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 #if INCLUDE_JFR
38 #include "jfr/jfr.hpp"
39 #endif // INCLUDE_JFR
40
41 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
42
43 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
44 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
45 bool ReferenceProcessor::_pending_list_uses_discovered_field = false;
46 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
47
48 void referenceProcessor_init() {
49 ReferenceProcessor::init_statics();
50 }
51
52 void ReferenceProcessor::init_statics() {
53 // We need a monotonically non-deccreasing time in ms but
54 // os::javaTimeMillis() does not guarantee monotonicity.
55 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
56
57 // Initialize the soft ref timestamp clock.
58 _soft_ref_timestamp_clock = now;
59 // Also update the soft ref clock in j.l.r.SoftReference
60 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
61
62 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
63 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
64 NOT_COMPILER2(LRUCurrentHeapPolicy());
65 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
66 vm_exit_during_initialization("Could not allocate reference policy object");
67 }
68 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
69 RefDiscoveryPolicy == ReferentBasedDiscovery,
70 "Unrecongnized RefDiscoveryPolicy");
71 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
72 }
73
74 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
75 #ifdef ASSERT
76 // Verify that we're not currently discovering refs
77 assert(!verify_disabled || !_discovering_refs, "nested call?");
78
79 if (check_no_refs) {
80 // Verify that the discovered lists are empty
81 verify_no_references_recorded();
82 }
83 #endif // ASSERT
84
85 // Someone could have modified the value of the static
86 // field in the j.l.r.SoftReference class that holds the
87 // soft reference timestamp clock using reflection or
88 // Unsafe between GCs. Unconditionally update the static
89 // field in ReferenceProcessor here so that we use the new
90 // value during reference discovery.
91
92 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
93 _discovering_refs = true;
94 }
95
96 ReferenceProcessor::ReferenceProcessor(MemRegion span,
97 bool mt_processing,
98 uint mt_processing_degree,
99 bool mt_discovery,
100 uint mt_discovery_degree,
101 bool atomic_discovery,
102 BoolObjectClosure* is_alive_non_header) :
103 _discovering_refs(false),
104 _enqueuing_is_done(false),
105 _is_alive_non_header(is_alive_non_header),
106 _processing_is_mt(mt_processing),
107 _next_id(0)
108 {
109 _span = span;
110 _discovery_is_atomic = atomic_discovery;
111 _discovery_is_mt = mt_discovery;
112 _num_q = MAX2(1U, mt_processing_degree);
113 _max_num_q = MAX2(_num_q, mt_discovery_degree);
114 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
115 _max_num_q * number_of_subclasses_of_ref(), mtGC);
116
117 if (_discovered_refs == NULL) {
118 vm_exit_during_initialization("Could not allocated RefProc Array");
119 }
120 _discoveredSoftRefs = &_discovered_refs[0];
121 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
122 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
123 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
124 _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q];
125
126 // Initialize all entries to NULL
127 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
128 _discovered_refs[i].set_head(NULL);
129 _discovered_refs[i].set_length(0);
130 }
131
132 setup_policy(false /* default soft ref policy */);
133 }
134
135 #ifndef PRODUCT
136 void ReferenceProcessor::verify_no_references_recorded() {
137 guarantee(!_discovering_refs, "Discovering refs?");
138 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
139 guarantee(_discovered_refs[i].is_empty(),
140 "Found non-empty discovered list");
141 }
142 }
143 #endif
144
145 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
146 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
147 if (UseCompressedOops) {
148 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
149 } else {
150 f->do_oop((oop*)_discovered_refs[i].adr_head());
151 }
152 }
153 }
154
155 void ReferenceProcessor::update_soft_ref_master_clock() {
156 // Update (advance) the soft ref master clock field. This must be done
157 // after processing the soft ref list.
158
159 // We need a monotonically non-deccreasing time in ms but
160 // os::javaTimeMillis() does not guarantee monotonicity.
161 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
162 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
163 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
164
165 NOT_PRODUCT(
166 if (now < _soft_ref_timestamp_clock) {
167 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
168 _soft_ref_timestamp_clock, now);
169 }
170 )
171 // The values of now and _soft_ref_timestamp_clock are set using
172 // javaTimeNanos(), which is guaranteed to be monotonically
173 // non-decreasing provided the underlying platform provides such
174 // a time source (and it is bug free).
175 // In product mode, however, protect ourselves from non-monotonicty.
176 if (now > _soft_ref_timestamp_clock) {
177 _soft_ref_timestamp_clock = now;
178 java_lang_ref_SoftReference::set_clock(now);
179 }
180 // Else leave clock stalled at its old value until time progresses
181 // past clock value.
182 }
183
184 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
185 size_t total = 0;
186 for (uint i = 0; i < _max_num_q; ++i) {
187 total += lists[i].length();
188 }
189 return total;
190 }
191
192 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
193 BoolObjectClosure* is_alive,
194 OopClosure* keep_alive,
195 VoidClosure* complete_gc,
196 AbstractRefProcTaskExecutor* task_executor,
197 GCTimer* gc_timer,
198 GCId gc_id) {
199 NOT_PRODUCT(verify_ok_to_handle_reflists());
200
201 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
202 // Stop treating discovered references specially.
203 disable_discovery();
204
205 // If discovery was concurrent, someone could have modified
206 // the value of the static field in the j.l.r.SoftReference
207 // class that holds the soft reference timestamp clock using
208 // reflection or Unsafe between when discovery was enabled and
209 // now. Unconditionally update the static field in ReferenceProcessor
210 // here so that we use the new value during processing of the
211 // discovered soft refs.
