Fri, 26 Sep 2014 17:48:10 -0400
8047125: (ref) More phantom object references
Reviewed-by: mchung, dfuchs, ahgross, jmasa, brutisso, mgerdin
Contributed-by: kim.barrett@oracle.com
1 /*
2 * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
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23 */
25 #include "precompiled.hpp"
26 #include "classfile/javaClasses.hpp"
27 #include "classfile/systemDictionary.hpp"
28 #include "gc_implementation/shared/gcTimer.hpp"
29 #include "gc_implementation/shared/gcTraceTime.hpp"
30 #include "gc_interface/collectedHeap.hpp"
31 #include "gc_interface/collectedHeap.inline.hpp"
32 #include "memory/referencePolicy.hpp"
33 #include "memory/referenceProcessor.hpp"
34 #include "oops/oop.inline.hpp"
35 #include "runtime/java.hpp"
36 #include "runtime/jniHandles.hpp"
38 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
40 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
41 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
42 bool ReferenceProcessor::_pending_list_uses_discovered_field = false;
43 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
45 void referenceProcessor_init() {
46 ReferenceProcessor::init_statics();
47 }
49 void ReferenceProcessor::init_statics() {
50 // We need a monotonically non-deccreasing time in ms but
51 // os::javaTimeMillis() does not guarantee monotonicity.
52 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
54 // Initialize the soft ref timestamp clock.
55 _soft_ref_timestamp_clock = now;
56 // Also update the soft ref clock in j.l.r.SoftReference
57 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
59 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
60 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
61 NOT_COMPILER2(LRUCurrentHeapPolicy());
62 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
63 vm_exit_during_initialization("Could not allocate reference policy object");
64 }
65 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
66 RefDiscoveryPolicy == ReferentBasedDiscovery,
67 "Unrecongnized RefDiscoveryPolicy");
68 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
69 }
71 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
72 #ifdef ASSERT
73 // Verify that we're not currently discovering refs
74 assert(!verify_disabled || !_discovering_refs, "nested call?");
76 if (check_no_refs) {
77 // Verify that the discovered lists are empty
78 verify_no_references_recorded();
79 }
80 #endif // ASSERT
82 // Someone could have modified the value of the static
83 // field in the j.l.r.SoftReference class that holds the
84 // soft reference timestamp clock using reflection or
85 // Unsafe between GCs. Unconditionally update the static
86 // field in ReferenceProcessor here so that we use the new
87 // value during reference discovery.
89 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
90 _discovering_refs = true;
91 }
93 ReferenceProcessor::ReferenceProcessor(MemRegion span,
94 bool mt_processing,
95 uint mt_processing_degree,
96 bool mt_discovery,
97 uint mt_discovery_degree,
98 bool atomic_discovery,
99 BoolObjectClosure* is_alive_non_header) :
100 _discovering_refs(false),
101 _enqueuing_is_done(false),
102 _is_alive_non_header(is_alive_non_header),
103 _processing_is_mt(mt_processing),
104 _next_id(0)
105 {
106 _span = span;
107 _discovery_is_atomic = atomic_discovery;
108 _discovery_is_mt = mt_discovery;
109 _num_q = MAX2(1U, mt_processing_degree);
110 _max_num_q = MAX2(_num_q, mt_discovery_degree);
111 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList,
112 _max_num_q * number_of_subclasses_of_ref(), mtGC);
114 if (_discovered_refs == NULL) {
115 vm_exit_during_initialization("Could not allocated RefProc Array");
116 }
117 _discoveredSoftRefs = &_discovered_refs[0];
118 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
119 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
120 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
121 _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q];
123 // Initialize all entries to NULL
124 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
125 _discovered_refs[i].set_head(NULL);
126 _discovered_refs[i].set_length(0);
127 }
129 setup_policy(false /* default soft ref policy */);
130 }
132 #ifndef PRODUCT
133 void ReferenceProcessor::verify_no_references_recorded() {
134 guarantee(!_discovering_refs, "Discovering refs?");
135 for (uint 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
142 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
143 for (uint 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 }
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.
156 // We need a monotonically non-deccreasing time in ms but
157 // os::javaTimeMillis() does not guarantee monotonicity.
158 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
159 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
160 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
162 NOT_PRODUCT(
163 if (now < _soft_ref_timestamp_clock) {
164 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
165 _soft_ref_timestamp_clock, now);
166 }
167 )
168 // The values of now and _soft_ref_timestamp_clock are set using
169 // javaTimeNanos(), which is guaranteed to be monotonically
170 // non-decreasing provided the underlying platform provides such
171 // a time source (and it is bug free).
172 // In product mode, however, protect ourselves from non-monotonicty.
173 if (now > _soft_ref_timestamp_clock) {
174 _soft_ref_timestamp_clock = now;
175 java_lang_ref_SoftReference::set_clock(now);
176 }
177 // Else leave clock stalled at its old value until time progresses
178 // past clock value.
179 }
181 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
182 size_t total = 0;
183 for (uint i = 0; i < _max_num_q; ++i) {
184 total += lists[i].length();
185 }
186 return total;
187 }
189 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
190 BoolObjectClosure* is_alive,
191 OopClosure* keep_alive,
192 VoidClosure* complete_gc,
193 AbstractRefProcTaskExecutor* task_executor,
194 GCTimer* gc_timer) {
195 NOT_PRODUCT(verify_ok_to_handle_reflists());
197 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
198 // Stop treating discovered references specially.
