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 AlwaysAliveClosure: public BoolObjectClosure {
282 public:
283 virtual bool do_object_b(oop obj) { return true; }
284 };
285
286 class CountHandleClosure: public OopClosure {
287 private:
288 int _count;
289 public:
290 CountHandleClosure(): _count(0) {}
291 void do_oop(oop* unused) { _count++; }
292 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
293 int count() { return _count; }
294 };
295 CountHandleClosure global_handle_count;
296 AlwaysAliveClosure always_alive;
297 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
298 return global_handle_count.count();
299 }
300 #endif
301
302 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
303 OopClosure* keep_alive,
304 VoidClosure* complete_gc) {
305 #ifndef PRODUCT
306 if (PrintGCDetails && PrintReferenceGC) {
307 unsigned int count = count_jni_refs();
308 gclog_or_tty->print(", %u refs", count);
309 }
310 #endif
311 JNIHandles::weak_oops_do(is_alive, keep_alive);
312 JFR_ONLY(Jfr::weak_oops_do(is_alive, keep_alive));
313 complete_gc->do_void();
314 }
315
316
317 template <class T>
318 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
319 AbstractRefProcTaskExecutor* task_executor) {
320
321 // Remember old value of pending references list
322 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
323 T old_pending_list_value = *pending_list_addr;
324
325 // Enqueue references that are not made active again, and
326 // clear the decks for the next collection (cycle).
327 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
328 // Do the post-barrier on pending_list_addr missed in
329 // enqueue_discovered_reflist.
330 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
331
332 // Stop treating discovered references specially.
333 ref->disable_discovery();
334
335 // Return true if new pending references were added
336 return old_pending_list_value != *pending_list_addr;
337 }
338
339 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
340 NOT_PRODUCT(verify_ok_to_handle_reflists());
341 if (UseCompressedOops) {
342 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
343 } else {
344 return enqueue_discovered_ref_helper<oop>(this, task_executor);
345 }
346 }
347
348 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
349 HeapWord* pending_list_addr) {
350 // Given a list of refs linked through the "discovered" field
351 // (java.lang.ref.Reference.discovered), self-loop their "next" field
352 // thus distinguishing them from active References, then
353 // prepend them to the pending list.
354 //
355 // The Java threads will see the Reference objects linked together through
356 // the discovered field. Instead of trying to do the write barrier updates
357 // in all places in the reference processor where we manipulate the discovered
358 // field we make sure to do the barrier here where we anyway iterate through
359 // all linked Reference objects. Note that it is important to not dirty any
360 // cards during reference processing since this will cause card table
361 // verification to fail for G1.
362 //
363 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
364 // the "next" field is used to chain the pending list, not the discovered
365 // field.
366 if (TraceReferenceGC && PrintGCDetails) {
367 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
368 INTPTR_FORMAT, (address)refs_list.head());
369 }
370
371 oop obj = NULL;
372 oop next_d = refs_list.head();
373 if (pending_list_uses_discovered_field()) { // New behavior
374 // Walk down the list, self-looping the next field
375 // so that the References are not considered active.
376 while (obj != next_d) {
377 obj = next_d;
378 assert(obj->is_instanceRef(), "should be reference object");
379 next_d = java_lang_ref_Reference::discovered(obj);
380 if (TraceReferenceGC && PrintGCDetails) {
381 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
382 (void *)obj, (void *)next_d);
383 }
384 assert(java_lang_ref_Reference::next(obj) == NULL,
385 "Reference not active; should not be discovered");
386 // Self-loop next, so as to make Ref not active.
387 java_lang_ref_Reference::set_next_raw(obj, obj);
388 if (next_d != obj) {
389 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
390 } else {
391 // This is the last object.
392 // Swap refs_list into pending_list_addr and
393 // set obj's discovered to what we read from pending_list_addr.
394 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
395 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
396 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
397 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
398 }
399 }
400 } else { // Old behaviour
401 // Walk down the list, copying the discovered field into
402 // the next field and clearing the discovered field.
403 while (obj != next_d) {
404 obj = next_d;
405 assert(obj->is_instanceRef(), "should be reference object");
406 next_d = java_lang_ref_Reference::discovered(obj);
407 if (TraceReferenceGC && PrintGCDetails) {
408 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
409 (void *)obj, (void *)next_d);
410 }
411 assert(java_lang_ref_Reference::next(obj) == NULL,
412 "The reference should not be enqueued");
413 if (next_d == obj) { // obj is last
414 // Swap refs_list into pendling_list_addr and
415 // set obj's next to what we read from pending_list_addr.
416 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
417 // Need oop_check on pending_list_addr above;
418 // see special oop-check code at the end of
419 // enqueue_discovered_reflists() further below.
