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-deccreasing 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 "Unrecongnized 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 _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q];
122
123 // Initialize all entries to NULL
124 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
125 _discovered_refs[i].set_head(NULL);
126 _discovered_refs[i].set_length(0);
127 }
128
129 setup_policy(false /* default soft ref policy */);
130 }
131
132 #ifndef PRODUCT
133 void ReferenceProcessor::verify_no_references_recorded() {
134 guarantee(!_discovering_refs, "Discovering refs?");
135 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
136 guarantee(_discovered_refs[i].is_empty(),
137 "Found non-empty discovered list");
138 }
139 }
140 #endif
141
142 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
143 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
144 if (UseCompressedOops) {
145 f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
146 } else {
147 f->do_oop((oop*)_discovered_refs[i].adr_head());
148 }
149 }
150 }
151
152 void ReferenceProcessor::update_soft_ref_master_clock() {
153 // Update (advance) the soft ref master clock field. This must be done
154 // after processing the soft ref list.
155
156 // We need a monotonically non-deccreasing time in ms but
157 // os::javaTimeMillis() does not guarantee monotonicity.
158 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
159 jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
160 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
161
162 NOT_PRODUCT(
163 if (now < _soft_ref_timestamp_clock) {
164 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
165 _soft_ref_timestamp_clock, now);
166 }
167 )
168 // The values of now and _soft_ref_timestamp_clock are set using
169 // javaTimeNanos(), which is guaranteed to be monotonically
170 // non-decreasing provided the underlying platform provides such
171 // a time source (and it is bug free).
172 // In product mode, however, protect ourselves from non-monotonicty.
173 if (now > _soft_ref_timestamp_clock) {
174 _soft_ref_timestamp_clock = now;
175 java_lang_ref_SoftReference::set_clock(now);
176 }
177 // Else leave clock stalled at its old value until time progresses
178 // past clock value.
179 }
180
181 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
182 size_t total = 0;
183 for (uint i = 0; i < _max_num_q; ++i) {
184 total += lists[i].length();
185 }
186 return total;
187 }
188
189 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
190 BoolObjectClosure* is_alive,
191 OopClosure* keep_alive,
192 VoidClosure* complete_gc,
193 AbstractRefProcTaskExecutor* task_executor,
194 GCTimer* gc_timer,
195 GCId gc_id) {
196 NOT_PRODUCT(verify_ok_to_handle_reflists());
197
198 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
199 // Stop treating discovered references specially.
200 disable_discovery();
201
202 // If discovery was concurrent, someone could have modified
203 // the value of the static field in the j.l.r.SoftReference
204 // class that holds the soft reference timestamp clock using
205 // reflection or Unsafe between when discovery was enabled and
206 // now. Unconditionally update the static field in ReferenceProcessor
207 // here so that we use the new value during processing of the
208 // discovered soft refs.
209
210 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
211
212 bool trace_time = PrintGCDetails && PrintReferenceGC;
213
214 // Soft references
215 size_t soft_count = 0;
216 {
217 GCTraceTime tt("SoftReference", trace_time, false, gc_timer, gc_id);
218 soft_count =
219 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
220 is_alive, keep_alive, complete_gc, task_executor);
221 }
222
223 update_soft_ref_master_clock();
224
225 // Weak references
226 size_t weak_count = 0;
227 {
228 GCTraceTime tt("WeakReference", trace_time, false, gc_timer, gc_id);
229 weak_count =
230 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
231 is_alive, keep_alive, complete_gc, task_executor);
232 }
233
234 // Final references
235 size_t final_count = 0;
236 {
237 GCTraceTime tt("FinalReference", trace_time, false, gc_timer, gc_id);
238 final_count =
239 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
240 is_alive, keep_alive, complete_gc, task_executor);
241 }
242
243 // Phantom references
244 size_t phantom_count = 0;
245 {
246 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer, gc_id);
247 phantom_count =
248 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
249 is_alive, keep_alive, complete_gc, task_executor);
250
251 // Process cleaners, but include them in phantom statistics. We expect
252 // Cleaner references to be temporary, and don't want to deal with
253 // possible incompatibilities arising from making it more visible.
