1 /*
2 * Copyright (c) 2001, 2011, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/javaClasses.hpp"
27 #include "classfile/systemDictionary.hpp"
28 #include "gc_interface/collectedHeap.hpp"
29 #include "gc_interface/collectedHeap.inline.hpp"
30 #include "memory/referencePolicy.hpp"
31 #include "memory/referenceProcessor.hpp"
32 #include "oops/oop.inline.hpp"
33 #include "runtime/java.hpp"
34 #include "runtime/jniHandles.hpp"
35
36 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
37 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
38 bool ReferenceProcessor::_pending_list_uses_discovered_field = false;
39 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
40
41 void referenceProcessor_init() {
42 ReferenceProcessor::init_statics();
43 }
44
45 void ReferenceProcessor::init_statics() {
46 // os::javaTimeMillis() does not guarantee monotonicity.
47 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
48
49 // Initialize the soft ref timestamp clock.
50 _soft_ref_timestamp_clock = now;
51 // Also update the soft ref clock in j.l.r.SoftReference
52 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
53
54 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
55 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
56 NOT_COMPILER2(LRUCurrentHeapPolicy());
57 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
58 vm_exit_during_initialization("Could not allocate reference policy object");
59 }
60 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
61 RefDiscoveryPolicy == ReferentBasedDiscovery,
62 "Unrecongnized RefDiscoveryPolicy");
63 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
64 }
65
66 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
67 #ifdef ASSERT
68 // Verify that we're not currently discovering refs
69 assert(!verify_disabled || !_discovering_refs, "nested call?");
70
71 if (check_no_refs) {
72 // Verify that the discovered lists are empty
73 verify_no_references_recorded();
74 }
75 #endif // ASSERT
76
77 // Someone could have modified the value of the static
78 // field in the j.l.r.SoftReference class that holds the
79 // soft reference timestamp clock using reflection or
80 // Unsafe between GCs. Unconditionally update the static
81 // field in ReferenceProcessor here so that we use the new
82 // value during reference discovery.
83
84 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
85 _discovering_refs = true;
86 }
87
88 ReferenceProcessor::ReferenceProcessor(MemRegion span,
89 bool mt_processing,
90 int mt_processing_degree,
91 bool mt_discovery,
92 int mt_discovery_degree,
93 bool atomic_discovery,
94 BoolObjectClosure* is_alive_non_header,
95 bool discovered_list_needs_barrier) :
96 _discovering_refs(false),
97 _enqueuing_is_done(false),
98 _is_alive_non_header(is_alive_non_header),
99 _discovered_list_needs_barrier(discovered_list_needs_barrier),
100 _bs(NULL),
101 _processing_is_mt(mt_processing),
102 _next_id(0)
103 {
104 _span = span;
105 _discovery_is_atomic = atomic_discovery;
106 _discovery_is_mt = mt_discovery;
107 _num_q = MAX2(1, mt_processing_degree);
108 _max_num_q = MAX2(_num_q, mt_discovery_degree);
109 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
110 _max_num_q * number_of_subclasses_of_ref());
111 if (_discovered_refs == NULL) {
112 vm_exit_during_initialization("Could not allocated RefProc Array");
113 }
114 _discoveredSoftRefs = &_discovered_refs[0];
115 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
116 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
117 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
118
119 // Initialize all entries to NULL
120 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
121 _discovered_refs[i].set_head(NULL);
122 _discovered_refs[i].set_length(0);
123 }
124
125 // If we do barriers, cache a copy of the barrier set.
126 if (discovered_list_needs_barrier) {
127 _bs = Universe::heap()->barrier_set();
128 }
129 setup_policy(false /* default soft ref policy */);
130 }
131
132 #ifndef PRODUCT
133 void ReferenceProcessor::verify_no_references_recorded() {
134 guarantee(!_discovering_refs, "Discovering refs?");
135 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
136 guarantee(_discovered_refs[i].is_empty(),
137 "Found non-empty discovered list");
138 }
139 }
140 #endif
141
142 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
143 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
144 if (UseCompressedOops) {
145 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
146 } else {
147 f->do_oop((oop*)_discovered_refs[i].adr_head());
148 }
149 }
150 }
151
152 void ReferenceProcessor::update_soft_ref_master_clock() {
153 // Update (advance) the soft ref master clock field. This must be done
154 // after processing the soft ref list.
155
156 // os::javaTimeMillis() does not guarantee monotonicity.
157 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
158 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
159 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
160
161 NOT_PRODUCT(
162 if (now < _soft_ref_timestamp_clock) {
163 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
164 _soft_ref_timestamp_clock, now);
165 }
166 )
167 // In product mode, protect ourselves from system time being adjusted
168 // externally and going backward; see note in the implementation of
169 // GenCollectedHeap::time_since_last_gc() for the right way to fix
170 // this uniformly throughout the VM; see bug-id 4741166. XXX
171 if (now > _soft_ref_timestamp_clock) {
172 _soft_ref_timestamp_clock = now;
173 java_lang_ref_SoftReference::set_clock(now);
174 }
175 // Else leave clock stalled at its old value until time progresses
176 // past clock value.
177 }
178
179 void ReferenceProcessor::process_discovered_references(
180 BoolObjectClosure* is_alive,
181 OopClosure* keep_alive,
182 VoidClosure* complete_gc,
183 AbstractRefProcTaskExecutor* task_executor) {
184 NOT_PRODUCT(verify_ok_to_handle_reflists());
185
186 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
187 // Stop treating discovered references specially.
