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 _max_num_q = mt_degree;
149 _discoveredSoftRefs = NEW_C_HEAP_ARRAY(DiscoveredList, _max_num_q * subclasses_of_ref);
150 if (_discoveredSoftRefs == NULL) {
151 vm_exit_during_initialization("Could not allocated RefProc Array");
152 }
153 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
154 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
155 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
156 assert(sentinel_ref() != NULL, "_sentinelRef is NULL");
157 // Initialized all entries to _sentinelRef
158 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
159 _discoveredSoftRefs[i].set_head(sentinel_ref());
160 _discoveredSoftRefs[i].set_length(0);
161 }
162 // If we do barreirs, cache a copy of the barrier set.
163 if (discovered_list_needs_barrier) {
164 _bs = Universe::heap()->barrier_set();
165 }
166 }
167
168 #ifndef PRODUCT
169 void ReferenceProcessor::verify_no_references_recorded() {
170 guarantee(!_discovering_refs, "Discovering refs?");
171 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
172 guarantee(_discoveredSoftRefs[i].empty(),
173 "Found non-empty discovered list");
174 }
175 }
176 #endif
177
178 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
179 // Should this instead be
180 // for (int i = 0; i < subclasses_of_ref; i++_ {
181 // for (int j = 0; j < _num_q; j++) {
182 // int index = i * _max_num_q + j;
183 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
184 if (UseCompressedOops) {
185 f->do_oop((narrowOop*)_discoveredSoftRefs[i].adr_head());
186 } else {
187 f->do_oop((oop*)_discoveredSoftRefs[i].adr_head());
188 }
189 }
190 }
191
192 void ReferenceProcessor::oops_do(OopClosure* f) {
193 f->do_oop(adr_sentinel_ref());
194 }
195
196 void ReferenceProcessor::update_soft_ref_master_clock() {
197 // Update (advance) the soft ref master clock field. This must be done
198 // after processing the soft ref list.
199 jlong now = os::javaTimeMillis();
200 jlong clock = java_lang_ref_SoftReference::clock();
201 NOT_PRODUCT(
202 if (now < clock) {
203 warning("time warp: %d to %d", clock, now);
204 }
205 )
206 // In product mode, protect ourselves from system time being adjusted
207 // externally and going backward; see note in the implementation of
208 // GenCollectedHeap::time_since_last_gc() for the right way to fix
209 // this uniformly throughout the VM; see bug-id 4741166. XXX
210 if (now > clock) {
211 java_lang_ref_SoftReference::set_clock(now);
212 }
213 // Else leave clock stalled at its old value until time progresses
214 // past clock value.
215 }
216
217 void ReferenceProcessor::process_discovered_references(
218 BoolObjectClosure* is_alive,
219 OopClosure* keep_alive,
220 VoidClosure* complete_gc,
221 AbstractRefProcTaskExecutor* task_executor) {
222 NOT_PRODUCT(verify_ok_to_handle_reflists());
223
224 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
225 // Stop treating discovered references specially.
226 disable_discovery();
227
228 bool trace_time = PrintGCDetails && PrintReferenceGC;
229 // Soft references
230 {
231 TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);
232 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
233 is_alive, keep_alive, complete_gc, task_executor);
234 }
235
236 update_soft_ref_master_clock();
237
238 // Weak references
239 {
240 TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);
241 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
242 is_alive, keep_alive, complete_gc, task_executor);
243 }
244
245 // Final references
246 {
247 TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);
248 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
249 is_alive, keep_alive, complete_gc, task_executor);
250 }
251
252 // Phantom references
253 {
254 TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);
255 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
256 is_alive, keep_alive, complete_gc, task_executor);
257 }
258
259 // Weak global JNI references. It would make more sense (semantically) to
260 // traverse these simultaneously with the regular weak references above, but
261 // that is not how the JDK1.2 specification is. See #4126360. Native code can
262 // thus use JNI weak references to circumvent the phantom references and
263 // resurrect a "post-mortem" object.
264 {
265 TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);
266 if (task_executor != NULL) {
267 task_executor->set_single_threaded_mode();
268 }
269 process_phaseJNI(is_alive, keep_alive, complete_gc);
270 }
271 }
272
273 #ifndef PRODUCT
274 // Calculate the number of jni handles.
275 uint ReferenceProcessor::count_jni_refs() {
276 class AlwaysAliveClosure: public BoolObjectClosure {
277 public:
278 virtual bool do_object_b(oop obj) { return true; }
279 virtual void do_object(oop obj) { assert(false, "Don't call"); }
280 };
281
282 class CountHandleClosure: public OopClosure {
283 private:
284 int _count;
285 public:
286 CountHandleClosure(): _count(0) {}
287 void do_oop(oop* unused) { _count++; }
288 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
289 int count() { return _count; }
290 };
291 CountHandleClosure global_handle_count;
292 AlwaysAliveClosure always_alive;
293 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
294 return global_handle_count.count();
295 }
296 #endif
297
298 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
299 OopClosure* keep_alive,
300 VoidClosure* complete_gc) {
301 #ifndef PRODUCT
302 if (PrintGCDetails && PrintReferenceGC) {
303 unsigned int count = count_jni_refs();
304 gclog_or_tty->print(", %u refs", count);
305 }
306 #endif
307 JNIHandles::weak_oops_do(is_alive, keep_alive);
308 // Finally remember to keep sentinel around
309 keep_alive->do_oop(adr_sentinel_ref());
310 complete_gc->do_void();
311 }
312
313
314 template <class T>
315 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
316 AbstractRefProcTaskExecutor* task_executor) {
317
318 // Remember old value of pending references list
319 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
320 T old_pending_list_value = *pending_list_addr;
321
322 // Enqueue references that are not made active again, and
323 // clear the decks for the next collection (cycle).