254 phantom_count +=
255 process_discovered_reflist(_discoveredCleanerRefs, NULL, true,
256 is_alive, keep_alive, complete_gc, task_executor);
257 }
258
259 // Weak global JNI references. It would make more sense (semantically) to
260 // traverse these simultaneously with the regular weak references above, but
261 // that is not how the JDK1.2 specification is. See #4126360. Native code can
262 // thus use JNI weak references to circumvent the phantom references and
263 // resurrect a "post-mortem" object.
264 {
265 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer, gc_id);
266 if (task_executor != NULL) {
267 task_executor->set_single_threaded_mode();
268 }
269 process_phaseJNI(is_alive, keep_alive, complete_gc);
270 }
271
272 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
273 }
274
275 #ifndef PRODUCT
276 // Calculate the number of jni handles.
277 uint ReferenceProcessor::count_jni_refs() {
278 class AlwaysAliveClosure: public BoolObjectClosure {
279 public:
280 virtual bool do_object_b(oop obj) { return true; }
281 };
282
283 class CountHandleClosure: public OopClosure {
284 private:
285 int _count;
286 public:
287 CountHandleClosure(): _count(0) {}
288 void do_oop(oop* unused) { _count++; }
289 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
290 int count() { return _count; }
291 };
292 CountHandleClosure global_handle_count;
293 AlwaysAliveClosure always_alive;
294 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
295 return global_handle_count.count();
296 }
297 #endif
298
299 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
300 OopClosure* keep_alive,
301 VoidClosure* complete_gc) {
302 #ifndef PRODUCT
303 if (PrintGCDetails && PrintReferenceGC) {
304 unsigned int count = count_jni_refs();
305 gclog_or_tty->print(", %u refs", count);
306 }
307 #endif
308 if (UseShenandoahGC) {
309 // Workaround bugs with JNI weak reference processing, by pessimistically
310 // assuming all JNI weak refs are alive. This effectively makes JNI weak refs
311 // non-reclaimable. // TODO: Fix this properly
312 class AlwaysAliveClosure: public BoolObjectClosure {
313 public:
314 virtual bool do_object_b(oop obj) { return true; }
315 };
316
317 AlwaysAliveClosure always_alive;
318 JNIHandles::weak_oops_do(&always_alive, keep_alive);
319 } else {
320 JNIHandles::weak_oops_do(is_alive, keep_alive);
321 }
322 complete_gc->do_void();
323 }
324
325
326 template <class T>
327 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
328 AbstractRefProcTaskExecutor* task_executor) {
329
330 // Remember old value of pending references list
331 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
332 T old_pending_list_value = *pending_list_addr;
333
334 // Enqueue references that are not made active again, and
335 // clear the decks for the next collection (cycle).
336 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
337 // Do the post-barrier on pending_list_addr missed in
338 // enqueue_discovered_reflist.
339 oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
340
341 // Stop treating discovered references specially.
342 ref->disable_discovery();
343
344 // Return true if new pending references were added
345 return ! oopDesc::safe_equals(old_pending_list_value, *pending_list_addr);
346 }
347
348 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
349 NOT_PRODUCT(verify_ok_to_handle_reflists());
350 if (UseCompressedOops) {
351 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
352 } else {
353 return enqueue_discovered_ref_helper<oop>(this, task_executor);
354 }
355 }
356
357 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
358 HeapWord* pending_list_addr) {
359 // Given a list of refs linked through the "discovered" field
360 // (java.lang.ref.Reference.discovered), self-loop their "next" field
361 // thus distinguishing them from active References, then
362 // prepend them to the pending list.
363 //
364 // The Java threads will see the Reference objects linked together through
365 // the discovered field. Instead of trying to do the write barrier updates
366 // in all places in the reference processor where we manipulate the discovered
367 // field we make sure to do the barrier here where we anyway iterate through
368 // all linked Reference objects. Note that it is important to not dirty any
369 // cards during reference processing since this will cause card table
370 // verification to fail for G1.
371 //
372 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
373 // the "next" field is used to chain the pending list, not the discovered
374 // field.
375 if (TraceReferenceGC && PrintGCDetails) {
376 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
377 INTPTR_FORMAT, (address)refs_list.head());
378 }
379
380 oop obj = NULL;
381 oop next_d = refs_list.head();
382 if (pending_list_uses_discovered_field()) { // New behavior
383 // Walk down the list, self-looping the next field
384 // so that the References are not considered active.
