1 /*
2 * Copyright (c) 2001, 2013, 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 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
39 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
40 bool ReferenceProcessor::_pending_list_uses_discovered_field = false;
41 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0;
42
43 void referenceProcessor_init() {
44 ReferenceProcessor::init_statics();
45 }
46
47 void ReferenceProcessor::init_statics() {
48 // We need a monotonically non-decreasing time in ms but
49 // os::javaTimeMillis() does not guarantee monotonicity.
50 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
51
52 // Initialize the soft ref timestamp clock.
53 _soft_ref_timestamp_clock = now;
54 // Also update the soft ref clock in j.l.r.SoftReference
55 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
56
57 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
58 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
59 NOT_COMPILER2(LRUCurrentHeapPolicy());
60 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
61 vm_exit_during_initialization("Could not allocate reference policy object");
62 }
63 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
64 RefDiscoveryPolicy == ReferentBasedDiscovery,
65 "Unrecongnized RefDiscoveryPolicy");
66 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
67 }
68
69 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
70 #ifdef ASSERT
71 // Verify that we're not currently discovering refs
72 assert(!verify_disabled || !_discovering_refs, "nested call?");
73
74 if (check_no_refs) {
75 // Verify that the discovered lists are empty
76 verify_no_references_recorded();
77 }
78 #endif // ASSERT
79
80 // Someone could have modified the value of the static
81 // field in the j.l.r.SoftReference class that holds the
82 // soft reference timestamp clock using reflection or
83 // Unsafe between GCs. Unconditionally update the static
84 // field in ReferenceProcessor here so that we use the new
85 // value during reference discovery.
86
87 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
88 _discovering_refs = true;
89 }
90
91 ReferenceProcessor::ReferenceProcessor(MemRegion span,
92 bool mt_processing,
93 uint mt_processing_degree,
94 bool mt_discovery,
95 uint mt_discovery_degree,
96 bool atomic_discovery,
97 BoolObjectClosure* is_alive_non_header,
98 bool discovered_list_needs_barrier) :
99 _discovering_refs(false),
100 _enqueuing_is_done(false),
101 _is_alive_non_header(is_alive_non_header),
102 _discovered_list_needs_barrier(discovered_list_needs_barrier),
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 NOT_PRODUCT(verify_ok_to_handle_reflists());
195
196 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
197 // Stop treating discovered references specially.
198 disable_discovery();
199
200 // If discovery was concurrent, someone could have modified
201 // the value of the static field in the j.l.r.SoftReference
202 // class that holds the soft reference timestamp clock using
203 // reflection or Unsafe between when discovery was enabled and
204 // now. Unconditionally update the static field in ReferenceProcessor
205 // here so that we use the new value during processing of the
206 // discovered soft refs.
207
208 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
209
210 bool trace_time = PrintGCDetails && PrintReferenceGC;
211
212 // Soft references
213 size_t soft_count = 0;
214 {
215 GCTraceTime tt("SoftReference", trace_time, false, gc_timer);
216 soft_count =
217 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
218 is_alive, keep_alive, complete_gc, task_executor);
219 }
220
221 update_soft_ref_master_clock();
222
223 // Weak references
224 size_t weak_count = 0;
225 {
226 GCTraceTime tt("WeakReference", trace_time, false, gc_timer);
227 weak_count =
228 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
229 is_alive, keep_alive, complete_gc, task_executor);
230 }
231
232 // Final references
233 size_t final_count = 0;
234 {
235 GCTraceTime tt("FinalReference", trace_time, false, gc_timer);
236 final_count =
237 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
238 is_alive, keep_alive, complete_gc, task_executor);
239 }
240
241 // Phantom references
242 size_t phantom_count = 0;
243 {
244 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer);
245 phantom_count =
246 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
247 is_alive, keep_alive, complete_gc, task_executor);
248 }
249
250 // Weak global JNI references. It would make more sense (semantically) to
251 // traverse these simultaneously with the regular weak references above, but
252 // that is not how the JDK1.2 specification is. See #4126360. Native code can
253 // thus use JNI weak references to circumvent the phantom references and
254 // resurrect a "post-mortem" object.
255 {
256 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
257 if (task_executor != NULL) {
258 task_executor->set_single_threaded_mode();
259 }
260 process_phaseJNI(is_alive, keep_alive, complete_gc);
261 }
262
263 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
264 }
265
266 #ifndef PRODUCT
267 // Calculate the number of jni handles.
268 uint ReferenceProcessor::count_jni_refs() {
269 class AlwaysAliveClosure: public BoolObjectClosure {
270 public:
271 virtual bool do_object_b(oop obj) { return true; }
272 };
273
274 class CountHandleClosure: public OopClosure {
275 private:
276 int _count;
277 public:
278 CountHandleClosure(): _count(0) {}
279 void do_oop(oop* unused) { _count++; }
280 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
281 int count() { return _count; }
282 };
283 CountHandleClosure global_handle_count;
284 AlwaysAliveClosure always_alive;
285 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
286 return global_handle_count.count();
287 }
288 #endif
289
290 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
291 OopClosure* keep_alive,
292 VoidClosure* complete_gc) {
293 #ifndef PRODUCT
294 if (PrintGCDetails && PrintReferenceGC) {
295 unsigned int count = count_jni_refs();
296 gclog_or_tty->print(", %u refs", count);
297 }
298 #endif
299 JNIHandles::weak_oops_do(is_alive, keep_alive);
300 complete_gc->do_void();
301 }
302
303
304 template <class T>
305 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
306 AbstractRefProcTaskExecutor* task_executor) {
307
308 // Remember old value of pending references list
309 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
310 T old_pending_list_value = *pending_list_addr;
311
312 // Enqueue references that are not made active again, and
313 // clear the decks for the next collection (cycle).