420 if (old == NULL) {
421 // obj should be made to point to itself, since
422 // pending list was empty.
423 java_lang_ref_Reference::set_next(obj, obj);
424 } else {
425 java_lang_ref_Reference::set_next(obj, old);
426 }
427 } else {
428 java_lang_ref_Reference::set_next(obj, next_d);
429 }
430 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
431 }
432 }
433 }
434
435 // Parallel enqueue task
436 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
437 public:
438 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
439 DiscoveredList discovered_refs[],
440 HeapWord* pending_list_addr,
441 int n_queues)
442 : EnqueueTask(ref_processor, discovered_refs,
443 pending_list_addr, n_queues)
444 { }
445
446 virtual void work(unsigned int work_id) {
447 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
448 // Simplest first cut: static partitioning.
449 int index = work_id;
450 // The increment on "index" must correspond to the maximum number of queues
451 // (n_queues) with which that ReferenceProcessor was created. That
452 // is because of the "clever" way the discovered references lists were
453 // allocated and are indexed into.
454 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
455 for (int j = 0;
456 j < ReferenceProcessor::number_of_subclasses_of_ref();
457 j++, index += _n_queues) {
458 _ref_processor.enqueue_discovered_reflist(
459 _refs_lists[index], _pending_list_addr);
460 _refs_lists[index].set_head(NULL);
461 _refs_lists[index].set_length(0);
462 }
463 }
464 };
465
466 // Enqueue references that are not made active again
467 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
468 AbstractRefProcTaskExecutor* task_executor) {
469 if (_processing_is_mt && task_executor != NULL) {
470 // Parallel code
471 RefProcEnqueueTask tsk(*this, _discovered_refs,
472 pending_list_addr, _max_num_q);
473 task_executor->execute(tsk);
474 } else {
475 // Serial code: call the parent class's implementation
476 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
477 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
478 _discovered_refs[i].set_head(NULL);
479 _discovered_refs[i].set_length(0);
480 }
481 }
482 }
483
484 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
485 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
486 oop discovered = java_lang_ref_Reference::discovered(_ref);
487 assert(_discovered_addr && discovered->is_oop_or_null(),
488 "discovered field is bad");
489 _next = discovered;
490 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
491 _referent = java_lang_ref_Reference::referent(_ref);
492 assert(Universe::heap()->is_in_reserved_or_null(_referent),
493 "Wrong oop found in java.lang.Reference object");
494 assert(allow_null_referent ?
495 _referent->is_oop_or_null()
496 : _referent->is_oop(),
497 "bad referent");
498 }
499
500 void DiscoveredListIterator::remove() {
501 assert(_ref->is_oop(), "Dropping a bad reference");
502 oop_store_raw(_discovered_addr, NULL);
503
504 // First _prev_next ref actually points into DiscoveredList (gross).
505 oop new_next;
506 if (_next == _ref) {
507 // At the end of the list, we should make _prev point to itself.
508 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
509 // and _prev will be NULL.
510 new_next = _prev;
511 } else {
512 new_next = _next;
513 }
514 // Remove Reference object from discovered list. Note that G1 does not need a
515 // pre-barrier here because we know the Reference has already been found/marked,
516 // that's how it ended up in the discovered list in the first place.
517 oop_store_raw(_prev_next, new_next);
518 NOT_PRODUCT(_removed++);
519 _refs_list.dec_length(1);
520 }
521
522 // Make the Reference object active again.
523 void DiscoveredListIterator::make_active() {
524 // The pre barrier for G1 is probably just needed for the old
525 // reference processing behavior. Should we guard this with
526 // ReferenceProcessor::pending_list_uses_discovered_field() ?
527 if (UseG1GC) {
528 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
529 if (UseCompressedOops) {
530 oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
531 } else {
532 oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
533 }
534 }
535 java_lang_ref_Reference::set_next_raw(_ref, NULL);
536 }
537
538 void DiscoveredListIterator::clear_referent() {
539 oop_store_raw(_referent_addr, NULL);
540 }
541
542 // NOTE: process_phase*() are largely similar, and at a high level
543 // merely iterate over the extant list applying a predicate to
544 // each of its elements and possibly removing that element from the
545 // list and applying some further closures to that element.
546 // We should consider the possibility of replacing these
547 // process_phase*() methods by abstracting them into
548 // a single general iterator invocation that receives appropriate
549 // closures that accomplish this work.
550
551 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
552 // referents are not alive, but that should be kept alive for policy reasons.
553 // Keep alive the transitive closure of all such referents.