324 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
325 // Do the oop-check on pending_list_addr missed in
326 // enqueue_discovered_reflist. We should probably
327 // do a raw oop_check so that future such idempotent
328 // oop_stores relying on the oop-check side-effect
329 // may be elided automatically and safely without
330 // affecting correctness.
331 oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
332
333 // Stop treating discovered references specially.
334 ref->disable_discovery();
335
336 // Return true if new pending references were added
337 return old_pending_list_value != *pending_list_addr;
338 }
339
340 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
341 NOT_PRODUCT(verify_ok_to_handle_reflists());
342 if (UseCompressedOops) {
343 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
344 } else {
345 return enqueue_discovered_ref_helper<oop>(this, task_executor);
346 }
347 }
348
349 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
350 HeapWord* pending_list_addr) {
351 // Given a list of refs linked through the "discovered" field
352 // (java.lang.ref.Reference.discovered) chain them through the
353 // "next" field (java.lang.ref.Reference.next) and prepend
354 // to the pending list.
355 if (TraceReferenceGC && PrintGCDetails) {
356 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
357 INTPTR_FORMAT, (address)refs_list.head());
358 }
359 oop obj = refs_list.head();
360 // Walk down the list, copying the discovered field into
361 // the next field and clearing it (except for the last
362 // non-sentinel object which is treated specially to avoid
363 // confusion with an active reference).
364 while (obj != sentinel_ref()) {
365 assert(obj->is_instanceRef(), "should be reference object");
366 oop next = java_lang_ref_Reference::discovered(obj);
367 if (TraceReferenceGC && PrintGCDetails) {
368 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next " INTPTR_FORMAT,
369 obj, next);
370 }
371 assert(java_lang_ref_Reference::next(obj) == NULL,
372 "The reference should not be enqueued");
373 if (next == sentinel_ref()) { // obj is last
374 // Swap refs_list into pendling_list_addr and
375 // set obj's next to what we read from pending_list_addr.
376 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
377 // Need oop_check on pending_list_addr above;
378 // see special oop-check code at the end of
379 // enqueue_discovered_reflists() further below.
380 if (old == NULL) {
381 // obj should be made to point to itself, since
382 // pending list was empty.
383 java_lang_ref_Reference::set_next(obj, obj);
384 } else {
385 java_lang_ref_Reference::set_next(obj, old);
386 }
387 } else {
388 java_lang_ref_Reference::set_next(obj, next);
389 }
390 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
391 obj = next;
392 }
393 }
394
395 // Parallel enqueue task
396 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
397 public:
398 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
399 DiscoveredList discovered_refs[],
400 HeapWord* pending_list_addr,
401 oop sentinel_ref,
402 int n_queues)
403 : EnqueueTask(ref_processor, discovered_refs,
404 pending_list_addr, sentinel_ref, n_queues)
405 { }
406
407 virtual void work(unsigned int work_id) {
408 assert(work_id < (unsigned int)_ref_processor.num_q(), "Index out-of-bounds");
409 // Simplest first cut: static partitioning.
410 int index = work_id;
411 // The increment on "index" must correspond to the maximum number of queues
412 // (n_queues) with which that ReferenceProcessor was created. That
413 // is because of the "clever" way the discovered references lists were
414 // allocated and are indexed into. That number is ParallelGCThreads
415 // currently. Assert that.
416 assert(_n_queues == (int) ParallelGCThreads, "Different number not expected");
417 for (int j = 0;
418 j < subclasses_of_ref;
419 j++, index += _n_queues) {
420 _ref_processor.enqueue_discovered_reflist(
421 _refs_lists[index], _pending_list_addr);
422 _refs_lists[index].set_head(_sentinel_ref);
423 _refs_lists[index].set_length(0);
424 }
425 }
426 };
427
428 // Enqueue references that are not made active again
429 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
430 AbstractRefProcTaskExecutor* task_executor) {
431 if (_processing_is_mt && task_executor != NULL) {
432 // Parallel code
433 RefProcEnqueueTask tsk(*this, _discoveredSoftRefs,
434 pending_list_addr, sentinel_ref(), _max_num_q);
435 task_executor->execute(tsk);
436 } else {
437 // Serial code: call the parent class's implementation
438 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
439 enqueue_discovered_reflist(_discoveredSoftRefs[i], pending_list_addr);
440 _discoveredSoftRefs[i].set_head(sentinel_ref());
441 _discoveredSoftRefs[i].set_length(0);
442 }
443 }
444 }
445
446 // Iterator for the list of discovered references.
447 class DiscoveredListIterator {
448 public:
449 inline DiscoveredListIterator(DiscoveredList& refs_list,
450 OopClosure* keep_alive,
451 BoolObjectClosure* is_alive);
452
453 // End Of List.
454 inline bool has_next() const { return _next != ReferenceProcessor::sentinel_ref(); }
455
456 // Get oop to the Reference object.
457 inline oop obj() const { return _ref; }
458
459 // Get oop to the referent object.
460 inline oop referent() const { return _referent; }
461
462 // Returns true if referent is alive.
463 inline bool is_referent_alive() const;
464
465 // Loads data for the current reference.
466 // The "allow_null_referent" argument tells us to allow for the possibility
467 // of a NULL referent in the discovered Reference object. This typically
468 // happens in the case of concurrent collectors that may have done the
469 // discovery concurrently, or interleaved, with mutator execution.
470 inline void load_ptrs(DEBUG_ONLY(bool allow_null_referent));
471
472 // Move to the next discovered reference.
473 inline void next();
474
475 // Remove the current reference from the list
476 inline void remove();
477
478 // Make the Reference object active again.
479 inline void make_active() { java_lang_ref_Reference::set_next(_ref, NULL); }
480
481 // Make the referent alive.