314 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
315 // Do the post-barrier on pending_list_addr missed in
316 // enqueue_discovered_reflist.
317 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
318
319 // Stop treating discovered references specially.
320 ref->disable_discovery();
321
322 // Return true if new pending references were added
323 return old_pending_list_value != *pending_list_addr;
324 }
325
326 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
327 NOT_PRODUCT(verify_ok_to_handle_reflists());
328 if (UseCompressedOops) {
329 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
330 } else {
331 return enqueue_discovered_ref_helper<oop>(this, task_executor);
332 }
333 }
334
335 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
336 HeapWord* pending_list_addr) {
337 // Given a list of refs linked through the "discovered" field
338 // (java.lang.ref.Reference.discovered), self-loop their "next" field
339 // thus distinguishing them from active References, then
340 // prepend them to the pending list.
341 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
342 // the "next" field is used to chain the pending list, not the discovered
343 // field.
344
345 if (TraceReferenceGC && PrintGCDetails) {
346 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
347 INTPTR_FORMAT, (address)refs_list.head());
348 }
349
350 oop obj = NULL;
351 oop next_d = refs_list.head();
352 if (pending_list_uses_discovered_field()) { // New behavior
353 // Walk down the list, self-looping the next field
354 // so that the References are not considered active.
355 while (obj != next_d) {
356 obj = next_d;
357 assert(obj->is_instanceRef(), "should be reference object");
358 next_d = java_lang_ref_Reference::discovered(obj);
359 if (TraceReferenceGC && PrintGCDetails) {
360 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
361 (void *)obj, (void *)next_d);
362 }
363 assert(java_lang_ref_Reference::next(obj) == NULL,
364 "Reference not active; should not be discovered");
365 // Self-loop next, so as to make Ref not active.
366 // Post-barrier not needed when looping to self.
367 java_lang_ref_Reference::set_next_raw(obj, obj);
368 if (next_d == obj) { // obj is last
369 // Swap refs_list into pending_list_addr and
370 // set obj's discovered to what we read from pending_list_addr.
371 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
372 // Need post-barrier on pending_list_addr above;
373 // see special post-barrier code at the end of
374 // enqueue_discovered_reflists() further below.
375 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
376 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
377 }
378 }
379 } else { // Old behavior
380 // Walk down the list, copying the discovered field into
381 // the next field and clearing the discovered field.
382 while (obj != next_d) {
383 obj = next_d;
384 assert(obj->is_instanceRef(), "should be reference object");
385 next_d = java_lang_ref_Reference::discovered(obj);
386 if (TraceReferenceGC && PrintGCDetails) {
387 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
388 (void *)obj, (void *)next_d);
389 }
390 assert(java_lang_ref_Reference::next(obj) == NULL,
391 "The reference should not be enqueued");
392 if (next_d == obj) { // obj is last
393 // Swap refs_list into pending_list_addr and
394 // set obj's next to what we read from pending_list_addr.
395 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
396 // Need oop_check on pending_list_addr above;
397 // see special oop-check code at the end of
398 // enqueue_discovered_reflists() further below.
399 if (old == NULL) {
400 // obj should be made to point to itself, since
401 // pending list was empty.
402 java_lang_ref_Reference::set_next(obj, obj);
403 } else {
404 java_lang_ref_Reference::set_next(obj, old);
405 }
406 } else {
407 java_lang_ref_Reference::set_next(obj, next_d);
408 }
409 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
410 }
411 }
412 }
413
414 // Parallel enqueue task
415 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
416 public:
417 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
418 DiscoveredList discovered_refs[],
419 HeapWord* pending_list_addr,
420 int n_queues)
421 : EnqueueTask(ref_processor, discovered_refs,
422 pending_list_addr, n_queues)
423 { }
424
425 virtual void work(unsigned int work_id) {
426 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
427 // Simplest first cut: static partitioning.
428 int index = work_id;
429 // The increment on "index" must correspond to the maximum number of queues
430 // (n_queues) with which that ReferenceProcessor was created. That
431 // is because of the "clever" way the discovered references lists were
432 // allocated and are indexed into.
433 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
434 for (int j = 0;
435 j < ReferenceProcessor::number_of_subclasses_of_ref();
436 j++, index += _n_queues) {
437 _ref_processor.enqueue_discovered_reflist(
438 _refs_lists[index], _pending_list_addr);
439 _refs_lists[index].set_head(NULL);
440 _refs_lists[index].set_length(0);
441 }
442 }
443 };
444
445 // Enqueue references that are not made active again
446 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
447 AbstractRefProcTaskExecutor* task_executor) {
448 if (_processing_is_mt && task_executor != NULL) {
449 // Parallel code
450 RefProcEnqueueTask tsk(*this, _discovered_refs,
451 pending_list_addr, _max_num_q);
452 task_executor->execute(tsk);
453 } else {
454 // Serial code: call the parent class's implementation
455 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
456 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
457 _discovered_refs[i].set_head(NULL);
458 _discovered_refs[i].set_length(0);
459 }
460 }
461 }
462
463 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
464 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
465 oop discovered = java_lang_ref_Reference::discovered(_ref);
466 assert(_discovered_addr && discovered->is_oop_or_null(),
467 "discovered field is bad");
468 _next = discovered;
469 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
470 _referent = java_lang_ref_Reference::referent(_ref);
471 assert(Universe::heap()->is_in_reserved_or_null(_referent),
472 "Wrong oop found in java.lang.Reference object");
473 assert(allow_null_referent ?
