy == NULL
949 assert(refs_lists != _discoveredSoftRefs,
950 "Policy must be specified for soft references.");
951 }
952
953 // Phase 2:
954 // . Traverse the list and remove any refs whose referents are alive.
955 if (mt_processing) {
956 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
957 task_executor->execute(phase2);
958 } else {
959 for (int i = 0; i < _max_num_q; i++) {
960 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
961 }
962 }
963
964 // Phase 3:
965 // . Traverse the list and process referents as appropriate.
966 if (mt_processing) {
967 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
968 task_executor->execute(phase3);
969 } else {
970 for (int i = 0; i < _max_num_q; i++) {
971 process_phase3(refs_lists[i], clear_referent,
972 is_alive, keep_alive, complete_gc);
973 }
974 }
975 }
976
977 void ReferenceProcessor::clean_up_discovered_references() {
978 // loop over the lists
979 // Should this instead be
980 // for (int i = 0; i < subclasses_of_ref(); i++) {
981 // for (int j = 0; j < _num_q; j++) {
982 // int index = i * _max_num_q + j;
983 for (int i = 0; i < _max_num_q * subclasses_of_ref(); i++) {
984 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
985 gclog_or_tty->print_cr(
986 "\nScrubbing %s discovered list of Null referents",
987 list_name(i));
988 }
989 clean_up_discovered_reflist(_discoveredSoftRefs[i]);
990 }
991 }
992
993 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
994 assert(!discovery_is_atomic(), "Else why call this method?");
995 DiscoveredListIterator iter(refs_list, NULL, NULL);
996 while (iter.has_next()) {
997 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
998 oop next = java_lang_ref_Reference::next(iter.obj());
999 assert(next->is_oop_or_null(), "bad next field");
1000 // If referent has been cleared or Reference is not active,
1001 // drop it.
1002 if (iter.referent() == NULL || next != NULL) {
1003 debug_only(
1004 if (PrintGCDetails && TraceReferenceGC) {
1005 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
1006 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
1007 " and referent: " INTPTR_FORMAT,
1008 iter.obj(), next, iter.referent());
1009 }
1010 )
1011 // Remove Reference object from list
1012 iter.remove();
1013 iter.move_to_next();
1014 } else {
1015 iter.next();
1016 }
1017 }
1018 NOT_PRODUCT(
1019 if (PrintGCDetails && TraceReferenceGC) {
1020 gclog_or_tty->print(
1021 " Removed %d Refs with NULL referents out of %d discovered Refs",
1022 iter.removed(), iter.processed());
1023 }
1024 )
1025 }
1026
1027 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1028 int id = 0;
1029 // Determine the queue index to use for this object.
1030 if (_discovery_is_mt) {
1031 // During a multi-threaded discovery phase,
1032 // each thread saves to its "own" list.
1033 Thread* thr = Thread::current();
1034 id = thr->as_Worker_thread()->id();
1035 } else {
1036 // single-threaded discovery, we save in round-robin
1037 // fashion to each of the lists.
1038 if (_processing_is_mt) {
1039 id = next_id();
1040 }
1041 }
1042 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1043
1044 // Get the discovered queue to which we will add
1045 DiscoveredList* list = NULL;
1046 switch (rt) {
1047 case REF_OTHER:
1048 // Unknown reference type, no special treatment
1049 break;
1050 case REF_SOFT:
1051 list = &_discoveredSoftRefs[id];
1052 break;
1053 case REF_WEAK:
1054 list = &_discoveredWeakRefs[id];
1055 break;
1056 case REF_FINAL:
1057 list = &_discoveredFinalRefs[id];
1058 break;
1059 case REF_PHANTOM:
1060 list = &_discoveredPhantomRefs[id];
1061 break;
1062 case REF_NONE:
1063 // we should not reach here if we are an instanceRefKlass
1064 default:
1065 ShouldNotReachHere();
1066 }
1067 if (TraceReferenceGC && PrintGCDetails) {
1068 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1069 }
1070 return list;
1071 }
1072
1073 inline void
1074 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1075 oop obj,
1076 HeapWord* discovered_addr) {
1077 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1078 // First we must make sure this object is only enqueued once. CAS in a non null
1079 // discovered_addr.
1080 oop current_head = refs_list.head();
1081
1082 // Note: In the case of G1, this specific pre-barrier is strictly
1083 // not necessary because the only case we are interested in
1084 // here is when *discovered_addr is NULL (see the CAS further below),
1085 // so this will expand to nothing. As a result, we have manually
1086 // elided this out for G1, but left in the test for some future
1087 // collector that might have need for a pre-barrier here.
