1 /*
2 * Copyright (c) 2001, 2012, 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 // We need a monotonically non-deccreasing time in ms but
47 // os::javaTimeMillis() does not guarantee monotonicity.
48 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
49
50 // Initialize the soft ref timestamp clock.
51 _soft_ref_timestamp_clock = now;
52 // Also update the soft ref clock in j.l.r.SoftReference
53 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
54
55 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
56 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
57 NOT_COMPILER2(LRUCurrentHeapPolicy());
58 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
59 vm_exit_during_initialization("Could not allocate reference policy object");
60 }
61 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
62 RefDiscoveryPolicy == ReferentBasedDiscovery,
63 "Unrecongnized RefDiscoveryPolicy");
64 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
65 }
66
67 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
68 #ifdef ASSERT
69 // Verify that we're not currently discovering refs
70 assert(!verify_disabled || !_discovering_refs, "nested call?");
71
72 if (check_no_refs) {
73 // Verify that the discovered lists are empty
74 verify_no_references_recorded();
75 }
76 #endif // ASSERT
77
78 // Someone could have modified the value of the static
79 // field in the j.l.r.SoftReference class that holds the
80 // soft reference timestamp clock using reflection or
81 // Unsafe between GCs. Unconditionally update the static
82 // field in ReferenceProcessor here so that we use the new
83 // value during reference discovery.
84
85 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
86 _discovering_refs = true;
87 }
88
89 ReferenceProcessor::ReferenceProcessor(MemRegion span,
90 bool mt_processing,
91 uint mt_processing_degree,
92 bool mt_discovery,
93 uint mt_discovery_degree,
94 bool atomic_discovery,
95 BoolObjectClosure* is_alive_non_header,
96 bool discovered_list_needs_barrier) :
97 _discovering_refs(false),
98 _enqueuing_is_done(false),
99 _is_alive_non_header(is_alive_non_header),
100 _discovered_list_needs_barrier(discovered_list_needs_barrier),
101 _bs(NULL),
102 _processing_is_mt(mt_processing),
103 _next_id(0)
104 {
105 _span = span;
106 _discovery_is_atomic = atomic_discovery;
107 _discovery_is_mt = mt_discovery;
108 _num_q = MAX2(1U, mt_processing_degree);
109 _max_num_q = MAX2(_num_q, mt_discovery_degree);
110 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
111 _max_num_q * number_of_subclasses_of_ref(), mtGC);
112
113 if (_discovered_refs == NULL) {
114 vm_exit_during_initialization("Could not allocated RefProc Array");
115 }
116 _discoveredSoftRefs = &_discovered_refs[0];
117 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
118 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
119 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
120
121 // Initialize all entries to NULL
122 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
123 _discovered_refs[i].set_head(NULL);
124 _discovered_refs[i].set_length(0);
125 }
126
127 // If we do barriers, cache a copy of the barrier set.
128 if (discovered_list_needs_barrier) {
129 _bs = Universe::heap()->barrier_set();
130 }
131 setup_policy(false /* default soft ref policy */);
132 }
133
134 #ifndef PRODUCT
135 void ReferenceProcessor::verify_no_references_recorded() {
136 guarantee(!_discovering_refs, "Discovering refs?");
137 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
138 guarantee(_discovered_refs[i].is_empty(),
139 "Found non-empty discovered list");
140 }
141 }
142 #endif
143
144 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
145 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
146 if (UseCompressedOops) {
147 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
148 } else {
149 f->do_oop((oop*)_discovered_refs[i].adr_head());
150 }
151 }
152 }
153
154 void ReferenceProcessor::update_soft_ref_master_clock() {
155 // Update (advance) the soft ref master clock field. This must be done
156 // after processing the soft ref list.
157
158 // We need a monotonically non-deccreasing time in ms but
159 // os::javaTimeMillis() does not guarantee monotonicity.
160 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
161 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
162 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
163
164 NOT_PRODUCT(
165 if (now < _soft_ref_timestamp_clock) {
166 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
167 _soft_ref_timestamp_clock, now);
168 }
169 )
170 // The values of now and _soft_ref_timestamp_clock are set using
171 // javaTimeNanos(), which is guaranteed to be monotonically
172 // non-decreasing provided the underlying platform provides such
173 // a time source (and it is bug free).
174 // In product mode, however, protect ourselves from non-monotonicty.
175 if (now > _soft_ref_timestamp_clock) {
176 _soft_ref_timestamp_clock = now;
177 java_lang_ref_SoftReference::set_clock(now);
178 }
179 // Else leave clock stalled at its old value until time progresses
180 // past clock value.
181 }
182
183 void ReferenceProcessor::process_discovered_references(
184 BoolObjectClosure* is_alive,
185 OopClosure* keep_alive,
186 VoidClosure* complete_gc,
187 AbstractRefProcTaskExecutor* task_executor) {
188 NOT_PRODUCT(verify_ok_to_handle_reflists());
189
190 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
191 // Stop treating discovered references specially.
192 disable_discovery();
193
194 // If discovery was concurrent, someone could have modified
195 // the value of the static field in the j.l.r.SoftReference
196 // class that holds the soft reference timestamp clock using
197 // reflection or Unsafe between when discovery was enabled and
198 // now. Unconditionally update the static field in ReferenceProcessor
199 // here so that we use the new value during processing of the
200 // discovered soft refs.
