1 /*
2 * Copyright (c) 2001, 2014, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/javaClasses.hpp"
27 #include "classfile/systemDictionary.hpp"
28 #include "gc_implementation/shared/gcTimer.hpp"
29 #include "gc_implementation/shared/gcTraceTime.hpp"
30 #include "gc_interface/collectedHeap.hpp"
31 #include "gc_interface/collectedHeap.inline.hpp"
32 #include "memory/referencePolicy.hpp"
33 #include "memory/referenceProcessor.hpp"
34 #include "oops/oop.inline.hpp"
35 #include "runtime/java.hpp"
36 #include "runtime/jniHandles.hpp"
37
38 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
39
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;
44
45 void referenceProcessor_init() {
46 ReferenceProcessor::init_statics();
47 }
48
49 void ReferenceProcessor::init_statics() {
50 // We need a monotonically non-decreasing time in ms but
51 // os::javaTimeMillis() does not guarantee monotonicity.
52 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
53
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);
58
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 "Unrecognized RefDiscoveryPolicy");
68 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
69 }
70
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?");
75
76 if (check_no_refs) {
77 // Verify that the discovered lists are empty
78 verify_no_references_recorded();
79 }
80 #endif // ASSERT
81
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.
88
89 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
90 _discovering_refs = true;
91 }
92
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);
113
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
122 // Initialize all entries to NULL
123 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
124 _discovered_refs[i].set_head(NULL);
125 _discovered_refs[i].set_length(0);
126 }
127
128 setup_policy(false /* default soft ref policy */);
129 }
130
131 #ifndef PRODUCT
132 void ReferenceProcessor::verify_no_references_recorded() {
133 guarantee(!_discovering_refs, "Discovering refs?");
134 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
135 guarantee(_discovered_refs[i].is_empty(),
136 "Found non-empty discovered list");
137 }
138 }
139 #endif
140
141 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
142 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
143 if (UseCompressedOops) {
144 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
145 } else {
146 f->do_oop((oop*)_discovered_refs[i].adr_head());
147 }
148 }
149 }
150
151 void ReferenceProcessor::update_soft_ref_master_clock() {
152 // Update (advance) the soft ref master clock field. This must be done
153 // after processing the soft ref list.
154
155 // We need a monotonically non-decreasing time in ms but
156 // os::javaTimeMillis() does not guarantee monotonicity.
157 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
158 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
159 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
160
161 NOT_PRODUCT(
162 if (now < _soft_ref_timestamp_clock) {
163 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
164 _soft_ref_timestamp_clock, now);
165 }
166 )
167 // The values of now and _soft_ref_timestamp_clock are set using
168 // javaTimeNanos(), which is guaranteed to be monotonically
169 // non-decreasing provided the underlying platform provides such
170 // a time source (and it is bug free).
171 // In product mode, however, protect ourselves from non-monotonicity.
172 if (now > _soft_ref_timestamp_clock) {
173 _soft_ref_timestamp_clock = now;
174 java_lang_ref_SoftReference::set_clock(now);
175 }
176 // Else leave clock stalled at its old value until time progresses
177 // past clock value.
178 }
179
180 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
181 size_t total = 0;
182 for (uint i = 0; i < _max_num_q; ++i) {
183 total += lists[i].length();
184 }
185 return total;
186 }
187
188 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
189 BoolObjectClosure* is_alive,
190 OopClosure* keep_alive,
191 VoidClosure* complete_gc,
192 AbstractRefProcTaskExecutor* task_executor,
193 GCTimer* gc_timer,
194 GCId gc_id) {
195 NOT_PRODUCT(verify_ok_to_handle_reflists());
196
197 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
198 // Stop treating discovered references specially.
199 disable_discovery();
200
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.
208
209 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
210
211 bool trace_time = PrintGCDetails && PrintReferenceGC;
212
213 // Soft references
214 size_t soft_count = 0;
215 {
216 GCTraceTime tt("SoftReference", trace_time, false, gc_timer, gc_id);
217 soft_count =
218 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
219 is_alive, keep_alive, complete_gc, task_executor);
220 }
221
222 update_soft_ref_master_clock();
223
224 // Weak references
225 size_t weak_count = 0;
226 {
227 GCTraceTime tt("WeakReference", trace_time, false, gc_timer, gc_id);
228 weak_count =
229 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
230 is_alive, keep_alive, complete_gc, task_executor);
231 }
232
233 // Final references
234 size_t final_count = 0;
235 {
236 GCTraceTime tt("FinalReference", trace_time, false, gc_timer, gc_id);
237 final_count =
238 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
239 is_alive, keep_alive, complete_gc, task_executor);
240 }
241
242 // Phantom references
243 size_t phantom_count = 0;
244 {
245 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer, gc_id);
246 phantom_count =
247 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
248 is_alive, keep_alive, complete_gc, task_executor);
249 }
250
251 // Weak global JNI references. It would make more sense (semantically) to
252 // traverse these simultaneously with the regular weak references above, but
253 // that is not how the JDK1.2 specification is. See #4126360. Native code can
254 // thus use JNI weak references to circumvent the phantom references and
255 // resurrect a "post-mortem" object.