1088 if (_discovered_list_needs_barrier && !UseG1GC) {
1089 if (UseCompressedOops) {
1090 _bs->write_ref_field_pre((narrowOop*)discovered_addr, current_head);
1091 } else {
1092 _bs->write_ref_field_pre((oop*)discovered_addr, current_head);
1093 }
1094 guarantee(false, "Need to check non-G1 collector");
1095 }
1096 oop retest = oopDesc::atomic_compare_exchange_oop(current_head, discovered_addr,
1097 NULL);
1098 if (retest == NULL) {
1099 // This thread just won the right to enqueue the object.
1100 // We have separate lists for enqueueing so no synchronization
1101 // is necessary.
1102 refs_list.set_head(obj);
1103 refs_list.inc_length(1);
1104 if (_discovered_list_needs_barrier) {
1105 _bs->write_ref_field((void*)discovered_addr, current_head);
1106 }
1107
1108 if (TraceReferenceGC) {
1109 gclog_or_tty->print_cr("Enqueued reference (mt) (" INTPTR_FORMAT ": %s)",
1110 obj, obj->blueprint()->internal_name());
1111 }
1112 } else {
1113 // If retest was non NULL, another thread beat us to it:
1114 // The reference has already been discovered...
1115 if (TraceReferenceGC) {
1116 gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",
1117 obj, obj->blueprint()->internal_name());
1118 }
1119 }
1120 }
1121
1122 #ifndef PRODUCT
1123 // Non-atomic (i.e. concurrent) discovery might allow us
1124 // to observe j.l.References with NULL referents, being those
1125 // cleared concurrently by mutators during (or after) discovery.
1126 void ReferenceProcessor::verify_referent(oop obj) {
1127 bool da = discovery_is_atomic();
1128 oop referent = java_lang_ref_Reference::referent(obj);
1129 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1130 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1131 INTPTR_FORMAT " during %satomic discovery ",
1132 (intptr_t)referent, (intptr_t)obj, da ? "" : "non-"));
1133 }
1134 #endif
1135
1136 // We mention two of several possible choices here:
1137 // #0: if the reference object is not in the "originating generation"
1138 // (or part of the heap being collected, indicated by our "span"
1139 // we don't treat it specially (i.e. we scan it as we would
1140 // a normal oop, treating its references as strong references).
1141 // This means that references can't be enqueued unless their
1142 // referent is also in the same span. This is the simplest,
1143 // most "local" and most conservative approach, albeit one
1144 // that may cause weak references to be enqueued least promptly.
1145 // We call this choice the "ReferenceBasedDiscovery" policy.
1146 // #1: the reference object may be in any generation (span), but if
1147 // the referent is in the generation (span) being currently collected
1148 // then we can discover the reference object, provided
1149 // the object has not already been discovered by
1150 // a different concurrently running collector (as may be the
1151 // case, for instance, if the reference object is in CMS and
1152 // the referent in DefNewGeneration), and provided the processing
1153 // of this reference object by the current collector will
1154 // appear atomic to every other collector in the system.
1155 // (Thus, for instance, a concurrent collector may not
1156 // discover references in other generations even if the
1157 // referent is in its own generation). This policy may,
1158 // in certain cases, enqueue references somewhat sooner than
1159 // might Policy #0 above, but at marginally increased cost
1160 // and complexity in processing these references.
1161 // We call this choice the "RefeferentBasedDiscovery" policy.
1162 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1163 // We enqueue references only if we are discovering refs
1164 // (rather than processing discovered refs).
1165 if (!_discovering_refs || !RegisterReferences) {
1166 return false;
1167 }
1168 // We only enqueue active references.
1169 oop next = java_lang_ref_Reference::next(obj);
1170 if (next != NULL) {
1171 return false;
1172 }
1173
1174 HeapWord* obj_addr = (HeapWord*)obj;
1175 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1176 !_span.contains(obj_addr)) {
1177 // Reference is not in the originating generation;
1178 // don't treat it specially (i.e. we want to scan it as a normal
1179 // object with strong references).
1180 return false;
1181 }
1182
1183 // We only enqueue references whose referents are not (yet) strongly
1184 // reachable.
1185 if (is_alive_non_header() != NULL) {
1186 verify_referent(obj);
1187 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1188 return false; // referent is reachable
1189 }
1190 }
1191 if (rt == REF_SOFT) {
1192 // For soft refs we can decide now if these are not
1193 // current candidates for clearing, in which case we
1194 // can mark through them now, rather than delaying that
1195 // to the reference-processing phase. Since all current
1196 // time-stamp policies advance the soft-ref clock only
1197 // at a major collection cycle, this is always currently
1198 // accurate.