554 void
555 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
556 ReferencePolicy* policy,
557 BoolObjectClosure* is_alive,
558 OopClosure* keep_alive,
559 VoidClosure* complete_gc) {
560 assert(policy != NULL, "Must have a non-NULL policy");
561 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
562 // Decide which softly reachable refs should be kept alive.
563 while (iter.has_next()) {
564 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
565 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
566 if (referent_is_dead &&
567 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
568 if (TraceReferenceGC) {
569 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
570 (void *)iter.obj(), iter.obj()->klass()->internal_name());
571 }
572 // Remove Reference object from list
573 iter.remove();
574 // Make the Reference object active again
575 iter.make_active();
576 // keep the referent around
577 iter.make_referent_alive();
578 iter.move_to_next();
579 } else {
580 iter.next();
581 }
582 }
583 // Close the reachable set
584 complete_gc->do_void();
585 NOT_PRODUCT(
586 if (PrintGCDetails && TraceReferenceGC) {
587 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
588 "discovered Refs by policy, from list " INTPTR_FORMAT,
589 iter.removed(), iter.processed(), (address)refs_list.head());
590 }
591 )
592 }
593
594 // Traverse the list and remove any Refs that are not active, or
595 // whose referents are either alive or NULL.
596 void
597 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
598 BoolObjectClosure* is_alive,
599 OopClosure* keep_alive) {
600 assert(discovery_is_atomic(), "Error");
601 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
602 while (iter.has_next()) {
603 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
604 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
605 assert(next == NULL, "Should not discover inactive Reference");
606 if (iter.is_referent_alive()) {
607 if (TraceReferenceGC) {
608 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
609 (void *)iter.obj(), iter.obj()->klass()->internal_name());
610 }
611 // The referent is reachable after all.
612 // Remove Reference object from list.
613 iter.remove();
614 // Update the referent pointer as necessary: Note that this
615 // should not entail any recursive marking because the
616 // referent must already have been traversed.
617 iter.make_referent_alive();
618 iter.move_to_next();
619 } else {
620 iter.next();
621 }
622 }
623 NOT_PRODUCT(
624 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
625 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
626 "Refs in discovered list " INTPTR_FORMAT,
627 iter.removed(), iter.processed(), (address)refs_list.head());
628 }
629 )
630 }
631
632 void
633 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
634 BoolObjectClosure* is_alive,
635 OopClosure* keep_alive,
636 VoidClosure* complete_gc) {
637 assert(!discovery_is_atomic(), "Error");
638 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
639 while (iter.has_next()) {
640 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
641 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
642 oop next = java_lang_ref_Reference::next(iter.obj());
643 if ((iter.referent() == NULL || iter.is_referent_alive() ||
644 next != NULL)) {
645 assert(next->is_oop_or_null(), "bad next field");
646 // Remove Reference object from list
647 iter.remove();
648 // Trace the cohorts
649 iter.make_referent_alive();
650 if (UseCompressedOops) {
651 keep_alive->do_oop((narrowOop*)next_addr);
652 } else {
653 keep_alive->do_oop((oop*)next_addr);
654 }
655 iter.move_to_next();
656 } else {
657 iter.next();
658 }
659 }
660 // Now close the newly reachable set
661 complete_gc->do_void();
662 NOT_PRODUCT(
663 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
664 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
665 "Refs in discovered list " INTPTR_FORMAT,
666 iter.removed(), iter.processed(), (address)refs_list.head());
667 }
668 )
669 }
670
671 // Traverse the list and process the referents, by either
672 // clearing them or keeping them (and their reachable
673 // closure) alive.
674 void
675 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
676 bool clear_referent,
677 BoolObjectClosure* is_alive,
678 OopClosure* keep_alive,
679 VoidClosure* complete_gc) {
680 ResourceMark rm;
681 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
682 while (iter.has_next()) {
683 iter.update_discovered();
684 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
685 if (clear_referent) {
686 // NULL out referent pointer
687 iter.clear_referent();
688 } else {
689 // keep the referent around
690 iter.make_referent_alive();
691 }
692 if (TraceReferenceGC) {
693 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
694 clear_referent ? "cleared " : "",
695 (void *)iter.obj(), iter.obj()->klass()->internal_name());
696 }
697 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
698 iter.next();
699 }
700 // Remember to update the next pointer of the last ref.