201
202 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
203
204 bool trace_time = PrintGCDetails && PrintReferenceGC;
205 // Soft references
206 {
207 TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);
208 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
209 is_alive, keep_alive, complete_gc, task_executor);
210 }
211
212 update_soft_ref_master_clock();
213
214 // Weak references
215 {
216 TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);
217 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
218 is_alive, keep_alive, complete_gc, task_executor);
219 }
220
221 // Final references
222 {
223 TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);
224 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
225 is_alive, keep_alive, complete_gc, task_executor);
226 }
227
228 // Phantom references
229 {
230 TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);
231 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
232 is_alive, keep_alive, complete_gc, task_executor);
233 }
234
235 // Weak global JNI references. It would make more sense (semantically) to
236 // traverse these simultaneously with the regular weak references above, but
237 // that is not how the JDK1.2 specification is. See #4126360. Native code can
238 // thus use JNI weak references to circumvent the phantom references and
239 // resurrect a "post-mortem" object.
240 {
241 TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);
242 if (task_executor != NULL) {
243 task_executor->set_single_threaded_mode();
244 }
245 process_phaseJNI(is_alive, keep_alive, complete_gc);
246 }
247 }
248
249 #ifndef PRODUCT
250 // Calculate the number of jni handles.
251 uint ReferenceProcessor::count_jni_refs() {
252 class AlwaysAliveClosure: public BoolObjectClosure {
253 public:
254 virtual bool do_object_b(oop obj) { return true; }
255 };
256
257 class CountHandleClosure: public OopClosure {
258 private:
259 int _count;
260 public:
261 CountHandleClosure(): _count(0) {}
262 void do_oop(oop* unused) { _count++; }
263 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
264 int count() { return _count; }
265 };
266 CountHandleClosure global_handle_count;
267 AlwaysAliveClosure always_alive;
268 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
269 return global_handle_count.count();
270 }
271 #endif
272
273 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
274 OopClosure* keep_alive,
275 VoidClosure* complete_gc) {
276 #ifndef PRODUCT
277 if (PrintGCDetails && PrintReferenceGC) {
278 unsigned int count = count_jni_refs();
279 gclog_or_tty->print(", %u refs", count);
280 }
281 #endif
282 JNIHandles::weak_oops_do(is_alive, keep_alive);
283 complete_gc->do_void();
284 }
285
286
287 template <class T>
288 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
289 AbstractRefProcTaskExecutor* task_executor) {
290
291 // Remember old value of pending references list
292 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
293 T old_pending_list_value = *pending_list_addr;
294
295 // Enqueue references that are not made active again, and
296 // clear the decks for the next collection (cycle).
297 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
298 // Do the oop-check on pending_list_addr missed in
299 // enqueue_discovered_reflist. We should probably
300 // do a raw oop_check so that future such idempotent
301 // oop_stores relying on the oop-check side-effect
302 // may be elided automatically and safely without
303 // affecting correctness.
304 oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
305
306 // Stop treating discovered references specially.
307 ref->disable_discovery();
308
309 // Return true if new pending references were added
310 return old_pending_list_value != *pending_list_addr;
311 }
312
313 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
314 NOT_PRODUCT(verify_ok_to_handle_reflists());
315 if (UseCompressedOops) {
316 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
317 } else {
318 return enqueue_discovered_ref_helper<oop>(this, task_executor);
319 }
320 }
321
322 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
323 HeapWord* pending_list_addr) {
324 // Given a list of refs linked through the "discovered" field
325 // (java.lang.ref.Reference.discovered), self-loop their "next" field
326 // thus distinguishing them from active References, then
327 // prepend them to the pending list.
328 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
329 // the "next" field is used to chain the pending list, not the discovered
330 // field.
331
332 if (TraceReferenceGC && PrintGCDetails) {
333 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
334 INTPTR_FORMAT, (address)refs_list.head());
335 }
336
337 oop obj = NULL;
338 oop next_d = refs_list.head();
339 if (pending_list_uses_discovered_field()) { // New behaviour
340 // Walk down the list, self-looping the next field
341 // so that the References are not considered active.
342 while (obj != next_d) {
343 obj = next_d;
344 assert(obj->is_instanceRef(), "should be reference object");
345 next_d = java_lang_ref_Reference::discovered(obj);
346 if (TraceReferenceGC && PrintGCDetails) {
347 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
348 obj, next_d);
349 }
350 assert(java_lang_ref_Reference::next(obj) == NULL,
351 "Reference not active; should not be discovered");
352 // Self-loop next, so as to make Ref not active.
353 java_lang_ref_Reference::set_next(obj, obj);
354 if (next_d == obj) { // obj is last
355 // Swap refs_list into pendling_list_addr and
356 // set obj's discovered to what we read from pending_list_addr.
357 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
358 // Need oop_check on pending_list_addr above;
359 // see special oop-check code at the end of
360 // enqueue_discovered_reflists() further below.
361 java_lang_ref_Reference::set_discovered(obj, old); // old may be NULL
362 }
363 }
364 } else { // Old behaviour
365 // Walk down the list, copying the discovered field into
366 // the next field and clearing the discovered field.
367 while (obj != next_d) {
368 obj = next_d;
369 assert(obj->is_instanceRef(), "should be reference object");
370 next_d = java_lang_ref_Reference::discovered(obj);
371 if (TraceReferenceGC && PrintGCDetails) {
372 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
373 obj, next_d);
374 }
375 assert(java_lang_ref_Reference::next(obj) == NULL,
376 "The reference should not be enqueued");
377 if (next_d == obj) { // obj is last
378 // Swap refs_list into pendling_list_addr and
379 // set obj's next to what we read from pending_list_addr.
380 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
381 // Need oop_check on pending_list_addr above;
382 // see special oop-check code at the end of
383 // enqueue_discovered_reflists() further below.