1199 if (!_current_soft_ref_policy->should_clear_reference(obj)) {
1200 return false;
1201 }
1202 }
1203
1204 ResourceMark rm; // Needed for tracing.
1205
1206 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1207 const oop discovered = java_lang_ref_Reference::discovered(obj);
1208 assert(discovered->is_oop_or_null(), "bad discovered field");
1209 if (discovered != NULL) {
1210 // The reference has already been discovered...
1211 if (TraceReferenceGC) {
1212 gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",
1213 obj, obj->blueprint()->internal_name());
1214 }
1215 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1216 // assumes that an object is not processed twice;
1217 // if it's been already discovered it must be on another
1218 // generation's discovered list; so we won't discover it.
1219 return false;
1220 } else {
1221 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1222 "Unrecognized policy");
1223 // Check assumption that an object is not potentially
1224 // discovered twice except by concurrent collectors that potentially
1225 // trace the same Reference object twice.
1226 assert(UseConcMarkSweepGC || UseG1GC,
1227 "Only possible with a concurrent marking collector");
1228 return true;
1229 }
1230 }
1231
1232 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1233 verify_referent(obj);
1234 // enqueue if and only if either:
1235 // reference is in our span or
1236 // we are an atomic collector and referent is in our span
1237 if (_span.contains(obj_addr) ||
1238 (discovery_is_atomic() &&
1239 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1240 // should_enqueue = true;
1241 } else {
1242 return false;
1243 }
1244 } else {
1245 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1246 _span.contains(obj_addr), "code inconsistency");
1247 }
1248
1249 // Get the right type of discovered queue head.
1250 DiscoveredList* list = get_discovered_list(rt);
1251 if (list == NULL) {
1252 return false; // nothing special needs to be done
1253 }
1254
1255 if (_discovery_is_mt) {
1256 add_to_discovered_list_mt(*list, obj, discovered_addr);
1257 } else {
1258 // If "_discovered_list_needs_barrier", we do write barriers when
1259 // updating the discovered reference list. Otherwise, we do a raw store
1260 // here: the field will be visited later when processing the discovered
1261 // references.
1262 oop current_head = list->head();
1263 // As in the case further above, since we are over-writing a NULL
1264 // pre-value, we can safely elide the pre-barrier here for the case of G1.
1265 assert(discovered == NULL, "control point invariant");
1266 if (_discovered_list_needs_barrier && !UseG1GC) { // safe to elide for G1
1267 if (UseCompressedOops) {
1268 _bs->write_ref_field_pre((narrowOop*)discovered_addr, current_head);
1269 } else {
1270 _bs->write_ref_field_pre((oop*)discovered_addr, current_head);
1271 }
1272 guarantee(false, "Need to check non-G1 collector");
1273 }
1274 oop_store_raw(discovered_addr, current_head);
1275 if (_discovered_list_needs_barrier) {
1276 _bs->write_ref_field((void*)discovered_addr, current_head);
1277 }
1278 list->set_head(obj);
1279 list->inc_length(1);
1280
1281 if (TraceReferenceGC) {
1282 gclog_or_tty->print_cr("Enqueued reference (" INTPTR_FORMAT ": %s)",
1283 obj, obj->blueprint()->internal_name());
1284 }
1285 }
1286 assert(obj->is_oop(), "Enqueued a bad reference");
1287 verify_referent(obj);
1288 return true;
1289 }
1290
1291 // Preclean the discovered references by removing those
1292 // whose referents are alive, and by marking from those that
1293 // are not active. These lists can be handled here
1294 // in any order and, indeed, concurrently.
