1 /*
2 * Copyright (c) 2001, 2014, 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_implementation/shared/gcTimer.hpp"
29 #include "gc_implementation/shared/gcTraceTime.hpp"
30 #include "gc_interface/collectedHeap.hpp"
31 #include "gc_interface/collectedHeap.inline.hpp"
32 #include "memory/referencePolicy.hpp"
33 #include "memory/referenceProcessor.hpp"
34 #include "oops/oop.inline.hpp"
35 #include "runtime/java.hpp"
36 #include "runtime/jniHandles.hpp"
37
38 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
39
40 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
41 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
42 bool ReferenceProcessor::_pending_list_uses_discovered_field = false;
43 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
44
45 void referenceProcessor_init() {
46 ReferenceProcessor::init_statics();
47 }
48
49 void ReferenceProcessor::init_statics() {
50 // We need a monotonically non-decreasing time in ms but
51 // os::javaTimeMillis() does not guarantee monotonicity.
52 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
53
54 // Initialize the soft ref timestamp clock.
55 _soft_ref_timestamp_clock = now;
56 // Also update the soft ref clock in j.l.r.SoftReference
57 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
58
59 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
60 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
61 NOT_COMPILER2(LRUCurrentHeapPolicy());
62 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
63 vm_exit_during_initialization("Could not allocate reference policy object");
64 }
65 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
66 RefDiscoveryPolicy == ReferentBasedDiscovery,
67 "Unrecognized RefDiscoveryPolicy");
68 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
69 }
70
71 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
72 #ifdef ASSERT
73 // Verify that we're not currently discovering refs
74 assert(!verify_disabled || !_discovering_refs, "nested call?");
75
76 if (check_no_refs) {
77 // Verify that the discovered lists are empty
78 verify_no_references_recorded();
79 }
80 #endif // ASSERT
81
82 // Someone could have modified the value of the static
83 // field in the j.l.r.SoftReference class that holds the
84 // soft reference timestamp clock using reflection or
85 // Unsafe between GCs. Unconditionally update the static
86 // field in ReferenceProcessor here so that we use the new
87 // value during reference discovery.
88
89 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
90 _discovering_refs = true;
91 }
92
93 ReferenceProcessor::ReferenceProcessor(MemRegion span,
94 bool mt_processing,
95 uint mt_processing_degree,
96 bool mt_discovery,
97 uint mt_discovery_degree,
98 bool atomic_discovery,
99 BoolObjectClosure* is_alive_non_header) :
100 _discovering_refs(false),
101 _enqueuing_is_done(false),
102 _is_alive_non_header(is_alive_non_header),
103 _processing_is_mt(mt_processing),
104 _next_id(0)
105 {
106 _span = span;
107 _discovery_is_atomic = atomic_discovery;
108 _discovery_is_mt = mt_discovery;
109 _num_q = MAX2(1U, mt_processing_degree);
110 _max_num_q = MAX2(_num_q, mt_discovery_degree);
111 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
112 _max_num_q * number_of_subclasses_of_ref(), mtGC);
113
114 if (_discovered_refs == NULL) {
115 vm_exit_during_initialization("Could not allocated RefProc Array");
116 }
117 _discoveredSoftRefs = &_discovered_refs[0];
118 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
119 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
120 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
121
122 // Initialize all entries to NULL
123 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
124 _discovered_refs[i].set_head(NULL);
125 _discovered_refs[i].set_length(0);
126 }
127
128 setup_policy(false /* default soft ref policy */);
129 }
130
131 #ifndef PRODUCT
132 void ReferenceProcessor::verify_no_references_recorded() {
133 guarantee(!_discovering_refs, "Discovering refs?");
134 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
135 guarantee(_discovered_refs[i].is_empty(),
136 "Found non-empty discovered list");
137 }
138 }
139 #endif
140
141 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
142 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
143 if (UseCompressedOops) {
144 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
145 } else {
146 f->do_oop((oop*)_discovered_refs[i].adr_head());
147 }
148 }
149 }
150
151 void ReferenceProcessor::update_soft_ref_master_clock() {
152 // Update (advance) the soft ref master clock field. This must be done
153 // after processing the soft ref list.
154
155 // We need a monotonically non-decreasing time in ms but
156 // os::javaTimeMillis() does not guarantee monotonicity.
157 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
158 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
159 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
160
161 NOT_PRODUCT(
162 if (now < _soft_ref_timestamp_clock) {
163 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
164 _soft_ref_timestamp_clock, now);
165 }
166 )
167 // The values of now and _soft_ref_timestamp_clock are set using
168 // javaTimeNanos(), which is guaranteed to be monotonically
169 // non-decreasing provided the underlying platform provides such
170 // a time source (and it is bug free).
171 // In product mode, however, protect ourselves from non-monotonicity.
172 if (now > _soft_ref_timestamp_clock) {
173 _soft_ref_timestamp_clock = now;
174 java_lang_ref_SoftReference::set_clock(now);
175 }
176 // Else leave clock stalled at its old value until time progresses
177 // past clock value.
178 }
179
180 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
181 size_t total = 0;
182 for (uint i = 0; i < _max_num_q; ++i) {
183 total += lists[i].length();
184 }
185 return total;
186 }
187
188 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
189 BoolObjectClosure* is_alive,
190 OopClosure* keep_alive,
191 VoidClosure* complete_gc,
192 AbstractRefProcTaskExecutor* task_executor,
193 GCTimer* gc_timer) {
194 NOT_PRODUCT(verify_ok_to_handle_reflists());
195
196 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
197 // Stop treating discovered references specially.