212
213 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
214
215 bool trace_time = PrintGCDetails && PrintReferenceGC;
216
217 // Soft references
218 size_t soft_count = 0;
219 {
220 GCTraceTime tt("SoftReference", trace_time, false, gc_timer, gc_id);
221 soft_count =
222 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
223 is_alive, keep_alive, complete_gc, task_executor);
224 }
225
226 update_soft_ref_master_clock();
227
228 // Weak references
229 size_t weak_count = 0;
230 {
231 GCTraceTime tt("WeakReference", trace_time, false, gc_timer, gc_id);
232 weak_count =
233 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
234 is_alive, keep_alive, complete_gc, task_executor);
235 }
236
237 // Final references
238 size_t final_count = 0;
239 {
240 GCTraceTime tt("FinalReference", trace_time, false, gc_timer, gc_id);
241 final_count =
242 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
243 is_alive, keep_alive, complete_gc, task_executor);
244 }
245
246 // Phantom references
247 size_t phantom_count = 0;
248 {
249 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer, gc_id);
250 phantom_count =
251 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
252 is_alive, keep_alive, complete_gc, task_executor);
253
254 // Process cleaners, but include them in phantom statistics. We expect
255 // Cleaner references to be temporary, and don't want to deal with
256 // possible incompatibilities arising from making it more visible.
257 phantom_count +=
258 process_discovered_reflist(_discoveredCleanerRefs, NULL, true,
259 is_alive, keep_alive, complete_gc, task_executor);
260 }
261
262 // Weak global JNI references. It would make more sense (semantically) to
263 // traverse these simultaneously with the regular weak references above, but
264 // that is not how the JDK1.2 specification is. See #4126360. Native code can
265 // thus use JNI weak references to circumvent the phantom references and
266 // resurrect a "post-mortem" object.
267 {
268 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer, gc_id);
269 if (task_executor != NULL) {
270 task_executor->set_single_threaded_mode();
271 }
272 process_phaseJNI(is_alive, keep_alive, complete_gc);
273 }
274
275 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
276 }
277
278 #ifndef PRODUCT
279 // Calculate the number of jni handles.
280 uint ReferenceProcessor::count_jni_refs() {
281 class CountHandleClosure: public OopClosure {
282 private:
283 int _count;
284 public:
285 CountHandleClosure(): _count(0) {}
286 void do_oop(oop* unused) { _count++; }
287 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
288 int count() { return _count; }
289 };
290 CountHandleClosure global_handle_count;
291 JNIHandles::weak_oops_do(&global_handle_count);
292 return global_handle_count.count();
293 }
294 #endif
295
296 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
297 OopClosure* keep_alive,
298 VoidClosure* complete_gc) {
299 #ifndef PRODUCT
300 if (PrintGCDetails && PrintReferenceGC) {
301 unsigned int count = count_jni_refs();
302 gclog_or_tty->print(", %u refs", count);
303 }
304 #endif
305 JNIHandles::weak_oops_do(is_alive, keep_alive);
306 JFR_ONLY(Jfr::weak_oops_do(is_alive, keep_alive));
307 complete_gc->do_void();
308 }
309
310
311 template <class T>
312 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
313 AbstractRefProcTaskExecutor* task_executor) {
314
315 // Remember old value of pending references list
316 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
317 T old_pending_list_value = *pending_list_addr;
318
319 // Enqueue references that are not made active again, and
320 // clear the decks for the next collection (cycle).
321 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
322 // Do the post-barrier on pending_list_addr missed in
323 // enqueue_discovered_reflist.
324 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
325
326 // Stop treating discovered references specially.
327 ref->disable_discovery();
328
329 // Return true if new pending references were added
330 return old_pending_list_value != *pending_list_addr;
331 }
332
333 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
334 NOT_PRODUCT(verify_ok_to_handle_reflists());
335 if (UseCompressedOops) {
336 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
337 } else {
338 return enqueue_discovered_ref_helper<oop>(this, task_executor);
339 }
340 }
341
342 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
343 HeapWord* pending_list_addr) {
344 // Given a list of refs linked through the "discovered" field
345 // (java.lang.ref.Reference.discovered), self-loop their "next" field
346 // thus distinguishing them from active References, then
347 // prepend them to the pending list.
348 //
349 // The Java threads will see the Reference objects linked together through
350 // the discovered field. Instead of trying to do the write barrier updates
351 // in all places in the reference processor where we manipulate the discovered
352 // field we make sure to do the barrier here where we anyway iterate through
353 // all linked Reference objects. Note that it is important to not dirty any
354 // cards during reference processing since this will cause card table
355 // verification to fail for G1.
356 //
357 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
358 // the "next" field is used to chain the pending list, not the discovered
359 // field.
360 if (TraceReferenceGC && PrintGCDetails) {
361 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
362 INTPTR_FORMAT, (address)refs_list.head());
363 }
364
365 oop obj = NULL;
366 oop next_d = refs_list.head();
367 if (pending_list_uses_discovered_field()) { // New behavior
368 // Walk down the list, self-looping the next field
369 // so that the References are not considered active.
370 while (obj != next_d) {
371 obj = next_d;
372 assert(obj->is_instanceRef(), "should be reference object");
373 next_d = java_lang_ref_Reference::discovered(obj);
374 if (TraceReferenceGC && PrintGCDetails) {
375 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
376 (void *)obj, (void *)next_d);
377 }
378 assert(java_lang_ref_Reference::next(obj) == NULL,
379 "Reference not active; should not be discovered");
380 // Self-loop next, so as to make Ref not active.