384 if (old == NULL) {
385 // obj should be made to point to itself, since
386 // pending list was empty.
387 java_lang_ref_Reference::set_next(obj, obj);
388 } else {
389 java_lang_ref_Reference::set_next(obj, old);
390 }
391 } else {
392 java_lang_ref_Reference::set_next(obj, next_d);
393 }
394 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
395 }
396 }
397 }
398
399 // Parallel enqueue task
400 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
401 public:
402 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
403 DiscoveredList discovered_refs[],
404 HeapWord* pending_list_addr,
405 int n_queues)
406 : EnqueueTask(ref_processor, discovered_refs,
407 pending_list_addr, n_queues)
408 { }
409
410 virtual void work(unsigned int work_id) {
411 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
412 // Simplest first cut: static partitioning.
413 int index = work_id;
414 // The increment on "index" must correspond to the maximum number of queues
415 // (n_queues) with which that ReferenceProcessor was created. That
416 // is because of the "clever" way the discovered references lists were
417 // allocated and are indexed into.
418 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
419 for (int j = 0;
420 j < ReferenceProcessor::number_of_subclasses_of_ref();
421 j++, index += _n_queues) {
422 _ref_processor.enqueue_discovered_reflist(
423 _refs_lists[index], _pending_list_addr);
424 _refs_lists[index].set_head(NULL);
425 _refs_lists[index].set_length(0);
426 }
427 }
428 };
429
430 // Enqueue references that are not made active again
431 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
432 AbstractRefProcTaskExecutor* task_executor) {
433 if (_processing_is_mt && task_executor != NULL) {
434 // Parallel code
435 RefProcEnqueueTask tsk(*this, _discovered_refs,
436 pending_list_addr, _max_num_q);
437 task_executor->execute(tsk);
438 } else {
439 // Serial code: call the parent class's implementation
440 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
441 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
442 _discovered_refs[i].set_head(NULL);
443 _discovered_refs[i].set_length(0);
444 }
445 }
446 }
447
448 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
449 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
450 oop discovered = java_lang_ref_Reference::discovered(_ref);
451 assert(_discovered_addr && discovered->is_oop_or_null(),
452 "discovered field is bad");
453 _next = discovered;
454 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
455 _referent = java_lang_ref_Reference::referent(_ref);
456 assert(Universe::heap()->is_in_reserved_or_null(_referent),
457 "Wrong oop found in java.lang.Reference object");
458 assert(allow_null_referent ?
459 _referent->is_oop_or_null()
460 : _referent->is_oop(),
461 "bad referent");
462 }
463
464 void DiscoveredListIterator::remove() {
465 assert(_ref->is_oop(), "Dropping a bad reference");
466 oop_store_raw(_discovered_addr, NULL);
467
468 // First _prev_next ref actually points into DiscoveredList (gross).
469 oop new_next;
470 if (_next == _ref) {
471 // At the end of the list, we should make _prev point to itself.
472 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
473 // and _prev will be NULL.
474 new_next = _prev;
475 } else {
476 new_next = _next;
477 }
478
479 if (UseCompressedOops) {
480 // Remove Reference object from list.
481 oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next);
482 } else {
483 // Remove Reference object from list.
484 oopDesc::store_heap_oop((oop*)_prev_next, new_next);
485 }
486 NOT_PRODUCT(_removed++);
487 _refs_list.dec_length(1);
488 }
489
490 // Make the Reference object active again.
491 void DiscoveredListIterator::make_active() {
492 // For G1 we don't want to use set_next - it
493 // will dirty the card for the next field of
494 // the reference object and will fail
495 // CT verification.
496 if (UseG1GC) {
497 BarrierSet* bs = oopDesc::bs();
498 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
499
500 if (UseCompressedOops) {
501 bs->write_ref_field_pre((narrowOop*)next_addr, NULL);
502 } else {
503 bs->write_ref_field_pre((oop*)next_addr, NULL);
504 }
505 java_lang_ref_Reference::set_next_raw(_ref, NULL);
506 } else {
507 java_lang_ref_Reference::set_next(_ref, NULL);
508 }
509 }
510
511 void DiscoveredListIterator::clear_referent() {
512 oop_store_raw(_referent_addr, NULL);
513 }
514
515 // NOTE: process_phase*() are largely similar, and at a high level
516 // merely iterate over the extant list applying a predicate to
517 // each of its elements and possibly removing that element from the
518 // list and applying some further closures to that element.
519 // We should consider the possibility of replacing these
520 // process_phase*() methods by abstracting them into
521 // a single general iterator invocation that receives appropriate
522 // closures that accomplish this work.
523
524 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
525 // referents are not alive, but that should be kept alive for policy reasons.
526 // Keep alive the transitive closure of all such referents.
527 void
528 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
529 ReferencePolicy* policy,
530 BoolObjectClosure* is_alive,
531 OopClosure* keep_alive,
532 VoidClosure* complete_gc) {
533 assert(policy != NULL, "Must have a non-NULL policy");
534 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
535 // Decide which softly reachable refs should be kept alive.