198 disable_discovery();
199
200 // If discovery was concurrent, someone could have modified
201 // the value of the static field in the j.l.r.SoftReference
202 // class that holds the soft reference timestamp clock using
203 // reflection or Unsafe between when discovery was enabled and
204 // now. Unconditionally update the static field in ReferenceProcessor
205 // here so that we use the new value during processing of the
206 // discovered soft refs.
207
208 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
209
210 bool trace_time = PrintGCDetails && PrintReferenceGC;
211
212 // Soft references
213 size_t soft_count = 0;
214 {
215 GCTraceTime tt("SoftReference", trace_time, false, gc_timer);
216 soft_count =
217 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
218 is_alive, keep_alive, complete_gc, task_executor);
219 }
220
221 update_soft_ref_master_clock();
222
223 // Weak references
224 size_t weak_count = 0;
225 {
226 GCTraceTime tt("WeakReference", trace_time, false, gc_timer);
227 weak_count =
228 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
229 is_alive, keep_alive, complete_gc, task_executor);
230 }
231
232 // Final references
233 size_t final_count = 0;
234 {
235 GCTraceTime tt("FinalReference", trace_time, false, gc_timer);
236 final_count =
237 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
238 is_alive, keep_alive, complete_gc, task_executor);
239 }
240
241 // Phantom references
242 size_t phantom_count = 0;
243 {
244 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer);
245 phantom_count =
246 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
247 is_alive, keep_alive, complete_gc, task_executor);
248 }
249
250 // Weak global JNI references. It would make more sense (semantically) to
251 // traverse these simultaneously with the regular weak references above, but
252 // that is not how the JDK1.2 specification is. See #4126360. Native code can
253 // thus use JNI weak references to circumvent the phantom references and
254 // resurrect a "post-mortem" object.
255 {
256 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
257 if (task_executor != NULL) {
258 task_executor->set_single_threaded_mode();
259 }
260 process_phaseJNI(is_alive, keep_alive, complete_gc);
261 }
262
263 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
264 }
265
266 #ifndef PRODUCT
267 // Calculate the number of jni handles.
268 uint ReferenceProcessor::count_jni_refs() {
269 class AlwaysAliveClosure: public BoolObjectClosure {
270 public:
271 virtual bool do_object_b(oop obj) { return true; }
272 };
273
274 class CountHandleClosure: public OopClosure {
275 private:
276 int _count;
277 public:
278 CountHandleClosure(): _count(0) {}
279 void do_oop(oop* unused) { _count++; }
280 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
281 int count() { return _count; }
282 };
283 CountHandleClosure global_handle_count;
284 AlwaysAliveClosure always_alive;
285 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
286 return global_handle_count.count();
287 }
288 #endif
289
290 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
291 OopClosure* keep_alive,
292 VoidClosure* complete_gc) {
293 #ifndef PRODUCT
294 if (PrintGCDetails && PrintReferenceGC) {
295 unsigned int count = count_jni_refs();
296 gclog_or_tty->print(", %u refs", count);
297 }
298 #endif
299 JNIHandles::weak_oops_do(is_alive, keep_alive);
300 complete_gc->do_void();
301 }
302
303
304 template <class T>
305 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
306 AbstractRefProcTaskExecutor* task_executor) {
307
308 // Remember old value of pending references list
309 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
310 T old_pending_list_value = *pending_list_addr;
311
312 // Enqueue references that are not made active again, and
313 // clear the decks for the next collection (cycle).
314 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
315 // Do the post-barrier on pending_list_addr missed in
316 // enqueue_discovered_reflist.
317 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
318
319 // Stop treating discovered references specially.
320 ref->disable_discovery();
321
322 // Return true if new pending references were added
323 return old_pending_list_value != *pending_list_addr;
324 }
325
326 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
327 NOT_PRODUCT(verify_ok_to_handle_reflists());
328 if (UseCompressedOops) {
329 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
330 } else {
331 return enqueue_discovered_ref_helper<oop>(this, task_executor);
332 }
333 }
334
335 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
336 HeapWord* pending_list_addr) {
337 // Given a list of refs linked through the "discovered" field
338 // (java.lang.ref.Reference.discovered), self-loop their "next" field
339 // thus distinguishing them from active References, then
340 // prepend them to the pending list.
341 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
342 // the "next" field is used to chain the pending list, not the discovered
343 // field.
344
345 if (TraceReferenceGC && PrintGCDetails) {
346 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
347 INTPTR_FORMAT, (address)refs_list.head());
348 }
349
350 oop obj = NULL;
351 oop next_d = refs_list.head();
352 if (pending_list_uses_discovered_field()) { // New behavior
353 // Walk down the list, self-looping the next field
354 // so that the References are not considered active.
355 while (obj != next_d) {
356 obj = next_d;
357 assert(obj->is_instanceRef(), "should be reference object");
358 next_d = java_lang_ref_Reference::discovered(obj);
359 if (TraceReferenceGC && PrintGCDetails) {
360 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
361 (void *)obj, (void *)next_d);
362 }
363 assert(java_lang_ref_Reference::next(obj) == NULL,
364 "Reference not active; should not be discovered");
365 // Self-loop next, so as to make Ref not active.
366 java_lang_ref_Reference::set_next_raw(obj, obj);
367 if (next_d != obj) {
368 // The Java threads will see the Reference objects linked together through
369 // the discovered field. Instead of trying to do the write barrier updates
370 // in all places in the reference processor where we manipulate the discovered
371 // field we make sure to do the barrier here where we anyway iterate through
372 // all linked Reference objects.