199 disable_discovery();
201 // If discovery was concurrent, someone could have modified
202 // the value of the static field in the j.l.r.SoftReference
203 // class that holds the soft reference timestamp clock using
204 // reflection or Unsafe between when discovery was enabled and
205 // now. Unconditionally update the static field in ReferenceProcessor
206 // here so that we use the new value during processing of the
207 // discovered soft refs.
209 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
211 bool trace_time = PrintGCDetails && PrintReferenceGC;
213 // Soft references
214 size_t soft_count = 0;
215 {
216 GCTraceTime tt("SoftReference", trace_time, false, gc_timer);
217 soft_count =
218 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
219 is_alive, keep_alive, complete_gc, task_executor);
220 }
222 update_soft_ref_master_clock();
224 // Weak references
225 size_t weak_count = 0;
226 {
227 GCTraceTime tt("WeakReference", trace_time, false, gc_timer);
228 weak_count =
229 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
230 is_alive, keep_alive, complete_gc, task_executor);
231 }
233 // Final references
234 size_t final_count = 0;
235 {
236 GCTraceTime tt("FinalReference", trace_time, false, gc_timer);
237 final_count =
238 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
239 is_alive, keep_alive, complete_gc, task_executor);
240 }
242 // Phantom references
243 size_t phantom_count = 0;
244 {
245 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer);
246 phantom_count =
247 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
248 is_alive, keep_alive, complete_gc, task_executor);
250 // Process cleaners, but include them in phantom statistics. We expect
251 // Cleaner references to be temporary, and don't want to deal with
252 // possible incompatibilities arising from making it more visible.
253 phantom_count +=
254 process_discovered_reflist(_discoveredCleanerRefs, NULL, false,
255 is_alive, keep_alive, complete_gc, task_executor);
256 }
258 // Weak global JNI references. It would make more sense (semantically) to
259 // traverse these simultaneously with the regular weak references above, but
260 // that is not how the JDK1.2 specification is. See #4126360. Native code can
261 // thus use JNI weak references to circumvent the phantom references and
262 // resurrect a "post-mortem" object.
263 {
264 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
265 if (task_executor != NULL) {
266 task_executor->set_single_threaded_mode();
267 }
268 process_phaseJNI(is_alive, keep_alive, complete_gc);
269 }
271 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
272 }
274 #ifndef PRODUCT
275 // Calculate the number of jni handles.
276 uint ReferenceProcessor::count_jni_refs() {
277 class AlwaysAliveClosure: public BoolObjectClosure {
278 public:
279 virtual bool do_object_b(oop obj) { return true; }
280 };
282 class CountHandleClosure: public OopClosure {
283 private:
284 int _count;
285 public:
286 CountHandleClosure(): _count(0) {}
287 void do_oop(oop* unused) { _count++; }
288 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
289 int count() { return _count; }
290 };
291 CountHandleClosure global_handle_count;
292 AlwaysAliveClosure always_alive;
293 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
294 return global_handle_count.count();
295 }
296 #endif
298 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
299 OopClosure* keep_alive,
300 VoidClosure* complete_gc) {
301 #ifndef PRODUCT
302 if (PrintGCDetails && PrintReferenceGC) {
303 unsigned int count = count_jni_refs();
304 gclog_or_tty->print(", %u refs", count);
305 }
306 #endif
307 JNIHandles::weak_oops_do(is_alive, keep_alive);
308 complete_gc->do_void();
309 }
312 template <class T>
313 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
314 AbstractRefProcTaskExecutor* task_executor) {
316 // Remember old value of pending references list
317 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
318 T old_pending_list_value = *pending_list_addr;
320 // Enqueue references that are not made active again, and
321 // clear the decks for the next collection (cycle).
322 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
323 // Do the post-barrier on pending_list_addr missed in
324 // enqueue_discovered_reflist.
325 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
327 // Stop treating discovered references specially.
328 ref->disable_discovery();
330 // Return true if new pending references were added
331 return old_pending_list_value != *pending_list_addr;
332 }
334 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
335 NOT_PRODUCT(verify_ok_to_handle_reflists());
336 if (UseCompressedOops) {
337 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
338 } else {
339 return enqueue_discovered_ref_helper<oop>(this, task_executor);
340 }
341 }
343 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
344 HeapWord* pending_list_addr) {
345 // Given a list of refs linked through the "discovered" field
346 // (java.lang.ref.Reference.discovered), self-loop their "next" field
347 // thus distinguishing them from active References, then
348 // prepend them to the pending list.
349 //
350 // The Java threads will see the Reference objects linked together through
351 // the discovered field. Instead of trying to do the write barrier updates
352 // in all places in the reference processor where we manipulate the discovered
353 // field we make sure to do the barrier here where we anyway iterate through
354 // all linked Reference objects. Note that it is important to not dirty any
355 // cards during reference processing since this will cause card table
356 // verification to fail for G1.
357 //
358 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
359 // the "next" field is used to chain the pending list, not the discovered
360 // field.
361 if (TraceReferenceGC && PrintGCDetails) {
362 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
363 INTPTR_FORMAT, (address)refs_list.head());
364 }
366 oop obj = NULL;
367 oop next_d = refs_list.head();
368 if (pending_list_uses_discovered_field()) { // New behavior
369 // Walk down the list, self-looping the next field
370 // so that the References are not considered active.
371 while (obj != next_d) {
372 obj = next_d;
373 assert(obj->is_instanceRef(), "should be reference object");
374 next_d = java_lang_ref_Reference::discovered(obj);
375 if (TraceReferenceGC && PrintGCDetails) {
376 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
377 (void *)obj, (void *)next_d);
378 }
379 assert(java_lang_ref_Reference::next(obj) == NULL,
380 "Reference not active; should not be discovered");
381 // Self-loop next, so as to make Ref not active.