256 {
257 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer, gc_id);
258 if (task_executor != NULL) {
259 task_executor->set_single_threaded_mode();
260 }
261 process_phaseJNI(is_alive, keep_alive, complete_gc);
262 }
263
264 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
265 }
266
267 #ifndef PRODUCT
268 // Calculate the number of jni handles.
269 uint ReferenceProcessor::count_jni_refs() {
270 class AlwaysAliveClosure: public BoolObjectClosure {
271 public:
272 virtual bool do_object_b(oop obj) { return true; }
273 };
274
275 class CountHandleClosure: public OopClosure {
276 private:
277 int _count;
278 public:
279 CountHandleClosure(): _count(0) {}
280 void do_oop(oop* unused) { _count++; }
281 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
282 int count() { return _count; }
283 };
284 CountHandleClosure global_handle_count;
285 AlwaysAliveClosure always_alive;
286 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
287 return global_handle_count.count();
288 }
289 #endif
290
291 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
292 OopClosure* keep_alive,
293 VoidClosure* complete_gc) {
294 #ifndef PRODUCT
295 if (PrintGCDetails && PrintReferenceGC) {
296 unsigned int count = count_jni_refs();
297 gclog_or_tty->print(", %u refs", count);
298 }
299 #endif
300 JNIHandles::weak_oops_do(is_alive, keep_alive);
301 complete_gc->do_void();
302 }
303
304
305 template <class T>
306 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
307 AbstractRefProcTaskExecutor* task_executor) {
308
309 // Remember old value of pending references list
310 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
311 T old_pending_list_value = *pending_list_addr;
312
313 // Enqueue references that are not made active again, and
314 // clear the decks for the next collection (cycle).
315 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
316 // Do the post-barrier on pending_list_addr missed in
317 // enqueue_discovered_reflist.
318 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
319
320 // Stop treating discovered references specially.
321 ref->disable_discovery();
322
323 // Return true if new pending references were added
324 return old_pending_list_value != *pending_list_addr;
325 }
326
327 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
328 NOT_PRODUCT(verify_ok_to_handle_reflists());
329 if (UseCompressedOops) {
330 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
331 } else {
332 return enqueue_discovered_ref_helper<oop>(this, task_executor);
333 }
334 }
335
336 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
337 HeapWord* pending_list_addr) {
338 // Given a list of refs linked through the "discovered" field
339 // (java.lang.ref.Reference.discovered), self-loop their "next" field
340 // thus distinguishing them from active References, then
341 // prepend them to the pending list.
342 //
343 // The Java threads will see the Reference objects linked together through
344 // the discovered field. Instead of trying to do the write barrier updates
345 // in all places in the reference processor where we manipulate the discovered
346 // field we make sure to do the barrier here where we anyway iterate through
347 // all linked Reference objects. Note that it is important to not dirty any
348 // cards during reference processing since this will cause card table
349 // verification to fail for G1.
350 //
351 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
352 // the "next" field is used to chain the pending list, not the discovered
353 // field.
354 if (TraceReferenceGC && PrintGCDetails) {
355 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
356 INTPTR_FORMAT, (address)refs_list.head());
357 }
358
359 oop obj = NULL;
360 oop next_d = refs_list.head();
361 if (pending_list_uses_discovered_field()) { // New behavior
362 // Walk down the list, self-looping the next field
363 // so that the References are not considered active.
364 while (obj != next_d) {
365 obj = next_d;
366 assert(obj->is_instanceRef(), "should be reference object");
367 next_d = java_lang_ref_Reference::discovered(obj);
368 if (TraceReferenceGC && PrintGCDetails) {
369 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
370 (void *)obj, (void *)next_d);
371 }
372 assert(java_lang_ref_Reference::next(obj) == NULL,
373 "Reference not active; should not be discovered");
374 // Self-loop next, so as to make Ref not active.
375 java_lang_ref_Reference::set_next_raw(obj, obj);
376 if (next_d != obj) {
377 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
378 } else {
379 // This is the last object.
380 // Swap refs_list into pending_list_addr and
381 // set obj's discovered to what we read from pending_list_addr.
382 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
383 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
384 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
385 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
386 }
387 }
388 } else { // Old behavior
389 // Walk down the list, copying the discovered field into
390 // the next field and clearing the discovered field.
391 while (obj != next_d) {
392 obj = next_d;
393 assert(obj->is_instanceRef(), "should be reference object");
394 next_d = java_lang_ref_Reference::discovered(obj);
395 if (TraceReferenceGC && PrintGCDetails) {
396 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
397 (void *)obj, (void *)next_d);
398 }
399 assert(java_lang_ref_Reference::next(obj) == NULL,
400 "The reference should not be enqueued");
401 if (next_d == obj) { // obj is last
402 // Swap refs_list into pending_list_addr and
403 // set obj's next to what we read from pending_list_addr.
404 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
405 // Need oop_check on pending_list_addr above;
406 // see special oop-check code at the end of
407 // enqueue_discovered_reflists() further below.
408 if (old == NULL) {
409 // obj should be made to point to itself, since
410 // pending list was empty.