474 _referent->is_oop_or_null()
475 : _referent->is_oop(),
476 "bad referent");
477 }
478
479 void DiscoveredListIterator::remove() {
480 assert(_ref->is_oop(), "Dropping a bad reference");
481 oop_store_raw(_discovered_addr, NULL);
482
483 // First _prev_next ref actually points into DiscoveredList (gross).
484 oop new_next;
485 if (_next == _ref) {
486 // At the end of the list, we should make _prev point to itself.
487 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
488 // and _prev will be NULL.
489 new_next = _prev;
490 } else {
491 new_next = _next;
492 }
493 // Remove Reference object from list.
494 oop_store_raw(_prev_next, new_next);
495 if (_discovered_list_needs_barrier && _prev_next != _refs_list.adr_head()) {
496 // Needs post-barrier and this is not the list head (which is not on the heap)
497 oopDesc::bs()->write_ref_field(_prev_next, new_next);
498 }
499 NOT_PRODUCT(_removed++);
500 _refs_list.dec_length(1);
501 }
502
503 // Make the Reference object active again.
504 void DiscoveredListIterator::make_active() {
505 // For G1 we don't want to use set_next - it
506 // will dirty the card for the next field of
507 // the reference object and will fail
508 // CT verification.
509 if (UseG1GC) {
510 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
511 if (UseCompressedOops) {
512 oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
513 } else {
514 oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
515 }
516 java_lang_ref_Reference::set_next_raw(_ref, NULL);
517 } else {
518 java_lang_ref_Reference::set_next(_ref, NULL);
519 }
520 }
521
522 void DiscoveredListIterator::clear_referent() {
523 oop_store_raw(_referent_addr, NULL);
524 }
525
526 // NOTE: process_phase*() are largely similar, and at a high level
527 // merely iterate over the extant list applying a predicate to
528 // each of its elements and possibly removing that element from the
529 // list and applying some further closures to that element.
530 // We should consider the possibility of replacing these
531 // process_phase*() methods by abstracting them into
532 // a single general iterator invocation that receives appropriate
533 // closures that accomplish this work.
534
535 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
536 // referents are not alive, but that should be kept alive for policy reasons.
537 // Keep alive the transitive closure of all such referents.
538 void
539 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
540 ReferencePolicy* policy,
541 BoolObjectClosure* is_alive,
542 OopClosure* keep_alive,
543 VoidClosure* complete_gc) {
544 assert(policy != NULL, "Must have a non-NULL policy");
545 DiscoveredListIterator iter(refs_list, keep_alive, is_alive, _discovered_list_needs_barrier);
546 // Decide which softly reachable refs should be kept alive.
547 while (iter.has_next()) {
548 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
549 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
550 if (referent_is_dead &&
551 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
552 if (TraceReferenceGC) {
553 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
554 (void *)iter.obj(), iter.obj()->klass()->internal_name());
555 }
556 // Remove Reference object from list
557 iter.remove();
558 // Make the Reference object active again
559 iter.make_active();
560 // keep the referent around
561 iter.make_referent_alive();
562 iter.move_to_next();
563 } else {
564 iter.next();
565 }
566 }
567 // Close the reachable set
568 complete_gc->do_void();
569 NOT_PRODUCT(
570 if (PrintGCDetails && TraceReferenceGC) {
571 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
572 "discovered Refs by policy, from list " INTPTR_FORMAT,
573 iter.removed(), iter.processed(), (address)refs_list.head());
574 }
575 )
576 }
577
578 // Traverse the list and remove any Refs that are not active, or
579 // whose referents are either alive or NULL.
580 void
581 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
582 BoolObjectClosure* is_alive,
583 OopClosure* keep_alive) {
584 assert(discovery_is_atomic(), "Error");
585 DiscoveredListIterator iter(refs_list, keep_alive, is_alive, _discovered_list_needs_barrier);
586 while (iter.has_next()) {
587 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
588 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
589 assert(next == NULL, "Should not discover inactive Reference");
590 if (iter.is_referent_alive()) {
591 if (TraceReferenceGC) {
592 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
593 (void *)iter.obj(), iter.obj()->klass()->internal_name());
594 }
595 // The referent is reachable after all.
596 // Remove Reference object from list.
597 iter.remove();
598 // Update the referent pointer as necessary: Note that this
599 // should not entail any recursive marking because the
600 // referent must already have been traversed.