381 java_lang_ref_Reference::set_next_raw(obj, obj);
382 if (next_d != obj) {
383 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
384 } else {
385 // This is the last object.
386 // Swap refs_list into pending_list_addr and
387 // set obj's discovered to what we read from pending_list_addr.
388 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
389 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
390 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
391 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
392 }
393 }
394 } else { // Old behaviour
395 // Walk down the list, copying the discovered field into
396 // the next field and clearing the discovered field.
397 while (obj != next_d) {
398 obj = next_d;
399 assert(obj->is_instanceRef(), "should be reference object");
400 next_d = java_lang_ref_Reference::discovered(obj);
401 if (TraceReferenceGC && PrintGCDetails) {
402 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
403 (void *)obj, (void *)next_d);
404 }
405 assert(java_lang_ref_Reference::next(obj) == NULL,
406 "The reference should not be enqueued");
407 if (next_d == obj) { // obj is last
408 // Swap refs_list into pendling_list_addr and
409 // set obj's next to what we read from pending_list_addr.
410 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
411 // Need oop_check on pending_list_addr above;
412 // see special oop-check code at the end of
413 // enqueue_discovered_reflists() further below.
414 if (old == NULL) {
415 // obj should be made to point to itself, since
416 // pending list was empty.
417 java_lang_ref_Reference::set_next(obj, obj);
418 } else {
419 java_lang_ref_Reference::set_next(obj, old);
420 }
421 } else {
422 java_lang_ref_Reference::set_next(obj, next_d);
423 }
424 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
425 }
426 }
427 }
428
429 // Parallel enqueue task
430 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
431 public:
432 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
433 DiscoveredList discovered_refs[],
434 HeapWord* pending_list_addr,
435 int n_queues)
436 : EnqueueTask(ref_processor, discovered_refs,
437 pending_list_addr, n_queues)
438 { }
439
440 virtual void work(unsigned int work_id) {
441 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
442 // Simplest first cut: static partitioning.
443 int index = work_id;
444 // The increment on "index" must correspond to the maximum number of queues
445 // (n_queues) with which that ReferenceProcessor was created. That
446 // is because of the "clever" way the discovered references lists were
447 // allocated and are indexed into.
448 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
449 for (int j = 0;
450 j < ReferenceProcessor::number_of_subclasses_of_ref();
451 j++, index += _n_queues) {
452 _ref_processor.enqueue_discovered_reflist(
453 _refs_lists[index], _pending_list_addr);
454 _refs_lists[index].set_head(NULL);
455 _refs_lists[index].set_length(0);
456 }
457 }
458 };
459
460 // Enqueue references that are not made active again
461 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
462 AbstractRefProcTaskExecutor* task_executor) {
463 if (_processing_is_mt && task_executor != NULL) {
464 // Parallel code
465 RefProcEnqueueTask tsk(*this, _discovered_refs,
466 pending_list_addr, _max_num_q);
467 task_executor->execute(tsk);
468 } else {
469 // Serial code: call the parent class's implementation
470 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
471 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
472 _discovered_refs[i].set_head(NULL);
473 _discovered_refs[i].set_length(0);
474 }
475 }
476 }
477
478 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
479 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
480 oop discovered = java_lang_ref_Reference::discovered(_ref);
481 assert(_discovered_addr && discovered->is_oop_or_null(),
482 "discovered field is bad");
483 _next = discovered;
484 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
485 _referent = java_lang_ref_Reference::referent(_ref);
486 assert(Universe::heap()->is_in_reserved_or_null(_referent),
487 "Wrong oop found in java.lang.Reference object");
488 assert(allow_null_referent ?
489 _referent->is_oop_or_null()
490 : _referent->is_oop(),
491 "bad referent");
492 }
493
494 void DiscoveredListIterator::remove() {
495 assert(_ref->is_oop(), "Dropping a bad reference");
496 oop_store_raw(_discovered_addr, NULL);
497
498 // First _prev_next ref actually points into DiscoveredList (gross).
499 oop new_next;
500 if (_next == _ref) {
501 // At the end of the list, we should make _prev point to itself.
502 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
503 // and _prev will be NULL.
504 new_next = _prev;
505 } else {
506 new_next = _next;
507 }
508 // Remove Reference object from discovered list. Note that G1 does not need a
509 // pre-barrier here because we know the Reference has already been found/marked,
510 // that's how it ended up in the discovered list in the first place.
511 oop_store_raw(_prev_next, new_next);
512 NOT_PRODUCT(_removed++);
513 _refs_list.dec_length(1);
514 }
515
516 // Make the Reference object active again.
517 void DiscoveredListIterator::make_active() {
518 // The pre barrier for G1 is probably just needed for the old
519 // reference processing behavior. Should we guard this with
520 // ReferenceProcessor::pending_list_uses_discovered_field() ?
521 if (UseG1GC) {
522 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
523 if (UseCompressedOops) {
524 oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
525 } else {
526 oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
527 }
528 }
529 java_lang_ref_Reference::set_next_raw(_ref, NULL);
530 }
531
532 void DiscoveredListIterator::clear_referent() {
533 oop_store_raw(_referent_addr, NULL);
534 }
535
536 // NOTE: process_phase*() are largely similar, and at a high level
537 // merely iterate over the extant list applying a predicate to
538 // each of its elements and possibly removing that element from the
539 // list and applying some further closures to that element.
540 // We should consider the possibility of replacing these
541 // process_phase*() methods by abstracting them into
542 // a single general iterator invocation that receives appropriate
543 // closures that accomplish this work.
544
545 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
546 // referents are not alive, but that should be kept alive for policy reasons.