482 inline void make_referent_alive() {
483 if (UseCompressedOops) {
484 _keep_alive->do_oop((narrowOop*)_referent_addr);
485 } else {
486 _keep_alive->do_oop((oop*)_referent_addr);
487 }
488 }
489
490 // Update the discovered field.
491 inline void update_discovered() {
492 // First _prev_next ref actually points into DiscoveredList (gross).
493 if (UseCompressedOops) {
494 _keep_alive->do_oop((narrowOop*)_prev_next);
495 } else {
496 _keep_alive->do_oop((oop*)_prev_next);
497 }
498 }
499
500 // NULL out referent pointer.
501 inline void clear_referent() { oop_store_raw(_referent_addr, NULL); }
502
503 // Statistics
504 NOT_PRODUCT(
505 inline size_t processed() const { return _processed; }
506 inline size_t removed() const { return _removed; }
507 )
508
509 inline void move_to_next();
510
511 private:
512 DiscoveredList& _refs_list;
513 HeapWord* _prev_next;
514 oop _ref;
515 HeapWord* _discovered_addr;
516 oop _next;
517 HeapWord* _referent_addr;
518 oop _referent;
519 OopClosure* _keep_alive;
520 BoolObjectClosure* _is_alive;
521 DEBUG_ONLY(
522 oop _first_seen; // cyclic linked list check
523 )
524 NOT_PRODUCT(
525 size_t _processed;
526 size_t _removed;
527 )
528 };
529
530 inline DiscoveredListIterator::DiscoveredListIterator(DiscoveredList& refs_list,
531 OopClosure* keep_alive,
532 BoolObjectClosure* is_alive)
533 : _refs_list(refs_list),
534 _prev_next(refs_list.adr_head()),
535 _ref(refs_list.head()),
536 #ifdef ASSERT
537 _first_seen(refs_list.head()),
538 #endif
539 #ifndef PRODUCT
540 _processed(0),
541 _removed(0),
542 #endif
543 _next(refs_list.head()),
544 _keep_alive(keep_alive),
545 _is_alive(is_alive)
546 { }
547
548 inline bool DiscoveredListIterator::is_referent_alive() const {
549 return _is_alive->do_object_b(_referent);
550 }
551
552 inline void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
553 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
554 oop discovered = java_lang_ref_Reference::discovered(_ref);
555 assert(_discovered_addr && discovered->is_oop_or_null(),
556 "discovered field is bad");
557 _next = discovered;
558 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
559 _referent = java_lang_ref_Reference::referent(_ref);
560 assert(Universe::heap()->is_in_reserved_or_null(_referent),
561 "Wrong oop found in java.lang.Reference object");
562 assert(allow_null_referent ?
563 _referent->is_oop_or_null()
564 : _referent->is_oop(),
565 "bad referent");
566 }
567
568 inline void DiscoveredListIterator::next() {
569 _prev_next = _discovered_addr;
570 move_to_next();
571 }
572
573 inline void DiscoveredListIterator::remove() {
574 assert(_ref->is_oop(), "Dropping a bad reference");
575 oop_store_raw(_discovered_addr, NULL);
576 // First _prev_next ref actually points into DiscoveredList (gross).
577 if (UseCompressedOops) {
578 // Remove Reference object from list.
579 oopDesc::encode_store_heap_oop_not_null((narrowOop*)_prev_next, _next);
580 } else {
581 // Remove Reference object from list.
582 oopDesc::store_heap_oop((oop*)_prev_next, _next);
583 }
584 NOT_PRODUCT(_removed++);
585 _refs_list.dec_length(1);
586 }
587
588 inline void DiscoveredListIterator::move_to_next() {
589 _ref = _next;
590 assert(_ref != _first_seen, "cyclic ref_list found");
591 NOT_PRODUCT(_processed++);
592 }
593
594 // NOTE: process_phase*() are largely similar, and at a high level
595 // merely iterate over the extant list applying a predicate to
596 // each of its elements and possibly removing that element from the
597 // list and applying some further closures to that element.
598 // We should consider the possibility of replacing these
599 // process_phase*() methods by abstracting them into
600 // a single general iterator invocation that receives appropriate
601 // closures that accomplish this work.
602
603 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
604 // referents are not alive, but that should be kept alive for policy reasons.
605 // Keep alive the transitive closure of all such referents.
606 void
607 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
608 ReferencePolicy* policy,
609 BoolObjectClosure* is_alive,
610 OopClosure* keep_alive,
611 VoidClosure* complete_gc) {
612 assert(policy != NULL, "Must have a non-NULL policy");
613 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
614 // Decide which softly reachable refs should be kept alive.
615 while (iter.has_next()) {
616 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
617 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
618 if (referent_is_dead && !policy->should_clear_reference(iter.obj())) {
619 if (TraceReferenceGC) {
620 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
621 iter.obj(), iter.obj()->blueprint()->internal_name());
622 }
623 // Remove Reference object from list
624 iter.remove();
625 // Make the Reference object active again
626 iter.make_active();
627 // keep the referent around
628 iter.make_referent_alive();
629 iter.move_to_next();
630 } else {
631 iter.next();
632 }
633 }
634 // Close the reachable set
635 complete_gc->do_void();
636 NOT_PRODUCT(
637 if (PrintGCDetails && TraceReferenceGC) {
638 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
639 "discovered Refs by policy list " INTPTR_FORMAT,
640 iter.removed(), iter.processed(), (address)refs_list.head());
641 }
642 )
643 }
644
645 // Traverse the list and remove any Refs that are not active, or
646 // whose referents are either alive or NULL.