373 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
374 } else {
375 // This is the last object.
376 // Swap refs_list into pending_list_addr and
377 // set obj's discovered to what we read from pending_list_addr.
378 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
379 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
380 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
381 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
382 }
383 }
384 } else { // Old behavior
385 // Walk down the list, copying the discovered field into
386 // the next field and clearing the discovered field.
387 while (obj != next_d) {
388 obj = next_d;
389 assert(obj->is_instanceRef(), "should be reference object");
390 next_d = java_lang_ref_Reference::discovered(obj);
391 if (TraceReferenceGC && PrintGCDetails) {
392 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
393 (void *)obj, (void *)next_d);
394 }
395 assert(java_lang_ref_Reference::next(obj) == NULL,
396 "The reference should not be enqueued");
397 if (next_d == obj) { // obj is last
398 // Swap refs_list into pending_list_addr and
399 // set obj's next to what we read from pending_list_addr.
400 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
401 // Need oop_check on pending_list_addr above;
402 // see special oop-check code at the end of
403 // enqueue_discovered_reflists() further below.
404 if (old == NULL) {
405 // obj should be made to point to itself, since
406 // pending list was empty.
407 java_lang_ref_Reference::set_next(obj, obj);
408 } else {
409 java_lang_ref_Reference::set_next(obj, old);
410 }
411 } else {
412 java_lang_ref_Reference::set_next(obj, next_d);
413 }
414 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
415 }
416 }
417 }
418
419 // Parallel enqueue task
420 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
421 public:
422 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
423 DiscoveredList discovered_refs[],
424 HeapWord* pending_list_addr,
425 int n_queues)
426 : EnqueueTask(ref_processor, discovered_refs,
427 pending_list_addr, n_queues)
428 { }
429
430 virtual void work(unsigned int work_id) {
431 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
432 // Simplest first cut: static partitioning.
433 int index = work_id;
434 // The increment on "index" must correspond to the maximum number of queues
435 // (n_queues) with which that ReferenceProcessor was created. That
436 // is because of the "clever" way the discovered references lists were
437 // allocated and are indexed into.
438 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
439 for (int j = 0;
440 j < ReferenceProcessor::number_of_subclasses_of_ref();
441 j++, index += _n_queues) {
442 _ref_processor.enqueue_discovered_reflist(
443 _refs_lists[index], _pending_list_addr);
444 _refs_lists[index].set_head(NULL);
445 _refs_lists[index].set_length(0);
446 }
447 }
448 };
449
450 // Enqueue references that are not made active again
451 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
452 AbstractRefProcTaskExecutor* task_executor) {
453 if (_processing_is_mt && task_executor != NULL) {
454 // Parallel code
455 RefProcEnqueueTask tsk(*this, _discovered_refs,
456 pending_list_addr, _max_num_q);
457 task_executor->execute(tsk);
458 } else {
459 // Serial code: call the parent class's implementation
460 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
461 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
462 _discovered_refs[i].set_head(NULL);
463 _discovered_refs[i].set_length(0);
464 }
465 }
466 }
467
468 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
469 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
470 oop discovered = java_lang_ref_Reference::discovered(_ref);
471 assert(_discovered_addr && discovered->is_oop_or_null(),
472 "discovered field is bad");
473 _next = discovered;
474 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
475 _referent = java_lang_ref_Reference::referent(_ref);
476 assert(Universe::heap()->is_in_reserved_or_null(_referent),
477 "Wrong oop found in java.lang.Reference object");
478 assert(allow_null_referent ?
479 _referent->is_oop_or_null()
480 : _referent->is_oop(),
481 "bad referent");
482 }
483
484 void DiscoveredListIterator::remove() {
485 assert(_ref->is_oop(), "Dropping a bad reference");
486 oop_store_raw(_discovered_addr, NULL);
487
488 // First _prev_next ref actually points into DiscoveredList (gross).
489 oop new_next;
490 if (_next == _ref) {
491 // At the end of the list, we should make _prev point to itself.
492 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
493 // and _prev will be NULL.
494 new_next = _prev;
495 } else {
496 new_next = _next;
497 }
498 // Remove Reference object from discovered list. Note that G1 does not need a
499 // pre-barrier here because we know the Reference has already been found/marked,
500 // that's how it ended up in the discovered list in the first place.
501 oop_store_raw(_prev_next, new_next);
502 NOT_PRODUCT(_removed++);
503 _refs_list.dec_length(1);
504 }
505
506 // Make the Reference object active again.
507 void DiscoveredListIterator::make_active() {
508 // The pre barrier for G1 is probably just needed for the old
509 // reference processing behavior. Should we guard this with
510 // ReferenceProcessor::pending_list_uses_discovered_field() ?
511 if (UseG1GC) {
512 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
513 if (UseCompressedOops) {
514 oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
515 } else {
516 oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
517 }
518 }
519 java_lang_ref_Reference::set_next_raw(_ref, NULL);
520 }
521
522 void DiscoveredListIterator::clear_referent() {
523 oop_store_raw(_referent_addr, NULL);
524 }
525
526 // NOTE: process_phase*() are largely similar, and at a high level
527 // merely iterate over the extant list applying a predicate to
528 // each of its elements and possibly removing that element from the
529 // list and applying some further closures to that element.