188 disable_discovery();
189
190 // If discovery was concurrent, someone could have modified
191 // the value of the static field in the j.l.r.SoftReference
192 // class that holds the soft reference timestamp clock using
193 // reflection or Unsafe between when discovery was enabled and
194 // now. Unconditionally update the static field in ReferenceProcessor
195 // here so that we use the new value during processing of the
196 // discovered soft refs.
197
198 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
199
200 bool trace_time = PrintGCDetails && PrintReferenceGC;
201 // Soft references
202 {
203 TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);
204 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
205 is_alive, keep_alive, complete_gc, task_executor);
206 }
207
208 update_soft_ref_master_clock();
209
210 // Weak references
211 {
212 TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);
213 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
214 is_alive, keep_alive, complete_gc, task_executor);
215 }
216
217 // Final references
218 {
219 TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);
220 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
221 is_alive, keep_alive, complete_gc, task_executor);
222 }
223
224 // Phantom references
225 {
226 TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);
227 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
228 is_alive, keep_alive, complete_gc, task_executor);
229 }
230
231 // Weak global JNI references. It would make more sense (semantically) to
232 // traverse these simultaneously with the regular weak references above, but
233 // that is not how the JDK1.2 specification is. See #4126360. Native code can
234 // thus use JNI weak references to circumvent the phantom references and
235 // resurrect a "post-mortem" object.
236 {
237 TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);
238 if (task_executor != NULL) {
239 task_executor->set_single_threaded_mode();
240 }
241 process_phaseJNI(is_alive, keep_alive, complete_gc);
242 }
243 }
244
245 #ifndef PRODUCT
246 // Calculate the number of jni handles.
247 uint ReferenceProcessor::count_jni_refs() {
248 class AlwaysAliveClosure: public BoolObjectClosure {
249 public:
250 virtual bool do_object_b(oop obj) { return true; }
251 virtual void do_object(oop obj) { assert(false, "Don't call"); }
252 };
253
254 class CountHandleClosure: public OopClosure {
255 private:
256 int _count;
257 public:
258 CountHandleClosure(): _count(0) {}
259 void do_oop(oop* unused) { _count++; }
260 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
261 int count() { return _count; }
262 };
263 CountHandleClosure global_handle_count;
264 AlwaysAliveClosure always_alive;
265 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
266 return global_handle_count.count();
267 }
268 #endif
269
270 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
271 OopClosure* keep_alive,
272 VoidClosure* complete_gc) {
273 #ifndef PRODUCT
274 if (PrintGCDetails && PrintReferenceGC) {
275 unsigned int count = count_jni_refs();
276 gclog_or_tty->print(", %u refs", count);
277 }
278 #endif
279 JNIHandles::weak_oops_do(is_alive, keep_alive);
280 complete_gc->do_void();
281 }
282
283
284 template <class T>
285 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
286 AbstractRefProcTaskExecutor* task_executor) {
287
288 // Remember old value of pending references list
289 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
290 T old_pending_list_value = *pending_list_addr;
291
292 // Enqueue references that are not made active again, and
293 // clear the decks for the next collection (cycle).
294 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
295 // Do the oop-check on pending_list_addr missed in
296 // enqueue_discovered_reflist. We should probably
297 // do a raw oop_check so that future such idempotent
298 // oop_stores relying on the oop-check side-effect
299 // may be elided automatically and safely without
300 // affecting correctness.
301 oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
302
303 // Stop treating discovered references specially.
304 ref->disable_discovery();
305
306 // Return true if new pending references were added
307 return old_pending_list_value != *pending_list_addr;
308 }
309
310 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
311 NOT_PRODUCT(verify_ok_to_handle_reflists());
312 if (UseCompressedOops) {
313 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
314 } else {
315 return enqueue_discovered_ref_helper<oop>(this, task_executor);
316 }
317 }
318
319 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
320 HeapWord* pending_list_addr) {
321 // Given a list of refs linked through the "discovered" field
322 // (java.lang.ref.Reference.discovered), self-loop their "next" field
323 // thus distinguishing them from active References, then
324 // prepend them to the pending list.
325 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
326 // the "next" field is used to chain the pending list, not the discovered
327 // field.
328
329 if (TraceReferenceGC && PrintGCDetails) {
330 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
331 INTPTR_FORMAT, (address)refs_list.head());
332 }
333
334 oop obj = NULL;
335 oop next_d = refs_list.head();
336 if (pending_list_uses_discovered_field()) { // New behaviour
337 // Walk down the list, self-looping the next field
338 // so that the References are not considered active.
339 while (obj != next_d) {
340 obj = next_d;
341 assert(obj->is_instanceRef(), "should be reference object");
342 next_d = java_lang_ref_Reference::discovered(obj);
343 if (TraceReferenceGC && PrintGCDetails) {
344 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
345 obj, next_d);
346 }
347 assert(java_lang_ref_Reference::next(obj) == NULL,
348 "Reference not active; should not be discovered");
349 // Self-loop next, so as to make Ref not active.
350 java_lang_ref_Reference::set_next(obj, obj);
351 if (next_d == obj) { // obj is last
352 // Swap refs_list into pendling_list_addr and
353 // set obj's discovered to what we read from pending_list_addr.
354 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
355 // Need oop_check on pending_list_addr above;
356 // see special oop-check code at the end of
357 // enqueue_discovered_reflists() further below.
358 java_lang_ref_Reference::set_discovered(obj, old); // old may be NULL
359 }
360 }
361 } else { // Old behaviour
362 // Walk down the list, copying the discovered field into
363 // the next field and clearing the discovered field.
364 while (obj != next_d) {
365 obj = next_d;
366 assert(obj->is_instanceRef(), "should be reference object");
367 next_d = java_lang_ref_Reference::discovered(obj);
368 if (TraceReferenceGC && PrintGCDetails) {
369 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
370 obj, next_d);
371 }
372 assert(java_lang_ref_Reference::next(obj) == NULL,
373 "The reference should not be enqueued");
374 if (next_d == obj) { // obj is last
375 // Swap refs_list into pendling_list_addr and
376 // set obj's next to what we read from pending_list_addr.