411 java_lang_ref_Reference::set_next(obj, obj);
412 } else {
413 java_lang_ref_Reference::set_next(obj, old);
414 }
415 } else {
416 java_lang_ref_Reference::set_next(obj, next_d);
417 }
418 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
419 }
420 }
421 }
422
423 // Parallel enqueue task
424 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
425 public:
426 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
427 DiscoveredList discovered_refs[],
428 HeapWord* pending_list_addr,
429 int n_queues)
430 : EnqueueTask(ref_processor, discovered_refs,
431 pending_list_addr, n_queues)
432 { }
433
434 virtual void work(unsigned int work_id) {
435 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
436 // Simplest first cut: static partitioning.
437 int index = work_id;
438 // The increment on "index" must correspond to the maximum number of queues
439 // (n_queues) with which that ReferenceProcessor was created. That
440 // is because of the "clever" way the discovered references lists were
441 // allocated and are indexed into.
442 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
443 for (int j = 0;
444 j < ReferenceProcessor::number_of_subclasses_of_ref();
445 j++, index += _n_queues) {
446 _ref_processor.enqueue_discovered_reflist(
447 _refs_lists[index], _pending_list_addr);
448 _refs_lists[index].set_head(NULL);
449 _refs_lists[index].set_length(0);
450 }
451 }
452 };
453
454 // Enqueue references that are not made active again
455 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
456 AbstractRefProcTaskExecutor* task_executor) {
457 if (_processing_is_mt && task_executor != NULL) {
458 // Parallel code
459 RefProcEnqueueTask tsk(*this, _discovered_refs,
460 pending_list_addr, _max_num_q);
461 task_executor->execute(tsk);
462 } else {
463 // Serial code: call the parent class's implementation
464 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
465 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
466 _discovered_refs[i].set_head(NULL);
467 _discovered_refs[i].set_length(0);
468 }
469 }
470 }
471
472 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
473 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
474 oop discovered = java_lang_ref_Reference::discovered(_ref);
475 assert(_discovered_addr && discovered->is_oop_or_null(),
476 err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
477 _next = discovered;
478 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
479 _referent = java_lang_ref_Reference::referent(_ref);
480 assert(Universe::heap()->is_in_reserved_or_null(_referent),
481 "Wrong oop found in java.lang.Reference object");
482 assert(allow_null_referent ?
483 _referent->is_oop_or_null()
484 : _referent->is_oop(),
485 err_msg("Expected an oop%s for referent field at " PTR_FORMAT,
486 (allow_null_referent ? " or NULL" : ""),
487 p2i(_referent)));
488 }
489
490 void DiscoveredListIterator::remove() {
491 assert(_ref->is_oop(), "Dropping a bad reference");
492 oop_store_raw(_discovered_addr, NULL);
493
494 // First _prev_next ref actually points into DiscoveredList (gross).
495 oop new_next;
496 if (_next == _ref) {
497 // At the end of the list, we should make _prev point to itself.
498 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
499 // and _prev will be NULL.
500 new_next = _prev;
501 } else {
502 new_next = _next;
503 }
504 // Remove Reference object from discovered list. Note that G1 does not need a
505 // pre-barrier here because we know the Reference has already been found/marked,
506 // that's how it ended up in the discovered list in the first place.
507 oop_store_raw(_prev_next, new_next);
508 NOT_PRODUCT(_removed++);
509 _refs_list.dec_length(1);
510 }
511
512 // Make the Reference object active again.
513 void DiscoveredListIterator::make_active() {
514 // The pre barrier for G1 is probably just needed for the old
515 // reference processing behavior. Should we guard this with
516 // ReferenceProcessor::pending_list_uses_discovered_field() ?
517 if (UseG1GC) {
518 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
519 if (UseCompressedOops) {
520 oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
521 } else {
522 oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
523 }
524 }
525 java_lang_ref_Reference::set_next_raw(_ref, NULL);
526 }
527
528 void DiscoveredListIterator::clear_referent() {
529 oop_store_raw(_referent_addr, NULL);
530 }
531
532 // NOTE: process_phase*() are largely similar, and at a high level
533 // merely iterate over the extant list applying a predicate to
534 // each of its elements and possibly removing that element from the
535 // list and applying some further closures to that element.
536 // We should consider the possibility of replacing these
537 // process_phase*() methods by abstracting them into
538 // a single general iterator invocation that receives appropriate
539 // closures that accomplish this work.
540
541 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
542 // referents are not alive, but that should be kept alive for policy reasons.
543 // Keep alive the transitive closure of all such referents.
544 void
545 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
546 ReferencePolicy* policy,
547 BoolObjectClosure* is_alive,
548 OopClosure* keep_alive,
549 VoidClosure* complete_gc) {
550 assert(policy != NULL, "Must have a non-NULL policy");
551 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
552 // Decide which softly reachable refs should be kept alive.
553 while (iter.has_next()) {
554 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
555 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
556 if (referent_is_dead &&
557 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
558 if (TraceReferenceGC) {
559 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
560 (void *)iter.obj(), iter.obj()->klass()->internal_name());
561 }
562 // Remove Reference object from list
563 iter.remove();
564 // Make the Reference object active again
565 iter.make_active();
566 // keep the referent around
567 iter.make_referent_alive();
568 iter.move_to_next();
569 } else {
570 iter.next();
571 }
572 }
573 // Close the reachable set
574 complete_gc->do_void();
575 NOT_PRODUCT(
576 if (PrintGCDetails && TraceReferenceGC) {
577 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
578 "discovered Refs by policy, from list " INTPTR_FORMAT,
579 iter.removed(), iter.processed(), (address)refs_list.head());
580 }
581 )
582 }
583
584 // Traverse the list and remove any Refs that are not active, or
585 // whose referents are either alive or NULL.