647 void
648 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
649 BoolObjectClosure* is_alive,
650 OopClosure* keep_alive) {
651 assert(discovery_is_atomic(), "Error");
652 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
653 while (iter.has_next()) {
654 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
655 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
656 assert(next == NULL, "Should not discover inactive Reference");
657 if (iter.is_referent_alive()) {
658 if (TraceReferenceGC) {
659 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
660 iter.obj(), iter.obj()->blueprint()->internal_name());
661 }
662 // The referent is reachable after all.
663 // Remove Reference object from list.
664 iter.remove();
665 // Update the referent pointer as necessary: Note that this
666 // should not entail any recursive marking because the
667 // referent must already have been traversed.
668 iter.make_referent_alive();
669 iter.move_to_next();
670 } else {
671 iter.next();
672 }
673 }
674 NOT_PRODUCT(
675 if (PrintGCDetails && TraceReferenceGC) {
676 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
677 "Refs in discovered list " INTPTR_FORMAT,
678 iter.removed(), iter.processed(), (address)refs_list.head());
679 }
680 )
681 }
682
683 void
684 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
685 BoolObjectClosure* is_alive,
686 OopClosure* keep_alive,
687 VoidClosure* complete_gc) {
688 assert(!discovery_is_atomic(), "Error");
689 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
690 while (iter.has_next()) {
691 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
692 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
693 oop next = java_lang_ref_Reference::next(iter.obj());
694 if ((iter.referent() == NULL || iter.is_referent_alive() ||
695 next != NULL)) {
696 assert(next->is_oop_or_null(), "bad next field");
697 // Remove Reference object from list
698 iter.remove();
699 // Trace the cohorts
700 iter.make_referent_alive();
701 if (UseCompressedOops) {
702 keep_alive->do_oop((narrowOop*)next_addr);
703 } else {
704 keep_alive->do_oop((oop*)next_addr);
705 }
706 iter.move_to_next();
707 } else {
708 iter.next();
709 }
710 }
711 // Now close the newly reachable set
712 complete_gc->do_void();
713 NOT_PRODUCT(
714 if (PrintGCDetails && TraceReferenceGC) {
715 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
716 "Refs in discovered list " INTPTR_FORMAT,
717 iter.removed(), iter.processed(), (address)refs_list.head());
718 }
719 )
720 }
721
722 // Traverse the list and process the referents, by either
723 // clearing them or keeping them (and their reachable
724 // closure) alive.
725 void
726 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
727 bool clear_referent,
728 BoolObjectClosure* is_alive,
729 OopClosure* keep_alive,
730 VoidClosure* complete_gc) {
731 ResourceMark rm;
732 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
733 while (iter.has_next()) {
734 iter.update_discovered();
735 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
736 if (clear_referent) {
737 // NULL out referent pointer
738 iter.clear_referent();
739 } else {
740 // keep the referent around
741 iter.make_referent_alive();
742 }
743 if (TraceReferenceGC) {
744 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
745 clear_referent ? "cleared " : "",
746 iter.obj(), iter.obj()->blueprint()->internal_name());
747 }
748 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
749 iter.next();
750 }
751 // Remember to keep sentinel pointer around
752 iter.update_discovered();
753 // Close the reachable set
754 complete_gc->do_void();
755 }
756
757 void
758 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
759 oop obj = refs_list.head();
760 while (obj != sentinel_ref()) {
761 oop discovered = java_lang_ref_Reference::discovered(obj);
762 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
763 obj = discovered;
764 }
765 refs_list.set_head(sentinel_ref());
766 refs_list.set_length(0);
767 }
768
769 void ReferenceProcessor::abandon_partial_discovery() {
770 // loop over the lists
771 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
772 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
773 gclog_or_tty->print_cr(
774 "\nAbandoning %s discovered list",
775 list_name(i));
776 }
777 abandon_partial_discovered_list(_discoveredSoftRefs[i]);
778 }
779 }
780
781 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
782 public:
783 RefProcPhase1Task(ReferenceProcessor& ref_processor,
784 DiscoveredList refs_lists[],
785 ReferencePolicy* policy,
786 bool marks_oops_alive)
787 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
788 _policy(policy)
789 { }
790 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
791 OopClosure& keep_alive,
792 VoidClosure& complete_gc)
793 {
794 Thread* thr = Thread::current();
795 int refs_list_index = ((WorkerThread*)thr)->id();
796 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
797 &is_alive, &keep_alive, &complete_gc);
798 }
799 private:
800 ReferencePolicy* _policy;
801 };
802
803 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
804 public:
805 RefProcPhase2Task(ReferenceProcessor& ref_processor,
806 DiscoveredList refs_lists[],
807 bool marks_oops_alive)
808 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
809 { }
810 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
811 OopClosure& keep_alive,
812 VoidClosure& complete_gc)
813 {
814 _ref_processor.process_phase2(_refs_lists[i],
815 &is_alive, &keep_alive, &complete_gc);
816 }
817 };
818
819 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
820 public:
821 RefProcPhase3Task(ReferenceProcessor& ref_processor,
822 DiscoveredList refs_lists[],
823 bool clear_referent,
824 bool marks_oops_alive)
825 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
826 _clear_referent(clear_referent)
827 { }
828 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
829 OopClosure& keep_alive,
830 VoidClosure& complete_gc)
831 {
832 // Don't use "refs_list_index" calculated in this way because
833 // balance_queues() has moved the Ref's into the first n queues.
834 // Thread* thr = Thread::current();
835 // int refs_list_index = ((WorkerThread*)thr)->id();
836 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
837 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
838 &is_alive, &keep_alive, &complete_gc);
839 }
840 private:
841 bool _clear_referent;
842 };
843
844 // Balances reference queues.
845 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
846 // queues[0, 1, ..., _num_q-1] because only the first _num_q
847 // corresponding to the active workers will be processed.