547 // Keep alive the transitive closure of all such referents.
548 void
549 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
550 ReferencePolicy* policy,
551 BoolObjectClosure* is_alive,
552 OopClosure* keep_alive,
553 VoidClosure* complete_gc) {
554 assert(policy != NULL, "Must have a non-NULL policy");
555 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
556 // Decide which softly reachable refs should be kept alive.
557 while (iter.has_next()) {
558 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
559 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
560 if (referent_is_dead &&
561 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
562 if (TraceReferenceGC) {
563 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
564 (void *)iter.obj(), iter.obj()->klass()->internal_name());
565 }
566 // Remove Reference object from list
567 iter.remove();
568 // Make the Reference object active again
569 iter.make_active();
570 // keep the referent around
571 iter.make_referent_alive();
572 iter.move_to_next();
573 } else {
574 iter.next();
575 }
576 }
577 // Close the reachable set
578 complete_gc->do_void();
579 NOT_PRODUCT(
580 if (PrintGCDetails && TraceReferenceGC) {
581 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
582 "discovered Refs by policy, from list " INTPTR_FORMAT,
583 iter.removed(), iter.processed(), (address)refs_list.head());
584 }
585 )
586 }
587
588 // Traverse the list and remove any Refs that are not active, or
589 // whose referents are either alive or NULL.
590 void
591 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
592 BoolObjectClosure* is_alive,
593 OopClosure* keep_alive) {
594 assert(discovery_is_atomic(), "Error");
595 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
596 while (iter.has_next()) {
597 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
598 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
599 assert(next == NULL, "Should not discover inactive Reference");
600 if (iter.is_referent_alive()) {
601 if (TraceReferenceGC) {
602 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
603 (void *)iter.obj(), iter.obj()->klass()->internal_name());
604 }
605 // The referent is reachable after all.
606 // Remove Reference object from list.
607 iter.remove();
608 // Update the referent pointer as necessary: Note that this
609 // should not entail any recursive marking because the
610 // referent must already have been traversed.
611 iter.make_referent_alive();
612 iter.move_to_next();
613 } else {
614 iter.next();
615 }
616 }
617 NOT_PRODUCT(
618 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
619 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
620 "Refs in discovered list " INTPTR_FORMAT,
621 iter.removed(), iter.processed(), (address)refs_list.head());
622 }
623 )
624 }
625
626 void
627 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
628 BoolObjectClosure* is_alive,
629 OopClosure* keep_alive,
630 VoidClosure* complete_gc) {
631 assert(!discovery_is_atomic(), "Error");
632 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
633 while (iter.has_next()) {
634 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
635 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
636 oop next = java_lang_ref_Reference::next(iter.obj());
637 if ((iter.referent() == NULL || iter.is_referent_alive() ||
638 next != NULL)) {
639 assert(next->is_oop_or_null(), "bad next field");
640 // Remove Reference object from list
641 iter.remove();
642 // Trace the cohorts
643 iter.make_referent_alive();
644 if (UseCompressedOops) {
645 keep_alive->do_oop((narrowOop*)next_addr);
646 } else {
647 keep_alive->do_oop((oop*)next_addr);
648 }
649 iter.move_to_next();
650 } else {
651 iter.next();
652 }
653 }
654 // Now close the newly reachable set
655 complete_gc->do_void();
656 NOT_PRODUCT(
657 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
658 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
659 "Refs in discovered list " INTPTR_FORMAT,
660 iter.removed(), iter.processed(), (address)refs_list.head());
661 }
662 )
663 }
664
665 // Traverse the list and process the referents, by either
666 // clearing them or keeping them (and their reachable
667 // closure) alive.
668 void
669 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
670 bool clear_referent,
671 BoolObjectClosure* is_alive,
672 OopClosure* keep_alive,
673 VoidClosure* complete_gc) {
674 ResourceMark rm;
675 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
676 while (iter.has_next()) {
677 iter.update_discovered();
678 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
679 if (clear_referent) {
680 // NULL out referent pointer
681 iter.clear_referent();
682 } else {
683 // keep the referent around
684 iter.make_referent_alive();
685 }
686 if (TraceReferenceGC) {
687 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
688 clear_referent ? "cleared " : "",
689 (void *)iter.obj(), iter.obj()->klass()->internal_name());
690 }
691 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
692 iter.next();
693 }
694 // Remember to update the next pointer of the last ref.
695 iter.update_discovered();
696 // Close the reachable set
697 complete_gc->do_void();
698 }
699
700 void
701 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
702 oop obj = NULL;
703 oop next = refs_list.head();
704 while (next != obj) {
705 obj = next;
706 next = java_lang_ref_Reference::discovered(obj);
707 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
708 }
709 refs_list.set_head(NULL);
710 refs_list.set_length(0);
711 }
712
713 void
714 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
715 clear_discovered_references(refs_list);
716 }
717
718 void ReferenceProcessor::abandon_partial_discovery() {
719 // loop over the lists
720 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
721 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
722 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
723 }
724 abandon_partial_discovered_list(_discovered_refs[i]);
725 }
726 }
727
728 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
729 public:
730 RefProcPhase1Task(ReferenceProcessor& ref_processor,
731 DiscoveredList refs_lists[],
732 ReferencePolicy* policy,
733 bool marks_oops_alive)
734 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
735 _policy(policy)
736 { }
737 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
738 OopClosure& keep_alive,
739 VoidClosure& complete_gc)
740 {
741 Thread* thr = Thread::current();
742 int refs_list_index = ((WorkerThread*)thr)->id();
743 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
744 &is_alive, &keep_alive, &complete_gc);
745 }
746 private:
747 ReferencePolicy* _policy;
748 };
749
750 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
751 public:
752 RefProcPhase2Task(ReferenceProcessor& ref_processor,
753 DiscoveredList refs_lists[],
754 bool marks_oops_alive)
755 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
756 { }
757 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
758 OopClosure& keep_alive,
759 VoidClosure& complete_gc)
760 {
761 _ref_processor.process_phase2(_refs_lists[i],
762 &is_alive, &keep_alive, &complete_gc);
763 }
764 };
765
766 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
767 public:
768 RefProcPhase3Task(ReferenceProcessor& ref_processor,
769 DiscoveredList refs_lists[],
770 bool clear_referent,
771 bool marks_oops_alive)
772 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
773 _clear_referent(clear_referent)
774 { }
775 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
776 OopClosure& keep_alive,
777 VoidClosure& complete_gc)
778 {
779 // Don't use "refs_list_index" calculated in this way because
780 // balance_queues() has moved the Ref's into the first n queues.