701 iter.update_discovered();
702 // Close the reachable set
703 complete_gc->do_void();
704 }
705
706 void
707 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
708 oop obj = NULL;
709 oop next = refs_list.head();
710 while (next != obj) {
711 obj = next;
712 next = java_lang_ref_Reference::discovered(obj);
713 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
714 }
715 refs_list.set_head(NULL);
716 refs_list.set_length(0);
717 }
718
719 void
720 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
721 clear_discovered_references(refs_list);
722 }
723
724 void ReferenceProcessor::abandon_partial_discovery() {
725 // loop over the lists
726 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
727 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
728 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
729 }
730 abandon_partial_discovered_list(_discovered_refs[i]);
731 }
732 }
733
734 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
735 public:
736 RefProcPhase1Task(ReferenceProcessor& ref_processor,
737 DiscoveredList refs_lists[],
738 ReferencePolicy* policy,
739 bool marks_oops_alive)
740 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
741 _policy(policy)
742 { }
743 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
744 OopClosure& keep_alive,
745 VoidClosure& complete_gc)
746 {
747 Thread* thr = Thread::current();
748 int refs_list_index = ((WorkerThread*)thr)->id();
749 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
750 &is_alive, &keep_alive, &complete_gc);
751 }
752 private:
753 ReferencePolicy* _policy;
754 };
755
756 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
757 public:
758 RefProcPhase2Task(ReferenceProcessor& ref_processor,
759 DiscoveredList refs_lists[],
760 bool marks_oops_alive)
761 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
762 { }
763 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
764 OopClosure& keep_alive,
765 VoidClosure& complete_gc)
766 {
767 _ref_processor.process_phase2(_refs_lists[i],
768 &is_alive, &keep_alive, &complete_gc);
769 }
770 };
771
772 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
773 public:
774 RefProcPhase3Task(ReferenceProcessor& ref_processor,
775 DiscoveredList refs_lists[],
776 bool clear_referent,
777 bool marks_oops_alive)
778 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
779 _clear_referent(clear_referent)
780 { }
781 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
782 OopClosure& keep_alive,
783 VoidClosure& complete_gc)
784 {
785 // Don't use "refs_list_index" calculated in this way because
786 // balance_queues() has moved the Ref's into the first n queues.
787 // Thread* thr = Thread::current();
788 // int refs_list_index = ((WorkerThread*)thr)->id();
789 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
790 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
791 &is_alive, &keep_alive, &complete_gc);
792 }
793 private:
794 bool _clear_referent;
795 };
796
797 // Balances reference queues.
798 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
799 // queues[0, 1, ..., _num_q-1] because only the first _num_q
800 // corresponding to the active workers will be processed.
801 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
802 {
803 // calculate total length
804 size_t total_refs = 0;
805 if (TraceReferenceGC && PrintGCDetails) {
806 gclog_or_tty->print_cr("\nBalance ref_lists ");
807 }
808
809 for (uint i = 0; i < _max_num_q; ++i) {
810 total_refs += ref_lists[i].length();
811 if (TraceReferenceGC && PrintGCDetails) {
812 gclog_or_tty->print("%d ", ref_lists[i].length());
813 }
814 }
815 if (TraceReferenceGC && PrintGCDetails) {
816 gclog_or_tty->print_cr(" = %d", total_refs);
817 }
818 size_t avg_refs = total_refs / _num_q + 1;
819 uint to_idx = 0;
820 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
821 bool move_all = false;
822 if (from_idx >= _num_q) {
823 move_all = ref_lists[from_idx].length() > 0;
824 }
825 while ((ref_lists[from_idx].length() > avg_refs) ||
826 move_all) {
827 assert(to_idx < _num_q, "Sanity Check!");
828 if (ref_lists[to_idx].length() < avg_refs) {
829 // move superfluous refs
830 size_t refs_to_move;
831 // Move all the Ref's if the from queue will not be processed.
832 if (move_all) {
833 refs_to_move = MIN2(ref_lists[from_idx].length(),
834 avg_refs - ref_lists[to_idx].length());
835 } else {
836 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
837 avg_refs - ref_lists[to_idx].length());
838 }
839
840 assert(refs_to_move > 0, "otherwise the code below will fail");
841
842 oop move_head = ref_lists[from_idx].head();
843 oop move_tail = move_head;
844 oop new_head = move_head;
845 // find an element to split the list on
846 for (size_t j = 0; j < refs_to_move; ++j) {
847 move_tail = new_head;
848 new_head = java_lang_ref_Reference::discovered(new_head);
849 }
850
851 // Add the chain to the to list.
852 if (ref_lists[to_idx].head() == NULL) {
853 // to list is empty. Make a loop at the end.
854 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
855 } else {
856 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
857 }
858 ref_lists[to_idx].set_head(move_head);
859 ref_lists[to_idx].inc_length(refs_to_move);
860
861 // Remove the chain from the from list.
862 if (move_tail == new_head) {
863 // We found the end of the from list.