848 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
849 {
850 // calculate total length
851 size_t total_refs = 0;
852 if (TraceReferenceGC && PrintGCDetails) {
853 gclog_or_tty->print_cr("\nBalance ref_lists ");
854 }
855
856 for (int i = 0; i < _max_num_q; ++i) {
857 total_refs += ref_lists[i].length();
858 if (TraceReferenceGC && PrintGCDetails) {
859 gclog_or_tty->print("%d ", ref_lists[i].length());
860 }
861 }
862 if (TraceReferenceGC && PrintGCDetails) {
863 gclog_or_tty->print_cr(" = %d", total_refs);
864 }
865 size_t avg_refs = total_refs / _num_q + 1;
866 int to_idx = 0;
867 for (int from_idx = 0; from_idx < _max_num_q; from_idx++) {
868 bool move_all = false;
869 if (from_idx >= _num_q) {
870 move_all = ref_lists[from_idx].length() > 0;
871 }
872 while ((ref_lists[from_idx].length() > avg_refs) ||
873 move_all) {
874 assert(to_idx < _num_q, "Sanity Check!");
875 if (ref_lists[to_idx].length() < avg_refs) {
876 // move superfluous refs
877 size_t refs_to_move;
878 // Move all the Ref's if the from queue will not be processed.
879 if (move_all) {
880 refs_to_move = MIN2(ref_lists[from_idx].length(),
881 avg_refs - ref_lists[to_idx].length());
882 } else {
883 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
884 avg_refs - ref_lists[to_idx].length());
885 }
886 oop move_head = ref_lists[from_idx].head();
887 oop move_tail = move_head;
888 oop new_head = move_head;
889 // find an element to split the list on
890 for (size_t j = 0; j < refs_to_move; ++j) {
891 move_tail = new_head;
892 new_head = java_lang_ref_Reference::discovered(new_head);
893 }
894 java_lang_ref_Reference::set_discovered(move_tail, ref_lists[to_idx].head());
895 ref_lists[to_idx].set_head(move_head);
896 ref_lists[to_idx].inc_length(refs_to_move);
897 ref_lists[from_idx].set_head(new_head);
898 ref_lists[from_idx].dec_length(refs_to_move);
899 if (ref_lists[from_idx].length() == 0) {
900 break;
901 }
902 } else {
903 to_idx = (to_idx + 1) % _num_q;
904 }
905 }
906 }
907 #ifdef ASSERT
908 size_t balanced_total_refs = 0;
909 for (int i = 0; i < _max_num_q; ++i) {
910 balanced_total_refs += ref_lists[i].length();
911 if (TraceReferenceGC && PrintGCDetails) {
912 gclog_or_tty->print("%d ", ref_lists[i].length());
913 }
914 }
915 if (TraceReferenceGC && PrintGCDetails) {
916 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
917 gclog_or_tty->flush();
918 }
919 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
920 #endif
921 }
922
923 void ReferenceProcessor::balance_all_queues() {
924 balance_queues(_discoveredSoftRefs);
925 balance_queues(_discoveredWeakRefs);
926 balance_queues(_discoveredFinalRefs);
927 balance_queues(_discoveredPhantomRefs);
928 }
929
930 void
931 ReferenceProcessor::process_discovered_reflist(
932 DiscoveredList refs_lists[],
933 ReferencePolicy* policy,
934 bool clear_referent,
935 BoolObjectClosure* is_alive,
936 OopClosure* keep_alive,
937 VoidClosure* complete_gc,
938 AbstractRefProcTaskExecutor* task_executor)
939 {
940 bool mt_processing = task_executor != NULL && _processing_is_mt;
941 // If discovery used MT and a dynamic number of GC threads, then
942 // the queues must be balanced for correctness if fewer than the
943 // maximum number of queues were used. The number of queue used
944 // during discovery may be different than the number to be used
945 // for processing so don't depend of _num_q < _max_num_q as part
946 // of the test.
947 bool must_balance = _discovery_is_mt;
948
949 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
950 must_balance) {
951 balance_queues(refs_lists);
952 }
953 if (PrintReferenceGC && PrintGCDetails) {
954 size_t total = 0;
955 for (int i = 0; i < _num_q; ++i) {
956 total += refs_lists[i].length();
957 }
958 gclog_or_tty->print(", %u refs", total);
959 }
960
961 // Phase 1 (soft refs only):
962 // . Traverse the list and remove any SoftReferences whose
963 // referents are not alive, but that should be kept alive for
964 // policy reasons. Keep alive the transitive closure of all
965 // such referents.
966 if (policy != NULL) {
967 if (mt_processing) {
968 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
969 task_executor->execute(phase1);
970 } else {
971 for (int i = 0; i < _num_q; i++) {
972 process_phase1(refs_lists[i], policy,
973 is_alive, keep_alive, complete_gc);
974 }
975 }
976 } else { // policy == NULL
977 assert(refs_lists != _discoveredSoftRefs,
978 "Policy must be specified for soft references.");
979 }
980
981 // Phase 2:
982 // . Traverse the list and remove any refs whose referents are alive.
983 if (mt_processing) {
984 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
985 task_executor->execute(phase2);
986 } else {
987 for (int i = 0; i < _num_q; i++) {
988 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
989 }
990 }
991
992 // Phase 3:
993 // . Traverse the list and process referents as appropriate.