586 void
587 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
588 BoolObjectClosure* is_alive,
589 OopClosure* keep_alive) {
590 assert(discovery_is_atomic(), "Error");
591 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
592 while (iter.has_next()) {
593 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
594 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
595 assert(next == NULL, "Should not discover inactive Reference");
596 if (iter.is_referent_alive()) {
597 if (TraceReferenceGC) {
598 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
599 (void *)iter.obj(), iter.obj()->klass()->internal_name());
600 }
601 // The referent is reachable after all.
602 // Remove Reference object from list.
603 iter.remove();
604 // Update the referent pointer as necessary: Note that this
605 // should not entail any recursive marking because the
606 // referent must already have been traversed.
607 iter.make_referent_alive();
608 iter.move_to_next();
609 } else {
610 iter.next();
611 }
612 }
613 NOT_PRODUCT(
614 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
615 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
616 "Refs in discovered list " INTPTR_FORMAT,
617 iter.removed(), iter.processed(), (address)refs_list.head());
618 }
619 )
620 }
621
622 void
623 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
624 BoolObjectClosure* is_alive,
625 OopClosure* keep_alive,
626 VoidClosure* complete_gc) {
627 assert(!discovery_is_atomic(), "Error");
628 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
629 while (iter.has_next()) {
630 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
631 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
632 oop next = java_lang_ref_Reference::next(iter.obj());
633 if ((iter.referent() == NULL || iter.is_referent_alive() ||
634 next != NULL)) {
635 assert(next->is_oop_or_null(), err_msg("Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next)));
636 // Remove Reference object from list
637 iter.remove();
638 // Trace the cohorts
639 iter.make_referent_alive();
640 if (UseCompressedOops) {
641 keep_alive->do_oop((narrowOop*)next_addr);
642 } else {
643 keep_alive->do_oop((oop*)next_addr);
644 }
645 iter.move_to_next();
646 } else {
647 iter.next();
648 }
649 }
650 // Now close the newly reachable set
651 complete_gc->do_void();
652 NOT_PRODUCT(
653 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
654 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
655 "Refs in discovered list " INTPTR_FORMAT,
656 iter.removed(), iter.processed(), (address)refs_list.head());
657 }
658 )
659 }
660
661 // Traverse the list and process the referents, by either
662 // clearing them or keeping them (and their reachable
663 // closure) alive.
664 void
665 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
666 bool clear_referent,
667 BoolObjectClosure* is_alive,
668 OopClosure* keep_alive,
669 VoidClosure* complete_gc) {
670 ResourceMark rm;
671 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
672 while (iter.has_next()) {
673 iter.update_discovered();
674 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
675 if (clear_referent) {
676 // NULL out referent pointer
677 iter.clear_referent();
678 } else {
679 // keep the referent around
680 iter.make_referent_alive();
681 }
682 if (TraceReferenceGC) {
683 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
684 clear_referent ? "cleared " : "",
685 (void *)iter.obj(), iter.obj()->klass()->internal_name());
686 }
687 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
688 iter.next();
689 }
690 // Remember to update the next pointer of the last ref.
691 iter.update_discovered();
692 // Close the reachable set
693 complete_gc->do_void();
694 }
695
696 void
697 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
698 oop obj = NULL;
699 oop next = refs_list.head();
700 while (next != obj) {
701 obj = next;
702 next = java_lang_ref_Reference::discovered(obj);
703 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
704 }
705 refs_list.set_head(NULL);
706 refs_list.set_length(0);
707 }
708
709 void
710 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
711 clear_discovered_references(refs_list);
712 }
713
714 void ReferenceProcessor::abandon_partial_discovery() {
715 // loop over the lists
716 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
717 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
718 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
719 }
720 abandon_partial_discovered_list(_discovered_refs[i]);
721 }
722 }
723
724 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
725 public:
726 RefProcPhase1Task(ReferenceProcessor& ref_processor,
727 DiscoveredList refs_lists[],
728 ReferencePolicy* policy,
729 bool marks_oops_alive)
730 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
731 _policy(policy)
732 { }
733 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
734 OopClosure& keep_alive,
735 VoidClosure& complete_gc)
736 {
737 Thread* thr = Thread::current();
738 int refs_list_index = ((WorkerThread*)thr)->id();
739 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
740 &is_alive, &keep_alive, &complete_gc);
741 }
742 private:
743 ReferencePolicy* _policy;
744 };
745
746 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
747 public:
748 RefProcPhase2Task(ReferenceProcessor& ref_processor,
749 DiscoveredList refs_lists[],
750 bool marks_oops_alive)
751 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
752 { }
753 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
754 OopClosure& keep_alive,
755 VoidClosure& complete_gc)
756 {
757 _ref_processor.process_phase2(_refs_lists[i],
758 &is_alive, &keep_alive, &complete_gc);
759 }
760 };
761
762 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
763 public:
764 RefProcPhase3Task(ReferenceProcessor& ref_processor,
765 DiscoveredList refs_lists[],
766 bool clear_referent,
767 bool marks_oops_alive)
768 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
769 _clear_referent(clear_referent)
770 { }
771 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
772 OopClosure& keep_alive,
773 VoidClosure& complete_gc)
774 {
775 // Don't use "refs_list_index" calculated in this way because
776 // balance_queues() has moved the Ref's into the first n queues.