601 iter.make_referent_alive();
602 iter.move_to_next();
603 } else {
604 iter.next();
605 }
606 }
607 NOT_PRODUCT(
608 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
609 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
610 "Refs in discovered list " INTPTR_FORMAT,
611 iter.removed(), iter.processed(), (address)refs_list.head());
612 }
613 )
614 }
615
616 void
617 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
618 BoolObjectClosure* is_alive,
619 OopClosure* keep_alive,
620 VoidClosure* complete_gc) {
621 assert(!discovery_is_atomic(), "Error");
622 DiscoveredListIterator iter(refs_list, keep_alive, is_alive, _discovered_list_needs_barrier);
623 while (iter.has_next()) {
624 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
625 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
626 oop next = java_lang_ref_Reference::next(iter.obj());
627 if ((iter.referent() == NULL || iter.is_referent_alive() ||
628 next != NULL)) {
629 assert(next->is_oop_or_null(), "bad next field");
630 // Remove Reference object from list
631 iter.remove();
632 // Trace the cohorts
633 iter.make_referent_alive();
634 if (UseCompressedOops) {
635 keep_alive->do_oop((narrowOop*)next_addr);
636 } else {
637 keep_alive->do_oop((oop*)next_addr);
638 }
639 iter.move_to_next();
640 } else {
641 iter.next();
642 }
643 }
644 // Now close the newly reachable set
645 complete_gc->do_void();
646 NOT_PRODUCT(
647 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
648 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
649 "Refs in discovered list " INTPTR_FORMAT,
650 iter.removed(), iter.processed(), (address)refs_list.head());
651 }
652 )
653 }
654
655 // Traverse the list and process the referents, by either
656 // clearing them or keeping them (and their reachable
657 // closure) alive.
658 void
659 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
660 bool clear_referent,
661 BoolObjectClosure* is_alive,
662 OopClosure* keep_alive,
663 VoidClosure* complete_gc) {
664 ResourceMark rm;
665 DiscoveredListIterator iter(refs_list, keep_alive, is_alive, _discovered_list_needs_barrier);
666 while (iter.has_next()) {
667 iter.update_discovered();
668 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
669 if (clear_referent) {
670 // NULL out referent pointer
671 iter.clear_referent();
672 } else {
673 // keep the referent around
674 iter.make_referent_alive();
675 }
676 if (TraceReferenceGC) {
677 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
678 clear_referent ? "cleared " : "",
679 (void *)iter.obj(), iter.obj()->klass()->internal_name());
680 }
681 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
682 iter.next();
683 }
684 // Remember to update the next pointer of the last ref.
685 iter.update_discovered();
686 // Close the reachable set
687 complete_gc->do_void();
688 }
689
690 void
691 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
692 oop obj = NULL;
693 oop next = refs_list.head();
694 while (next != obj) {
695 obj = next;
696 next = java_lang_ref_Reference::discovered(obj);
697 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
698 }
699 refs_list.set_head(NULL);
700 refs_list.set_length(0);
701 }
702
703 void
704 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
705 clear_discovered_references(refs_list);
706 }
707
708 void ReferenceProcessor::abandon_partial_discovery() {
709 // loop over the lists
710 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
711 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
712 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
713 }
714 abandon_partial_discovered_list(_discovered_refs[i]);
715 }
716 }
717
718 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
719 public:
720 RefProcPhase1Task(ReferenceProcessor& ref_processor,
721 DiscoveredList refs_lists[],
722 ReferencePolicy* policy,
723 bool marks_oops_alive)
724 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
725 _policy(policy)
726 { }
727 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
728 OopClosure& keep_alive,
729 VoidClosure& complete_gc)
730 {
731 Thread* thr = Thread::current();
732 int refs_list_index = ((WorkerThread*)thr)->id();
733 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
734 &is_alive, &keep_alive, &complete_gc);
735 }
736 private:
737 ReferencePolicy* _policy;
738 };
739
740 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
741 public:
742 RefProcPhase2Task(ReferenceProcessor& ref_processor,
743 DiscoveredList refs_lists[],
744 bool marks_oops_alive)
745 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
746 { }
747 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
748 OopClosure& keep_alive,
749 VoidClosure& complete_gc)
750 {
751 _ref_processor.process_phase2(_refs_lists[i],
752 &is_alive, &keep_alive, &complete_gc);
753 }
754 };
755
756 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
757 public:
758 RefProcPhase3Task(ReferenceProcessor& ref_processor,
759 DiscoveredList refs_lists[],
760 bool clear_referent,
761 bool marks_oops_alive)
762 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
763 _clear_referent(clear_referent)
764 { }
765 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
766 OopClosure& keep_alive,
767 VoidClosure& complete_gc)
768 {
769 // Don't use "refs_list_index" calculated in this way because
770 // balance_queues() has moved the Ref's into the first n queues.
771 // Thread* thr = Thread::current();
772 // int refs_list_index = ((WorkerThread*)thr)->id();
773 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
774 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
775 &is_alive, &keep_alive, &complete_gc);
776 }
777 private:
778 bool _clear_referent;
779 };
780
781 void ReferenceProcessor::set_discovered(oop ref, oop value) {
782 java_lang_ref_Reference::set_discovered_raw(ref, value);
783 if (_discovered_list_needs_barrier) {
784 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(ref), value);
785 }
786 }
787
788 // Balances reference queues.
789 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
790 // queues[0, 1, ..., _num_q-1] because only the first _num_q
791 // corresponding to the active workers will be processed.