994 if (mt_processing) {
995 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
996 task_executor->execute(phase3);
997 } else {
998 for (int i = 0; i < _num_q; i++) {
999 process_phase3(refs_lists[i], clear_referent,
1000 is_alive, keep_alive, complete_gc);
1001 }
1002 }
1003 }
1004
1005 void ReferenceProcessor::clean_up_discovered_references() {
1006 // loop over the lists
1007 // Should this instead be
1008 // for (int i = 0; i < subclasses_of_ref; i++_ {
1009 // for (int j = 0; j < _num_q; j++) {
1010 // int index = i * _max_num_q + j;
1011 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
1012 if (TraceReferenceGC && PrintGCDetails && ((i % _num_q) == 0)) {
1013 gclog_or_tty->print_cr(
1014 "\nScrubbing %s discovered list of Null referents",
1015 list_name(i));
1016 }
1017 clean_up_discovered_reflist(_discoveredSoftRefs[i]);
1018 }
1019 }
1020
1021 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
1022 assert(!discovery_is_atomic(), "Else why call this method?");
1023 DiscoveredListIterator iter(refs_list, NULL, NULL);
1024 while (iter.has_next()) {
1025 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1026 oop next = java_lang_ref_Reference::next(iter.obj());
1027 assert(next->is_oop_or_null(), "bad next field");
1028 // If referent has been cleared or Reference is not active,
1029 // drop it.
1030 if (iter.referent() == NULL || next != NULL) {
1031 debug_only(
1032 if (PrintGCDetails && TraceReferenceGC) {
1033 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
1034 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
1035 " and referent: " INTPTR_FORMAT,
1036 iter.obj(), next, iter.referent());
1037 }
1038 )
1039 // Remove Reference object from list
1040 iter.remove();
1041 iter.move_to_next();
1042 } else {
1043 iter.next();
1044 }
1045 }
1046 NOT_PRODUCT(
1047 if (PrintGCDetails && TraceReferenceGC) {
1048 gclog_or_tty->print(
1049 " Removed %d Refs with NULL referents out of %d discovered Refs",
1050 iter.removed(), iter.processed());
1051 }
1052 )
1053 }
1054
1055 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1056 int id = 0;
1057 // Determine the queue index to use for this object.
1058 if (_discovery_is_mt) {
1059 // During a multi-threaded discovery phase,
1060 // each thread saves to its "own" list.
1061 Thread* thr = Thread::current();
1062 assert(thr->is_GC_task_thread(),
1063 "Dubious cast from Thread* to WorkerThread*?");
1064 id = ((WorkerThread*)thr)->id();
1065 } else {
1066 // single-threaded discovery, we save in round-robin
1067 // fashion to each of the lists.
1068 if (_processing_is_mt) {
1069 id = next_id();
1070 }
1071 }
1072 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1073
1074 // Get the discovered queue to which we will add
1075 DiscoveredList* list = NULL;
1076 switch (rt) {
1077 case REF_OTHER:
1078 // Unknown reference type, no special treatment
1079 break;
1080 case REF_SOFT:
1081 list = &_discoveredSoftRefs[id];
1082 break;
1083 case REF_WEAK:
1084 list = &_discoveredWeakRefs[id];
1085 break;
1086 case REF_FINAL:
1087 list = &_discoveredFinalRefs[id];
1088 break;
1089 case REF_PHANTOM:
1090 list = &_discoveredPhantomRefs[id];
1091 break;
1092 case REF_NONE:
1093 // we should not reach here if we are an instanceRefKlass
1094 default:
1095 ShouldNotReachHere();
1096 }
1097 if (TraceReferenceGC && PrintGCDetails) {
1098 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT,
1099 id, list);
1100 }
1101 return list;
1102 }
1103
1104 inline void
1105 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1106 oop obj,
1107 HeapWord* discovered_addr) {
1108 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1109 // First we must make sure this object is only enqueued once. CAS in a non null
1110 // discovered_addr.
1111 oop current_head = refs_list.head();
1112
1113 // Note: In the case of G1, this specific pre-barrier is strictly
1114 // not necessary because the only case we are interested in
1115 // here is when *discovered_addr is NULL (see the CAS further below),
1116 // so this will expand to nothing. As a result, we have manually
1117 // elided this out for G1, but left in the test for some future
1118 // collector that might have need for a pre-barrier here.
1119 if (_discovered_list_needs_barrier && !UseG1GC) {
1120 if (UseCompressedOops) {
1121 _bs->write_ref_field_pre((narrowOop*)discovered_addr, current_head);
1122 } else {
1123 _bs->write_ref_field_pre((oop*)discovered_addr, current_head);
1124 }
1125 guarantee(false, "Need to check non-G1 collector");
1126 }
1127 oop retest = oopDesc::atomic_compare_exchange_oop(current_head, discovered_addr,
1128 NULL);
1129 if (retest == NULL) {
1130 // This thread just won the right to enqueue the object.
1131 // We have separate lists for enqueueing so no synchronization
1132 // is necessary.
1133 refs_list.set_head(obj);
1134 refs_list.inc_length(1);
1135 if (_discovered_list_needs_barrier) {
1136 _bs->write_ref_field((void*)discovered_addr, current_head);
1137 }
1138 } else {
1139 // If retest was non NULL, another thread beat us to it:
1140 // The reference has already been discovered...
1141 if (TraceReferenceGC) {
1142 gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",
1143 obj, obj->blueprint()->internal_name());
1144 }
1145 }
1146 }
1147
1148 // We mention two of several possible choices here:
1149 // #0: if the reference object is not in the "originating generation"
1150 // (or part of the heap being collected, indicated by our "span"
1151 // we don't treat it specially (i.e. we scan it as we would
1152 // a normal oop, treating its references as strong references).
1153 // This means that references can't be enqueued unless their
1154 // referent is also in the same span. This is the simplest,
1155 // most "local" and most conservative approach, albeit one
1156 // that may cause weak references to be enqueued least promptly.
1157 // We call this choice the "ReferenceBasedDiscovery" policy.
1158 // #1: the reference object may be in any generation (span), but if
1159 // the referent is in the generation (span) being currently collected
1160 // then we can discover the reference object, provided
1161 // the object has not already been discovered by
1162 // a different concurrently running collector (as may be the
1163 // case, for instance, if the reference object is in CMS and
1164 // the referent in DefNewGeneration), and provided the processing
1165 // of this reference object by the current collector will
1166 // appear atomic to every other collector in the system.