864 ref_lists[from_idx].set_head(NULL);
865 } else {
866 ref_lists[from_idx].set_head(new_head);
867 }
868 ref_lists[from_idx].dec_length(refs_to_move);
869 if (ref_lists[from_idx].length() == 0) {
870 break;
871 }
872 } else {
873 to_idx = (to_idx + 1) % _num_q;
874 }
875 }
876 }
877 #ifdef ASSERT
878 size_t balanced_total_refs = 0;
879 for (uint i = 0; i < _max_num_q; ++i) {
880 balanced_total_refs += ref_lists[i].length();
881 if (TraceReferenceGC && PrintGCDetails) {
882 gclog_or_tty->print("%d ", ref_lists[i].length());
883 }
884 }
885 if (TraceReferenceGC && PrintGCDetails) {
886 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
887 gclog_or_tty->flush();
888 }
889 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
890 #endif
891 }
892
893 void ReferenceProcessor::balance_all_queues() {
894 balance_queues(_discoveredSoftRefs);
895 balance_queues(_discoveredWeakRefs);
896 balance_queues(_discoveredFinalRefs);
897 balance_queues(_discoveredPhantomRefs);
898 balance_queues(_discoveredCleanerRefs);
899 }
900
901 size_t
902 ReferenceProcessor::process_discovered_reflist(
903 DiscoveredList refs_lists[],
904 ReferencePolicy* policy,
905 bool clear_referent,
906 BoolObjectClosure* is_alive,
907 OopClosure* keep_alive,
908 VoidClosure* complete_gc,
909 AbstractRefProcTaskExecutor* task_executor)
910 {
911 bool mt_processing = task_executor != NULL && _processing_is_mt;
912 // If discovery used MT and a dynamic number of GC threads, then
913 // the queues must be balanced for correctness if fewer than the
914 // maximum number of queues were used. The number of queue used
915 // during discovery may be different than the number to be used
916 // for processing so don't depend of _num_q < _max_num_q as part
917 // of the test.
918 bool must_balance = _discovery_is_mt;
919
920 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
921 must_balance) {
922 balance_queues(refs_lists);
923 }
924
925 size_t total_list_count = total_count(refs_lists);
926
927 if (PrintReferenceGC && PrintGCDetails) {
928 gclog_or_tty->print(", %u refs", total_list_count);
929 }
930
931 // Phase 1 (soft refs only):
932 // . Traverse the list and remove any SoftReferences whose
933 // referents are not alive, but that should be kept alive for
934 // policy reasons. Keep alive the transitive closure of all
935 // such referents.
936 if (policy != NULL) {
937 if (mt_processing) {
938 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
939 task_executor->execute(phase1);
940 } else {
941 for (uint i = 0; i < _max_num_q; i++) {
942 process_phase1(refs_lists[i], policy,
943 is_alive, keep_alive, complete_gc);
944 }
945 }
946 } else { // policy == NULL
947 assert(refs_lists != _discoveredSoftRefs,
948 "Policy must be specified for soft references.");
949 }
950
951 // Phase 2:
952 // . Traverse the list and remove any refs whose referents are alive.
953 if (mt_processing) {
954 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
955 task_executor->execute(phase2);
956 } else {
957 for (uint i = 0; i < _max_num_q; i++) {
958 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
959 }
960 }
961
962 // Phase 3:
963 // . Traverse the list and process referents as appropriate.
964 if (mt_processing) {
965 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
966 task_executor->execute(phase3);
967 } else {
968 for (uint i = 0; i < _max_num_q; i++) {
969 process_phase3(refs_lists[i], clear_referent,
970 is_alive, keep_alive, complete_gc);
971 }
972 }
973
974 return total_list_count;
975 }
976
977 void ReferenceProcessor::clean_up_discovered_references() {
978 // loop over the lists
979 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
980 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
981 gclog_or_tty->print_cr(
982 "\nScrubbing %s discovered list of Null referents",
983 list_name(i));
984 }
985 clean_up_discovered_reflist(_discovered_refs[i]);
986 }
987 }
988
989 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
990 assert(!discovery_is_atomic(), "Else why call this method?");
991 DiscoveredListIterator iter(refs_list, NULL, NULL);
992 while (iter.has_next()) {
993 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
994 oop next = java_lang_ref_Reference::next(iter.obj());
995 assert(next->is_oop_or_null(), "bad next field");
996 // If referent has been cleared or Reference is not active,
997 // drop it.