382 java_lang_ref_Reference::set_next_raw(obj, obj);
383 if (next_d != obj) {
384 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
385 } else {
386 // This is the last object.
387 // Swap refs_list into pending_list_addr and
388 // set obj's discovered to what we read from pending_list_addr.
389 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
390 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
391 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
392 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
393 }
394 }
395 } else { // Old behaviour
396 // Walk down the list, copying the discovered field into
397 // the next field and clearing the discovered field.
398 while (obj != next_d) {
399 obj = next_d;
400 assert(obj->is_instanceRef(), "should be reference object");
401 next_d = java_lang_ref_Reference::discovered(obj);
402 if (TraceReferenceGC && PrintGCDetails) {
403 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
404 (void *)obj, (void *)next_d);
405 }
406 assert(java_lang_ref_Reference::next(obj) == NULL,
407 "The reference should not be enqueued");
408 if (next_d == obj) { // obj is last
409 // Swap refs_list into pendling_list_addr and
410 // set obj's next to what we read from pending_list_addr.
411 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
412 // Need oop_check on pending_list_addr above;
413 // see special oop-check code at the end of
414 // enqueue_discovered_reflists() further below.
415 if (old == NULL) {
416 // obj should be made to point to itself, since
417 // pending list was empty.
418 java_lang_ref_Reference::set_next(obj, obj);
419 } else {
420 java_lang_ref_Reference::set_next(obj, old);
421 }
422 } else {
423 java_lang_ref_Reference::set_next(obj, next_d);
424 }
425 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
426 }
427 }
428 }
430 // Parallel enqueue task
431 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
432 public:
433 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
434 DiscoveredList discovered_refs[],
435 HeapWord* pending_list_addr,
436 int n_queues)
437 : EnqueueTask(ref_processor, discovered_refs,
438 pending_list_addr, n_queues)
439 { }
441 virtual void work(unsigned int work_id) {
442 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
443 // Simplest first cut: static partitioning.
444 int index = work_id;
445 // The increment on "index" must correspond to the maximum number of queues
446 // (n_queues) with which that ReferenceProcessor was created. That
447 // is because of the "clever" way the discovered references lists were
448 // allocated and are indexed into.
449 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
450 for (int j = 0;
451 j < ReferenceProcessor::number_of_subclasses_of_ref();
452 j++, index += _n_queues) {
453 _ref_processor.enqueue_discovered_reflist(
454 _refs_lists[index], _pending_list_addr);
455 _refs_lists[index].set_head(NULL);
456 _refs_lists[index].set_length(0);
457 }
458 }
459 };
461 // Enqueue references that are not made active again
462 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
463 AbstractRefProcTaskExecutor* task_executor) {
464 if (_processing_is_mt && task_executor != NULL) {
465 // Parallel code
466 RefProcEnqueueTask tsk(*this, _discovered_refs,
467 pending_list_addr, _max_num_q);
468 task_executor->execute(tsk);
469 } else {
470 // Serial code: call the parent class's implementation
471 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
472 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
473 _discovered_refs[i].set_head(NULL);
474 _discovered_refs[i].set_length(0);
475 }
476 }
477 }
479 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
480 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
481 oop discovered = java_lang_ref_Reference::discovered(_ref);
482 assert(_discovered_addr && discovered->is_oop_or_null(),
483 "discovered field is bad");
484 _next = discovered;
485 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
486 _referent = java_lang_ref_Reference::referent(_ref);
487 assert(Universe::heap()->is_in_reserved_or_null(_referent),
488 "Wrong oop found in java.lang.Reference object");
489 assert(allow_null_referent ?
490 _referent->is_oop_or_null()
491 : _referent->is_oop(),
492 "bad referent");
493 }
495 void DiscoveredListIterator::remove() {
496 assert(_ref->is_oop(), "Dropping a bad reference");
497 oop_store_raw(_discovered_addr, NULL);
499 // First _prev_next ref actually points into DiscoveredList (gross).
500 oop new_next;
501 if (_next == _ref) {
502 // At the end of the list, we should make _prev point to itself.
503 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
504 // and _prev will be NULL.
505 new_next = _prev;
506 } else {
507 new_next = _next;
508 }
509 // Remove Reference object from discovered list. Note that G1 does not need a
510 // pre-barrier here because we know the Reference has already been found/marked,
511 // that's how it ended up in the discovered list in the first place.
512 oop_store_raw(_prev_next, new_next);
513 NOT_PRODUCT(_removed++);
514 _refs_list.dec_length(1);
515 }
517 // Make the Reference object active again.
518 void DiscoveredListIterator::make_active() {
519 // The pre barrier for G1 is probably just needed for the old
520 // reference processing behavior. Should we guard this with
521 // ReferenceProcessor::pending_list_uses_discovered_field() ?
522 if (UseG1GC) {
523 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
524 if (UseCompressedOops) {
525 oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
526 } else {
527 oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
528 }
529 }
530 java_lang_ref_Reference::set_next_raw(_ref, NULL);
531 }
533 void DiscoveredListIterator::clear_referent() {
534 oop_store_raw(_referent_addr, NULL);
535 }
537 // NOTE: process_phase*() are largely similar, and at a high level
538 // merely iterate over the extant list applying a predicate to
539 // each of its elements and possibly removing that element from the
540 // list and applying some further closures to that element.