792 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
793 {
794 // calculate total length
795 size_t total_refs = 0;
796 if (TraceReferenceGC && PrintGCDetails) {
797 gclog_or_tty->print_cr("\nBalance ref_lists ");
798 }
799
800 for (uint i = 0; i < _max_num_q; ++i) {
801 total_refs += ref_lists[i].length();
802 if (TraceReferenceGC && PrintGCDetails) {
803 gclog_or_tty->print("%d ", ref_lists[i].length());
804 }
805 }
806 if (TraceReferenceGC && PrintGCDetails) {
807 gclog_or_tty->print_cr(" = %d", total_refs);
808 }
809 size_t avg_refs = total_refs / _num_q + 1;
810 uint to_idx = 0;
811 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
812 bool move_all = false;
813 if (from_idx >= _num_q) {
814 move_all = ref_lists[from_idx].length() > 0;
815 }
816 while ((ref_lists[from_idx].length() > avg_refs) ||
817 move_all) {
818 assert(to_idx < _num_q, "Sanity Check!");
819 if (ref_lists[to_idx].length() < avg_refs) {
820 // move superfluous refs
821 size_t refs_to_move;
822 // Move all the Ref's if the from queue will not be processed.
823 if (move_all) {
824 refs_to_move = MIN2(ref_lists[from_idx].length(),
825 avg_refs - ref_lists[to_idx].length());
826 } else {
827 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
828 avg_refs - ref_lists[to_idx].length());
829 }
830
831 assert(refs_to_move > 0, "otherwise the code below will fail");
832
833 oop move_head = ref_lists[from_idx].head();
834 oop move_tail = move_head;
835 oop new_head = move_head;
836 // find an element to split the list on
837 for (size_t j = 0; j < refs_to_move; ++j) {
838 move_tail = new_head;
839 new_head = java_lang_ref_Reference::discovered(new_head);
840 }
841
842 // Add the chain to the to list.
843 if (ref_lists[to_idx].head() == NULL) {
844 // to list is empty. Make a loop at the end.
845 set_discovered(move_tail, move_tail);
846 } else {
847 set_discovered(move_tail, ref_lists[to_idx].head());
848 }
849 ref_lists[to_idx].set_head(move_head);
850 ref_lists[to_idx].inc_length(refs_to_move);
851
852 // Remove the chain from the from list.
853 if (move_tail == new_head) {
854 // We found the end of the from list.
855 ref_lists[from_idx].set_head(NULL);
856 } else {
857 ref_lists[from_idx].set_head(new_head);
858 }
859 ref_lists[from_idx].dec_length(refs_to_move);
860 if (ref_lists[from_idx].length() == 0) {
861 break;
862 }
863 } else {
864 to_idx = (to_idx + 1) % _num_q;
865 }
866 }
867 }
868 #ifdef ASSERT
869 size_t balanced_total_refs = 0;
870 for (uint i = 0; i < _max_num_q; ++i) {
871 balanced_total_refs += ref_lists[i].length();
872 if (TraceReferenceGC && PrintGCDetails) {
873 gclog_or_tty->print("%d ", ref_lists[i].length());
874 }
875 }
876 if (TraceReferenceGC && PrintGCDetails) {
877 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
878 gclog_or_tty->flush();
879 }
880 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
881 #endif
882 }
883
884 void ReferenceProcessor::balance_all_queues() {
885 balance_queues(_discoveredSoftRefs);
886 balance_queues(_discoveredWeakRefs);
887 balance_queues(_discoveredFinalRefs);
888 balance_queues(_discoveredPhantomRefs);
889 }
890
891 size_t
892 ReferenceProcessor::process_discovered_reflist(
893 DiscoveredList refs_lists[],
894 ReferencePolicy* policy,
895 bool clear_referent,
896 BoolObjectClosure* is_alive,
897 OopClosure* keep_alive,
898 VoidClosure* complete_gc,
899 AbstractRefProcTaskExecutor* task_executor)
900 {
901 bool mt_processing = task_executor != NULL && _processing_is_mt;
902 // If discovery used MT and a dynamic number of GC threads, then
903 // the queues must be balanced for correctness if fewer than the
904 // maximum number of queues were used. The number of queue used
905 // during discovery may be different than the number to be used
906 // for processing so don't depend of _num_q < _max_num_q as part
907 // of the test.
908 bool must_balance = _discovery_is_mt;
909
910 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
911 must_balance) {
912 balance_queues(refs_lists);
913 }
914
915 size_t total_list_count = total_count(refs_lists);
916
917 if (PrintReferenceGC && PrintGCDetails) {
918 gclog_or_tty->print(", %u refs", total_list_count);
919 }
920
921 // Phase 1 (soft refs only):
922 // . Traverse the list and remove any SoftReferences whose
923 // referents are not alive, but that should be kept alive for
924 // policy reasons. Keep alive the transitive closure of all
925 // such referents.
926 if (policy != NULL) {
927 if (mt_processing) {
928 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
929 task_executor->execute(phase1);
930 } else {
931 for (uint i = 0; i < _max_num_q; i++) {
932 process_phase1(refs_lists[i], policy,
933 is_alive, keep_alive, complete_gc);
934 }
935 }
936 } else { // policy == NULL
937 assert(refs_lists != _discoveredSoftRefs,
938 "Policy must be specified for soft references.");
939 }
940
941 // Phase 2:
942 // . Traverse the list and remove any refs whose referents are alive.
943 if (mt_processing) {
944 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
945 task_executor->execute(phase2);
946 } else {
947 for (uint i = 0; i < _max_num_q; i++) {
948 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
949 }
950 }
951
952 // Phase 3:
953 // . Traverse the list and process referents as appropriate.