777 // Thread* thr = Thread::current();
778 // int refs_list_index = ((WorkerThread*)thr)->id();
779 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
780 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
781 &is_alive, &keep_alive, &complete_gc);
782 }
783 private:
784 bool _clear_referent;
785 };
786
787 // Balances reference queues.
788 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
789 // queues[0, 1, ..., _num_q-1] because only the first _num_q
790 // corresponding to the active workers will be processed.
791 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
792 {
793 // calculate total length
794 size_t total_refs = 0;
795 if (TraceReferenceGC && PrintGCDetails) {
796 gclog_or_tty->print_cr("\nBalance ref_lists ");
797 }
798
799 for (uint i = 0; i < _max_num_q; ++i) {
800 total_refs += ref_lists[i].length();
801 if (TraceReferenceGC && PrintGCDetails) {
802 gclog_or_tty->print("%d ", ref_lists[i].length());
803 }
804 }
805 if (TraceReferenceGC && PrintGCDetails) {
806 gclog_or_tty->print_cr(" = %d", total_refs);
807 }
808 size_t avg_refs = total_refs / _num_q + 1;
809 uint to_idx = 0;
810 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
811 bool move_all = false;
812 if (from_idx >= _num_q) {
813 move_all = ref_lists[from_idx].length() > 0;
814 }
815 while ((ref_lists[from_idx].length() > avg_refs) ||
816 move_all) {
817 assert(to_idx < _num_q, "Sanity Check!");
818 if (ref_lists[to_idx].length() < avg_refs) {
819 // move superfluous refs
820 size_t refs_to_move;
821 // Move all the Ref's if the from queue will not be processed.
822 if (move_all) {
823 refs_to_move = MIN2(ref_lists[from_idx].length(),
824 avg_refs - ref_lists[to_idx].length());
825 } else {
826 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
827 avg_refs - ref_lists[to_idx].length());
828 }
829
830 assert(refs_to_move > 0, "otherwise the code below will fail");
831
832 oop move_head = ref_lists[from_idx].head();
833 oop move_tail = move_head;
834 oop new_head = move_head;
835 // find an element to split the list on
836 for (size_t j = 0; j < refs_to_move; ++j) {
837 move_tail = new_head;
838 new_head = java_lang_ref_Reference::discovered(new_head);
839 }
840
841 // Add the chain to the to list.
842 if (ref_lists[to_idx].head() == NULL) {
843 // to list is empty. Make a loop at the end.
844 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
845 } else {
846 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
847 }
848 ref_lists[to_idx].set_head(move_head);
849 ref_lists[to_idx].inc_length(refs_to_move);
850
851 // Remove the chain from the from list.
852 if (move_tail == new_head) {
853 // We found the end of the from list.
854 ref_lists[from_idx].set_head(NULL);
855 } else {
856 ref_lists[from_idx].set_head(new_head);
857 }
858 ref_lists[from_idx].dec_length(refs_to_move);
859 if (ref_lists[from_idx].length() == 0) {
860 break;
861 }
862 } else {
863 to_idx = (to_idx + 1) % _num_q;
864 }
865 }
866 }
867 #ifdef ASSERT
868 size_t balanced_total_refs = 0;
869 for (uint i = 0; i < _max_num_q; ++i) {
870 balanced_total_refs += ref_lists[i].length();
871 if (TraceReferenceGC && PrintGCDetails) {
872 gclog_or_tty->print("%d ", ref_lists[i].length());
873 }
874 }
875 if (TraceReferenceGC && PrintGCDetails) {
876 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
877 gclog_or_tty->flush();
878 }
879 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
880 #endif
881 }
882
883 void ReferenceProcessor::balance_all_queues() {
884 balance_queues(_discoveredSoftRefs);
885 balance_queues(_discoveredWeakRefs);
886 balance_queues(_discoveredFinalRefs);
887 balance_queues(_discoveredPhantomRefs);
888 }
889
890 size_t
891 ReferenceProcessor::process_discovered_reflist(
892 DiscoveredList refs_lists[],
893 ReferencePolicy* policy,
894 bool clear_referent,
895 BoolObjectClosure* is_alive,
896 OopClosure* keep_alive,
897 VoidClosure* complete_gc,
898 AbstractRefProcTaskExecutor* task_executor)
899 {
900 bool mt_processing = task_executor != NULL && _processing_is_mt;
901 // If discovery used MT and a dynamic number of GC threads, then
902 // the queues must be balanced for correctness if fewer than the
903 // maximum number of queues were used. The number of queue used
904 // during discovery may be different than the number to be used
905 // for processing so don't depend of _num_q < _max_num_q as part
906 // of the test.