541 // We should consider the possibility of replacing these
542 // process_phase*() methods by abstracting them into
543 // a single general iterator invocation that receives appropriate
544 // closures that accomplish this work.
546 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
547 // referents are not alive, but that should be kept alive for policy reasons.
548 // Keep alive the transitive closure of all such referents.
549 void
550 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
551 ReferencePolicy* policy,
552 BoolObjectClosure* is_alive,
553 OopClosure* keep_alive,
554 VoidClosure* complete_gc) {
555 assert(policy != NULL, "Must have a non-NULL policy");
556 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
557 // Decide which softly reachable refs should be kept alive.
558 while (iter.has_next()) {
559 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
560 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
561 if (referent_is_dead &&
562 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
563 if (TraceReferenceGC) {
564 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
565 (void *)iter.obj(), iter.obj()->klass()->internal_name());
566 }
567 // Remove Reference object from list
568 iter.remove();
569 // Make the Reference object active again
570 iter.make_active();
571 // keep the referent around
572 iter.make_referent_alive();
573 iter.move_to_next();
574 } else {
575 iter.next();
576 }
577 }
578 // Close the reachable set
579 complete_gc->do_void();
580 NOT_PRODUCT(
581 if (PrintGCDetails && TraceReferenceGC) {
582 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
583 "discovered Refs by policy, from list " INTPTR_FORMAT,
584 iter.removed(), iter.processed(), (address)refs_list.head());
585 }
586 )
587 }
589 // Traverse the list and remove any Refs that are not active, or
590 // whose referents are either alive or NULL.
591 void
592 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
593 BoolObjectClosure* is_alive,
594 OopClosure* keep_alive) {
595 assert(discovery_is_atomic(), "Error");
596 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
597 while (iter.has_next()) {
598 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
599 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
600 assert(next == NULL, "Should not discover inactive Reference");
601 if (iter.is_referent_alive()) {
602 if (TraceReferenceGC) {
603 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
604 (void *)iter.obj(), iter.obj()->klass()->internal_name());
605 }
606 // The referent is reachable after all.
607 // Remove Reference object from list.
608 iter.remove();
609 // Update the referent pointer as necessary: Note that this
610 // should not entail any recursive marking because the
611 // referent must already have been traversed.
612 iter.make_referent_alive();
613 iter.move_to_next();
614 } else {
615 iter.next();
616 }
617 }
618 NOT_PRODUCT(
619 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
620 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
621 "Refs in discovered list " INTPTR_FORMAT,
622 iter.removed(), iter.processed(), (address)refs_list.head());
623 }
624 )
625 }
627 void
628 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
629 BoolObjectClosure* is_alive,
630 OopClosure* keep_alive,
631 VoidClosure* complete_gc) {
632 assert(!discovery_is_atomic(), "Error");
633 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
634 while (iter.has_next()) {
635 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
636 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
637 oop next = java_lang_ref_Reference::next(iter.obj());
638 if ((iter.referent() == NULL || iter.is_referent_alive() ||
639 next != NULL)) {
640 assert(next->is_oop_or_null(), "bad next field");
641 // Remove Reference object from list
642 iter.remove();
643 // Trace the cohorts
644 iter.make_referent_alive();
645 if (UseCompressedOops) {
646 keep_alive->do_oop((narrowOop*)next_addr);
647 } else {
648 keep_alive->do_oop((oop*)next_addr);
649 }
650 iter.move_to_next();
651 } else {
652 iter.next();
653 }
654 }
655 // Now close the newly reachable set
656 complete_gc->do_void();
657 NOT_PRODUCT(
658 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
659 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
660 "Refs in discovered list " INTPTR_FORMAT,
661 iter.removed(), iter.processed(), (address)refs_list.head());
662 }
663 )
664 }
666 // Traverse the list and process the referents, by either
667 // clearing them or keeping them (and their reachable
668 // closure) alive.
669 void
670 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
671 bool clear_referent,
672 BoolObjectClosure* is_alive,
673 OopClosure* keep_alive,
674 VoidClosure* complete_gc) {
675 ResourceMark rm;
676 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
677 while (iter.has_next()) {
678 iter.update_discovered();
679 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
680 if (clear_referent) {
681 // NULL out referent pointer
682 iter.clear_referent();
683 } else {
684 // keep the referent around
685 iter.make_referent_alive();
686 }
687 if (TraceReferenceGC) {
688 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
689 clear_referent ? "cleared " : "",
690 (void *)iter.obj(), iter.obj()->klass()->internal_name());
691 }
692 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
693 iter.next();
694 }
695 // Remember to update the next pointer of the last ref.