954 if (mt_processing) {
955 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
956 task_executor->execute(phase3);
957 } else {
958 for (uint i = 0; i < _max_num_q; i++) {
959 process_phase3(refs_lists[i], clear_referent,
960 is_alive, keep_alive, complete_gc);
961 }
962 }
963
964 return total_list_count;
965 }
966
967 void ReferenceProcessor::clean_up_discovered_references() {
968 // loop over the lists
969 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
970 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
971 gclog_or_tty->print_cr(
972 "\nScrubbing %s discovered list of Null referents",
973 list_name(i));
974 }
975 clean_up_discovered_reflist(_discovered_refs[i]);
976 }
977 }
978
979 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
980 assert(!discovery_is_atomic(), "Else why call this method?");
981 DiscoveredListIterator iter(refs_list, NULL, NULL, _discovered_list_needs_barrier);
982 while (iter.has_next()) {
983 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
984 oop next = java_lang_ref_Reference::next(iter.obj());
985 assert(next->is_oop_or_null(), "bad next field");
986 // If referent has been cleared or Reference is not active,
987 // drop it.
988 if (iter.referent() == NULL || next != NULL) {
989 debug_only(
990 if (PrintGCDetails && TraceReferenceGC) {
991 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
992 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
993 " and referent: " INTPTR_FORMAT,
994 (void *)iter.obj(), (void *)next, (void *)iter.referent());
995 }
996 )
997 // Remove Reference object from list
998 iter.remove();
999 iter.move_to_next();
1000 } else {
1001 iter.next();
1002 }
1003 }
1004 NOT_PRODUCT(
1005 if (PrintGCDetails && TraceReferenceGC) {
1006 gclog_or_tty->print(
1007 " Removed %d Refs with NULL referents out of %d discovered Refs",
1008 iter.removed(), iter.processed());
1009 }
1010 )
1011 }
1012
1013 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1014 uint id = 0;
1015 // Determine the queue index to use for this object.
1016 if (_discovery_is_mt) {
1017 // During a multi-threaded discovery phase,
1018 // each thread saves to its "own" list.
1019 Thread* thr = Thread::current();
1020 id = thr->as_Worker_thread()->id();
1021 } else {
1022 // single-threaded discovery, we save in round-robin
1023 // fashion to each of the lists.
1024 if (_processing_is_mt) {
1025 id = next_id();
1026 }
1027 }
1028 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1029
1030 // Get the discovered queue to which we will add
1031 DiscoveredList* list = NULL;
1032 switch (rt) {
1033 case REF_OTHER:
1034 // Unknown reference type, no special treatment
1035 break;
1036 case REF_SOFT:
1037 list = &_discoveredSoftRefs[id];
1038 break;
1039 case REF_WEAK:
1040 list = &_discoveredWeakRefs[id];
1041 break;
1042 case REF_FINAL:
1043 list = &_discoveredFinalRefs[id];
1044 break;
1045 case REF_PHANTOM:
1046 list = &_discoveredPhantomRefs[id];
1047 break;
1048 case REF_NONE:
1049 // we should not reach here if we are an InstanceRefKlass
1050 default:
1051 ShouldNotReachHere();
1052 }
1053 if (TraceReferenceGC && PrintGCDetails) {
1054 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1055 }
1056 return list;
1057 }
1058
1059 inline void
1060 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1061 oop obj,
1062 HeapWord* discovered_addr) {
1063 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1064 // First we must make sure this object is only enqueued once. CAS in a non null
1065 // discovered_addr.
1066 oop current_head = refs_list.head();
1067 // The last ref must have its discovered field pointing to itself.
1068 oop next_discovered = (current_head != NULL) ? current_head : obj;
1069
1070 // Note: In the case of G1, this specific pre-barrier is strictly
1071 // not necessary because the only case we are interested in
1072 // here is when *discovered_addr is NULL (see the CAS further below),
1073 // so this will expand to nothing. As a result, we have manually
1074 // elided this out for G1, but left in the test for some future
1075 // collector that might have need for a pre-barrier here, e.g.:-
1076 // oopDesc::bs()->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1077 assert(!_discovered_list_needs_barrier || UseG1GC,
1078 "Need to check non-G1 collector: "
1079 "may need a pre-write-barrier for CAS from NULL below");
1080 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1081 NULL);
1082 if (retest == NULL) {
1083 // This thread just won the right to enqueue the object.
1084 // We have separate lists for enqueueing, so no synchronization
1085 // is necessary.
1086 refs_list.set_head(obj);
1087 refs_list.inc_length(1);
1088 if (_discovered_list_needs_barrier) {
1089 oopDesc::bs()->write_ref_field((void*)discovered_addr, next_discovered);
1090 }
1091
1092 if (TraceReferenceGC) {
1093 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1094 (void *)obj, obj->klass()->internal_name());
1095 }
1096 } else {
1097 // If retest was non NULL, another thread beat us to it:
1098 // The reference has already been discovered...
1099 if (TraceReferenceGC) {
1100 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1101 (void *)obj, obj->klass()->internal_name());
1102 }
1103 }
1104 }
1105
1106 #ifndef PRODUCT
1107 // Non-atomic (i.e. concurrent) discovery might allow us
1108 // to observe j.l.References with NULL referents, being those
1109 // cleared concurrently by mutators during (or after) discovery.