377 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
378 // Need oop_check on pending_list_addr above;
379 // see special oop-check code at the end of
380 // enqueue_discovered_reflists() further below.
381 if (old == NULL) {
382 // obj should be made to point to itself, since
383 // pending list was empty.
384 java_lang_ref_Reference::set_next(obj, obj);
385 } else {
386 java_lang_ref_Reference::set_next(obj, old);
387 }
388 } else {
389 java_lang_ref_Reference::set_next(obj, next_d);
390 }
391 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
392 }
393 }
394 }
395
396 // Parallel enqueue task
397 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
398 public:
399 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
400 DiscoveredList discovered_refs[],
401 HeapWord* pending_list_addr,
402 int n_queues)
403 : EnqueueTask(ref_processor, discovered_refs,
404 pending_list_addr, n_queues)
405 { }
406
407 virtual void work(unsigned int work_id) {
408 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
409 // Simplest first cut: static partitioning.
410 int index = work_id;
411 // The increment on "index" must correspond to the maximum number of queues
412 // (n_queues) with which that ReferenceProcessor was created. That
413 // is because of the "clever" way the discovered references lists were
414 // allocated and are indexed into.
415 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
416 for (int j = 0;
417 j < ReferenceProcessor::number_of_subclasses_of_ref();
418 j++, index += _n_queues) {
419 _ref_processor.enqueue_discovered_reflist(
420 _refs_lists[index], _pending_list_addr);
421 _refs_lists[index].set_head(NULL);
422 _refs_lists[index].set_length(0);
423 }
424 }
425 };
426
427 // Enqueue references that are not made active again
428 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
429 AbstractRefProcTaskExecutor* task_executor) {
430 if (_processing_is_mt && task_executor != NULL) {
431 // Parallel code
432 RefProcEnqueueTask tsk(*this, _discovered_refs,
433 pending_list_addr, _max_num_q);
434 task_executor->execute(tsk);
435 } else {
436 // Serial code: call the parent class's implementation
437 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
438 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
439 _discovered_refs[i].set_head(NULL);
440 _discovered_refs[i].set_length(0);
441 }
442 }
443 }
444
445 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
446 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
447 oop discovered = java_lang_ref_Reference::discovered(_ref);
448 assert(_discovered_addr && discovered->is_oop_or_null(),
449 "discovered field is bad");
450 _next = discovered;
451 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
452 _referent = java_lang_ref_Reference::referent(_ref);
453 assert(Universe::heap()->is_in_reserved_or_null(_referent),
454 "Wrong oop found in java.lang.Reference object");
455 assert(allow_null_referent ?
456 _referent->is_oop_or_null()
457 : _referent->is_oop(),
458 "bad referent");
459 }
460
461 void DiscoveredListIterator::remove() {
462 assert(_ref->is_oop(), "Dropping a bad reference");
463 oop_store_raw(_discovered_addr, NULL);
464
465 // First _prev_next ref actually points into DiscoveredList (gross).
466 oop new_next;
467 if (_next == _ref) {
468 // At the end of the list, we should make _prev point to itself.
469 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
470 // and _prev will be NULL.
471 new_next = _prev;
472 } else {
473 new_next = _next;
474 }
475
476 if (UseCompressedOops) {
477 // Remove Reference object from list.
478 oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next);
479 } else {
480 // Remove Reference object from list.
481 oopDesc::store_heap_oop((oop*)_prev_next, new_next);
482 }
483 NOT_PRODUCT(_removed++);
484 _refs_list.dec_length(1);
485 }
486
487 // Make the Reference object active again.
488 void DiscoveredListIterator::make_active() {
489 // For G1 we don't want to use set_next - it
490 // will dirty the card for the next field of
491 // the reference object and will fail
492 // CT verification.
493 if (UseG1GC) {
494 BarrierSet* bs = oopDesc::bs();
495 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
496
497 if (UseCompressedOops) {
498 bs->write_ref_field_pre((narrowOop*)next_addr, NULL);
499 } else {
500 bs->write_ref_field_pre((oop*)next_addr, NULL);
501 }
502 java_lang_ref_Reference::set_next_raw(_ref, NULL);
503 } else {
504 java_lang_ref_Reference::set_next(_ref, NULL);
505 }
506 }
507
508 void DiscoveredListIterator::clear_referent() {
509 oop_store_raw(_referent_addr, NULL);
510 }
511
512 // NOTE: process_phase*() are largely similar, and at a high level
513 // merely iterate over the extant list applying a predicate to
514 // each of its elements and possibly removing that element from the
515 // list and applying some further closures to that element.
516 // We should consider the possibility of replacing these
517 // process_phase*() methods by abstracting them into
518 // a single general iterator invocation that receives appropriate
519 // closures that accomplish this work.
520
521 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
522 // referents are not alive, but that should be kept alive for policy reasons.
523 // Keep alive the transitive closure of all such referents.
524 void
525 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
526 ReferencePolicy* policy,
527 BoolObjectClosure* is_alive,
528 OopClosure* keep_alive,
529 VoidClosure* complete_gc) {
530 assert(policy != NULL, "Must have a non-NULL policy");
531 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
532 // Decide which softly reachable refs should be kept alive.