530 // We should consider the possibility of replacing these
531 // process_phase*() methods by abstracting them into
532 // a single general iterator invocation that receives appropriate
533 // closures that accomplish this work.
534
535 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
536 // referents are not alive, but that should be kept alive for policy reasons.
537 // Keep alive the transitive closure of all such referents.
538 void
539 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
540 ReferencePolicy* policy,
541 BoolObjectClosure* is_alive,
542 OopClosure* keep_alive,
543 VoidClosure* complete_gc) {
544 assert(policy != NULL, "Must have a non-NULL policy");
545 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
546 // Decide which softly reachable refs should be kept alive.
547 while (iter.has_next()) {
548 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
549 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
550 if (referent_is_dead &&
551 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
552 if (TraceReferenceGC) {
553 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
554 (void *)iter.obj(), iter.obj()->klass()->internal_name());
555 }
556 // Remove Reference object from list
557 iter.remove();
558 // Make the Reference object active again
559 iter.make_active();
560 // keep the referent around
561 iter.make_referent_alive();
562 iter.move_to_next();
563 } else {
564 iter.next();
565 }
566 }
567 // Close the reachable set
568 complete_gc->do_void();
569 NOT_PRODUCT(
570 if (PrintGCDetails && TraceReferenceGC) {
571 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
572 "discovered Refs by policy, from list " INTPTR_FORMAT,
573 iter.removed(), iter.processed(), (address)refs_list.head());
574 }
575 )
576 }
577
578 // Traverse the list and remove any Refs that are not active, or
579 // whose referents are either alive or NULL.
580 void
581 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
582 BoolObjectClosure* is_alive,
583 OopClosure* keep_alive) {
584 assert(discovery_is_atomic(), "Error");
585 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
586 while (iter.has_next()) {
587 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
588 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
589 assert(next == NULL, "Should not discover inactive Reference");
590 if (iter.is_referent_alive()) {
591 if (TraceReferenceGC) {
592 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
593 (void *)iter.obj(), iter.obj()->klass()->internal_name());
594 }
595 // The referent is reachable after all.
596 // Remove Reference object from list.
597 iter.remove();
598 // Update the referent pointer as necessary: Note that this
599 // should not entail any recursive marking because the
600 // referent must already have been traversed.
601 iter.make_referent_alive();
602 iter.move_to_next();
603 } else {
604 iter.next();
605 }
606 }
607 NOT_PRODUCT(
608 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
609 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
610 "Refs in discovered list " INTPTR_FORMAT,
611 iter.removed(), iter.processed(), (address)refs_list.head());
612 }
613 )
614 }
615
616 void
617 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
618 BoolObjectClosure* is_alive,
619 OopClosure* keep_alive,
620 VoidClosure* complete_gc) {
621 assert(!discovery_is_atomic(), "Error");
622 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
623 while (iter.has_next()) {
624 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
625 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
626 oop next = java_lang_ref_Reference::next(iter.obj());
627 if ((iter.referent() == NULL || iter.is_referent_alive() ||
628 next != NULL)) {
629 assert(next->is_oop_or_null(), "bad next field");
630 // Remove Reference object from list
631 iter.remove();
632 // Trace the cohorts
633 iter.make_referent_alive();
634 if (UseCompressedOops) {
635 keep_alive->do_oop((narrowOop*)next_addr);
636 } else {
637 keep_alive->do_oop((oop*)next_addr);
638 }
639 iter.move_to_next();
640 } else {
641 iter.next();
642 }
643 }
644 // Now close the newly reachable set
645 complete_gc->do_void();
646 NOT_PRODUCT(
647 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
648 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
649 "Refs in discovered list " INTPTR_FORMAT,
650 iter.removed(), iter.processed(), (address)refs_list.head());
651 }
652 )
653 }
654
655 // Traverse the list and process the referents, by either
656 // clearing them or keeping them (and their reachable
657 // closure) alive.
658 void
659 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
660 bool clear_referent,
661 BoolObjectClosure* is_alive,
662 OopClosure* keep_alive,
663 VoidClosure* complete_gc) {
664 ResourceMark rm;
665 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
666 while (iter.has_next()) {
667 iter.update_discovered();
668 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
669 if (clear_referent) {
670 // NULL out referent pointer
671 iter.clear_referent();
672 } else {
673 // keep the referent around
674 iter.make_referent_alive();
675 }
676 if (TraceReferenceGC) {
677 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
678 clear_referent ? "cleared " : "",
679 (void *)iter.obj(), iter.obj()->klass()->internal_name());
680 }
681 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
682 iter.next();
683 }
684 // Remember to update the next pointer of the last ref.