907 bool must_balance = _discovery_is_mt;
908
909 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
910 must_balance) {
911 balance_queues(refs_lists);
912 }
913
914 size_t total_list_count = total_count(refs_lists);
915
916 if (PrintReferenceGC && PrintGCDetails) {
917 gclog_or_tty->print(", %u refs", total_list_count);
918 }
919
920 // Phase 1 (soft refs only):
921 // . Traverse the list and remove any SoftReferences whose
922 // referents are not alive, but that should be kept alive for
923 // policy reasons. Keep alive the transitive closure of all
924 // such referents.
925 if (policy != NULL) {
926 if (mt_processing) {
927 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
928 task_executor->execute(phase1);
929 } else {
930 for (uint i = 0; i < _max_num_q; i++) {
931 process_phase1(refs_lists[i], policy,
932 is_alive, keep_alive, complete_gc);
933 }
934 }
935 } else { // policy == NULL
936 assert(refs_lists != _discoveredSoftRefs,
937 "Policy must be specified for soft references.");
938 }
939
940 // Phase 2:
941 // . Traverse the list and remove any refs whose referents are alive.
942 if (mt_processing) {
943 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
944 task_executor->execute(phase2);
945 } else {
946 for (uint i = 0; i < _max_num_q; i++) {
947 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
948 }
949 }
950
951 // Phase 3:
952 // . Traverse the list and process referents as appropriate.
953 if (mt_processing) {
954 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
955 task_executor->execute(phase3);
956 } else {
957 for (uint i = 0; i < _max_num_q; i++) {
958 process_phase3(refs_lists[i], clear_referent,
959 is_alive, keep_alive, complete_gc);
960 }
961 }
962
963 return total_list_count;
964 }
965
966 void ReferenceProcessor::clean_up_discovered_references() {
967 // loop over the lists
968 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
969 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
970 gclog_or_tty->print_cr(
971 "\nScrubbing %s discovered list of Null referents",
972 list_name(i));
973 }
974 clean_up_discovered_reflist(_discovered_refs[i]);
975 }
976 }
977
978 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
979 assert(!discovery_is_atomic(), "Else why call this method?");
980 DiscoveredListIterator iter(refs_list, NULL, NULL);
981 while (iter.has_next()) {
982 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
983 oop next = java_lang_ref_Reference::next(iter.obj());
984 assert(next->is_oop_or_null(), err_msg("Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next)));
985 // If referent has been cleared or Reference is not active,
986 // drop it.
987 if (iter.referent() == NULL || next != NULL) {
988 debug_only(
989 if (PrintGCDetails && TraceReferenceGC) {
990 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
991 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
992 " and referent: " INTPTR_FORMAT,
993 (void *)iter.obj(), (void *)next, (void *)iter.referent());
994 }
995 )
996 // Remove Reference object from list
997 iter.remove();
998 iter.move_to_next();
999 } else {
1000 iter.next();
1001 }
1002 }
1003 NOT_PRODUCT(
1004 if (PrintGCDetails && TraceReferenceGC) {
1005 gclog_or_tty->print(
1006 " Removed %d Refs with NULL referents out of %d discovered Refs",
1007 iter.removed(), iter.processed());
1008 }
1009 )
1010 }
1011
1012 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1013 uint id = 0;
1014 // Determine the queue index to use for this object.
1015 if (_discovery_is_mt) {
1016 // During a multi-threaded discovery phase,
1017 // each thread saves to its "own" list.
1018 Thread* thr = Thread::current();
1019 id = thr->as_Worker_thread()->id();
1020 } else {
1021 // single-threaded discovery, we save in round-robin
1022 // fashion to each of the lists.
1023 if (_processing_is_mt) {
1024 id = next_id();
1025 }
1026 }
1027 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1028
1029 // Get the discovered queue to which we will add
1030 DiscoveredList* list = NULL;
1031 switch (rt) {
1032 case REF_OTHER:
1033 // Unknown reference type, no special treatment
1034 break;
1035 case REF_SOFT:
1036 list = &_discoveredSoftRefs[id];
1037 break;
1038 case REF_WEAK:
1039 list = &_discoveredWeakRefs[id];
1040 break;
1041 case REF_FINAL:
1042 list = &_discoveredFinalRefs[id];
1043 break;
1044 case REF_PHANTOM:
1045 list = &_discoveredPhantomRefs[id];
1046 break;
1047 case REF_NONE:
1048 // we should not reach here if we are an InstanceRefKlass
1049 default:
1050 ShouldNotReachHere();
1051 }
1052 if (TraceReferenceGC && PrintGCDetails) {
1053 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1054 }
1055 return list;
1056 }
1057
1058 inline void
1059 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1060 oop obj,
1061 HeapWord* discovered_addr) {
1062 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1063 // First we must make sure this object is only enqueued once. CAS in a non null
1064 // discovered_addr.
1065 oop current_head = refs_list.head();
1066 // The last ref must have its discovered field pointing to itself.
1067 oop next_discovered = (current_head != NULL) ? current_head : obj;
1068
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
1078 if (TraceReferenceGC) {
1079 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1080 (void *)obj, obj->klass()->internal_name());
1081 }
1082 } else {
1083 // If retest was non NULL, another thread beat us to it:
1084 // The reference has already been discovered...