533 while (iter.has_next()) {
534 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
535 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
536 if (referent_is_dead &&
537 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
538 if (TraceReferenceGC) {
539 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
540 iter.obj(), iter.obj()->blueprint()->internal_name());
541 }
542 // Remove Reference object from list
543 iter.remove();
544 // Make the Reference object active again
545 iter.make_active();
546 // keep the referent around
547 iter.make_referent_alive();
548 iter.move_to_next();
549 } else {
550 iter.next();
551 }
552 }
553 // Close the reachable set
554 complete_gc->do_void();
555 NOT_PRODUCT(
556 if (PrintGCDetails && TraceReferenceGC) {
557 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
558 "discovered Refs by policy, from list " INTPTR_FORMAT,
559 iter.removed(), iter.processed(), (address)refs_list.head());
560 }
561 )
562 }
563
564 // Traverse the list and remove any Refs that are not active, or
565 // whose referents are either alive or NULL.
566 void
567 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
568 BoolObjectClosure* is_alive,
569 OopClosure* keep_alive) {
570 assert(discovery_is_atomic(), "Error");
571 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
572 while (iter.has_next()) {
573 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
574 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
575 assert(next == NULL, "Should not discover inactive Reference");
576 if (iter.is_referent_alive()) {
577 if (TraceReferenceGC) {
578 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
579 iter.obj(), iter.obj()->blueprint()->internal_name());
580 }
581 // The referent is reachable after all.
582 // Remove Reference object from list.
583 iter.remove();
584 // Update the referent pointer as necessary: Note that this
585 // should not entail any recursive marking because the
586 // referent must already have been traversed.
587 iter.make_referent_alive();
588 iter.move_to_next();
589 } else {
590 iter.next();
591 }
592 }
593 NOT_PRODUCT(
594 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
595 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
596 "Refs in discovered list " INTPTR_FORMAT,
597 iter.removed(), iter.processed(), (address)refs_list.head());
598 }
599 )
600 }
601
602 void
603 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
604 BoolObjectClosure* is_alive,
605 OopClosure* keep_alive,
606 VoidClosure* complete_gc) {
607 assert(!discovery_is_atomic(), "Error");
608 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
609 while (iter.has_next()) {
610 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
611 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
612 oop next = java_lang_ref_Reference::next(iter.obj());
613 if ((iter.referent() == NULL || iter.is_referent_alive() ||
614 next != NULL)) {
615 assert(next->is_oop_or_null(), "bad next field");
616 // Remove Reference object from list
617 iter.remove();
618 // Trace the cohorts
619 iter.make_referent_alive();
620 if (UseCompressedOops) {
621 keep_alive->do_oop((narrowOop*)next_addr);
622 } else {
623 keep_alive->do_oop((oop*)next_addr);
624 }
625 iter.move_to_next();
626 } else {
627 iter.next();
628 }
629 }
630 // Now close the newly reachable set
631 complete_gc->do_void();
632 NOT_PRODUCT(
633 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
634 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
635 "Refs in discovered list " INTPTR_FORMAT,
636 iter.removed(), iter.processed(), (address)refs_list.head());
637 }
638 )
639 }
640
641 // Traverse the list and process the referents, by either
642 // clearing them or keeping them (and their reachable
643 // closure) alive.
644 void
645 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
646 bool clear_referent,
647 BoolObjectClosure* is_alive,
648 OopClosure* keep_alive,
649 VoidClosure* complete_gc) {
650 ResourceMark rm;
651 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
652 while (iter.has_next()) {
653 iter.update_discovered();
654 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
655 if (clear_referent) {
656 // NULL out referent pointer
657 iter.clear_referent();
658 } else {
659 // keep the referent around
660 iter.make_referent_alive();
661 }
662 if (TraceReferenceGC) {
663 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
664 clear_referent ? "cleared " : "",
665 iter.obj(), iter.obj()->blueprint()->internal_name());
666 }
667 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
668 iter.next();
669 }
670 // Remember to update the next pointer of the last ref.
671 iter.update_discovered();
672 // Close the reachable set
673 complete_gc->do_void();
674 }
675
676 void
677 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
678 oop obj = NULL;
679 oop next = refs_list.head();
680 while (next != obj) {
681 obj = next;
682 next = java_lang_ref_Reference::discovered(obj);
683 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
684 }
685 refs_list.set_head(NULL);
686 refs_list.set_length(0);
687 }
688
689 void
690 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
691 clear_discovered_references(refs_list);
692 }
693
694 void ReferenceProcessor::abandon_partial_discovery() {
695 // loop over the lists
696 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
697 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
698 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
699 }
700 abandon_partial_discovered_list(_discovered_refs[i]);
701 }
702 }
703
704 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
705 public:
706 RefProcPhase1Task(ReferenceProcessor& ref_processor,
707 DiscoveredList refs_lists[],
708 ReferencePolicy* policy,
709 bool marks_oops_alive)
710 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
711 _policy(policy)
712 { }
713 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
714 OopClosure& keep_alive,
715 VoidClosure& complete_gc)
716 {
717 Thread* thr = Thread::current();
718 int refs_list_index = ((WorkerThread*)thr)->id();
719 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
720 &is_alive, &keep_alive, &complete_gc);
721 }
722 private:
723 ReferencePolicy* _policy;
724 };
725
726 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
727 public:
728 RefProcPhase2Task(ReferenceProcessor& ref_processor,
729 DiscoveredList refs_lists[],
730 bool marks_oops_alive)
731 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
732 { }
733 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
734 OopClosure& keep_alive,
735 VoidClosure& complete_gc)
736 {
737 _ref_processor.process_phase2(_refs_lists[i],
738 &is_alive, &keep_alive, &complete_gc);
739 }
740 };
741
742 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
743 public:
744 RefProcPhase3Task(ReferenceProcessor& ref_processor,
745 DiscoveredList refs_lists[],
746 bool clear_referent,
747 bool marks_oops_alive)
748 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
749 _clear_referent(clear_referent)
750 { }
751 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
752 OopClosure& keep_alive,
753 VoidClosure& complete_gc)
754 {
755 // Don't use "refs_list_index" calculated in this way because
756 // balance_queues() has moved the Ref's into the first n queues.