685 iter.update_discovered();
686 // Close the reachable set
687 complete_gc->do_void();
688 }
689
690 void
691 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
692 oop obj = NULL;
693 oop next = refs_list.head();
694 while (next != obj) {
695 obj = next;
696 next = java_lang_ref_Reference::discovered(obj);
697 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
698 }
699 refs_list.set_head(NULL);
700 refs_list.set_length(0);
701 }
702
703 void
704 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
705 clear_discovered_references(refs_list);
706 }
707
708 void ReferenceProcessor::abandon_partial_discovery() {
709 // loop over the lists
710 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
711 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
712 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
713 }
714 abandon_partial_discovered_list(_discovered_refs[i]);
715 }
716 }
717
718 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
719 public:
720 RefProcPhase1Task(ReferenceProcessor& ref_processor,
721 DiscoveredList refs_lists[],
722 ReferencePolicy* policy,
723 bool marks_oops_alive)
724 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
725 _policy(policy)
726 { }
727 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
728 OopClosure& keep_alive,
729 VoidClosure& complete_gc)
730 {
731 Thread* thr = Thread::current();
732 int refs_list_index = ((WorkerThread*)thr)->id();
733 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
734 &is_alive, &keep_alive, &complete_gc);
735 }
736 private:
737 ReferencePolicy* _policy;
738 };
739
740 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
741 public:
742 RefProcPhase2Task(ReferenceProcessor& ref_processor,
743 DiscoveredList refs_lists[],
744 bool marks_oops_alive)
745 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
746 { }
747 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
748 OopClosure& keep_alive,
749 VoidClosure& complete_gc)
750 {
751 _ref_processor.process_phase2(_refs_lists[i],
752 &is_alive, &keep_alive, &complete_gc);
753 }
754 };
755
756 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
757 public:
758 RefProcPhase3Task(ReferenceProcessor& ref_processor,
759 DiscoveredList refs_lists[],
760 bool clear_referent,
761 bool marks_oops_alive)
762 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
763 _clear_referent(clear_referent)
764 { }
765 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
766 OopClosure& keep_alive,
767 VoidClosure& complete_gc)
768 {
769 // Don't use "refs_list_index" calculated in this way because
770 // balance_queues() has moved the Ref's into the first n queues.
771 // Thread* thr = Thread::current();
772 // int refs_list_index = ((WorkerThread*)thr)->id();
773 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
774 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
775 &is_alive, &keep_alive, &complete_gc);
776 }
777 private:
778 bool _clear_referent;
779 };
780
781 // Balances reference queues.
782 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
783 // queues[0, 1, ..., _num_q-1] because only the first _num_q
784 // corresponding to the active workers will be processed.
785 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
786 {
787 // calculate total length
788 size_t total_refs = 0;
789 if (TraceReferenceGC && PrintGCDetails) {
790 gclog_or_tty->print_cr("\nBalance ref_lists ");
791 }
792
793 for (uint i = 0; i < _max_num_q; ++i) {
794 total_refs += ref_lists[i].length();
795 if (TraceReferenceGC && PrintGCDetails) {
796 gclog_or_tty->print("%d ", ref_lists[i].length());
797 }
798 }
799 if (TraceReferenceGC && PrintGCDetails) {
800 gclog_or_tty->print_cr(" = %d", total_refs);
801 }
802 size_t avg_refs = total_refs / _num_q + 1;
803 uint to_idx = 0;
804 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
805 bool move_all = false;
806 if (from_idx >= _num_q) {
807 move_all = ref_lists[from_idx].length() > 0;
808 }
809 while ((ref_lists[from_idx].length() > avg_refs) ||
810 move_all) {
811 assert(to_idx < _num_q, "Sanity Check!");
812 if (ref_lists[to_idx].length() < avg_refs) {
813 // move superfluous refs
814 size_t refs_to_move;
815 // Move all the Ref's if the from queue will not be processed.
816 if (move_all) {
817 refs_to_move = MIN2(ref_lists[from_idx].length(),
818 avg_refs - ref_lists[to_idx].length());
819 } else {
820 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
821 avg_refs - ref_lists[to_idx].length());
822 }
823
824 assert(refs_to_move > 0, "otherwise the code below will fail");
825
826 oop move_head = ref_lists[from_idx].head();
827 oop move_tail = move_head;
828 oop new_head = move_head;
829 // find an element to split the list on
830 for (size_t j = 0; j < refs_to_move; ++j) {
831 move_tail = new_head;
832 new_head = java_lang_ref_Reference::discovered(new_head);
833 }
834
835 // Add the chain to the to list.
836 if (ref_lists[to_idx].head() == NULL) {
837 // to list is empty. Make a loop at the end.
838 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
839 } else {
840 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
841 }
842 ref_lists[to_idx].set_head(move_head);
843 ref_lists[to_idx].inc_length(refs_to_move);
844
845 // Remove the chain from the from list.
846 if (move_tail == new_head) {
847 // We found the end of the from list.
848 ref_lists[from_idx].set_head(NULL);
849 } else {
850 ref_lists[from_idx].set_head(new_head);
851 }
852 ref_lists[from_idx].dec_length(refs_to_move);
853 if (ref_lists[from_idx].length() == 0) {
854 break;
855 }
856 } else {
857 to_idx = (to_idx + 1) % _num_q;
858 }
859 }
860 }
861 #ifdef ASSERT
862 size_t balanced_total_refs = 0;
863 for (uint i = 0; i < _max_num_q; ++i) {
864 balanced_total_refs += ref_lists[i].length();
865 if (TraceReferenceGC && PrintGCDetails) {
866 gclog_or_tty->print("%d ", ref_lists[i].length());
867 }
868 }
869 if (TraceReferenceGC && PrintGCDetails) {
870 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
871 gclog_or_tty->flush();
872 }
873 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
874 #endif
875 }
876
877 void ReferenceProcessor::balance_all_queues() {
878 balance_queues(_discoveredSoftRefs);
879 balance_queues(_discoveredWeakRefs);
880 balance_queues(_discoveredFinalRefs);
881 balance_queues(_discoveredPhantomRefs);
882 }
883
884 size_t
885 ReferenceProcessor::process_discovered_reflist(
886 DiscoveredList refs_lists[],
887 ReferencePolicy* policy,
888 bool clear_referent,
889 BoolObjectClosure* is_alive,
890 OopClosure* keep_alive,
891 VoidClosure* complete_gc,
892 AbstractRefProcTaskExecutor* task_executor)
893 {
894 bool mt_processing = task_executor != NULL && _processing_is_mt;
895 // If discovery used MT and a dynamic number of GC threads, then
896 // the queues must be balanced for correctness if fewer than the
897 // maximum number of queues were used. The number of queue used
898 // during discovery may be different than the number to be used
899 // for processing so don't depend of _num_q < _max_num_q as part
900 // of the test.