536 while (iter.has_next()) {
537 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
538 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
539 if (referent_is_dead &&
540 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
541 if (TraceReferenceGC) {
542 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
543 iter.obj(), iter.obj()->klass()->internal_name());
544 }
545 // Remove Reference object from list
546 iter.remove();
547 // Make the Reference object active again
548 iter.make_active();
549 // keep the referent around
550 iter.make_referent_alive();
551 iter.move_to_next();
552 } else {
553 iter.next();
554 }
555 }
556 // Close the reachable set
557 complete_gc->do_void();
558 NOT_PRODUCT(
559 if (PrintGCDetails && TraceReferenceGC) {
560 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
561 "discovered Refs by policy, from list " INTPTR_FORMAT,
562 iter.removed(), iter.processed(), (address)refs_list.head());
563 }
564 )
565 }
566
567 // Traverse the list and remove any Refs that are not active, or
568 // whose referents are either alive or NULL.
569 void
570 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
571 BoolObjectClosure* is_alive,
572 OopClosure* keep_alive) {
573 assert(discovery_is_atomic(), "Error");
574 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
575 while (iter.has_next()) {
576 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
577 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
578 assert(next == NULL, "Should not discover inactive Reference");
579 if (iter.is_referent_alive()) {
580 if (TraceReferenceGC) {
581 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
582 iter.obj(), iter.obj()->klass()->internal_name());
583 }
584 // The referent is reachable after all.
585 // Remove Reference object from list.
586 iter.remove();
587 // Update the referent pointer as necessary: Note that this
588 // should not entail any recursive marking because the
589 // referent must already have been traversed.
590 iter.make_referent_alive();
591 iter.move_to_next();
592 } else {
593 iter.next();
594 }
595 }
596 NOT_PRODUCT(
597 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
598 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
599 "Refs in discovered list " INTPTR_FORMAT,
600 iter.removed(), iter.processed(), (address)refs_list.head());
601 }
602 )
603 }
604
605 void
606 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
607 BoolObjectClosure* is_alive,
608 OopClosure* keep_alive,
609 VoidClosure* complete_gc) {
610 assert(!discovery_is_atomic(), "Error");
611 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
612 while (iter.has_next()) {
613 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
614 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
615 oop next = java_lang_ref_Reference::next(iter.obj());
616 if ((iter.referent() == NULL || iter.is_referent_alive() ||
617 next != NULL)) {
618 assert(next->is_oop_or_null(), "bad next field");
619 // Remove Reference object from list
620 iter.remove();
621 // Trace the cohorts
622 iter.make_referent_alive();
623 if (UseCompressedOops) {
624 keep_alive->do_oop((narrowOop*)next_addr);
625 } else {
626 keep_alive->do_oop((oop*)next_addr);
627 }
628 iter.move_to_next();
629 } else {
630 iter.next();
631 }
632 }
633 // Now close the newly reachable set
634 complete_gc->do_void();
635 NOT_PRODUCT(
636 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
637 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
638 "Refs in discovered list " INTPTR_FORMAT,
639 iter.removed(), iter.processed(), (address)refs_list.head());
640 }
641 )
642 }
643
644 // Traverse the list and process the referents, by either
645 // clearing them or keeping them (and their reachable
646 // closure) alive.
647 void
648 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
649 bool clear_referent,
650 BoolObjectClosure* is_alive,
651 OopClosure* keep_alive,
652 VoidClosure* complete_gc) {
653 ResourceMark rm;
654 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
655 while (iter.has_next()) {
656 iter.update_discovered();
657 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
658 if (clear_referent) {
659 // NULL out referent pointer
660 iter.clear_referent();
661 } else {
662 // keep the referent around
663 iter.make_referent_alive();
664 }
665 if (TraceReferenceGC) {
666 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
667 clear_referent ? "cleared " : "",
668 iter.obj(), iter.obj()->klass()->internal_name());
669 }
670 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
671 iter.next();
672 }
673 // Remember to update the next pointer of the last ref.
674 iter.update_discovered();
675 // Close the reachable set
676 complete_gc->do_void();
677 }
678
679 void
680 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
681 oop obj = NULL;
682 oop next = refs_list.head();
683 while (next != obj) {
684 obj = next;
685 next = java_lang_ref_Reference::discovered(obj);
686 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
687 }
688 refs_list.set_head(NULL);
689 refs_list.set_length(0);
690 }
691
692 void
693 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
694 clear_discovered_references(refs_list);
695 }
696
697 void ReferenceProcessor::abandon_partial_discovery() {
698 // loop over the lists
699 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
700 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
701 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
702 }
703 abandon_partial_discovered_list(_discovered_refs[i]);
704 }
705 }
706
707 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
708 public:
709 RefProcPhase1Task(ReferenceProcessor& ref_processor,
710 DiscoveredList refs_lists[],
711 ReferencePolicy* policy,
712 bool marks_oops_alive)
713 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
714 _policy(policy)
715 { }
716 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
717 OopClosure& keep_alive,
718 VoidClosure& complete_gc)
719 {
720 Thread* thr = Thread::current();
721 int refs_list_index = ((WorkerThread*)thr)->id();
722 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
723 &is_alive, &keep_alive, &complete_gc);
724 }
725 private:
726 ReferencePolicy* _policy;
727 };
728
729 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
730 public:
731 RefProcPhase2Task(ReferenceProcessor& ref_processor,
732 DiscoveredList refs_lists[],
733 bool marks_oops_alive)
734 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
735 { }
736 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
737 OopClosure& keep_alive,
738 VoidClosure& complete_gc)
739 {
740 _ref_processor.process_phase2(_refs_lists[i],
741 &is_alive, &keep_alive, &complete_gc);
742 }
743 };
744
745 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
746 public:
747 RefProcPhase3Task(ReferenceProcessor& ref_processor,
748 DiscoveredList refs_lists[],
749 bool clear_referent,
750 bool marks_oops_alive)
751 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
752 _clear_referent(clear_referent)
753 { }
754 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
755 OopClosure& keep_alive,
756 VoidClosure& complete_gc)
757 {
758 // Don't use "refs_list_index" calculated in this way because
759 // balance_queues() has moved the Ref's into the first n queues.