696 iter.update_discovered();
697 // Close the reachable set
698 complete_gc->do_void();
699 }
701 void
702 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
703 oop obj = NULL;
704 oop next = refs_list.head();
705 while (next != obj) {
706 obj = next;
707 next = java_lang_ref_Reference::discovered(obj);
708 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
709 }
710 refs_list.set_head(NULL);
711 refs_list.set_length(0);
712 }
714 void
715 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
716 clear_discovered_references(refs_list);
717 }
719 void ReferenceProcessor::abandon_partial_discovery() {
720 // loop over the lists
721 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
722 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
723 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
724 }
725 abandon_partial_discovered_list(_discovered_refs[i]);
726 }
727 }
729 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
730 public:
731 RefProcPhase1Task(ReferenceProcessor& ref_processor,
732 DiscoveredList refs_lists[],
733 ReferencePolicy* policy,
734 bool marks_oops_alive)
735 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
736 _policy(policy)
737 { }
738 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
739 OopClosure& keep_alive,
740 VoidClosure& complete_gc)
741 {
742 Thread* thr = Thread::current();
743 int refs_list_index = ((WorkerThread*)thr)->id();
744 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
745 &is_alive, &keep_alive, &complete_gc);
746 }
747 private:
748 ReferencePolicy* _policy;
749 };
751 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
752 public:
753 RefProcPhase2Task(ReferenceProcessor& ref_processor,
754 DiscoveredList refs_lists[],
755 bool marks_oops_alive)
756 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
757 { }
758 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
759 OopClosure& keep_alive,
760 VoidClosure& complete_gc)
761 {
762 _ref_processor.process_phase2(_refs_lists[i],
763 &is_alive, &keep_alive, &complete_gc);
764 }
765 };
767 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
768 public:
769 RefProcPhase3Task(ReferenceProcessor& ref_processor,
770 DiscoveredList refs_lists[],
771 bool clear_referent,
772 bool marks_oops_alive)
773 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
774 _clear_referent(clear_referent)
775 { }
776 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
777 OopClosure& keep_alive,
778 VoidClosure& complete_gc)
779 {
780 // Don't use "refs_list_index" calculated in this way because
781 // balance_queues() has moved the Ref's into the first n queues.
782 // Thread* thr = Thread::current();
783 // int refs_list_index = ((WorkerThread*)thr)->id();
784 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
785 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
786 &is_alive, &keep_alive, &complete_gc);
787 }
788 private:
789 bool _clear_referent;
790 };
792 // Balances reference queues.
793 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
794 // queues[0, 1, ..., _num_q-1] because only the first _num_q
795 // corresponding to the active workers will be processed.
796 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
797 {
798 // calculate total length
799 size_t total_refs = 0;
800 if (TraceReferenceGC && PrintGCDetails) {
801 gclog_or_tty->print_cr("\nBalance ref_lists ");
802 }
804 for (uint i = 0; i < _max_num_q; ++i) {
805 total_refs += ref_lists[i].length();
806 if (TraceReferenceGC && PrintGCDetails) {
807 gclog_or_tty->print("%d ", ref_lists[i].length());
808 }
809 }
810 if (TraceReferenceGC && PrintGCDetails) {
811 gclog_or_tty->print_cr(" = %d", total_refs);
812 }
813 size_t avg_refs = total_refs / _num_q + 1;
814 uint to_idx = 0;
815 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
816 bool move_all = false;
817 if (from_idx >= _num_q) {
818 move_all = ref_lists[from_idx].length() > 0;
819 }
820 while ((ref_lists[from_idx].length() > avg_refs) ||
821 move_all) {
822 assert(to_idx < _num_q, "Sanity Check!");
823 if (ref_lists[to_idx].length() < avg_refs) {
824 // move superfluous refs
825 size_t refs_to_move;
826 // Move all the Ref's if the from queue will not be processed.
827 if (move_all) {
828 refs_to_move = MIN2(ref_lists[from_idx].length(),
829 avg_refs - ref_lists[to_idx].length());
830 } else {
831 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
832 avg_refs - ref_lists[to_idx].length());
833 }
835 assert(refs_to_move > 0, "otherwise the code below will fail");
837 oop move_head = ref_lists[from_idx].head();
838 oop move_tail = move_head;
839 oop new_head = move_head;
840 // find an element to split the list on
841 for (size_t j = 0; j < refs_to_move; ++j) {
842 move_tail = new_head;
843 new_head = java_lang_ref_Reference::discovered(new_head);
844 }
846 // Add the chain to the to list.
847 if (ref_lists[to_idx].head() == NULL) {
848 // to list is empty. Make a loop at the end.
849 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
850 } else {
851 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
852 }
853 ref_lists[to_idx].set_head(move_head);
854 ref_lists[to_idx].inc_length(refs_to_move);
856 // Remove the chain from the from list.
857 if (move_tail == new_head) {
858 // We found the end of the from list.
859 ref_lists[from_idx].set_head(NULL);
860 } else {
861 ref_lists[from_idx].set_head(new_head);
862 }
863 ref_lists[from_idx].dec_length(refs_to_move);
864 if (ref_lists[from_idx].length() == 0) {
865 break;
866 }
867 } else {
868 to_idx = (to_idx + 1) % _num_q;
869 }
870 }
871 }
872 #ifdef ASSERT
873 size_t balanced_total_refs = 0;
874 for (uint i = 0; i < _max_num_q; ++i) {
875 balanced_total_refs += ref_lists[i].length();
876 if (TraceReferenceGC && PrintGCDetails) {
877 gclog_or_tty->print("%d ", ref_lists[i].length());
878 }
879 }
880 if (TraceReferenceGC && PrintGCDetails) {
881 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
882 gclog_or_tty->flush();
883 }
884 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
885 #endif
886 }
888 void ReferenceProcessor::balance_all_queues() {
889 balance_queues(_discoveredSoftRefs);
890 balance_queues(_discoveredWeakRefs);
891 balance_queues(_discoveredFinalRefs);
892 balance_queues(_discoveredPhantomRefs);
893 balance_queues(_discoveredCleanerRefs);
894 }
896 size_t
897 ReferenceProcessor::process_discovered_reflist(
898 DiscoveredList refs_lists[],
899 ReferencePolicy* policy,
900 bool clear_referent,
901 BoolObjectClosure* is_alive,
902 OopClosure* keep_alive,
903 VoidClosure* complete_gc,
904 AbstractRefProcTaskExecutor* task_executor)
905 {
906 bool mt_processing = task_executor != NULL && _processing_is_mt;
907 // If discovery used MT and a dynamic number of GC threads, then
908 // the queues must be balanced for correctness if fewer than the
909 // maximum number of queues were used. The number of queue used
910 // during discovery may be different than the number to be used
911 // for processing so don't depend of _num_q < _max_num_q as part
912 // of the test.