901 bool must_balance = _discovery_is_mt;
902
903 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
904 must_balance) {
905 balance_queues(refs_lists);
906 }
907
908 size_t total_list_count = total_count(refs_lists);
909
910 if (PrintReferenceGC && PrintGCDetails) {
911 gclog_or_tty->print(", %u refs", total_list_count);
912 }
913
914 // Phase 1 (soft refs only):
915 // . Traverse the list and remove any SoftReferences whose
916 // referents are not alive, but that should be kept alive for
917 // policy reasons. Keep alive the transitive closure of all
918 // such referents.
919 if (policy != NULL) {
920 if (mt_processing) {
921 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
922 task_executor->execute(phase1);
923 } else {
924 for (uint i = 0; i < _max_num_q; i++) {
925 process_phase1(refs_lists[i], policy,
926 is_alive, keep_alive, complete_gc);
927 }
928 }
929 } else { // policy == NULL
930 assert(refs_lists != _discoveredSoftRefs,
931 "Policy must be specified for soft references.");
932 }
933
934 // Phase 2:
935 // . Traverse the list and remove any refs whose referents are alive.
936 if (mt_processing) {
937 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
938 task_executor->execute(phase2);
939 } else {
940 for (uint i = 0; i < _max_num_q; i++) {
941 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
942 }
943 }
944
945 // Phase 3:
946 // . Traverse the list and process referents as appropriate.
947 if (mt_processing) {
948 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
949 task_executor->execute(phase3);
950 } else {
951 for (uint i = 0; i < _max_num_q; i++) {
952 process_phase3(refs_lists[i], clear_referent,
953 is_alive, keep_alive, complete_gc);
954 }
955 }
956
957 return total_list_count;
958 }
959
960 void ReferenceProcessor::clean_up_discovered_references() {
961 // loop over the lists
962 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
963 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
964 gclog_or_tty->print_cr(
965 "\nScrubbing %s discovered list of Null referents",
966 list_name(i));
967 }
968 clean_up_discovered_reflist(_discovered_refs[i]);
969 }
970 }
971
972 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
973 assert(!discovery_is_atomic(), "Else why call this method?");
974 DiscoveredListIterator iter(refs_list, NULL, NULL);
975 while (iter.has_next()) {
976 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
977 oop next = java_lang_ref_Reference::next(iter.obj());
978 assert(next->is_oop_or_null(), "bad next field");
979 // If referent has been cleared or Reference is not active,
980 // drop it.
981 if (iter.referent() == NULL || next != NULL) {
982 debug_only(
983 if (PrintGCDetails && TraceReferenceGC) {
984 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
985 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
986 " and referent: " INTPTR_FORMAT,
987 (void *)iter.obj(), (void *)next, (void *)iter.referent());
988 }
989 )
990 // Remove Reference object from list
991 iter.remove();
992 iter.move_to_next();
993 } else {
994 iter.next();
995 }
996 }
997 NOT_PRODUCT(
998 if (PrintGCDetails && TraceReferenceGC) {
999 gclog_or_tty->print(
1000 " Removed %d Refs with NULL referents out of %d discovered Refs",
1001 iter.removed(), iter.processed());
1002 }
1003 )
1004 }
1005
1006 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1007 uint id = 0;
1008 // Determine the queue index to use for this object.
1009 if (_discovery_is_mt) {
1010 // During a multi-threaded discovery phase,
1011 // each thread saves to its "own" list.
1012 Thread* thr = Thread::current();
1013 id = thr->as_Worker_thread()->id();
1014 } else {
1015 // single-threaded discovery, we save in round-robin
1016 // fashion to each of the lists.
1017 if (_processing_is_mt) {
1018 id = next_id();
1019 }
1020 }
1021 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1022
1023 // Get the discovered queue to which we will add
1024 DiscoveredList* list = NULL;
1025 switch (rt) {
1026 case REF_OTHER:
1027 // Unknown reference type, no special treatment
1028 break;
1029 case REF_SOFT:
1030 list = &_discoveredSoftRefs[id];
1031 break;
1032 case REF_WEAK:
1033 list = &_discoveredWeakRefs[id];
1034 break;
1035 case REF_FINAL:
1036 list = &_discoveredFinalRefs[id];
1037 break;
1038 case REF_PHANTOM:
1039 list = &_discoveredPhantomRefs[id];
1040 break;
1041 case REF_NONE:
1042 // we should not reach here if we are an InstanceRefKlass
1043 default:
1044 ShouldNotReachHere();
1045 }
1046 if (TraceReferenceGC && PrintGCDetails) {
1047 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1048 }
1049 return list;
1050 }
1051
1052 inline void
1053 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1054 oop obj,
1055 HeapWord* discovered_addr) {
1056 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1057 // First we must make sure this object is only enqueued once. CAS in a non null
1058 // discovered_addr.
1059 oop current_head = refs_list.head();
1060 // The last ref must have its discovered field pointing to itself.