760 // Thread* thr = Thread::current();
761 // int refs_list_index = ((WorkerThread*)thr)->id();
762 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
763 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
764 &is_alive, &keep_alive, &complete_gc);
765 }
766 private:
767 bool _clear_referent;
768 };
769
770 void ReferenceProcessor::set_discovered(oop ref, oop value) {
771 if (_discovered_list_needs_barrier) {
772 java_lang_ref_Reference::set_discovered(ref, value);
773 } else {
774 java_lang_ref_Reference::set_discovered_raw(ref, value);
775 }
776 }
777
778 // Balances reference queues.
779 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
780 // queues[0, 1, ..., _num_q-1] because only the first _num_q
781 // corresponding to the active workers will be processed.
782 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
783 {
784 // calculate total length
785 size_t total_refs = 0;
786 if (TraceReferenceGC && PrintGCDetails) {
787 gclog_or_tty->print_cr("\nBalance ref_lists ");
788 }
789
790 for (uint i = 0; i < _max_num_q; ++i) {
791 total_refs += ref_lists[i].length();
792 if (TraceReferenceGC && PrintGCDetails) {
793 gclog_or_tty->print("%d ", ref_lists[i].length());
794 }
795 }
796 if (TraceReferenceGC && PrintGCDetails) {
797 gclog_or_tty->print_cr(" = %d", total_refs);
798 }
799 size_t avg_refs = total_refs / _num_q + 1;
800 uint to_idx = 0;
801 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
802 bool move_all = false;
803 if (from_idx >= _num_q) {
804 move_all = ref_lists[from_idx].length() > 0;
805 }
806 while ((ref_lists[from_idx].length() > avg_refs) ||
807 move_all) {
808 assert(to_idx < _num_q, "Sanity Check!");
809 if (ref_lists[to_idx].length() < avg_refs) {
810 // move superfluous refs
811 size_t refs_to_move;
812 // Move all the Ref's if the from queue will not be processed.
813 if (move_all) {
814 refs_to_move = MIN2(ref_lists[from_idx].length(),
815 avg_refs - ref_lists[to_idx].length());
816 } else {
817 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
818 avg_refs - ref_lists[to_idx].length());
819 }
820
821 assert(refs_to_move > 0, "otherwise the code below will fail");
822
823 oop move_head = ref_lists[from_idx].head();
824 oop move_tail = move_head;
825 oop new_head = move_head;
826 // find an element to split the list on
827 for (size_t j = 0; j < refs_to_move; ++j) {
828 move_tail = new_head;
829 new_head = java_lang_ref_Reference::discovered(new_head);
830 }
831
832 // Add the chain to the to list.
833 if (ref_lists[to_idx].head() == NULL) {
834 // to list is empty. Make a loop at the end.
835 set_discovered(move_tail, move_tail);
836 } else {
837 set_discovered(move_tail, ref_lists[to_idx].head());
838 }
839 ref_lists[to_idx].set_head(move_head);
840 ref_lists[to_idx].inc_length(refs_to_move);
841
842 // Remove the chain from the from list.
843 if (move_tail == new_head) {
844 // We found the end of the from list.
845 ref_lists[from_idx].set_head(NULL);
846 } else {
847 ref_lists[from_idx].set_head(new_head);
848 }
849 ref_lists[from_idx].dec_length(refs_to_move);
850 if (ref_lists[from_idx].length() == 0) {
851 break;
852 }
853 } else {
854 to_idx = (to_idx + 1) % _num_q;
855 }
856 }
857 }
858 #ifdef ASSERT
859 size_t balanced_total_refs = 0;
860 for (uint i = 0; i < _max_num_q; ++i) {
861 balanced_total_refs += ref_lists[i].length();
862 if (TraceReferenceGC && PrintGCDetails) {
863 gclog_or_tty->print("%d ", ref_lists[i].length());
864 }
865 }
866 if (TraceReferenceGC && PrintGCDetails) {
867 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
868 gclog_or_tty->flush();
869 }
870 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
871 #endif
872 }
873
874 void ReferenceProcessor::balance_all_queues() {
875 balance_queues(_discoveredSoftRefs);
876 balance_queues(_discoveredWeakRefs);
877 balance_queues(_discoveredFinalRefs);
878 balance_queues(_discoveredPhantomRefs);
879 }
880
881 void
882 ReferenceProcessor::process_discovered_reflist(
883 DiscoveredList refs_lists[],
884 ReferencePolicy* policy,
885 bool clear_referent,
886 BoolObjectClosure* is_alive,
887 OopClosure* keep_alive,
888 VoidClosure* complete_gc,
889 AbstractRefProcTaskExecutor* task_executor)
890 {
891 bool mt_processing = task_executor != NULL && _processing_is_mt;
892 // If discovery used MT and a dynamic number of GC threads, then
893 // the queues must be balanced for correctness if fewer than the
894 // maximum number of queues were used. The number of queue used
895 // during discovery may be different than the number to be used
896 // for processing so don't depend of _num_q < _max_num_q as part
897 // of the test.
898 bool must_balance = _discovery_is_mt;
899
900 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
901 must_balance) {
902 balance_queues(refs_lists);
903 }
904 if (PrintReferenceGC && PrintGCDetails) {
905 size_t total = 0;
906 for (uint i = 0; i < _max_num_q; ++i) {
907 total += refs_lists[i].length();
908 }
909 gclog_or_tty->print(", %u refs", total);
910 }
911
912 // Phase 1 (soft refs only):
913 // . Traverse the list and remove any SoftReferences whose
914 // referents are not alive, but that should be kept alive for
915 // policy reasons. Keep alive the transitive closure of all
916 // such referents.