913 bool must_balance = _discovery_is_mt;
915 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
916 must_balance) {
917 balance_queues(refs_lists);
918 }
920 size_t total_list_count = total_count(refs_lists);
922 if (PrintReferenceGC && PrintGCDetails) {
923 gclog_or_tty->print(", %u refs", total_list_count);
924 }
926 // Phase 1 (soft refs only):
927 // . Traverse the list and remove any SoftReferences whose
928 // referents are not alive, but that should be kept alive for
929 // policy reasons. Keep alive the transitive closure of all
930 // such referents.
931 if (policy != NULL) {
932 if (mt_processing) {
933 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
934 task_executor->execute(phase1);
935 } else {
936 for (uint i = 0; i < _max_num_q; i++) {
937 process_phase1(refs_lists[i], policy,
938 is_alive, keep_alive, complete_gc);
939 }
940 }
941 } else { // policy == NULL
942 assert(refs_lists != _discoveredSoftRefs,
943 "Policy must be specified for soft references.");
944 }
946 // Phase 2:
947 // . Traverse the list and remove any refs whose referents are alive.
948 if (mt_processing) {
949 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
950 task_executor->execute(phase2);
951 } else {
952 for (uint i = 0; i < _max_num_q; i++) {
953 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
954 }
955 }
957 // Phase 3:
958 // . Traverse the list and process referents as appropriate.
959 if (mt_processing) {
960 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
961 task_executor->execute(phase3);
962 } else {
963 for (uint i = 0; i < _max_num_q; i++) {
964 process_phase3(refs_lists[i], clear_referent,
965 is_alive, keep_alive, complete_gc);
966 }
967 }
969 return total_list_count;
970 }
972 void ReferenceProcessor::clean_up_discovered_references() {
973 // loop over the lists
974 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
975 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
976 gclog_or_tty->print_cr(
977 "\nScrubbing %s discovered list of Null referents",
978 list_name(i));
979 }
980 clean_up_discovered_reflist(_discovered_refs[i]);
981 }
982 }
984 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
985 assert(!discovery_is_atomic(), "Else why call this method?");
986 DiscoveredListIterator iter(refs_list, NULL, NULL);
987 while (iter.has_next()) {
988 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
989 oop next = java_lang_ref_Reference::next(iter.obj());
990 assert(next->is_oop_or_null(), "bad next field");
991 // If referent has been cleared or Reference is not active,
992 // drop it.
993 if (iter.referent() == NULL || next != NULL) {
994 debug_only(
995 if (PrintGCDetails && TraceReferenceGC) {
996 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
997 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
998 " and referent: " INTPTR_FORMAT,
999 (void *)iter.obj(), (void *)next, (void *)iter.referent());
1000 }
1001 )
1002 // Remove Reference object from list
1003 iter.remove();
1004 iter.move_to_next();
1005 } else {
1006 iter.next();
1007 }
1008 }
1009 NOT_PRODUCT(
1010 if (PrintGCDetails && TraceReferenceGC) {
1011 gclog_or_tty->print(
1012 " Removed %d Refs with NULL referents out of %d discovered Refs",
1013 iter.removed(), iter.processed());
1014 }
1015 )
1016 }
1018 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1019 uint id = 0;
1020 // Determine the queue index to use for this object.
1021 if (_discovery_is_mt) {
1022 // During a multi-threaded discovery phase,
1023 // each thread saves to its "own" list.
1024 Thread* thr = Thread::current();
1025 id = thr->as_Worker_thread()->id();
1026 } else {
1027 // single-threaded discovery, we save in round-robin
1028 // fashion to each of the lists.
1029 if (_processing_is_mt) {
1030 id = next_id();
1031 }
1032 }
1033 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1035 // Get the discovered queue to which we will add
1036 DiscoveredList* list = NULL;
1037 switch (rt) {
1038 case REF_OTHER:
1039 // Unknown reference type, no special treatment
1040 break;
1041 case REF_SOFT:
1042 list = &_discoveredSoftRefs[id];
1043 break;
1044 case REF_WEAK:
1045 list = &_discoveredWeakRefs[id];
1046 break;
1047 case REF_FINAL:
1048 list = &_discoveredFinalRefs[id];
1049 break;
1050 case REF_PHANTOM:
1051 list = &_discoveredPhantomRefs[id];
1052 break;
1053 case REF_CLEANER:
1054 list = &_discoveredCleanerRefs[id];
1055 break;
1056 case REF_NONE:
1057 // we should not reach here if we are an InstanceRefKlass
1058 default:
1059 ShouldNotReachHere();
1060 }
1061 if (TraceReferenceGC && PrintGCDetails) {
1062 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1063 }
1064 return list;
1065 }
1067 inline void
1068 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1069 oop obj,
1070 HeapWord* discovered_addr) {
1071 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1072 // First we must make sure this object is only enqueued once. CAS in a non null
1073 // discovered_addr.
1074 oop current_head = refs_list.head();
1075 // The last ref must have its discovered field pointing to itself.
1076 oop next_discovered = (current_head != NULL) ? current_head : obj;
1078 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1079 NULL);
1080 if (retest == NULL) {
1081 // This thread just won the right to enqueue the object.