1061 oop next_discovered = (current_head != NULL) ? current_head : obj;
1062
1063 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1064 NULL);
1065 if (retest == NULL) {
1066 // This thread just won the right to enqueue the object.
1067 // We have separate lists for enqueueing, so no synchronization
1068 // is necessary.
1069 refs_list.set_head(obj);
1070 refs_list.inc_length(1);
1071
1072 if (TraceReferenceGC) {
1073 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1074 (void *)obj, obj->klass()->internal_name());
1075 }
1076 } else {
1077 // If retest was non NULL, another thread beat us to it:
1078 // The reference has already been discovered...
1079 if (TraceReferenceGC) {
1080 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1081 (void *)obj, obj->klass()->internal_name());
1082 }
1083 }
1084 }
1085
1086 #ifndef PRODUCT
1087 // Non-atomic (i.e. concurrent) discovery might allow us
1088 // to observe j.l.References with NULL referents, being those
1089 // cleared concurrently by mutators during (or after) discovery.
1090 void ReferenceProcessor::verify_referent(oop obj) {
1091 bool da = discovery_is_atomic();
1092 oop referent = java_lang_ref_Reference::referent(obj);
1093 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1094 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1095 INTPTR_FORMAT " during %satomic discovery ",
1096 (void *)referent, (void *)obj, da ? "" : "non-"));
1097 }
1098 #endif
1099
1100 // We mention two of several possible choices here:
1101 // #0: if the reference object is not in the "originating generation"
1102 // (or part of the heap being collected, indicated by our "span"
1103 // we don't treat it specially (i.e. we scan it as we would
1104 // a normal oop, treating its references as strong references).
1105 // This means that references can't be discovered unless their
1106 // referent is also in the same span. This is the simplest,
1107 // most "local" and most conservative approach, albeit one
1108 // that may cause weak references to be enqueued least promptly.
1109 // We call this choice the "ReferenceBasedDiscovery" policy.
1110 // #1: the reference object may be in any generation (span), but if
1111 // the referent is in the generation (span) being currently collected
1112 // then we can discover the reference object, provided
1113 // the object has not already been discovered by
1114 // a different concurrently running collector (as may be the
1115 // case, for instance, if the reference object is in CMS and
1116 // the referent in DefNewGeneration), and provided the processing
1117 // of this reference object by the current collector will
1118 // appear atomic to every other collector in the system.
1119 // (Thus, for instance, a concurrent collector may not
1120 // discover references in other generations even if the
1121 // referent is in its own generation). This policy may,
1122 // in certain cases, enqueue references somewhat sooner than
1123 // might Policy #0 above, but at marginally increased cost
1124 // and complexity in processing these references.
1125 // We call this choice the "RefeferentBasedDiscovery" policy.
1126 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1127 // Make sure we are discovering refs (rather than processing discovered refs).
1128 if (!_discovering_refs || !RegisterReferences) {
1129 return false;
1130 }
1131 // We only discover active references.
1132 oop next = java_lang_ref_Reference::next(obj);
1133 if (next != NULL) { // Ref is no longer active
1134 return false;
1135 }
1136
1137 HeapWord* obj_addr = (HeapWord*)obj;
1138 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1139 !_span.contains(obj_addr)) {
1140 // Reference is not in the originating generation;
1141 // don't treat it specially (i.e. we want to scan it as a normal
1142 // object with strong references).
1143 return false;
1144 }
1145
1146 // We only discover references whose referents are not (yet)
1147 // known to be strongly reachable.
1148 if (is_alive_non_header() != NULL) {
1149 verify_referent(obj);
1150 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1151 return false; // referent is reachable
1152 }
1153 }
1154 if (rt == REF_SOFT) {
1155 // For soft refs we can decide now if these are not
1156 // current candidates for clearing, in which case we
1157 // can mark through them now, rather than delaying that
1158 // to the reference-processing phase. Since all current
1159 // time-stamp policies advance the soft-ref clock only
1160 // at a major collection cycle, this is always currently
1161 // accurate.
1162 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1163 return false;
1164 }
1165 }
1166
1167 ResourceMark rm; // Needed for tracing.
1168
1169 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1170 const oop discovered = java_lang_ref_Reference::discovered(obj);
1171 assert(discovered->is_oop_or_null(), "bad discovered field");
1172 if (discovered != NULL) {
1173 // The reference has already been discovered...
1174 if (TraceReferenceGC) {
1175 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1176 (void *)obj, obj->klass()->internal_name());
1177 }
1178 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1179 // assumes that an object is not processed twice;
1180 // if it's been already discovered it must be on another
1181 // generation's discovered list; so we won't discover it.
1182 return false;
1183 } else {
1184 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1185 "Unrecognized policy");
1186 // Check assumption that an object is not potentially
1187 // discovered twice except by concurrent collectors that potentially
1188 // trace the same Reference object twice.
1189 assert(UseConcMarkSweepGC || UseG1GC,
1190 "Only possible with a concurrent marking collector");
1191 return true;
1192 }
1193 }
1194
1195 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1196 verify_referent(obj);
1197 // Discover if and only if EITHER:
1198 // .. reference is in our span, OR
1199 // .. we are an atomic collector and referent is in our span
1200 if (_span.contains(obj_addr) ||
1201 (discovery_is_atomic() &&
1202 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1203 // should_enqueue = true;
1204 } else {
1205 return false;
1206 }
1207 } else {
1208 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1209 _span.contains(obj_addr), "code inconsistency");
1210 }
1211
1212 // Get the right type of discovered queue head.