917 if (policy != NULL) {
918 if (mt_processing) {
919 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
920 task_executor->execute(phase1);
921 } else {
922 for (uint i = 0; i < _max_num_q; i++) {
923 process_phase1(refs_lists[i], policy,
924 is_alive, keep_alive, complete_gc);
925 }
926 }
927 } else { // policy == NULL
928 assert(refs_lists != _discoveredSoftRefs,
929 "Policy must be specified for soft references.");
930 }
931
932 // Phase 2:
933 // . Traverse the list and remove any refs whose referents are alive.
934 if (mt_processing) {
935 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
936 task_executor->execute(phase2);
937 } else {
938 for (uint i = 0; i < _max_num_q; i++) {
939 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
940 }
941 }
942
943 // Phase 3:
944 // . Traverse the list and process referents as appropriate.
945 if (mt_processing) {
946 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
947 task_executor->execute(phase3);
948 } else {
949 for (uint i = 0; i < _max_num_q; i++) {
950 process_phase3(refs_lists[i], clear_referent,
951 is_alive, keep_alive, complete_gc);
952 }
953 }
954 }
955
956 void ReferenceProcessor::clean_up_discovered_references() {
957 // loop over the lists
958 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
959 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
960 gclog_or_tty->print_cr(
961 "\nScrubbing %s discovered list of Null referents",
962 list_name(i));
963 }
964 clean_up_discovered_reflist(_discovered_refs[i]);
965 }
966 }
967
968 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
969 assert(!discovery_is_atomic(), "Else why call this method?");
970 DiscoveredListIterator iter(refs_list, NULL, NULL);
971 while (iter.has_next()) {
972 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
973 oop next = java_lang_ref_Reference::next(iter.obj());
974 assert(next->is_oop_or_null(), "bad next field");
975 // If referent has been cleared or Reference is not active,
976 // drop it.
977 if (iter.referent() == NULL || next != NULL) {
978 debug_only(
979 if (PrintGCDetails && TraceReferenceGC) {
980 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
981 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
982 " and referent: " INTPTR_FORMAT,
983 iter.obj(), next, iter.referent());
984 }
985 )
986 // Remove Reference object from list
987 iter.remove();
988 iter.move_to_next();
989 } else {
990 iter.next();
991 }
992 }
993 NOT_PRODUCT(
994 if (PrintGCDetails && TraceReferenceGC) {
995 gclog_or_tty->print(
996 " Removed %d Refs with NULL referents out of %d discovered Refs",
997 iter.removed(), iter.processed());
998 }
999 )
1000 }
1001
1002 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1003 uint id = 0;
1004 // Determine the queue index to use for this object.
1005 if (_discovery_is_mt) {
1006 // During a multi-threaded discovery phase,
1007 // each thread saves to its "own" list.
1008 Thread* thr = Thread::current();
1009 id = thr->as_Worker_thread()->id();
1010 } else {
1011 // single-threaded discovery, we save in round-robin
1012 // fashion to each of the lists.
1013 if (_processing_is_mt) {
1014 id = next_id();
1015 }
1016 }
1017 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1018
1019 // Get the discovered queue to which we will add
1020 DiscoveredList* list = NULL;
1021 switch (rt) {
1022 case REF_OTHER:
1023 // Unknown reference type, no special treatment
1024 break;
1025 case REF_SOFT:
1026 list = &_discoveredSoftRefs[id];
1027 break;
1028 case REF_WEAK:
1029 list = &_discoveredWeakRefs[id];
1030 break;
1031 case REF_FINAL:
1032 list = &_discoveredFinalRefs[id];
1033 break;
1034 case REF_PHANTOM:
1035 list = &_discoveredPhantomRefs[id];
1036 break;
1037 case REF_NONE:
1038 // we should not reach here if we are an InstanceRefKlass
1039 default:
1040 ShouldNotReachHere();
1041 }
1042 if (TraceReferenceGC && PrintGCDetails) {
1043 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1044 }
1045 return list;
1046 }
1047
1048 inline void
1049 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1050 oop obj,
1051 HeapWord* discovered_addr) {
1052 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1053 // First we must make sure this object is only enqueued once. CAS in a non null
1054 // discovered_addr.
1055 oop current_head = refs_list.head();
1056 // The last ref must have its discovered field pointing to itself.
1057 oop next_discovered = (current_head != NULL) ? current_head : obj;
1058
1059 // Note: In the case of G1, this specific pre-barrier is strictly
1060 // not necessary because the only case we are interested in
1061 // here is when *discovered_addr is NULL (see the CAS further below),
1062 // so this will expand to nothing. As a result, we have manually
1063 // elided this out for G1, but left in the test for some future
1064 // collector that might have need for a pre-barrier here, e.g.:-
1065 // _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1066 assert(!_discovered_list_needs_barrier || UseG1GC,
1067 "Need to check non-G1 collector: "
1068 "may need a pre-write-barrier for CAS from NULL below");
1069 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1070 NULL);
1071 if (retest == NULL) {
1072 // This thread just won the right to enqueue the object.
1073 // We have separate lists for enqueueing, so no synchronization
1074 // is necessary.