1167 // (Thus, for instance, a concurrent collector may not
1168 // discover references in other generations even if the
1169 // referent is in its own generation). This policy may,
1170 // in certain cases, enqueue references somewhat sooner than
1171 // might Policy #0 above, but at marginally increased cost
1172 // and complexity in processing these references.
1173 // We call this choice the "RefeferentBasedDiscovery" policy.
1174 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1175 // We enqueue references only if we are discovering refs
1176 // (rather than processing discovered refs).
1177 if (!_discovering_refs || !RegisterReferences) {
1178 return false;
1179 }
1180 // We only enqueue active references.
1181 oop next = java_lang_ref_Reference::next(obj);
1182 if (next != NULL) {
1183 return false;
1184 }
1185
1186 HeapWord* obj_addr = (HeapWord*)obj;
1187 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1188 !_span.contains(obj_addr)) {
1189 // Reference is not in the originating generation;
1190 // don't treat it specially (i.e. we want to scan it as a normal
1191 // object with strong references).
1192 return false;
1193 }
1194
1195 // We only enqueue references whose referents are not (yet) strongly
1196 // reachable.
1197 if (is_alive_non_header() != NULL) {
1198 oop referent = java_lang_ref_Reference::referent(obj);
1199 // In the case of non-concurrent discovery, the last
1200 // disjunct below should hold. It may not hold in the
1201 // case of concurrent discovery because mutators may
1202 // concurrently clear() a Reference.
1203 assert(UseConcMarkSweepGC || UseG1GC || referent != NULL,
1204 "Refs with null referents already filtered");
1205 if (is_alive_non_header()->do_object_b(referent)) {
1206 return false; // referent is reachable
1207 }
1208 }
1209 if (rt == REF_SOFT) {
1210 // For soft refs we can decide now if these are not
1211 // current candidates for clearing, in which case we
1212 // can mark through them now, rather than delaying that
1213 // to the reference-processing phase. Since all current
1214 // time-stamp policies advance the soft-ref clock only
1215 // at a major collection cycle, this is always currently
1216 // accurate.
1217 if (!_current_soft_ref_policy->should_clear_reference(obj)) {
1218 return false;
1219 }
1220 }
1221
1222 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1223 const oop discovered = java_lang_ref_Reference::discovered(obj);
1224 assert(discovered->is_oop_or_null(), "bad discovered field");
1225 if (discovered != NULL) {
1226 // The reference has already been discovered...
1227 if (TraceReferenceGC) {
1228 gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",
1229 obj, obj->blueprint()->internal_name());
1230 }
1231 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1232 // assumes that an object is not processed twice;
1233 // if it's been already discovered it must be on another
1234 // generation's discovered list; so we won't discover it.
1235 return false;
1236 } else {
1237 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1238 "Unrecognized policy");
1239 // Check assumption that an object is not potentially
1240 // discovered twice except by concurrent collectors that potentially
1241 // trace the same Reference object twice.
1242 assert(UseConcMarkSweepGC,
1243 "Only possible with an incremental-update concurrent collector");
1244 return true;
1245 }
1246 }
1247
1248 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1249 oop referent = java_lang_ref_Reference::referent(obj);
1250 assert(referent->is_oop(), "bad referent");
1251 // enqueue if and only if either:
1252 // reference is in our span or
1253 // we are an atomic collector and referent is in our span
1254 if (_span.contains(obj_addr) ||
1255 (discovery_is_atomic() && _span.contains(referent))) {
1256 // should_enqueue = true;
1257 } else {
1258 return false;
1259 }
1260 } else {
1261 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1262 _span.contains(obj_addr), "code inconsistency");
1263 }
1264
1265 // Get the right type of discovered queue head.
1266 DiscoveredList* list = get_discovered_list(rt);
1267 if (list == NULL) {
1268 return false; // nothing special needs to be done
1269 }
1270
1271 if (_discovery_is_mt) {
1272 add_to_discovered_list_mt(*list, obj, discovered_addr);
1273 } else {
1274 // If "_discovered_list_needs_barrier", we do write barriers when
1275 // updating the discovered reference list. Otherwise, we do a raw store
1276 // here: the field will be visited later when processing the discovered
1277 // references.
1278 oop current_head = list->head();
1279 // As in the case further above, since we are over-writing a NULL
1280 // pre-value, we can safely elide the pre-barrier here for the case of G1.
1281 assert(discovered == NULL, "control point invariant");
1282 if (_discovered_list_needs_barrier && !UseG1GC) { // safe to elide for G1
1283 if (UseCompressedOops) {
1284 _bs->write_ref_field_pre((narrowOop*)discovered_addr, current_head);
1285 } else {
1286 _bs->write_ref_field_pre((oop*)discovered_addr, current_head);
1287 }
1288 guarantee(false, "Need to check non-G1 collector");
1289 }
1290 oop_store_raw(discovered_addr, current_head);
1291 if (_discovered_list_needs_barrier) {
1292 _bs->write_ref_field((void*)discovered_addr, current_head);
1293 }
1294 list->set_head(obj);
1295 list->inc_length(1);
1296 }
1297
1298 // In the MT discovery case, it is currently possible to see
1299 // the following message multiple times if several threads
1300 // discover a reference about the same time. Only one will
1301 // however have actually added it to the disocvered queue.
1302 // One could let add_to_discovered_list_mt() return an
1303 // indication for success in queueing (by 1 thread) or
1304 // failure (by all other threads), but I decided the extra
1305 // code was not worth the effort for something that is
1306 // only used for debugging support.