781 // Thread* thr = Thread::current();
782 // int refs_list_index = ((WorkerThread*)thr)->id();
783 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
784 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
785 &is_alive, &keep_alive, &complete_gc);
786 }
787 private:
788 bool _clear_referent;
789 };
790
791 // Balances reference queues.
792 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
793 // queues[0, 1, ..., _num_q-1] because only the first _num_q
794 // corresponding to the active workers will be processed.
795 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
796 {
797 // calculate total length
798 size_t total_refs = 0;
799 if (TraceReferenceGC && PrintGCDetails) {
800 gclog_or_tty->print_cr("\nBalance ref_lists ");
801 }
802
803 for (uint i = 0; i < _max_num_q; ++i) {
804 total_refs += ref_lists[i].length();
805 if (TraceReferenceGC && PrintGCDetails) {
806 gclog_or_tty->print("%d ", ref_lists[i].length());
807 }
808 }
809 if (TraceReferenceGC && PrintGCDetails) {
810 gclog_or_tty->print_cr(" = %d", total_refs);
811 }
812 size_t avg_refs = total_refs / _num_q + 1;
813 uint to_idx = 0;
814 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
815 bool move_all = false;
816 if (from_idx >= _num_q) {
817 move_all = ref_lists[from_idx].length() > 0;
818 }
819 while ((ref_lists[from_idx].length() > avg_refs) ||
820 move_all) {
821 assert(to_idx < _num_q, "Sanity Check!");
822 if (ref_lists[to_idx].length() < avg_refs) {
823 // move superfluous refs
824 size_t refs_to_move;
825 // Move all the Ref's if the from queue will not be processed.
826 if (move_all) {
827 refs_to_move = MIN2(ref_lists[from_idx].length(),
828 avg_refs - ref_lists[to_idx].length());
829 } else {
830 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
831 avg_refs - ref_lists[to_idx].length());
832 }
833
834 assert(refs_to_move > 0, "otherwise the code below will fail");
835
836 oop move_head = ref_lists[from_idx].head();
837 oop move_tail = move_head;
838 oop new_head = move_head;
839 // find an element to split the list on
840 for (size_t j = 0; j < refs_to_move; ++j) {
841 move_tail = new_head;
842 new_head = java_lang_ref_Reference::discovered(new_head);
843 }
844
845 // Add the chain to the to list.
846 if (ref_lists[to_idx].head() == NULL) {
847 // to list is empty. Make a loop at the end.
848 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
849 } else {
850 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
851 }
852 ref_lists[to_idx].set_head(move_head);
853 ref_lists[to_idx].inc_length(refs_to_move);
854
855 // Remove the chain from the from list.
856 if (move_tail == new_head) {
857 // We found the end of the from list.
858 ref_lists[from_idx].set_head(NULL);
859 } else {
860 ref_lists[from_idx].set_head(new_head);
861 }
862 ref_lists[from_idx].dec_length(refs_to_move);
863 if (ref_lists[from_idx].length() == 0) {
864 break;
865 }
866 } else {
867 to_idx = (to_idx + 1) % _num_q;
868 }
869 }
870 }
871 #ifdef ASSERT
872 size_t balanced_total_refs = 0;
873 for (uint i = 0; i < _max_num_q; ++i) {
874 balanced_total_refs += ref_lists[i].length();
875 if (TraceReferenceGC && PrintGCDetails) {
876 gclog_or_tty->print("%d ", ref_lists[i].length());
877 }
878 }
879 if (TraceReferenceGC && PrintGCDetails) {
880 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
881 gclog_or_tty->flush();
882 }
883 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
884 #endif
885 }
886
887 void ReferenceProcessor::balance_all_queues() {
888 balance_queues(_discoveredSoftRefs);
889 balance_queues(_discoveredWeakRefs);
890 balance_queues(_discoveredFinalRefs);
891 balance_queues(_discoveredPhantomRefs);
892 balance_queues(_discoveredCleanerRefs);
893 }
894
895 size_t
896 ReferenceProcessor::process_discovered_reflist(
897 DiscoveredList refs_lists[],
898 ReferencePolicy* policy,
899 bool clear_referent,
900 BoolObjectClosure* is_alive,
901 OopClosure* keep_alive,
902 VoidClosure* complete_gc,
903 AbstractRefProcTaskExecutor* task_executor)
904 {
905 bool mt_processing = task_executor != NULL && _processing_is_mt;
906 // If discovery used MT and a dynamic number of GC threads, then
907 // the queues must be balanced for correctness if fewer than the
908 // maximum number of queues were used. The number of queue used
909 // during discovery may be different than the number to be used
910 // for processing so don't depend of _num_q < _max_num_q as part
911 // of the test.