1082 // We have separate lists for enqueueing, so no synchronization
1083 // is necessary.
1084 refs_list.set_head(obj);
1085 refs_list.inc_length(1);
1087 if (TraceReferenceGC) {
1088 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1089 (void *)obj, obj->klass()->internal_name());
1090 }
1091 } else {
1092 // If retest was non NULL, another thread beat us to it:
1093 // The reference has already been discovered...
1094 if (TraceReferenceGC) {
1095 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1096 (void *)obj, obj->klass()->internal_name());
1097 }
1098 }
1099 }
1101 #ifndef PRODUCT
1102 // Non-atomic (i.e. concurrent) discovery might allow us
1103 // to observe j.l.References with NULL referents, being those
1104 // cleared concurrently by mutators during (or after) discovery.
1105 void ReferenceProcessor::verify_referent(oop obj) {
1106 bool da = discovery_is_atomic();
1107 oop referent = java_lang_ref_Reference::referent(obj);
1108 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1109 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1110 INTPTR_FORMAT " during %satomic discovery ",
1111 (void *)referent, (void *)obj, da ? "" : "non-"));
1112 }
1113 #endif
1115 // We mention two of several possible choices here:
1116 // #0: if the reference object is not in the "originating generation"
1117 // (or part of the heap being collected, indicated by our "span"
1118 // we don't treat it specially (i.e. we scan it as we would
1119 // a normal oop, treating its references as strong references).
1120 // This means that references can't be discovered unless their
1121 // referent is also in the same span. This is the simplest,
1122 // most "local" and most conservative approach, albeit one
1123 // that may cause weak references to be enqueued least promptly.
1124 // We call this choice the "ReferenceBasedDiscovery" policy.
1125 // #1: the reference object may be in any generation (span), but if
1126 // the referent is in the generation (span) being currently collected
1127 // then we can discover the reference object, provided
1128 // the object has not already been discovered by
1129 // a different concurrently running collector (as may be the
1130 // case, for instance, if the reference object is in CMS and
1131 // the referent in DefNewGeneration), and provided the processing
1132 // of this reference object by the current collector will
1133 // appear atomic to every other collector in the system.
1134 // (Thus, for instance, a concurrent collector may not
1135 // discover references in other generations even if the
1136 // referent is in its own generation). This policy may,
1137 // in certain cases, enqueue references somewhat sooner than
1138 // might Policy #0 above, but at marginally increased cost
1139 // and complexity in processing these references.
1140 // We call this choice the "RefeferentBasedDiscovery" policy.
1141 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1142 // Make sure we are discovering refs (rather than processing discovered refs).
1143 if (!_discovering_refs || !RegisterReferences) {
1144 return false;
1145 }
1146 // We only discover active references.
1147 oop next = java_lang_ref_Reference::next(obj);
1148 if (next != NULL) { // Ref is no longer active
1149 return false;
1150 }
1152 HeapWord* obj_addr = (HeapWord*)obj;
1153 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1154 !_span.contains(obj_addr)) {
1155 // Reference is not in the originating generation;
1156 // don't treat it specially (i.e. we want to scan it as a normal
1157 // object with strong references).
1158 return false;
1159 }
1161 // We only discover references whose referents are not (yet)
1162 // known to be strongly reachable.
1163 if (is_alive_non_header() != NULL) {
1164 verify_referent(obj);
1165 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1166 return false; // referent is reachable
1167 }
1168 }
1169 if (rt == REF_SOFT) {
1170 // For soft refs we can decide now if these are not
1171 // current candidates for clearing, in which case we
1172 // can mark through them now, rather than delaying that
1173 // to the reference-processing phase. Since all current
1174 // time-stamp policies advance the soft-ref clock only
1175 // at a major collection cycle, this is always currently
1176 // accurate.
1177 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1178 return false;
1179 }
1180 }
1182 ResourceMark rm; // Needed for tracing.
1184 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1185 const oop discovered = java_lang_ref_Reference::discovered(obj);
1186 assert(discovered->is_oop_or_null(), "bad discovered field");
1187 if (discovered != NULL) {
1188 // The reference has already been discovered...
1189 if (TraceReferenceGC) {
1190 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1191 (void *)obj, obj->klass()->internal_name());
1192 }
1193 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1194 // assumes that an object is not processed twice;
1195 // if it's been already discovered it must be on another
1196 // generation's discovered list; so we won't discover it.
1197 return false;
1198 } else {
1199 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1200 "Unrecognized policy");
1201 // Check assumption that an object is not potentially
1202 // discovered twice except by concurrent collectors that potentially
1203 // trace the same Reference object twice.
1204 assert(UseConcMarkSweepGC || UseG1GC,
1205 "Only possible with a concurrent marking collector");
1206 return true;
1207 }
1208 }
1210 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1211 verify_referent(obj);
1212 // Discover if and only if EITHER:
1213 // .. reference is in our span, OR
1214 // .. we are an atomic collector and referent is in our span
1215 if (_span.contains(obj_addr) ||
1216 (discovery_is_atomic() &&
1217 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1218 // should_enqueue = true;
1219 } else {
1220 return false;
1221 }
1222 } else {
1223 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1224 _span.contains(obj_addr), "code inconsistency");
1225 }
1227 // Get the right type of discovered queue head.
1228 DiscoveredList* list = get_discovered_list(rt);
1229 if (list == NULL) {
1230 return false; // nothing special needs to be done
1231 }
1233 if (_discovery_is_mt) {
1234 add_to_discovered_list_mt(*list, obj, discovered_addr);
1235 } else {
1236 // We do a raw store here: the field will be visited later when processing
1237 // the discovered references.