757 // Thread* thr = Thread::current();
758 // int refs_list_index = ((WorkerThread*)thr)->id();
759 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
760 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
761 &is_alive, &keep_alive, &complete_gc);
762 }
763 private:
764 bool _clear_referent;
765 };
766
767 void ReferenceProcessor::set_discovered(oop ref, oop value) {
768 if (_discovered_list_needs_barrier) {
769 java_lang_ref_Reference::set_discovered(ref, value);
770 } else {
771 java_lang_ref_Reference::set_discovered_raw(ref, value);
772 }
773 }
774
775 // Balances reference queues.
776 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
777 // queues[0, 1, ..., _num_q-1] because only the first _num_q
778 // corresponding to the active workers will be processed.
779 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
780 {
781 // calculate total length
782 size_t total_refs = 0;
783 if (TraceReferenceGC && PrintGCDetails) {
784 gclog_or_tty->print_cr("\nBalance ref_lists ");
785 }
786
787 for (int i = 0; i < _max_num_q; ++i) {
788 total_refs += ref_lists[i].length();
789 if (TraceReferenceGC && PrintGCDetails) {
790 gclog_or_tty->print("%d ", ref_lists[i].length());
791 }
792 }
793 if (TraceReferenceGC && PrintGCDetails) {
794 gclog_or_tty->print_cr(" = %d", total_refs);
795 }
796 size_t avg_refs = total_refs / _num_q + 1;
797 int to_idx = 0;
798 for (int from_idx = 0; from_idx < _max_num_q; from_idx++) {
799 bool move_all = false;
800 if (from_idx >= _num_q) {
801 move_all = ref_lists[from_idx].length() > 0;
802 }
803 while ((ref_lists[from_idx].length() > avg_refs) ||
804 move_all) {
805 assert(to_idx < _num_q, "Sanity Check!");
806 if (ref_lists[to_idx].length() < avg_refs) {
807 // move superfluous refs
808 size_t refs_to_move;
809 // Move all the Ref's if the from queue will not be processed.
810 if (move_all) {
811 refs_to_move = MIN2(ref_lists[from_idx].length(),
812 avg_refs - ref_lists[to_idx].length());
813 } else {
814 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
815 avg_refs - ref_lists[to_idx].length());
816 }
817
818 assert(refs_to_move > 0, "otherwise the code below will fail");
819
820 oop move_head = ref_lists[from_idx].head();
821 oop move_tail = move_head;
822 oop new_head = move_head;
823 // find an element to split the list on
824 for (size_t j = 0; j < refs_to_move; ++j) {
825 move_tail = new_head;
826 new_head = java_lang_ref_Reference::discovered(new_head);
827 }
828
829 // Add the chain to the to list.
830 if (ref_lists[to_idx].head() == NULL) {
831 // to list is empty. Make a loop at the end.
832 set_discovered(move_tail, move_tail);
833 } else {
834 set_discovered(move_tail, ref_lists[to_idx].head());
835 }
836 ref_lists[to_idx].set_head(move_head);
837 ref_lists[to_idx].inc_length(refs_to_move);
838
839 // Remove the chain from the from list.
840 if (move_tail == new_head) {
841 // We found the end of the from list.
842 ref_lists[from_idx].set_head(NULL);
843 } else {
844 ref_lists[from_idx].set_head(new_head);
845 }
846 ref_lists[from_idx].dec_length(refs_to_move);
847 if (ref_lists[from_idx].length() == 0) {
848 break;
849 }
850 } else {
851 to_idx = (to_idx + 1) % _num_q;
852 }
853 }
854 }
855 #ifdef ASSERT
856 size_t balanced_total_refs = 0;
857 for (int i = 0; i < _max_num_q; ++i) {
858 balanced_total_refs += ref_lists[i].length();
859 if (TraceReferenceGC && PrintGCDetails) {
860 gclog_or_tty->print("%d ", ref_lists[i].length());
861 }
862 }
863 if (TraceReferenceGC && PrintGCDetails) {
864 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
865 gclog_or_tty->flush();
866 }
867 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
868 #endif
869 }
870
871 void ReferenceProcessor::balance_all_queues() {
872 balance_queues(_discoveredSoftRefs);
873 balance_queues(_discoveredWeakRefs);
874 balance_queues(_discoveredFinalRefs);
875 balance_queues(_discoveredPhantomRefs);
876 }
877
878 void
879 ReferenceProcessor::process_discovered_reflist(
880 DiscoveredList refs_lists[],
881 ReferencePolicy* policy,
882 bool clear_referent,
883 BoolObjectClosure* is_alive,
884 OopClosure* keep_alive,
885 VoidClosure* complete_gc,
886 AbstractRefProcTaskExecutor* task_executor)
887 {
888 bool mt_processing = task_executor != NULL && _processing_is_mt;
889 // If discovery used MT and a dynamic number of GC threads, then
890 // the queues must be balanced for correctness if fewer than the
891 // maximum number of queues were used. The number of queue used
892 // during discovery may be different than the number to be used
893 // for processing so don't depend of _num_q < _max_num_q as part
894 // of the test.