998 if (iter.referent() == NULL || next != NULL) {
999 debug_only(
1000 if (PrintGCDetails && TraceReferenceGC) {
1001 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
1002 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
1003 " and referent: " INTPTR_FORMAT,
1004 (void *)iter.obj(), (void *)next, (void *)iter.referent());
1005 }
1006 )
1007 // Remove Reference object from list
1008 iter.remove();
1009 iter.move_to_next();
1010 } else {
1011 iter.next();
1012 }
1013 }
1014 NOT_PRODUCT(
1015 if (PrintGCDetails && TraceReferenceGC) {
1016 gclog_or_tty->print(
1017 " Removed %d Refs with NULL referents out of %d discovered Refs",
1018 iter.removed(), iter.processed());
1019 }
1020 )
1021 }
1022
1023 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1024 uint id = 0;
1025 // Determine the queue index to use for this object.
1026 if (_discovery_is_mt) {
1027 // During a multi-threaded discovery phase,
1028 // each thread saves to its "own" list.
1029 Thread* thr = Thread::current();
1030 id = thr->as_Worker_thread()->id();
1031 } else {
1032 // single-threaded discovery, we save in round-robin
1033 // fashion to each of the lists.
1034 if (_processing_is_mt) {
1035 id = next_id();
1036 }
1037 }
1038 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1039
1040 // Get the discovered queue to which we will add
1041 DiscoveredList* list = NULL;
1042 switch (rt) {
1043 case REF_OTHER:
1044 // Unknown reference type, no special treatment
1045 break;
1046 case REF_SOFT:
1047 list = &_discoveredSoftRefs[id];
1048 break;
1049 case REF_WEAK:
1050 list = &_discoveredWeakRefs[id];
1051 break;
1052 case REF_FINAL:
1053 list = &_discoveredFinalRefs[id];
1054 break;
1055 case REF_PHANTOM:
1056 list = &_discoveredPhantomRefs[id];
1057 break;
1058 case REF_CLEANER:
1059 list = &_discoveredCleanerRefs[id];
1060 break;
1061 case REF_NONE:
1062 // we should not reach here if we are an InstanceRefKlass
1063 default:
1064 ShouldNotReachHere();
1065 }
1066 if (TraceReferenceGC && PrintGCDetails) {
1067 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1068 }
1069 return list;
1070 }
1071
1072 inline void
1073 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1074 oop obj,
1075 HeapWord* discovered_addr) {
1076 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1077 // First we must make sure this object is only enqueued once. CAS in a non null
1078 // discovered_addr.
1079 oop current_head = refs_list.head();
1080 // The last ref must have its discovered field pointing to itself.
1081 oop next_discovered = (current_head != NULL) ? current_head : obj;
1082
1083 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1084 NULL);
1085 if (retest == NULL) {
1086 // This thread just won the right to enqueue the object.
1087 // We have separate lists for enqueueing, so no synchronization
1088 // is necessary.
1089 refs_list.set_head(obj);
1090 refs_list.inc_length(1);
1091
1092 if (TraceReferenceGC) {
1093 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1094 (void *)obj, obj->klass()->internal_name());
1095 }
1096 } else {
1097 // If retest was non NULL, another thread beat us to it:
1098 // The reference has already been discovered...
1099 if (TraceReferenceGC) {
1100 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1101 (void *)obj, obj->klass()->internal_name());
1102 }
1103 }
1104 }
1105
1106 #ifndef PRODUCT
1107 // Non-atomic (i.e. concurrent) discovery might allow us
1108 // to observe j.l.References with NULL referents, being those
1109 // cleared concurrently by mutators during (or after) discovery.
1110 void ReferenceProcessor::verify_referent(oop obj) {
1111 bool da = discovery_is_atomic();
1112 oop referent = java_lang_ref_Reference::referent(obj);
1113 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1114 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1115 INTPTR_FORMAT " during %satomic discovery ",
1116 (void *)referent, (void *)obj, da ? "" : "non-"));
1117 }
1118 #endif
1119
1120 // We mention two of several possible choices here:
1121 // #0: if the reference object is not in the "originating generation"
1122 // (or part of the heap being collected, indicated by our "span"
1123 // we don't treat it specially (i.e. we scan it as we would
1124 // a normal oop, treating its references as strong references).
1125 // This means that references can't be discovered unless their
1126 // referent is also in the same span. This is the simplest,
1127 // most "local" and most conservative approach, albeit one
1128 // that may cause weak references to be enqueued least promptly.
1129 // We call this choice the "ReferenceBasedDiscovery" policy.
1130 // #1: the reference object may be in any generation (span), but if
1131 // the referent is in the generation (span) being currently collected
1132 // then we can discover the reference object, provided
1133 // the object has not already been discovered by
1134 // a different concurrently running collector (as may be the
1135 // case, for instance, if the reference object is in CMS and
1136 // the referent in DefNewGeneration), and provided the processing
1137 // of this reference object by the current collector will
1138 // appear atomic to every other collector in the system.