1085 if (TraceReferenceGC) {
1086 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1087 (void *)obj, obj->klass()->internal_name());
1088 }
1089 }
1090 }
1091
1092 #ifndef PRODUCT
1093 // Non-atomic (i.e. concurrent) discovery might allow us
1094 // to observe j.l.References with NULL referents, being those
1095 // cleared concurrently by mutators during (or after) discovery.
1096 void ReferenceProcessor::verify_referent(oop obj) {
1097 bool da = discovery_is_atomic();
1098 oop referent = java_lang_ref_Reference::referent(obj);
1099 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1100 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1101 INTPTR_FORMAT " during %satomic discovery ",
1102 (void *)referent, (void *)obj, da ? "" : "non-"));
1103 }
1104 #endif
1105
1106 // We mention two of several possible choices here:
1107 // #0: if the reference object is not in the "originating generation"
1108 // (or part of the heap being collected, indicated by our "span"
1109 // we don't treat it specially (i.e. we scan it as we would
1110 // a normal oop, treating its references as strong references).
1111 // This means that references can't be discovered unless their
1112 // referent is also in the same span. This is the simplest,
1113 // most "local" and most conservative approach, albeit one
1114 // that may cause weak references to be enqueued least promptly.
1115 // We call this choice the "ReferenceBasedDiscovery" policy.
1116 // #1: the reference object may be in any generation (span), but if
1117 // the referent is in the generation (span) being currently collected
1118 // then we can discover the reference object, provided
1119 // the object has not already been discovered by
1120 // a different concurrently running collector (as may be the
1121 // case, for instance, if the reference object is in CMS and
1122 // the referent in DefNewGeneration), and provided the processing
1123 // of this reference object by the current collector will
1124 // appear atomic to every other collector in the system.
1125 // (Thus, for instance, a concurrent collector may not
1126 // discover references in other generations even if the
1127 // referent is in its own generation). This policy may,
1128 // in certain cases, enqueue references somewhat sooner than
1129 // might Policy #0 above, but at marginally increased cost
1130 // and complexity in processing these references.
1131 // We call this choice the "RefeferentBasedDiscovery" policy.
1132 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1133 // Make sure we are discovering refs (rather than processing discovered refs).
1134 if (!_discovering_refs || !RegisterReferences) {
1135 return false;
1136 }
1137 // We only discover active references.
1138 oop next = java_lang_ref_Reference::next(obj);
1139 if (next != NULL) { // Ref is no longer active
1140 return false;
1141 }
1142
1143 HeapWord* obj_addr = (HeapWord*)obj;
1144 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1145 !_span.contains(obj_addr)) {
1146 // Reference is not in the originating generation;
1147 // don't treat it specially (i.e. we want to scan it as a normal
1148 // object with strong references).
1149 return false;
1150 }
1151
1152 // We only discover references whose referents are not (yet)
1153 // known to be strongly reachable.
1154 if (is_alive_non_header() != NULL) {
1155 verify_referent(obj);
1156 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1157 return false; // referent is reachable
1158 }
1159 }
1160 if (rt == REF_SOFT) {
1161 // For soft refs we can decide now if these are not
1162 // current candidates for clearing, in which case we
1163 // can mark through them now, rather than delaying that
1164 // to the reference-processing phase. Since all current
1165 // time-stamp policies advance the soft-ref clock only
1166 // at a major collection cycle, this is always currently
1167 // accurate.
1168 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1169 return false;
1170 }
1171 }
1172
1173 ResourceMark rm; // Needed for tracing.
1174
1175 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1176 const oop discovered = java_lang_ref_Reference::discovered(obj);
1177 assert(discovered->is_oop_or_null(), err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));
1178 if (discovered != NULL) {
1179 // The reference has already been discovered...
1180 if (TraceReferenceGC) {
1181 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1182 (void *)obj, obj->klass()->internal_name());
1183 }
1184 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1185 // assumes that an object is not processed twice;
1186 // if it's been already discovered it must be on another
1187 // generation's discovered list; so we won't discover it.
1188 return false;
1189 } else {
1190 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1191 "Unrecognized policy");
1192 // Check assumption that an object is not potentially
1193 // discovered twice except by concurrent collectors that potentially
1194 // trace the same Reference object twice.
1195 assert(UseConcMarkSweepGC || UseG1GC,
1196 "Only possible with a concurrent marking collector");
1197 return true;
1198 }
1199 }
1200
1201 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1202 verify_referent(obj);
1203 // Discover if and only if EITHER:
1204 // .. reference is in our span, OR
1205 // .. we are an atomic collector and referent is in our span
1206 if (_span.contains(obj_addr) ||
1207 (discovery_is_atomic() &&
1208 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1209 // should_enqueue = true;
1210 } else {
1211 return false;
1212 }
1213 } else {
1214 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1215 _span.contains(obj_addr), "code inconsistency");
1216 }
1217
1218 // Get the right type of discovered queue head.
1219 DiscoveredList* list = get_discovered_list(rt);
1220 if (list == NULL) {
1221 return false; // nothing special needs to be done
1222 }
1223
1224 if (_discovery_is_mt) {
1225 add_to_discovered_list_mt(*list, obj, discovered_addr);
1226 } else {
1227 // We do a raw store here: the field will be visited later when processing
1228 // the discovered references.