1110 void ReferenceProcessor::verify_referent(oop obj) {
1111 bool da = discovery_is_atomic();
1112 oop referent = java_lang_ref_Reference::referent(obj);
1113 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1114 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1115 INTPTR_FORMAT " during %satomic discovery ",
1116 (void *)referent, (void *)obj, da ? "" : "non-"));
1117 }
1118 #endif
1119
1120 // We mention two of several possible choices here:
1121 // #0: if the reference object is not in the "originating generation"
1122 // (or part of the heap being collected, indicated by our "span"
1123 // we don't treat it specially (i.e. we scan it as we would
1124 // a normal oop, treating its references as strong references).
1125 // This means that references can't be discovered unless their
1126 // referent is also in the same span. This is the simplest,
1127 // most "local" and most conservative approach, albeit one
1128 // that may cause weak references to be enqueued least promptly.
1129 // We call this choice the "ReferenceBasedDiscovery" policy.
1130 // #1: the reference object may be in any generation (span), but if
1131 // the referent is in the generation (span) being currently collected
1132 // then we can discover the reference object, provided
1133 // the object has not already been discovered by
1134 // a different concurrently running collector (as may be the
1135 // case, for instance, if the reference object is in CMS and
1136 // the referent in DefNewGeneration), and provided the processing
1137 // of this reference object by the current collector will
1138 // appear atomic to every other collector in the system.
1139 // (Thus, for instance, a concurrent collector may not
1140 // discover references in other generations even if the
1141 // referent is in its own generation). This policy may,
1142 // in certain cases, enqueue references somewhat sooner than
1143 // might Policy #0 above, but at marginally increased cost
1144 // and complexity in processing these references.
1145 // We call this choice the "RefeferentBasedDiscovery" policy.
1146 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1147 // Make sure we are discovering refs (rather than processing discovered refs).
1148 if (!_discovering_refs || !RegisterReferences) {
1149 return false;
1150 }
1151 // We only discover active references.
1152 oop next = java_lang_ref_Reference::next(obj);
1153 if (next != NULL) { // Ref is no longer active
1154 return false;
1155 }
1156
1157 HeapWord* obj_addr = (HeapWord*)obj;
1158 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1159 !_span.contains(obj_addr)) {
1160 // Reference is not in the originating generation;
1161 // don't treat it specially (i.e. we want to scan it as a normal
1162 // object with strong references).
1163 return false;
1164 }
1165
1166 // We only discover references whose referents are not (yet)
1167 // known to be strongly reachable.
1168 if (is_alive_non_header() != NULL) {
1169 verify_referent(obj);
1170 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1171 return false; // referent is reachable
1172 }
1173 }
1174 if (rt == REF_SOFT) {
1175 // For soft refs we can decide now if these are not
1176 // current candidates for clearing, in which case we
1177 // can mark through them now, rather than delaying that
1178 // to the reference-processing phase. Since all current
1179 // time-stamp policies advance the soft-ref clock only
1180 // at a major collection cycle, this is always currently
1181 // accurate.
1182 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1183 return false;
1184 }
1185 }
1186
1187 ResourceMark rm; // Needed for tracing.
1188
1189 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1190 const oop discovered = java_lang_ref_Reference::discovered(obj);
1191 assert(discovered->is_oop_or_null(), "bad discovered field");
1192 if (discovered != NULL) {
1193 // The reference has already been discovered...
1194 if (TraceReferenceGC) {
1195 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1196 (void *)obj, obj->klass()->internal_name());
1197 }
1198 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1199 // assumes that an object is not processed twice;
1200 // if it's been already discovered it must be on another
1201 // generation's discovered list; so we won't discover it.
1202 return false;
1203 } else {
1204 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1205 "Unrecognized policy");
1206 // Check assumption that an object is not potentially
1207 // discovered twice except by concurrent collectors that potentially
1208 // trace the same Reference object twice.
1209 assert(UseConcMarkSweepGC || UseG1GC,
1210 "Only possible with a concurrent marking collector");
1211 return true;
1212 }
1213 }
1214
1215 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1216 verify_referent(obj);
1217 // Discover if and only if EITHER:
1218 // .. reference is in our span, OR
1219 // .. we are an atomic collector and referent is in our span
1220 if (_span.contains(obj_addr) ||
1221 (discovery_is_atomic() &&
1222 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1223 // should_enqueue = true;
1224 } else {
1225 return false;
1226 }
1227 } else {
1228 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1229 _span.contains(obj_addr), "code inconsistency");
1230 }
1231
1232 // Get the right type of discovered queue head.
1233 DiscoveredList* list = get_discovered_list(rt);
1234 if (list == NULL) {
1235 return false; // nothing special needs to be done
1236 }
1237
1238 if (_discovery_is_mt) {
1239 add_to_discovered_list_mt(*list, obj, discovered_addr);
1240 } else {
1241 // If "_discovered_list_needs_barrier", we do write barriers when
1242 // updating the discovered reference list. Otherwise, we do a raw store
1243 // here: the field will be visited later when processing the discovered
1244 // references.
1245 oop current_head = list->head();
1246 // The last ref must have its discovered field pointing to itself.
1247 oop next_discovered = (current_head != NULL) ? current_head : obj;
1248
1249 // As in the case further above, since we are over-writing a NULL
1250 // pre-value, we can safely elide the pre-barrier here for the case of G1.