1139 // (Thus, for instance, a concurrent collector may not
1140 // discover references in other generations even if the
1141 // referent is in its own generation). This policy may,
1142 // in certain cases, enqueue references somewhat sooner than
1143 // might Policy #0 above, but at marginally increased cost
1144 // and complexity in processing these references.
1145 // We call this choice the "RefeferentBasedDiscovery" policy.
1146 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1147 // Make sure we are discovering refs (rather than processing discovered refs).
1148 if (!_discovering_refs || !RegisterReferences) {
1149 return false;
1150 }
1151 // We only discover active references.
1152 oop next = java_lang_ref_Reference::next(obj);
1153 if (next != NULL) { // Ref is no longer active
1154 return false;
1155 }
1156
1157 HeapWord* obj_addr = (HeapWord*)obj;
1158 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1159 !_span.contains(obj_addr)) {
1160 // Reference is not in the originating generation;
1161 // don't treat it specially (i.e. we want to scan it as a normal
1162 // object with strong references).
1163 return false;
1164 }
1165
1166 // We only discover references whose referents are not (yet)
1167 // known to be strongly reachable.
1168 if (is_alive_non_header() != NULL) {
1169 verify_referent(obj);
1170 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1171 return false; // referent is reachable
1172 }
1173 }
1174 if (rt == REF_SOFT) {
1175 // For soft refs we can decide now if these are not
1176 // current candidates for clearing, in which case we
1177 // can mark through them now, rather than delaying that
1178 // to the reference-processing phase. Since all current
1179 // time-stamp policies advance the soft-ref clock only
1180 // at a major collection cycle, this is always currently
1181 // accurate.
1182 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1183 return false;
1184 }
1185 }
1186
1187 ResourceMark rm; // Needed for tracing.
1188
1189 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1190 const oop discovered = java_lang_ref_Reference::discovered(obj);
1191 assert(discovered->is_oop_or_null(), "bad discovered field");
1192 if (discovered != NULL) {
1193 // The reference has already been discovered...
1194 if (TraceReferenceGC) {
1195 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1196 (void *)obj, obj->klass()->internal_name());
1197 }
1198 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1199 // assumes that an object is not processed twice;
1200 // if it's been already discovered it must be on another
1201 // generation's discovered list; so we won't discover it.
1202 return false;
1203 } else {
1204 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1205 "Unrecognized policy");
1206 // Check assumption that an object is not potentially
1207 // discovered twice except by concurrent collectors that potentially
1208 // trace the same Reference object twice.
1209 assert(UseConcMarkSweepGC || UseG1GC,
1210 "Only possible with a concurrent marking collector");
1211 return true;
1212 }
1213 }
1214
1215 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1216 verify_referent(obj);
1217 // Discover if and only if EITHER:
1218 // .. reference is in our span, OR
1219 // .. we are an atomic collector and referent is in our span
1220 if (_span.contains(obj_addr) ||
1221 (discovery_is_atomic() &&
1222 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1223 // should_enqueue = true;
1224 } else {
1225 return false;
1226 }
1227 } else {
1228 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1229 _span.contains(obj_addr), "code inconsistency");
1230 }
1231
1232 // Get the right type of discovered queue head.
1233 DiscoveredList* list = get_discovered_list(rt);
1234 if (list == NULL) {
1235 return false; // nothing special needs to be done
1236 }
1237
1238 if (_discovery_is_mt) {
1239 add_to_discovered_list_mt(*list, obj, discovered_addr);
1240 } else {
1241 // We do a raw store here: the field will be visited later when processing
1242 // the discovered references.
1243 oop current_head = list->head();
1244 // The last ref must have its discovered field pointing to itself.
1245 oop next_discovered = (current_head != NULL) ? current_head : obj;
1246
1247 assert(discovered == NULL, "control point invariant");
1248 oop_store_raw(discovered_addr, next_discovered);
1249 list->set_head(obj);
1250 list->inc_length(1);
1251
1252 if (TraceReferenceGC) {
1253 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1254 (void *)obj, obj->klass()->internal_name());
1255 }
1256 }
1257 assert(obj->is_oop(), "Discovered a bad reference");
1258 verify_referent(obj);
1259 return true;
1260 }
1261
1262 // Preclean the discovered references by removing those
1263 // whose referents are alive, and by marking from those that
1264 // are not active. These lists can be handled here
1265 // in any order and, indeed, concurrently.