1213 DiscoveredList* list = get_discovered_list(rt);
1214 if (list == NULL) {
1215 return false; // nothing special needs to be done
1216 }
1217
1218 if (_discovery_is_mt) {
1219 add_to_discovered_list_mt(*list, obj, discovered_addr);
1220 } else {
1221 // We do a raw store here: the field will be visited later when processing
1222 // the discovered references.
1223 oop current_head = list->head();
1224 // The last ref must have its discovered field pointing to itself.
1225 oop next_discovered = (current_head != NULL) ? current_head : obj;
1226
1227 assert(discovered == NULL, "control point invariant");
1228 oop_store_raw(discovered_addr, next_discovered);
1229 list->set_head(obj);
1230 list->inc_length(1);
1231
1232 if (TraceReferenceGC) {
1233 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1234 (void *)obj, obj->klass()->internal_name());
1235 }
1236 }
1237 assert(obj->is_oop(), "Discovered a bad reference");
1238 verify_referent(obj);
1239 return true;
1240 }
1241
1242 // Preclean the discovered references by removing those
1243 // whose referents are alive, and by marking from those that
1244 // are not active. These lists can be handled here
1245 // in any order and, indeed, concurrently.
1246 void ReferenceProcessor::preclean_discovered_references(
1247 BoolObjectClosure* is_alive,
1248 OopClosure* keep_alive,
1249 VoidClosure* complete_gc,
1250 YieldClosure* yield,
1251 GCTimer* gc_timer) {
1252
1253 NOT_PRODUCT(verify_ok_to_handle_reflists());
1254
1255 // Soft references
1256 {
1257 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1258 false, gc_timer);
1259 for (uint i = 0; i < _max_num_q; i++) {
1260 if (yield->should_return()) {
1261 return;
1262 }
1263 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1264 keep_alive, complete_gc, yield);
1265 }
1266 }
1267
1268 // Weak references
1269 {
1270 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1271 false, gc_timer);
1272 for (uint i = 0; i < _max_num_q; i++) {
1273 if (yield->should_return()) {
1274 return;
1275 }
1276 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1277 keep_alive, complete_gc, yield);
1278 }
1279 }
1280
1281 // Final references
1282 {
1283 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1284 false, gc_timer);
1285 for (uint i = 0; i < _max_num_q; i++) {
1286 if (yield->should_return()) {
1287 return;
1288 }
1289 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1290 keep_alive, complete_gc, yield);
1291 }
1292 }
1293
1294 // Phantom references
1295 {
1296 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1297 false, gc_timer);
1298 for (uint i = 0; i < _max_num_q; i++) {
1299 if (yield->should_return()) {
1300 return;
1301 }
1302 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1303 keep_alive, complete_gc, yield);
1304 }
1305 }
1306 }
1307
1308 // Walk the given discovered ref list, and remove all reference objects
1309 // whose referents are still alive, whose referents are NULL or which
1310 // are not active (have a non-NULL next field). NOTE: When we are
1311 // thus precleaning the ref lists (which happens single-threaded today),
1312 // we do not disable refs discovery to honor the correct semantics of
1313 // java.lang.Reference. As a result, we need to be careful below
1314 // that ref removal steps interleave safely with ref discovery steps
1315 // (in this thread).
1316 void
1317 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1318 BoolObjectClosure* is_alive,
1319 OopClosure* keep_alive,
1320 VoidClosure* complete_gc,
1321 YieldClosure* yield) {
1322 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1323 while (iter.has_next()) {
1324 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1325 oop obj = iter.obj();
1326 oop next = java_lang_ref_Reference::next(obj);
1327 if (iter.referent() == NULL || iter.is_referent_alive() ||
1328 next != NULL) {
1329 // The referent has been cleared, or is alive, or the Reference is not
1330 // active; we need to trace and mark its cohort.
1331 if (TraceReferenceGC) {
1332 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1333 (void *)iter.obj(), iter.obj()->klass()->internal_name());
1334 }
1335 // Remove Reference object from list
1336 iter.remove();
1337 // Keep alive its cohort.
1338 iter.make_referent_alive();
1339 if (UseCompressedOops) {
1340 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1341 keep_alive->do_oop(next_addr);
1342 } else {
1343 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1344 keep_alive->do_oop(next_addr);
1345 }
1346 iter.move_to_next();
1347 } else {
1348 iter.next();
1349 }
1350 }
1351 // Close the reachable set
1352 complete_gc->do_void();
1353
1354 NOT_PRODUCT(
1355 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1356 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1357 "Refs in discovered list " INTPTR_FORMAT,
1358 iter.removed(), iter.processed(), (address)refs_list.head());
1359 }
1360 )
1361 }
1362
1363 const char* ReferenceProcessor::list_name(uint i) {
1364 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1365 "Out of bounds index");
1366
1367 int j = i / _max_num_q;
1368 switch (j) {
1369 case 0: return "SoftRef";
1370 case 1: return "WeakRef";
1371 case 2: return "FinalRef";
1372 case 3: return "PhantomRef";
1373 }
1374 ShouldNotReachHere();
1375 return NULL;
1376 }
1377
1378 #ifndef PRODUCT
1379 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1380 // empty for now
1381 }
1382 #endif
1383
1384 #ifndef PRODUCT
1385 void ReferenceProcessor::clear_discovered_references() {
1386 guarantee(!_discovering_refs, "Discovering refs?");
1387 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1388 clear_discovered_references(_discovered_refs[i]);
1389 }
1390 }
1391
1392 #endif // PRODUCT