1307 if (TraceReferenceGC) {
1308 oop referent = java_lang_ref_Reference::referent(obj);
1309 if (PrintGCDetails) {
1310 gclog_or_tty->print_cr("Enqueued reference (" INTPTR_FORMAT ": %s)",
1311 obj, obj->blueprint()->internal_name());
1312 }
1313 assert(referent->is_oop(), "Enqueued a bad referent");
1314 }
1315 assert(obj->is_oop(), "Enqueued a bad reference");
1316 return true;
1317 }
1318
1319 // Preclean the discovered references by removing those
1320 // whose referents are alive, and by marking from those that
1321 // are not active. These lists can be handled here
1322 // in any order and, indeed, concurrently.
1323 void ReferenceProcessor::preclean_discovered_references(
1324 BoolObjectClosure* is_alive,
1325 OopClosure* keep_alive,
1326 VoidClosure* complete_gc,
1327 YieldClosure* yield,
1328 bool should_unload_classes) {
1329
1330 NOT_PRODUCT(verify_ok_to_handle_reflists());
1331
1332 #ifdef ASSERT
1333 bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC ||
1334 CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
1335 ExplicitGCInvokesConcurrentAndUnloadsClasses &&
1336 UseConcMarkSweepGC && should_unload_classes;
1337 RememberKlassesChecker mx(must_remember_klasses);
1338 #endif
1339 // Soft references
1340 {
1341 TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1342 false, gclog_or_tty);
1343 for (int i = 0; i < _max_num_q; i++) {
1344 if (yield->should_return()) {
1345 return;
1346 }
1347 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1348 keep_alive, complete_gc, yield);
1349 }
1350 }
1351
1352 // Weak references
1353 {
1354 TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1355 false, gclog_or_tty);
1356 for (int i = 0; i < _num_q; i++) {
1357 if (yield->should_return()) {
1358 return;
1359 }
1360 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1361 keep_alive, complete_gc, yield);
1362 }
1363 }
1364
1365 // Final references
1366 {
1367 TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1368 false, gclog_or_tty);
1369 for (int i = 0; i < _num_q; i++) {
1370 if (yield->should_return()) {
1371 return;
1372 }
1373 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1374 keep_alive, complete_gc, yield);
1375 }
1376 }
1377
1378 // Phantom references
1379 {
1380 TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1381 false, gclog_or_tty);
1382 for (int i = 0; i < _num_q; i++) {
1383 if (yield->should_return()) {
1384 return;
1385 }
1386 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1387 keep_alive, complete_gc, yield);
1388 }
1389 }
1390 }
1391
1392 // Walk the given discovered ref list, and remove all reference objects
1393 // whose referents are still alive, whose referents are NULL or which
1394 // are not active (have a non-NULL next field). NOTE: When we are
1395 // thus precleaning the ref lists (which happens single-threaded today),
1396 // we do not disable refs discovery to honour the correct semantics of
1397 // java.lang.Reference. As a result, we need to be careful below
1398 // that ref removal steps interleave safely with ref discovery steps
1399 // (in this thread).
1400 void
1401 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1402 BoolObjectClosure* is_alive,
1403 OopClosure* keep_alive,
1404 VoidClosure* complete_gc,
1405 YieldClosure* yield) {
1406 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1407 while (iter.has_next()) {
1408 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1409 oop obj = iter.obj();
1410 oop next = java_lang_ref_Reference::next(obj);
1411 if (iter.referent() == NULL || iter.is_referent_alive() ||
1412 next != NULL) {
1413 // The referent has been cleared, or is alive, or the Reference is not
1414 // active; we need to trace and mark its cohort.
1415 if (TraceReferenceGC) {
1416 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1417 iter.obj(), iter.obj()->blueprint()->internal_name());
1418 }
1419 // Remove Reference object from list
1420 iter.remove();
1421 // Keep alive its cohort.
1422 iter.make_referent_alive();
1423 if (UseCompressedOops) {
1424 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1425 keep_alive->do_oop(next_addr);
1426 } else {
1427 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1428 keep_alive->do_oop(next_addr);
1429 }
1430 iter.move_to_next();
1431 } else {
1432 iter.next();
1433 }
1434 }
1435 // Close the reachable set
1436 complete_gc->do_void();
1437
1438 NOT_PRODUCT(
1439 if (PrintGCDetails && PrintReferenceGC) {
1440 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1441 "Refs in discovered list " INTPTR_FORMAT,
1442 iter.removed(), iter.processed(), (address)refs_list.head());
1443 }
1444 )
1445 }
1446
1447 const char* ReferenceProcessor::list_name(int i) {
1448 assert(i >= 0 && i <= _max_num_q * subclasses_of_ref, "Out of bounds index");
1449 int j = i / _max_num_q;
1450 switch (j) {
1451 case 0: return "SoftRef";
1452 case 1: return "WeakRef";
1453 case 2: return "FinalRef";
1454 case 3: return "PhantomRef";
1455 }
1456 ShouldNotReachHere();
1457 return NULL;
1458 }
1459
1460 #ifndef PRODUCT
1461 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1462 // empty for now
1463 }
1464 #endif
1465
1466 void ReferenceProcessor::verify() {
1467 guarantee(sentinel_ref() != NULL && sentinel_ref()->is_oop(), "Lost _sentinelRef");
1468 }
1469
1470 #ifndef PRODUCT
1471 void ReferenceProcessor::clear_discovered_references() {
1472 guarantee(!_discovering_refs, "Discovering refs?");
1473 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
1474 oop obj = _discoveredSoftRefs[i].head();
1475 while (obj != sentinel_ref()) {
1476 oop next = java_lang_ref_Reference::discovered(obj);
1477 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
1478 obj = next;
1479 }
1480 _discoveredSoftRefs[i].set_head(sentinel_ref());
1481 _discoveredSoftRefs[i].set_length(0);
1482 }
1483 }
1484 #endif // PRODUCT