912 bool must_balance = _discovery_is_mt;
913
914 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
915 must_balance) {
916 balance_queues(refs_lists);
917 }
918
919 size_t total_list_count = total_count(refs_lists);
920
921 if (PrintReferenceGC && PrintGCDetails) {
922 gclog_or_tty->print(", %u refs", total_list_count);
923 }
924
925 // Phase 1 (soft refs only):
926 // . Traverse the list and remove any SoftReferences whose
927 // referents are not alive, but that should be kept alive for
928 // policy reasons. Keep alive the transitive closure of all
929 // such referents.
930 if (policy != NULL) {
931 if (mt_processing) {
932 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
933 task_executor->execute(phase1);
934 } else {
935 for (uint i = 0; i < _max_num_q; i++) {
936 process_phase1(refs_lists[i], policy,
937 is_alive, keep_alive, complete_gc);
938 }
939 }
940 } else { // policy == NULL
941 assert(refs_lists != _discoveredSoftRefs,
942 "Policy must be specified for soft references.");
943 }
944
945 // Phase 2:
946 // . Traverse the list and remove any refs whose referents are alive.
947 if (mt_processing) {
948 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
949 task_executor->execute(phase2);
950 } else {
951 for (uint i = 0; i < _max_num_q; i++) {
952 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
953 }
954 }
955
956 // Phase 3:
957 // . Traverse the list and process referents as appropriate.
958 if (mt_processing) {
959 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
960 task_executor->execute(phase3);
961 } else {
962 for (uint i = 0; i < _max_num_q; i++) {
963 process_phase3(refs_lists[i], clear_referent,
964 is_alive, keep_alive, complete_gc);
965 }
966 }
967
968 return total_list_count;
969 }
970
971 void ReferenceProcessor::clean_up_discovered_references() {
972 // loop over the lists
973 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
974 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
975 gclog_or_tty->print_cr(
976 "\nScrubbing %s discovered list of Null referents",
977 list_name(i));
978 }
979 clean_up_discovered_reflist(_discovered_refs[i]);
980 }
981 }
982
983 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
984 assert(!discovery_is_atomic(), "Else why call this method?");
985 DiscoveredListIterator iter(refs_list, NULL, NULL);
986 while (iter.has_next()) {
987 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
988 oop next = java_lang_ref_Reference::next(iter.obj());
989 assert(next->is_oop_or_null(), "bad next field");
990 // If referent has been cleared or Reference is not active,
991 // drop it.
992 if (iter.referent() == NULL || next != NULL) {
993 debug_only(
994 if (PrintGCDetails && TraceReferenceGC) {
995 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
996 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
997 " and referent: " INTPTR_FORMAT,
998 (void *)iter.obj(), (void *)next, (void *)iter.referent());
999 }
1000 )
1001 // Remove Reference object from list
1002 iter.remove();
1003 iter.move_to_next();
1004 } else {
1005 iter.next();
1006 }
1007 }
1008 NOT_PRODUCT(
1009 if (PrintGCDetails && TraceReferenceGC) {
1010 gclog_or_tty->print(
1011 " Removed %d Refs with NULL referents out of %d discovered Refs",
1012 iter.removed(), iter.processed());
1013 }
1014 )
1015 }
1016
1017 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1018 uint id = 0;
1019 // Determine the queue index to use for this object.
1020 if (_discovery_is_mt) {
1021 // During a multi-threaded discovery phase,
1022 // each thread saves to its "own" list.
1023 Thread* thr = Thread::current();
1024 id = thr->as_Worker_thread()->id();
1025 } else {
1026 // single-threaded discovery, we save in round-robin
1027 // fashion to each of the lists.
1028 if (_processing_is_mt) {
1029 id = next_id();
1030 }
1031 }
1032 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1033
1034 // Get the discovered queue to which we will add
1035 DiscoveredList* list = NULL;
1036 switch (rt) {
1037 case REF_OTHER:
1038 // Unknown reference type, no special treatment
1039 break;
1040 case REF_SOFT:
1041 list = &_discoveredSoftRefs[id];
1042 break;
1043 case REF_WEAK:
1044 list = &_discoveredWeakRefs[id];
1045 break;
1046 case REF_FINAL:
1047 list = &_discoveredFinalRefs[id];
1048 break;
1049 case REF_PHANTOM:
1050 list = &_discoveredPhantomRefs[id];
1051 break;
1052 case REF_CLEANER:
1053 list = &_discoveredCleanerRefs[id];
1054 break;
1055 case REF_NONE:
1056 // we should not reach here if we are an InstanceRefKlass
1057 default:
1058 ShouldNotReachHere();
1059 }
1060 if (TraceReferenceGC && PrintGCDetails) {
1061 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1062 }
1063 return list;
1064 }
1065
1066 inline void
1067 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1068 oop obj,
1069 HeapWord* discovered_addr) {
1070 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1071 // First we must make sure this object is only enqueued once. CAS in a non null
1072 // discovered_addr.
1073 oop current_head = refs_list.head();
1074 // The last ref must have its discovered field pointing to itself.
1075 oop next_discovered = (current_head != NULL) ? current_head : obj;
1076
1077 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1078 NULL);
1079 if (retest == NULL) {
1080 // This thread just won the right to enqueue the object.
1081 // We have separate lists for enqueueing, so no synchronization
1082 // is necessary.