895 bool must_balance = _discovery_is_mt;
896
897 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
898 must_balance) {
899 balance_queues(refs_lists);
900 }
901 if (PrintReferenceGC && PrintGCDetails) {
902 size_t total = 0;
903 for (int i = 0; i < _max_num_q; ++i) {
904 total += refs_lists[i].length();
905 }
906 gclog_or_tty->print(", %u refs", total);
907 }
908
909 // Phase 1 (soft refs only):
910 // . Traverse the list and remove any SoftReferences whose
911 // referents are not alive, but that should be kept alive for
912 // policy reasons. Keep alive the transitive closure of all
913 // such referents.
914 if (policy != NULL) {
915 if (mt_processing) {
916 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
917 task_executor->execute(phase1);
918 } else {
919 for (int i = 0; i < _max_num_q; i++) {
920 process_phase1(refs_lists[i], policy,
921 is_alive, keep_alive, complete_gc);
922 }
923 }
924 } else { // policy == NULL
925 assert(refs_lists != _discoveredSoftRefs,
926 "Policy must be specified for soft references.");
927 }
928
929 // Phase 2:
930 // . Traverse the list and remove any refs whose referents are alive.
931 if (mt_processing) {
932 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
933 task_executor->execute(phase2);
934 } else {
935 for (int i = 0; i < _max_num_q; i++) {
936 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
937 }
938 }
939
940 // Phase 3:
941 // . Traverse the list and process referents as appropriate.
942 if (mt_processing) {
943 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
944 task_executor->execute(phase3);
945 } else {
946 for (int i = 0; i < _max_num_q; i++) {
947 process_phase3(refs_lists[i], clear_referent,
948 is_alive, keep_alive, complete_gc);
949 }
950 }
951 }
952
953 void ReferenceProcessor::clean_up_discovered_references() {
954 // loop over the lists
955 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
956 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
957 gclog_or_tty->print_cr(
958 "\nScrubbing %s discovered list of Null referents",
959 list_name(i));
960 }
961 clean_up_discovered_reflist(_discovered_refs[i]);
962 }
963 }
964
965 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
966 assert(!discovery_is_atomic(), "Else why call this method?");
967 DiscoveredListIterator iter(refs_list, NULL, NULL);
968 while (iter.has_next()) {
969 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
970 oop next = java_lang_ref_Reference::next(iter.obj());
971 assert(next->is_oop_or_null(), "bad next field");
972 // If referent has been cleared or Reference is not active,
973 // drop it.
974 if (iter.referent() == NULL || next != NULL) {
975 debug_only(
976 if (PrintGCDetails && TraceReferenceGC) {
977 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
978 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
979 " and referent: " INTPTR_FORMAT,
980 iter.obj(), next, iter.referent());
981 }
982 )
983 // Remove Reference object from list
984 iter.remove();
985 iter.move_to_next();
986 } else {
987 iter.next();
988 }
989 }
990 NOT_PRODUCT(
991 if (PrintGCDetails && TraceReferenceGC) {
992 gclog_or_tty->print(
993 " Removed %d Refs with NULL referents out of %d discovered Refs",
994 iter.removed(), iter.processed());
995 }
996 )
997 }
998
999 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1000 int id = 0;
1001 // Determine the queue index to use for this object.
1002 if (_discovery_is_mt) {
1003 // During a multi-threaded discovery phase,
1004 // each thread saves to its "own" list.
1005 Thread* thr = Thread::current();
1006 id = thr->as_Worker_thread()->id();
1007 } else {
1008 // single-threaded discovery, we save in round-robin
1009 // fashion to each of the lists.
1010 if (_processing_is_mt) {
1011 id = next_id();
1012 }
1013 }
1014 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1015
1016 // Get the discovered queue to which we will add
1017 DiscoveredList* list = NULL;
1018 switch (rt) {
1019 case REF_OTHER:
1020 // Unknown reference type, no special treatment
1021 break;
1022 case REF_SOFT:
1023 list = &_discoveredSoftRefs[id];
1024 break;
1025 case REF_WEAK:
1026 list = &_discoveredWeakRefs[id];
1027 break;
1028 case REF_FINAL:
1029 list = &_discoveredFinalRefs[id];
1030 break;
1031 case REF_PHANTOM:
1032 list = &_discoveredPhantomRefs[id];
1033 break;
1034 case REF_NONE:
1035 // we should not reach here if we are an instanceRefKlass
1036 default:
1037 ShouldNotReachHere();
1038 }
1039 if (TraceReferenceGC && PrintGCDetails) {
1040 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1041 }
1042 return list;
1043 }
1044
1045 inline void
1046 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1047 oop obj,
1048 HeapWord* discovered_addr) {
1049 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1050 // First we must make sure this object is only enqueued once. CAS in a non null
1051 // discovered_addr.
1052 oop current_head = refs_list.head();
1053 // The last ref must have its discovered field pointing to itself.
1054 oop next_discovered = (current_head != NULL) ? current_head : obj;
1055
1056 // Note: In the case of G1, this specific pre-barrier is strictly
1057 // not necessary because the only case we are interested in
1058 // here is when *discovered_addr is NULL (see the CAS further below),
1059 // so this will expand to nothing. As a result, we have manually
1060 // elided this out for G1, but left in the test for some future
1061 // collector that might have need for a pre-barrier here, e.g.:-
1062 // _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1063 assert(!_discovered_list_needs_barrier || UseG1GC,
1064 "Need to check non-G1 collector: "
1065 "may need a pre-write-barrier for CAS from NULL below");
1066 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1067 NULL);
1068 if (retest == NULL) {
1069 // This thread just won the right to enqueue the object.
1070 // We have separate lists for enqueueing, so no synchronization
1071 // is necessary.
1072 refs_list.set_head(obj);
1073 refs_list.inc_length(1);
1074 if (_discovered_list_needs_barrier) {
1075 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1076 }
1077
1078 if (TraceReferenceGC) {
1079 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1080 obj, obj->blueprint()->internal_name());
1081 }
1082 } else {
1083 // If retest was non NULL, another thread beat us to it:
1084 // The reference has already been discovered...