1075 refs_list.set_head(obj);
1076 refs_list.inc_length(1);
1077 if (_discovered_list_needs_barrier) {
1078 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1079 }
1080
1081 if (TraceReferenceGC) {
1082 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1083 obj, obj->klass()->internal_name());
1084 }
1085 } else {
1086 // If retest was non NULL, another thread beat us to it:
1087 // The reference has already been discovered...
1088 if (TraceReferenceGC) {
1089 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1090 obj, obj->klass()->internal_name());
1091 }
1092 }
1093 }
1094
1095 #ifndef PRODUCT
1096 // Non-atomic (i.e. concurrent) discovery might allow us
1097 // to observe j.l.References with NULL referents, being those
1098 // cleared concurrently by mutators during (or after) discovery.
1099 void ReferenceProcessor::verify_referent(oop obj) {
1100 bool da = discovery_is_atomic();
1101 oop referent = java_lang_ref_Reference::referent(obj);
1102 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1103 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1104 INTPTR_FORMAT " during %satomic discovery ",
1105 (intptr_t)referent, (intptr_t)obj, da ? "" : "non-"));
1106 }
1107 #endif
1108
1109 // We mention two of several possible choices here:
1110 // #0: if the reference object is not in the "originating generation"
1111 // (or part of the heap being collected, indicated by our "span"
1112 // we don't treat it specially (i.e. we scan it as we would
1113 // a normal oop, treating its references as strong references).
1114 // This means that references can't be discovered unless their
1115 // referent is also in the same span. This is the simplest,
1116 // most "local" and most conservative approach, albeit one
1117 // that may cause weak references to be enqueued least promptly.
1118 // We call this choice the "ReferenceBasedDiscovery" policy.
1119 // #1: the reference object may be in any generation (span), but if
1120 // the referent is in the generation (span) being currently collected
1121 // then we can discover the reference object, provided
1122 // the object has not already been discovered by
1123 // a different concurrently running collector (as may be the
1124 // case, for instance, if the reference object is in CMS and
1125 // the referent in DefNewGeneration), and provided the processing
1126 // of this reference object by the current collector will
1127 // appear atomic to every other collector in the system.
1128 // (Thus, for instance, a concurrent collector may not
1129 // discover references in other generations even if the
1130 // referent is in its own generation). This policy may,
1131 // in certain cases, enqueue references somewhat sooner than
1132 // might Policy #0 above, but at marginally increased cost
1133 // and complexity in processing these references.
1134 // We call this choice the "RefeferentBasedDiscovery" policy.
1135 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1136 // Make sure we are discovering refs (rather than processing discovered refs).
1137 if (!_discovering_refs || !RegisterReferences) {
1138 return false;
1139 }
1140 // We only discover active references.
1141 oop next = java_lang_ref_Reference::next(obj);
1142 if (next != NULL) { // Ref is no longer active
1143 return false;
1144 }
1145
1146 HeapWord* obj_addr = (HeapWord*)obj;
1147 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1148 !_span.contains(obj_addr)) {
1149 // Reference is not in the originating generation;
1150 // don't treat it specially (i.e. we want to scan it as a normal
1151 // object with strong references).
1152 return false;
1153 }
1154
1155 // We only discover references whose referents are not (yet)
1156 // known to be strongly reachable.
1157 if (is_alive_non_header() != NULL) {
1158 verify_referent(obj);
1159 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1160 return false; // referent is reachable
1161 }
1162 }
1163 if (rt == REF_SOFT) {
1164 // For soft refs we can decide now if these are not
1165 // current candidates for clearing, in which case we
1166 // can mark through them now, rather than delaying that
1167 // to the reference-processing phase. Since all current
1168 // time-stamp policies advance the soft-ref clock only
1169 // at a major collection cycle, this is always currently
1170 // accurate.
1171 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1172 return false;
1173 }
1174 }
1175
1176 ResourceMark rm; // Needed for tracing.
1177
1178 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1179 const oop discovered = java_lang_ref_Reference::discovered(obj);
1180 assert(discovered->is_oop_or_null(), "bad discovered field");
1181 if (discovered != NULL) {
1182 // The reference has already been discovered...
1183 if (TraceReferenceGC) {
1184 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1185 obj, obj->klass()->internal_name());
1186 }
1187 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1188 // assumes that an object is not processed twice;
1189 // if it's been already discovered it must be on another
1190 // generation's discovered list; so we won't discover it.
1191 return false;
1192 } else {
1193 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1194 "Unrecognized policy");
1195 // Check assumption that an object is not potentially
1196 // discovered twice except by concurrent collectors that potentially
1197 // trace the same Reference object twice.
1198 assert(UseConcMarkSweepGC || UseG1GC,
1199 "Only possible with a concurrent marking collector");
1200 return true;
1201 }
1202 }
1203
1204 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1205 verify_referent(obj);
1206 // Discover if and only if EITHER:
1207 // .. reference is in our span, OR
1208 // .. we are an atomic collector and referent is in our span
1209 if (_span.contains(obj_addr) ||
1210 (discovery_is_atomic() &&
1211 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1212 // should_enqueue = true;
1213 } else {
1214 return false;
1215 }
1216 } else {
1217 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1218 _span.contains(obj_addr), "code inconsistency");
1219 }
1220
1221 // Get the right type of discovered queue head.
1222 DiscoveredList* list = get_discovered_list(rt);
1223 if (list == NULL) {
1224 return false; // nothing special needs to be done
1225 }
1226
1227 if (_discovery_is_mt) {
1228 add_to_discovered_list_mt(*list, obj, discovered_addr);
1229 } else {
1230 // If "_discovered_list_needs_barrier", we do write barriers when
1231 // updating the discovered reference list. Otherwise, we do a raw store
1232 // here: the field will be visited later when processing the discovered
1233 // references.