1083 refs_list.set_head(obj);
1084 refs_list.inc_length(1);
1085
1086 if (TraceReferenceGC) {
1087 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1088 (void *)obj, obj->klass()->internal_name());
1089 }
1090 } else {
1091 // If retest was non NULL, another thread beat us to it:
1092 // The reference has already been discovered...
1093 if (TraceReferenceGC) {
1094 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1095 (void *)obj, obj->klass()->internal_name());
1096 }
1097 }
1098 }
1099
1100 #ifndef PRODUCT
1101 // Non-atomic (i.e. concurrent) discovery might allow us
1102 // to observe j.l.References with NULL referents, being those
1103 // cleared concurrently by mutators during (or after) discovery.
1104 void ReferenceProcessor::verify_referent(oop obj) {
1105 bool da = discovery_is_atomic();
1106 oop referent = java_lang_ref_Reference::referent(obj);
1107 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1108 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1109 INTPTR_FORMAT " during %satomic discovery ",
1110 (void *)referent, (void *)obj, da ? "" : "non-"));
1111 }
1112 #endif
1113
1114 // We mention two of several possible choices here:
1115 // #0: if the reference object is not in the "originating generation"
1116 // (or part of the heap being collected, indicated by our "span"
1117 // we don't treat it specially (i.e. we scan it as we would
1118 // a normal oop, treating its references as strong references).
1119 // This means that references can't be discovered unless their
1120 // referent is also in the same span. This is the simplest,
1121 // most "local" and most conservative approach, albeit one
1122 // that may cause weak references to be enqueued least promptly.
1123 // We call this choice the "ReferenceBasedDiscovery" policy.
1124 // #1: the reference object may be in any generation (span), but if
1125 // the referent is in the generation (span) being currently collected
1126 // then we can discover the reference object, provided
1127 // the object has not already been discovered by
1128 // a different concurrently running collector (as may be the
1129 // case, for instance, if the reference object is in CMS and
1130 // the referent in DefNewGeneration), and provided the processing
1131 // of this reference object by the current collector will
1132 // appear atomic to every other collector in the system.
1133 // (Thus, for instance, a concurrent collector may not
1134 // discover references in other generations even if the
1135 // referent is in its own generation). This policy may,
1136 // in certain cases, enqueue references somewhat sooner than
1137 // might Policy #0 above, but at marginally increased cost
1138 // and complexity in processing these references.
1139 // We call this choice the "RefeferentBasedDiscovery" policy.
1140 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1141 // Make sure we are discovering refs (rather than processing discovered refs).
1142 if (!_discovering_refs || !RegisterReferences) {
1143 return false;
1144 }
1145 // We only discover active references.
1146 oop next = java_lang_ref_Reference::next(obj);
1147 if (next != NULL) { // Ref is no longer active
1148 return false;
1149 }
1150
1151 HeapWord* obj_addr = (HeapWord*)obj;
1152 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1153 !_span.contains(obj_addr)) {
1154 // Reference is not in the originating generation;
1155 // don't treat it specially (i.e. we want to scan it as a normal
1156 // object with strong references).
1157 return false;
1158 }
1159
1160 // We only discover references whose referents are not (yet)
1161 // known to be strongly reachable.
1162 if (is_alive_non_header() != NULL) {
1163 verify_referent(obj);
1164 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1165 return false; // referent is reachable
1166 }
1167 }
1168 if (rt == REF_SOFT) {
1169 // For soft refs we can decide now if these are not
1170 // current candidates for clearing, in which case we
1171 // can mark through them now, rather than delaying that
1172 // to the reference-processing phase. Since all current
1173 // time-stamp policies advance the soft-ref clock only
1174 // at a major collection cycle, this is always currently
1175 // accurate.
1176 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1177 return false;
1178 }
1179 }
1180
1181 ResourceMark rm; // Needed for tracing.
1182
1183 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1184 const oop discovered = java_lang_ref_Reference::discovered(obj);
1185 assert(discovered->is_oop_or_null(), "bad discovered field");
1186 if (discovered != NULL) {
1187 // The reference has already been discovered...
1188 if (TraceReferenceGC) {
1189 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1190 (void *)obj, obj->klass()->internal_name());
1191 }
1192 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1193 // assumes that an object is not processed twice;
1194 // if it's been already discovered it must be on another
1195 // generation's discovered list; so we won't discover it.
1196 return false;
1197 } else {
1198 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1199 "Unrecognized policy");
1200 // Check assumption that an object is not potentially
1201 // discovered twice except by concurrent collectors that potentially
1202 // trace the same Reference object twice.
1203 assert(UseConcMarkSweepGC || UseG1GC,
1204 "Only possible with a concurrent marking collector");
1205 return true;
1206 }
1207 }
1208
1209 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1210 verify_referent(obj);
1211 // Discover if and only if EITHER:
1212 // .. reference is in our span, OR
1213 // .. we are an atomic collector and referent is in our span
1214 if (_span.contains(obj_addr) ||
1215 (discovery_is_atomic() &&
1216 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1217 // should_enqueue = true;
1218 } else {
1219 return false;
1220 }
1221 } else {
1222 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1223 _span.contains(obj_addr), "code inconsistency");
1224 }
1225
1226 // Get the right type of discovered queue head.
1227 DiscoveredList* list = get_discovered_list(rt);
1228 if (list == NULL) {
1229 return false; // nothing special needs to be done
1230 }
1231
1232 if (_discovery_is_mt) {
1233 add_to_discovered_list_mt(*list, obj, discovered_addr);
1234 } else {
1235 // We do a raw store here: the field will be visited later when processing
1236 // the discovered references.
1237 oop current_head = list->head();
1238 // The last ref must have its discovered field pointing to itself.