1266 void ReferenceProcessor::preclean_discovered_references(
1267 BoolObjectClosure* is_alive,
1268 OopClosure* keep_alive,
1269 VoidClosure* complete_gc,
1270 YieldClosure* yield,
1271 GCTimer* gc_timer,
1272 GCId gc_id) {
1273
1274 NOT_PRODUCT(verify_ok_to_handle_reflists());
1275
1276 // Soft references
1277 {
1278 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1279 false, gc_timer, gc_id);
1280 for (uint i = 0; i < _max_num_q; i++) {
1281 if (yield->should_return()) {
1282 return;
1283 }
1284 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1285 keep_alive, complete_gc, yield);
1286 }
1287 }
1288
1289 // Weak references
1290 {
1291 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1292 false, gc_timer, gc_id);
1293 for (uint i = 0; i < _max_num_q; i++) {
1294 if (yield->should_return()) {
1295 return;
1296 }
1297 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1298 keep_alive, complete_gc, yield);
1299 }
1300 }
1301
1302 // Final references
1303 {
1304 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1305 false, gc_timer, gc_id);
1306 for (uint i = 0; i < _max_num_q; i++) {
1307 if (yield->should_return()) {
1308 return;
1309 }
1310 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1311 keep_alive, complete_gc, yield);
1312 }
1313 }
1314
1315 // Phantom references
1316 {
1317 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1318 false, gc_timer, gc_id);
1319 for (uint i = 0; i < _max_num_q; i++) {
1320 if (yield->should_return()) {
1321 return;
1322 }
1323 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1324 keep_alive, complete_gc, yield);
1325 }
1326
1327 // Cleaner references. Included in timing for phantom references. We
1328 // expect Cleaner references to be temporary, and don't want to deal with
1329 // possible incompatibilities arising from making it more visible.
1330 for (uint i = 0; i < _max_num_q; i++) {
1331 if (yield->should_return()) {
1332 return;
1333 }
1334 preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1335 keep_alive, complete_gc, yield);
1336 }
1337 }
1338 }
1339
1340 // Walk the given discovered ref list, and remove all reference objects
1341 // whose referents are still alive, whose referents are NULL or which
1342 // are not active (have a non-NULL next field). NOTE: When we are
1343 // thus precleaning the ref lists (which happens single-threaded today),
1344 // we do not disable refs discovery to honour the correct semantics of
1345 // java.lang.Reference. As a result, we need to be careful below
1346 // that ref removal steps interleave safely with ref discovery steps
1347 // (in this thread).
1348 void
1349 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1350 BoolObjectClosure* is_alive,
1351 OopClosure* keep_alive,
1352 VoidClosure* complete_gc,
1353 YieldClosure* yield) {
1354 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1355 while (iter.has_next()) {
1356 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1357 oop obj = iter.obj();
1358 oop next = java_lang_ref_Reference::next(obj);
1359 if (iter.referent() == NULL || iter.is_referent_alive() ||
1360 next != NULL) {
1361 // The referent has been cleared, or is alive, or the Reference is not
1362 // active; we need to trace and mark its cohort.
1363 if (TraceReferenceGC) {
1364 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1365 (void *)iter.obj(), iter.obj()->klass()->internal_name());
1366 }
1367 // Remove Reference object from list
1368 iter.remove();
1369 // Keep alive its cohort.
1370 iter.make_referent_alive();
1371 if (UseCompressedOops) {
1372 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1373 keep_alive->do_oop(next_addr);
1374 } else {
1375 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1376 keep_alive->do_oop(next_addr);
1377 }
1378 iter.move_to_next();
1379 } else {
1380 iter.next();
1381 }
1382 }
1383 // Close the reachable set
1384 complete_gc->do_void();
1385
1386 NOT_PRODUCT(
1387 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1388 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1389 "Refs in discovered list " INTPTR_FORMAT,
1390 iter.removed(), iter.processed(), (address)refs_list.head());
1391 }
1392 )
1393 }
1394
1395 const char* ReferenceProcessor::list_name(uint i) {
1396 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1397 "Out of bounds index");
1398
1399 int j = i / _max_num_q;
1400 switch (j) {
1401 case 0: return "SoftRef";
1402 case 1: return "WeakRef";
1403 case 2: return "FinalRef";
1404 case 3: return "PhantomRef";
1405 case 4: return "CleanerRef";
1406 }
1407 ShouldNotReachHere();
1408 return NULL;
1409 }
1410
1411 #ifndef PRODUCT
1412 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1413 // empty for now
1414 }
1415 #endif
1416
1417 #ifndef PRODUCT
1418 void ReferenceProcessor::clear_discovered_references() {
1419 guarantee(!_discovering_refs, "Discovering refs?");
1420 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1421 clear_discovered_references(_discovered_refs[i]);
1422 }
1423 }
1424
1425 #endif // PRODUCT