1238 oop current_head = list->head();
1239 // The last ref must have its discovered field pointing to itself.
1240 oop next_discovered = (current_head != NULL) ? current_head : obj;
1242 assert(discovered == NULL, "control point invariant");
1243 oop_store_raw(discovered_addr, next_discovered);
1244 list->set_head(obj);
1245 list->inc_length(1);
1247 if (TraceReferenceGC) {
1248 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1249 (void *)obj, obj->klass()->internal_name());
1250 }
1251 }
1252 assert(obj->is_oop(), "Discovered a bad reference");
1253 verify_referent(obj);
1254 return true;
1255 }
1257 // Preclean the discovered references by removing those
1258 // whose referents are alive, and by marking from those that
1259 // are not active. These lists can be handled here
1260 // in any order and, indeed, concurrently.
1261 void ReferenceProcessor::preclean_discovered_references(
1262 BoolObjectClosure* is_alive,
1263 OopClosure* keep_alive,
1264 VoidClosure* complete_gc,
1265 YieldClosure* yield,
1266 GCTimer* gc_timer) {
1268 NOT_PRODUCT(verify_ok_to_handle_reflists());
1270 // Soft references
1271 {
1272 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1273 false, gc_timer);
1274 for (uint i = 0; i < _max_num_q; i++) {
1275 if (yield->should_return()) {
1276 return;
1277 }
1278 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1279 keep_alive, complete_gc, yield);
1280 }
1281 }
1283 // Weak references
1284 {
1285 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1286 false, gc_timer);
1287 for (uint i = 0; i < _max_num_q; i++) {
1288 if (yield->should_return()) {
1289 return;
1290 }
1291 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1292 keep_alive, complete_gc, yield);
1293 }
1294 }
1296 // Final references
1297 {
1298 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1299 false, gc_timer);
1300 for (uint i = 0; i < _max_num_q; i++) {
1301 if (yield->should_return()) {
1302 return;
1303 }
1304 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1305 keep_alive, complete_gc, yield);
1306 }
1307 }
1309 // Phantom references
1310 {
1311 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1312 false, gc_timer);
1313 for (uint i = 0; i < _max_num_q; i++) {
1314 if (yield->should_return()) {
1315 return;
1316 }
1317 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1318 keep_alive, complete_gc, yield);
1319 }
1321 // Cleaner references. Included in timing for phantom references. We
1322 // expect Cleaner references to be temporary, and don't want to deal with
1323 // possible incompatibilities arising from making it more visible.
1324 for (uint i = 0; i < _max_num_q; i++) {
1325 if (yield->should_return()) {
1326 return;
1327 }
1328 preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1329 keep_alive, complete_gc, yield);
1330 }
1331 }
1332 }
1334 // Walk the given discovered ref list, and remove all reference objects
1335 // whose referents are still alive, whose referents are NULL or which
1336 // are not active (have a non-NULL next field). NOTE: When we are
1337 // thus precleaning the ref lists (which happens single-threaded today),
1338 // we do not disable refs discovery to honour the correct semantics of
1339 // java.lang.Reference. As a result, we need to be careful below
1340 // that ref removal steps interleave safely with ref discovery steps
1341 // (in this thread).
1342 void
1343 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1344 BoolObjectClosure* is_alive,
1345 OopClosure* keep_alive,
1346 VoidClosure* complete_gc,
1347 YieldClosure* yield) {
1348 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1349 while (iter.has_next()) {
1350 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1351 oop obj = iter.obj();
1352 oop next = java_lang_ref_Reference::next(obj);
1353 if (iter.referent() == NULL || iter.is_referent_alive() ||
1354 next != NULL) {
1355 // The referent has been cleared, or is alive, or the Reference is not
1356 // active; we need to trace and mark its cohort.
1357 if (TraceReferenceGC) {
1358 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1359 (void *)iter.obj(), iter.obj()->klass()->internal_name());
1360 }
1361 // Remove Reference object from list
1362 iter.remove();
1363 // Keep alive its cohort.
1364 iter.make_referent_alive();
1365 if (UseCompressedOops) {
1366 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1367 keep_alive->do_oop(next_addr);
1368 } else {
1369 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1370 keep_alive->do_oop(next_addr);
1371 }
1372 iter.move_to_next();
1373 } else {
1374 iter.next();
1375 }
1376 }
1377 // Close the reachable set
1378 complete_gc->do_void();
1380 NOT_PRODUCT(
1381 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1382 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1383 "Refs in discovered list " INTPTR_FORMAT,
1384 iter.removed(), iter.processed(), (address)refs_list.head());
1385 }
1386 )
1387 }
1389 const char* ReferenceProcessor::list_name(uint i) {
1390 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1391 "Out of bounds index");
1393 int j = i / _max_num_q;
1394 switch (j) {
1395 case 0: return "SoftRef";
1396 case 1: return "WeakRef";
1397 case 2: return "FinalRef";
1398 case 3: return "PhantomRef";
1399 case 4: return "CleanerRef";
1400 }
1401 ShouldNotReachHere();
1402 return NULL;
1403 }
1405 #ifndef PRODUCT
1406 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1407 // empty for now
1408 }
1409 #endif
1411 #ifndef PRODUCT
1412 void ReferenceProcessor::clear_discovered_references() {
1413 guarantee(!_discovering_refs, "Discovering refs?");
1414 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1415 clear_discovered_references(_discovered_refs[i]);
1416 }
1417 }
1419 #endif // PRODUCT