1239 oop next_discovered = (current_head != NULL) ? current_head : obj;
1240
1241 assert(discovered == NULL, "control point invariant");
1242 oop_store_raw(discovered_addr, next_discovered);
1243 list->set_head(obj);
1244 list->inc_length(1);
1245
1246 if (TraceReferenceGC) {
1247 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1248 (void *)obj, obj->klass()->internal_name());
1249 }
1250 }
1251 assert(obj->is_oop(), "Discovered a bad reference");
1252 verify_referent(obj);
1253 return true;
1254 }
1255
1256 // Preclean the discovered references by removing those
1257 // whose referents are alive, and by marking from those that
1258 // are not active. These lists can be handled here
1259 // in any order and, indeed, concurrently.
1260 void ReferenceProcessor::preclean_discovered_references(
1261 BoolObjectClosure* is_alive,
1262 OopClosure* keep_alive,
1263 VoidClosure* complete_gc,
1264 YieldClosure* yield,
1265 GCTimer* gc_timer,
1266 GCId gc_id) {
1267
1268 NOT_PRODUCT(verify_ok_to_handle_reflists());
1269
1270 // Soft references
1271 {
1272 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1273 false, gc_timer, gc_id);
1274 for (uint i = 0; i < _max_num_q; i++) {
1275 if (yield->should_return()) {
1276 return;
1277 }
1278 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1279 keep_alive, complete_gc, yield);
1280 }
1281 }
1282
1283 // Weak references
1284 {
1285 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1286 false, gc_timer, gc_id);
1287 for (uint i = 0; i < _max_num_q; i++) {
1288 if (yield->should_return()) {
1289 return;
1290 }
1291 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1292 keep_alive, complete_gc, yield);
1293 }
1294 }
1295
1296 // Final references
1297 {
1298 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1299 false, gc_timer, gc_id);
1300 for (uint i = 0; i < _max_num_q; i++) {
1301 if (yield->should_return()) {
1302 return;
1303 }
1304 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1305 keep_alive, complete_gc, yield);
1306 }
1307 }
1308
1309 // Phantom references
1310 {
1311 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1312 false, gc_timer, gc_id);
1313 for (uint i = 0; i < _max_num_q; i++) {
1314 if (yield->should_return()) {
1315 return;
1316 }
1317 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1318 keep_alive, complete_gc, yield);
1319 }
1320
1321 // Cleaner references. Included in timing for phantom references. We
1322 // expect Cleaner references to be temporary, and don't want to deal with
1323 // possible incompatibilities arising from making it more visible.
1324 for (uint i = 0; i < _max_num_q; i++) {
1325 if (yield->should_return()) {
1326 return;
1327 }
1328 preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1329 keep_alive, complete_gc, yield);
1330 }
1331 }
1332 }
1333
1334 // Walk the given discovered ref list, and remove all reference objects
1335 // whose referents are still alive, whose referents are NULL or which
1336 // are not active (have a non-NULL next field). NOTE: When we are
1337 // thus precleaning the ref lists (which happens single-threaded today),
1338 // we do not disable refs discovery to honour the correct semantics of
1339 // java.lang.Reference. As a result, we need to be careful below
1340 // that ref removal steps interleave safely with ref discovery steps
1341 // (in this thread).
1342 void
1343 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1344 BoolObjectClosure* is_alive,
1345 OopClosure* keep_alive,
1346 VoidClosure* complete_gc,
1347 YieldClosure* yield) {
1348 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1349 while (iter.has_next()) {
1350 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1351 oop obj = iter.obj();
1352 oop next = java_lang_ref_Reference::next(obj);
1353 if (iter.referent() == NULL || iter.is_referent_alive() ||
1354 next != NULL) {
1355 // The referent has been cleared, or is alive, or the Reference is not
1356 // active; we need to trace and mark its cohort.
1357 if (TraceReferenceGC) {
1358 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1359 (void *)iter.obj(), iter.obj()->klass()->internal_name());
1360 }
1361 // Remove Reference object from list
1362 iter.remove();
1363 // Keep alive its cohort.
1364 iter.make_referent_alive();
1365 if (UseCompressedOops) {
1366 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1367 keep_alive->do_oop(next_addr);
1368 } else {
1369 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1370 keep_alive->do_oop(next_addr);
1371 }
1372 iter.move_to_next();
1373 } else {
1374 iter.next();
1375 }
1376 }
1377 // Close the reachable set
1378 complete_gc->do_void();
1379
1380 NOT_PRODUCT(
1381 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1382 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1383 "Refs in discovered list " INTPTR_FORMAT,
1384 iter.removed(), iter.processed(), (address)refs_list.head());
1385 }
1386 )
1387 }
1388
1389 const char* ReferenceProcessor::list_name(uint i) {
1390 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1391 "Out of bounds index");
1392
1393 int j = i / _max_num_q;
1394 switch (j) {
1395 case 0: return "SoftRef";
1396 case 1: return "WeakRef";
1397 case 2: return "FinalRef";
1398 case 3: return "PhantomRef";
1399 case 4: return "CleanerRef";
1400 }
1401 ShouldNotReachHere();
1402 return NULL;
1403 }
1404
1405 #ifndef PRODUCT
1406 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1407 // empty for now
1408 }
1409 #endif
1410
1411 #ifndef PRODUCT
1412 void ReferenceProcessor::clear_discovered_references() {
1413 guarantee(!_discovering_refs, "Discovering refs?");
1414 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1415 clear_discovered_references(_discovered_refs[i]);
1416 }
1417 }
1418
1419 #endif // PRODUCT