1085 if (TraceReferenceGC) {
1086 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1087 obj, obj->blueprint()->internal_name());
1088 }
1089 }
1090 }
1091
1092 #ifndef PRODUCT
1093 // Non-atomic (i.e. concurrent) discovery might allow us
1094 // to observe j.l.References with NULL referents, being those
1095 // cleared concurrently by mutators during (or after) discovery.
1096 void ReferenceProcessor::verify_referent(oop obj) {
1097 bool da = discovery_is_atomic();
1098 oop referent = java_lang_ref_Reference::referent(obj);
1099 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1100 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1101 INTPTR_FORMAT " during %satomic discovery ",
1102 (intptr_t)referent, (intptr_t)obj, da ? "" : "non-"));
1103 }
1104 #endif
1105
1106 // We mention two of several possible choices here:
1107 // #0: if the reference object is not in the "originating generation"
1108 // (or part of the heap being collected, indicated by our "span"
1109 // we don't treat it specially (i.e. we scan it as we would
1110 // a normal oop, treating its references as strong references).
1111 // This means that references can't be discovered unless their
1112 // referent is also in the same span. This is the simplest,
1113 // most "local" and most conservative approach, albeit one
1114 // that may cause weak references to be enqueued least promptly.
1115 // We call this choice the "ReferenceBasedDiscovery" policy.
1116 // #1: the reference object may be in any generation (span), but if
1117 // the referent is in the generation (span) being currently collected
1118 // then we can discover the reference object, provided
1119 // the object has not already been discovered by
1120 // a different concurrently running collector (as may be the
1121 // case, for instance, if the reference object is in CMS and
1122 // the referent in DefNewGeneration), and provided the processing
1123 // of this reference object by the current collector will
1124 // appear atomic to every other collector in the system.
1125 // (Thus, for instance, a concurrent collector may not
1126 // discover references in other generations even if the
1127 // referent is in its own generation). This policy may,
1128 // in certain cases, enqueue references somewhat sooner than
1129 // might Policy #0 above, but at marginally increased cost
1130 // and complexity in processing these references.
1131 // We call this choice the "RefeferentBasedDiscovery" policy.
1132 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1133 // Make sure we are discovering refs (rather than processing discovered refs).
1134 if (!_discovering_refs || !RegisterReferences) {
1135 return false;
1136 }
1137 // We only discover active references.
1138 oop next = java_lang_ref_Reference::next(obj);
1139 if (next != NULL) { // Ref is no longer active
1140 return false;
1141 }
1142
1143 HeapWord* obj_addr = (HeapWord*)obj;
1144 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1145 !_span.contains(obj_addr)) {
1146 // Reference is not in the originating generation;
1147 // don't treat it specially (i.e. we want to scan it as a normal
1148 // object with strong references).
1149 return false;
1150 }
1151
1152 // We only discover references whose referents are not (yet)
1153 // known to be strongly reachable.
1154 if (is_alive_non_header() != NULL) {
1155 verify_referent(obj);
1156 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1157 return false; // referent is reachable
1158 }
1159 }
1160 if (rt == REF_SOFT) {
1161 // For soft refs we can decide now if these are not
1162 // current candidates for clearing, in which case we
1163 // can mark through them now, rather than delaying that
1164 // to the reference-processing phase. Since all current
1165 // time-stamp policies advance the soft-ref clock only
1166 // at a major collection cycle, this is always currently
1167 // accurate.
1168 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1169 return false;
1170 }
1171 }
1172
1173 ResourceMark rm; // Needed for tracing.
1174
1175 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1176 const oop discovered = java_lang_ref_Reference::discovered(obj);
1177 assert(discovered->is_oop_or_null(), "bad discovered field");
1178 if (discovered != NULL) {
1179 // The reference has already been discovered...
1180 if (TraceReferenceGC) {
1181 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1182 obj, obj->blueprint()->internal_name());
1183 }
1184 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1185 // assumes that an object is not processed twice;
1186 // if it's been already discovered it must be on another
1187 // generation's discovered list; so we won't discover it.
1188 return false;
1189 } else {
1190 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1191 "Unrecognized policy");
1192 // Check assumption that an object is not potentially
1193 // discovered twice except by concurrent collectors that potentially
1194 // trace the same Reference object twice.
1195 assert(UseConcMarkSweepGC || UseG1GC,
1196 "Only possible with a concurrent marking collector");
1197 return true;
1198 }
1199 }
1200
1201 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1202 verify_referent(obj);
1203 // Discover if and only if EITHER:
1204 // .. reference is in our span, OR
1205 // .. we are an atomic collector and referent is in our span
1206 if (_span.contains(obj_addr) ||
1207 (discovery_is_atomic() &&
1208 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1209 // should_enqueue = true;
1210 } else {
1211 return false;
1212 }
1213 } else {
1214 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1215 _span.contains(obj_addr), "code inconsistency");
1216 }
1217
1218 // Get the right type of discovered queue head.
1219 DiscoveredList* list = get_discovered_list(rt);
1220 if (list == NULL) {
1221 return false; // nothing special needs to be done
1222 }
1223
1224 if (_discovery_is_mt) {
1225 add_to_discovered_list_mt(*list, obj, discovered_addr);
1226 } else {
1227 // If "_discovered_list_needs_barrier", we do write barriers when
1228 // updating the discovered reference list. Otherwise, we do a raw store
1229 // here: the field will be visited later when processing the discovered
1230 // references.
1231 oop current_head = list->head();
1232 // The last ref must have its discovered field pointing to itself.