1251 // e.g.:- oopDesc::bs()->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1252 assert(discovered == NULL, "control point invariant");
1253 assert(!_discovered_list_needs_barrier || UseG1GC,
1254 "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
1255 oop_store_raw(discovered_addr, next_discovered);
1256 if (_discovered_list_needs_barrier) {
1257 oopDesc::bs()->write_ref_field((void*)discovered_addr, next_discovered);
1258 }
1259 list->set_head(obj);
1260 list->inc_length(1);
1261
1262 if (TraceReferenceGC) {
1263 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1264 (void *)obj, obj->klass()->internal_name());
1265 }
1266 }
1267 assert(obj->is_oop(), "Discovered a bad reference");
1268 verify_referent(obj);
1269 return true;
1270 }
1271
1272 // Preclean the discovered references by removing those
1273 // whose referents are alive, and by marking from those that
1274 // are not active. These lists can be handled here
1275 // in any order and, indeed, concurrently.
1276 void ReferenceProcessor::preclean_discovered_references(
1277 BoolObjectClosure* is_alive,
1278 OopClosure* keep_alive,
1279 VoidClosure* complete_gc,
1280 YieldClosure* yield,
1281 GCTimer* gc_timer) {
1282
1283 NOT_PRODUCT(verify_ok_to_handle_reflists());
1284
1285 // Soft references
1286 {
1287 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1288 false, gc_timer);
1289 for (uint i = 0; i < _max_num_q; i++) {
1290 if (yield->should_return()) {
1291 return;
1292 }
1293 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1294 keep_alive, complete_gc, yield);
1295 }
1296 }
1297
1298 // Weak references
1299 {
1300 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1301 false, gc_timer);
1302 for (uint i = 0; i < _max_num_q; i++) {
1303 if (yield->should_return()) {
1304 return;
1305 }
1306 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1307 keep_alive, complete_gc, yield);
1308 }
1309 }
1310
1311 // Final references
1312 {
1313 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1314 false, gc_timer);
1315 for (uint i = 0; i < _max_num_q; i++) {
1316 if (yield->should_return()) {
1317 return;
1318 }
1319 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1320 keep_alive, complete_gc, yield);
1321 }
1322 }
1323
1324 // Phantom references
1325 {
1326 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1327 false, gc_timer);
1328 for (uint i = 0; i < _max_num_q; i++) {
1329 if (yield->should_return()) {
1330 return;
1331 }
1332 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1333 keep_alive, complete_gc, yield);
1334 }
1335 }
1336 }
1337
1338 // Walk the given discovered ref list, and remove all reference objects
1339 // whose referents are still alive, whose referents are NULL or which
1340 // are not active (have a non-NULL next field). NOTE: When we are
1341 // thus precleaning the ref lists (which happens single-threaded today),
1342 // we do not disable refs discovery to honor the correct semantics of
1343 // java.lang.Reference. As a result, we need to be careful below
1344 // that ref removal steps interleave safely with ref discovery steps
1345 // (in this thread).
1346 void
1347 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1348 BoolObjectClosure* is_alive,
1349 OopClosure* keep_alive,
1350 VoidClosure* complete_gc,
1351 YieldClosure* yield) {
1352 DiscoveredListIterator iter(refs_list, keep_alive, is_alive, _discovered_list_needs_barrier);
1353 while (iter.has_next()) {
1354 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1355 oop obj = iter.obj();
1356 oop next = java_lang_ref_Reference::next(obj);
1357 if (iter.referent() == NULL || iter.is_referent_alive() ||
1358 next != NULL) {
1359 // The referent has been cleared, or is alive, or the Reference is not
1360 // active; we need to trace and mark its cohort.
1361 if (TraceReferenceGC) {
1362 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1363 (void *)iter.obj(), iter.obj()->klass()->internal_name());
1364 }
1365 // Remove Reference object from list
1366 iter.remove();
1367 // Keep alive its cohort.
1368 iter.make_referent_alive();
1369 if (UseCompressedOops) {
1370 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1371 keep_alive->do_oop(next_addr);
1372 } else {
1373 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1374 keep_alive->do_oop(next_addr);
1375 }
1376 iter.move_to_next();
1377 } else {
1378 iter.next();
1379 }
1380 }
1381 // Close the reachable set
1382 complete_gc->do_void();
1383
1384 NOT_PRODUCT(
1385 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1386 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1387 "Refs in discovered list " INTPTR_FORMAT,
1388 iter.removed(), iter.processed(), (address)refs_list.head());
1389 }
1390 )
1391 }
1392
1393 const char* ReferenceProcessor::list_name(uint i) {
1394 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1395 "Out of bounds index");
1396
1397 int j = i / _max_num_q;
1398 switch (j) {
1399 case 0: return "SoftRef";
1400 case 1: return "WeakRef";
1401 case 2: return "FinalRef";
1402 case 3: return "PhantomRef";
1403 }
1404 ShouldNotReachHere();
1405 return NULL;
1406 }
1407
1408 #ifndef PRODUCT
1409 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1410 // empty for now
1411 }
1412 #endif
1413
1414 #ifndef PRODUCT
1415 void ReferenceProcessor::clear_discovered_references() {
1416 guarantee(!_discovering_refs, "Discovering refs?");
1417 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1418 clear_discovered_references(_discovered_refs[i]);
1419 }
1420 }
1421
1422 #endif // PRODUCT