1234 oop current_head = list->head();
1235 // The last ref must have its discovered field pointing to itself.
1236 oop next_discovered = (current_head != NULL) ? current_head : obj;
1237
1238 // As in the case further above, since we are over-writing a NULL
1239 // pre-value, we can safely elide the pre-barrier here for the case of G1.
1240 // e.g.:- _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1241 assert(discovered == NULL, "control point invariant");
1242 assert(!_discovered_list_needs_barrier || UseG1GC,
1243 "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
1244 oop_store_raw(discovered_addr, next_discovered);
1245 if (_discovered_list_needs_barrier) {
1246 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1247 }
1248 list->set_head(obj);
1249 list->inc_length(1);
1250
1251 if (TraceReferenceGC) {
1252 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1253 obj, obj->klass()->internal_name());
1254 }
1255 }
1256 assert(obj->is_oop(), "Discovered a bad reference");
1257 verify_referent(obj);
1258 return true;
1259 }
1260
1261 // Preclean the discovered references by removing those
1262 // whose referents are alive, and by marking from those that
1263 // are not active. These lists can be handled here
1264 // in any order and, indeed, concurrently.
1265 void ReferenceProcessor::preclean_discovered_references(
1266 BoolObjectClosure* is_alive,
1267 OopClosure* keep_alive,
1268 VoidClosure* complete_gc,
1269 YieldClosure* yield) {
1270
1271 NOT_PRODUCT(verify_ok_to_handle_reflists());
1272
1273 // Soft references
1274 {
1275 TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1276 false, gclog_or_tty);
1277 for (uint i = 0; i < _max_num_q; i++) {
1278 if (yield->should_return()) {
1279 return;
1280 }
1281 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1282 keep_alive, complete_gc, yield);
1283 }
1284 }
1285
1286 // Weak references
1287 {
1288 TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1289 false, gclog_or_tty);
1290 for (uint i = 0; i < _max_num_q; i++) {
1291 if (yield->should_return()) {
1292 return;
1293 }
1294 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1295 keep_alive, complete_gc, yield);
1296 }
1297 }
1298
1299 // Final references
1300 {
1301 TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1302 false, gclog_or_tty);
1303 for (uint i = 0; i < _max_num_q; i++) {
1304 if (yield->should_return()) {
1305 return;
1306 }
1307 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1308 keep_alive, complete_gc, yield);
1309 }
1310 }
1311
1312 // Phantom references
1313 {
1314 TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1315 false, gclog_or_tty);
1316 for (uint i = 0; i < _max_num_q; i++) {
1317 if (yield->should_return()) {
1318 return;
1319 }
1320 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1321 keep_alive, complete_gc, yield);
1322 }
1323 }
1324 }
1325
1326 // Walk the given discovered ref list, and remove all reference objects
1327 // whose referents are still alive, whose referents are NULL or which
1328 // are not active (have a non-NULL next field). NOTE: When we are
1329 // thus precleaning the ref lists (which happens single-threaded today),
1330 // we do not disable refs discovery to honour the correct semantics of
1331 // java.lang.Reference. As a result, we need to be careful below
1332 // that ref removal steps interleave safely with ref discovery steps
1333 // (in this thread).
1334 void
1335 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1336 BoolObjectClosure* is_alive,
1337 OopClosure* keep_alive,
1338 VoidClosure* complete_gc,
1339 YieldClosure* yield) {
1340 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1341 while (iter.has_next()) {
1342 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1343 oop obj = iter.obj();
1344 oop next = java_lang_ref_Reference::next(obj);
1345 if (iter.referent() == NULL || iter.is_referent_alive() ||
1346 next != NULL) {
1347 // The referent has been cleared, or is alive, or the Reference is not
1348 // active; we need to trace and mark its cohort.
1349 if (TraceReferenceGC) {
1350 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1351 iter.obj(), iter.obj()->klass()->internal_name());
1352 }
1353 // Remove Reference object from list
1354 iter.remove();
1355 // Keep alive its cohort.
1356 iter.make_referent_alive();
1357 if (UseCompressedOops) {
1358 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1359 keep_alive->do_oop(next_addr);
1360 } else {
1361 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1362 keep_alive->do_oop(next_addr);
1363 }
1364 iter.move_to_next();
1365 } else {
1366 iter.next();
1367 }
1368 }
1369 // Close the reachable set
1370 complete_gc->do_void();
1371
1372 NOT_PRODUCT(
1373 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1374 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1375 "Refs in discovered list " INTPTR_FORMAT,
1376 iter.removed(), iter.processed(), (address)refs_list.head());
1377 }
1378 )
1379 }
1380
1381 const char* ReferenceProcessor::list_name(uint i) {
1382 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1383 "Out of bounds index");
1384
1385 int j = i / _max_num_q;
1386 switch (j) {
1387 case 0: return "SoftRef";
1388 case 1: return "WeakRef";
1389 case 2: return "FinalRef";
1390 case 3: return "PhantomRef";
1391 }
1392 ShouldNotReachHere();
1393 return NULL;
1394 }
1395
1396 #ifndef PRODUCT
1397 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1398 // empty for now
1399 }
1400 #endif
1401
1402 #ifndef PRODUCT
1403 void ReferenceProcessor::clear_discovered_references() {
1404 guarantee(!_discovering_refs, "Discovering refs?");
1405 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1406 clear_discovered_references(_discovered_refs[i]);
1407 }
1408 }
1409
1410 #endif // PRODUCT