1233 oop next_discovered = (current_head != NULL) ? current_head : obj;
1234
1235 // As in the case further above, since we are over-writing a NULL
1236 // pre-value, we can safely elide the pre-barrier here for the case of G1.
1237 // e.g.:- _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1238 assert(discovered == NULL, "control point invariant");
1239 assert(!_discovered_list_needs_barrier || UseG1GC,
1240 "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
1241 oop_store_raw(discovered_addr, next_discovered);
1242 if (_discovered_list_needs_barrier) {
1243 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1244 }
1245 list->set_head(obj);
1246 list->inc_length(1);
1247
1248 if (TraceReferenceGC) {
1249 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1250 obj, obj->blueprint()->internal_name());
1251 }
1252 }
1253 assert(obj->is_oop(), "Discovered a bad reference");
1254 verify_referent(obj);
1255 return true;
1256 }
1257
1258 // Preclean the discovered references by removing those
1259 // whose referents are alive, and by marking from those that
1260 // are not active. These lists can be handled here
1261 // in any order and, indeed, concurrently.
1262 void ReferenceProcessor::preclean_discovered_references(
1263 BoolObjectClosure* is_alive,
1264 OopClosure* keep_alive,
1265 VoidClosure* complete_gc,
1266 YieldClosure* yield,
1267 bool should_unload_classes) {
1268
1269 NOT_PRODUCT(verify_ok_to_handle_reflists());
1270
1271 #ifdef ASSERT
1272 bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC ||
1273 CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
1274 ExplicitGCInvokesConcurrentAndUnloadsClasses &&
1275 UseConcMarkSweepGC && should_unload_classes;
1276 RememberKlassesChecker mx(must_remember_klasses);
1277 #endif
1278 // Soft references
1279 {
1280 TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1281 false, gclog_or_tty);
1282 for (int i = 0; i < _max_num_q; i++) {
1283 if (yield->should_return()) {
1284 return;
1285 }
1286 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1287 keep_alive, complete_gc, yield);
1288 }
1289 }
1290
1291 // Weak references
1292 {
1293 TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1294 false, gclog_or_tty);
1295 for (int i = 0; i < _max_num_q; i++) {
1296 if (yield->should_return()) {
1297 return;
1298 }
1299 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1300 keep_alive, complete_gc, yield);
1301 }
1302 }
1303
1304 // Final references
1305 {
1306 TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1307 false, gclog_or_tty);
1308 for (int i = 0; i < _max_num_q; i++) {
1309 if (yield->should_return()) {
1310 return;
1311 }
1312 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1313 keep_alive, complete_gc, yield);
1314 }
1315 }
1316
1317 // Phantom references
1318 {
1319 TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1320 false, gclog_or_tty);
1321 for (int i = 0; i < _max_num_q; i++) {
1322 if (yield->should_return()) {
1323 return;
1324 }
1325 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1326 keep_alive, complete_gc, yield);
1327 }
1328 }
1329 }
1330
1331 // Walk the given discovered ref list, and remove all reference objects
1332 // whose referents are still alive, whose referents are NULL or which
1333 // are not active (have a non-NULL next field). NOTE: When we are
1334 // thus precleaning the ref lists (which happens single-threaded today),
1335 // we do not disable refs discovery to honour the correct semantics of
1336 // java.lang.Reference. As a result, we need to be careful below
1337 // that ref removal steps interleave safely with ref discovery steps
1338 // (in this thread).
1339 void
1340 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1341 BoolObjectClosure* is_alive,
1342 OopClosure* keep_alive,
1343 VoidClosure* complete_gc,
1344 YieldClosure* yield) {
1345 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1346 while (iter.has_next()) {
1347 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1348 oop obj = iter.obj();
1349 oop next = java_lang_ref_Reference::next(obj);
1350 if (iter.referent() == NULL || iter.is_referent_alive() ||
1351 next != NULL) {
1352 // The referent has been cleared, or is alive, or the Reference is not
1353 // active; we need to trace and mark its cohort.
1354 if (TraceReferenceGC) {
1355 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1356 iter.obj(), iter.obj()->blueprint()->internal_name());
1357 }
1358 // Remove Reference object from list
1359 iter.remove();
1360 // Keep alive its cohort.
1361 iter.make_referent_alive();
1362 if (UseCompressedOops) {
1363 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1364 keep_alive->do_oop(next_addr);
1365 } else {
1366 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1367 keep_alive->do_oop(next_addr);
1368 }
1369 iter.move_to_next();
1370 } else {
1371 iter.next();
1372 }
1373 }
1374 // Close the reachable set
1375 complete_gc->do_void();
1376
1377 NOT_PRODUCT(
1378 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1379 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1380 "Refs in discovered list " INTPTR_FORMAT,
1381 iter.removed(), iter.processed(), (address)refs_list.head());
1382 }
1383 )
1384 }
1385
1386 const char* ReferenceProcessor::list_name(int i) {
1387 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1388 "Out of bounds index");
1389
1390 int j = i / _max_num_q;
1391 switch (j) {
1392 case 0: return "SoftRef";
1393 case 1: return "WeakRef";
1394 case 2: return "FinalRef";
1395 case 3: return "PhantomRef";
1396 }
1397 ShouldNotReachHere();
1398 return NULL;
1399 }
1400
1401 #ifndef PRODUCT
1402 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1403 // empty for now
1404 }
1405 #endif
1406
1407 #ifndef PRODUCT
1408 void ReferenceProcessor::clear_discovered_references() {
1409 guarantee(!_discovering_refs, "Discovering refs?");
1410 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1411 clear_discovered_references(_discovered_refs[i]);
1412 }
1413 }
1414
1415 #endif // PRODUCT