1229 oop current_head = list->head();
1230 // The last ref must have its discovered field pointing to itself.
1231 oop next_discovered = (current_head != NULL) ? current_head : obj;
1232
1233 assert(discovered == NULL, "control point invariant");
1234 oop_store_raw(discovered_addr, next_discovered);
1235 list->set_head(obj);
1236 list->inc_length(1);
1237
1238 if (TraceReferenceGC) {
1239 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1240 (void *)obj, obj->klass()->internal_name());
1241 }
1242 }
1243 assert(obj->is_oop(), "Discovered a bad reference");
1244 verify_referent(obj);
1245 return true;
1246 }
1247
1248 // Preclean the discovered references by removing those
1249 // whose referents are alive, and by marking from those that
1250 // are not active. These lists can be handled here
1251 // in any order and, indeed, concurrently.
1252 void ReferenceProcessor::preclean_discovered_references(
1253 BoolObjectClosure* is_alive,
1254 OopClosure* keep_alive,
1255 VoidClosure* complete_gc,
1256 YieldClosure* yield,
1257 GCTimer* gc_timer,
1258 GCId gc_id) {
1259
1260 NOT_PRODUCT(verify_ok_to_handle_reflists());
1261
1262 // Soft references
1263 {
1264 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1265 false, gc_timer, gc_id);
1266 for (uint i = 0; i < _max_num_q; i++) {
1267 if (yield->should_return()) {
1268 return;
1269 }
1270 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1271 keep_alive, complete_gc, yield);
1272 }
1273 }
1274
1275 // Weak references
1276 {
1277 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1278 false, gc_timer, gc_id);
1279 for (uint i = 0; i < _max_num_q; i++) {
1280 if (yield->should_return()) {
1281 return;
1282 }
1283 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1284 keep_alive, complete_gc, yield);
1285 }
1286 }
1287
1288 // Final references
1289 {
1290 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1291 false, gc_timer, gc_id);
1292 for (uint i = 0; i < _max_num_q; i++) {
1293 if (yield->should_return()) {
1294 return;
1295 }
1296 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1297 keep_alive, complete_gc, yield);
1298 }
1299 }
1300
1301 // Phantom references
1302 {
1303 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1304 false, gc_timer, gc_id);
1305 for (uint i = 0; i < _max_num_q; i++) {
1306 if (yield->should_return()) {
1307 return;
1308 }
1309 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1310 keep_alive, complete_gc, yield);
1311 }
1312 }
1313 }
1314
1315 // Walk the given discovered ref list, and remove all reference objects
1316 // whose referents are still alive, whose referents are NULL or which
1317 // are not active (have a non-NULL next field). NOTE: When we are
1318 // thus precleaning the ref lists (which happens single-threaded today),
1319 // we do not disable refs discovery to honor the correct semantics of
1320 // java.lang.Reference. As a result, we need to be careful below
1321 // that ref removal steps interleave safely with ref discovery steps
1322 // (in this thread).
1323 void
1324 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1325 BoolObjectClosure* is_alive,
1326 OopClosure* keep_alive,
1327 VoidClosure* complete_gc,
1328 YieldClosure* yield) {
1329 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1330 while (iter.has_next()) {
1331 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1332 oop obj = iter.obj();
1333 oop next = java_lang_ref_Reference::next(obj);
1334 if (iter.referent() == NULL || iter.is_referent_alive() ||
1335 next != NULL) {
1336 // The referent has been cleared, or is alive, or the Reference is not
1337 // active; we need to trace and mark its cohort.
1338 if (TraceReferenceGC) {
1339 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1340 (void *)iter.obj(), iter.obj()->klass()->internal_name());
1341 }
1342 // Remove Reference object from list
1343 iter.remove();
1344 // Keep alive its cohort.
1345 iter.make_referent_alive();
1346 if (UseCompressedOops) {
1347 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1348 keep_alive->do_oop(next_addr);
1349 } else {
1350 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1351 keep_alive->do_oop(next_addr);
1352 }
1353 iter.move_to_next();
1354 } else {
1355 iter.next();
1356 }
1357 }
1358 // Close the reachable set
1359 complete_gc->do_void();
1360
1361 NOT_PRODUCT(
1362 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1363 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1364 "Refs in discovered list " INTPTR_FORMAT,
1365 iter.removed(), iter.processed(), (address)refs_list.head());
1366 }
1367 )
1368 }
1369
1370 const char* ReferenceProcessor::list_name(uint i) {
1371 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1372 "Out of bounds index");
1373
1374 int j = i / _max_num_q;
1375 switch (j) {
1376 case 0: return "SoftRef";
1377 case 1: return "WeakRef";
1378 case 2: return "FinalRef";
1379 case 3: return "PhantomRef";
1380 }
1381 ShouldNotReachHere();
1382 return NULL;
1383 }
1384
1385 #ifndef PRODUCT
1386 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1387 // empty for now
1388 }
1389 #endif
1390
1391 #ifndef PRODUCT
1392 void ReferenceProcessor::clear_discovered_references() {
1393 guarantee(!_discovering_refs, "Discovering refs?");
1394 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1395 clear_discovered_references(_discovered_refs[i]);
1396 }
1397 }
1398
1399 #endif // PRODUCT