1295 void ReferenceProcessor::preclean_discovered_references(
1296 BoolObjectClosure* is_alive,
1297 OopClosure* keep_alive,
1298 VoidClosure* complete_gc,
1299 YieldClosure* yield,
1300 bool should_unload_classes) {
1301
1302 NOT_PRODUCT(verify_ok_to_handle_reflists());
1303
1304 #ifdef ASSERT
1305 bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC ||
1306 CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
1307 ExplicitGCInvokesConcurrentAndUnloadsClasses &&
1308 UseConcMarkSweepGC && should_unload_classes;
1309 RememberKlassesChecker mx(must_remember_klasses);
1310 #endif
1311 // Soft references
1312 {
1313 TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1314 false, gclog_or_tty);
1315 for (int i = 0; i < _max_num_q; i++) {
1316 if (yield->should_return()) {
1317 return;
1318 }
1319 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1320 keep_alive, complete_gc, yield);
1321 }
1322 }
1323
1324 // Weak references
1325 {
1326 TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1327 false, gclog_or_tty);
1328 for (int i = 0; i < _max_num_q; i++) {
1329 if (yield->should_return()) {
1330 return;
1331 }
1332 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1333 keep_alive, complete_gc, yield);
1334 }
1335 }
1336
1337 // Final references
1338 {
1339 TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1340 false, gclog_or_tty);
1341 for (int i = 0; i < _max_num_q; i++) {
1342 if (yield->should_return()) {
1343 return;
1344 }
1345 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1346 keep_alive, complete_gc, yield);
1347 }
1348 }
1349
1350 // Phantom references
1351 {
1352 TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1353 false, gclog_or_tty);
1354 for (int i = 0; i < _max_num_q; i++) {
1355 if (yield->should_return()) {
1356 return;
1357 }
1358 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1359 keep_alive, complete_gc, yield);
1360 }
1361 }
1362 }
1363
1364 // Walk the given discovered ref list, and remove all reference objects
1365 // whose referents are still alive, whose referents are NULL or which
1366 // are not active (have a non-NULL next field). NOTE: When we are
1367 // thus precleaning the ref lists (which happens single-threaded today),
1368 // we do not disable refs discovery to honour the correct semantics of
1369 // java.lang.Reference. As a result, we need to be careful below
1370 // that ref removal steps interleave safely with ref discovery steps
1371 // (in this thread).
1372 void
1373 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1374 BoolObjectClosure* is_alive,
1375 OopClosure* keep_alive,
1376 VoidClosure* complete_gc,
1377 YieldClosure* yield) {
1378 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1379 while (iter.has_next()) {
1380 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1381 oop obj = iter.obj();
1382 oop next = java_lang_ref_Reference::next(obj);
1383 if (iter.referent() == NULL || iter.is_referent_alive() ||
1384 next != NULL) {
1385 // The referent has been cleared, or is alive, or the Reference is not
1386 // active; we need to trace and mark its cohort.
1387 if (TraceReferenceGC) {
1388 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1389 iter.obj(), iter.obj()->blueprint()->internal_name());
1390 }
1391 // Remove Reference object from list
1392 iter.remove();
1393 // Keep alive its cohort.
1394 iter.make_referent_alive();
1395 if (UseCompressedOops) {
1396 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1397 keep_alive->do_oop(next_addr);
1398 } else {
1399 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1400 keep_alive->do_oop(next_addr);
1401 }
1402 iter.move_to_next();
1403 } else {
1404 iter.next();
1405 }
1406 }
1407 // Close the reachable set
1408 complete_gc->do_void();
1409
1410 NOT_PRODUCT(
1411 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1412 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1413 "Refs in discovered list " INTPTR_FORMAT,
1414 iter.removed(), iter.processed(), (address)refs_list.head());
1415 }
1416 )
1417 }
1418
1419 const char* ReferenceProcessor::list_name(int i) {
1420 assert(i >= 0 && i <= _max_num_q * subclasses_of_ref(), "Out of bounds index");
1421 int j = i / _max_num_q;
1422 switch (j) {
1423 case 0: return "SoftRef";
1424 case 1: return "WeakRef";
1425 case 2: return "FinalRef";
1426 case 3: return "PhantomRef";
1427 }
1428 ShouldNotReachHere();
1429 return NULL;
1430 }
1431
1432 #ifndef PRODUCT
1433 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1434 // empty for now
1435 }
1436 #endif
1437
1438 void ReferenceProcessor::verify() {
1439 guarantee(sentinel_ref() != NULL && sentinel_ref()->is_oop(), "Lost _sentinelRef");
1440 }
1441
1442 #ifndef PRODUCT
1443 void ReferenceProcessor::clear_discovered_references() {
1444 guarantee(!_discovering_refs, "Discovering refs?");
1445 for (int i = 0; i < _max_num_q * subclasses_of_ref(); i++) {
1446 oop obj = _discoveredSoftRefs[i].head();
1447 while (obj != sentinel_ref()) {
1448 oop next = java_lang_ref_Reference::discovered(obj);
1449 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
1450 obj = next;
1451 }
1452 _discoveredSoftRefs[i].set_head(sentinel_ref());
1453 _discoveredSoftRefs[i].set_length(0);
1454 }
1455 }
1456 #endif // PRODUCT