385 while (! oopDesc::safe_equals(obj, next_d)) {
386 obj = next_d;
387 assert(obj->is_instanceRef(), "should be reference object");
388 next_d = java_lang_ref_Reference::discovered(obj);
389 if (TraceReferenceGC && PrintGCDetails) {
390 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
391 (void *)obj, (void *)next_d);
392 }
393 assert(java_lang_ref_Reference::next(obj) == NULL,
394 "Reference not active; should not be discovered");
395 // Self-loop next, so as to make Ref not active.
396 java_lang_ref_Reference::set_next_raw(obj, obj);
397 if (! oopDesc::safe_equals(next_d, obj)) {
398 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
399 } else {
400 // This is the last object.
401 // Swap refs_list into pending_list_addr and
402 // set obj's discovered to what we read from pending_list_addr.
403 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
404 // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
405 java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
406 oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
407 }
408 }
409 } else { // Old behaviour
410 // Walk down the list, copying the discovered field into
411 // the next field and clearing the discovered field.
412 while (obj != next_d) {
413 obj = next_d;
414 assert(obj->is_instanceRef(), "should be reference object");
415 next_d = java_lang_ref_Reference::discovered(obj);
416 if (TraceReferenceGC && PrintGCDetails) {
417 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
418 (void *)obj, (void *)next_d);
419 }
420 assert(java_lang_ref_Reference::next(obj) == NULL,
421 "The reference should not be enqueued");
422 if (next_d == obj) { // obj is last
423 // Swap refs_list into pendling_list_addr and
424 // set obj's next to what we read from pending_list_addr.
425 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
426 // Need oop_check on pending_list_addr above;
427 // see special oop-check code at the end of
428 // enqueue_discovered_reflists() further below.
429 if (old == NULL) {
430 // obj should be made to point to itself, since
431 // pending list was empty.
432 java_lang_ref_Reference::set_next(obj, obj);
433 } else {
434 java_lang_ref_Reference::set_next(obj, old);
435 }
436 } else {
437 java_lang_ref_Reference::set_next(obj, next_d);
438 }
439 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
440 }
441 }
442 }
443
444 // Parallel enqueue task
445 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
446 public:
447 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
448 DiscoveredList discovered_refs[],
449 HeapWord* pending_list_addr,
450 int n_queues)
451 : EnqueueTask(ref_processor, discovered_refs,
452 pending_list_addr, n_queues)
453 { }
454
455 virtual void work(unsigned int work_id) {
456 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
457 // Simplest first cut: static partitioning.
458 int index = work_id;
459 // The increment on "index" must correspond to the maximum number of queues
460 // (n_queues) with which that ReferenceProcessor was created. That
461 // is because of the "clever" way the discovered references lists were
462 // allocated and are indexed into.
463 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
464 for (int j = 0;
465 j < ReferenceProcessor::number_of_subclasses_of_ref();
466 j++, index += _n_queues) {
467 _ref_processor.enqueue_discovered_reflist(
468 _refs_lists[index], _pending_list_addr);
469 _refs_lists[index].set_head(NULL);
470 _refs_lists[index].set_length(0);
471 }
472 }
473 };
474
475 // Enqueue references that are not made active again
476 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
477 AbstractRefProcTaskExecutor* task_executor) {
478 if (_processing_is_mt && task_executor != NULL) {
479 // Parallel code
480 RefProcEnqueueTask tsk(*this, _discovered_refs,
481 pending_list_addr, _max_num_q);
482 task_executor->execute(tsk);
483 } else {
484 // Serial code: call the parent class's implementation
485 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
486 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
487 _discovered_refs[i].set_head(NULL);
488 _discovered_refs[i].set_length(0);
489 }
490 }
491 }
492
493 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
494 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
495 oop discovered = java_lang_ref_Reference::discovered(_ref);
496 assert(_discovered_addr && discovered->is_oop_or_null(),
497 "discovered field is bad");
498 _next = discovered;
499 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
500 _referent = java_lang_ref_Reference::referent(_ref);
501 assert(Universe::heap()->is_in_reserved_or_null(_referent),
502 "Wrong oop found in java.lang.Reference object");
503 assert(allow_null_referent ?
504 _referent->is_oop_or_null()
505 : _referent->is_oop(),
506 "bad referent");
507 }
508
509 void DiscoveredListIterator::remove() {
510 assert(_ref->is_oop(), "Dropping a bad reference");
511 oop_store_raw(_discovered_addr, NULL);
512
513 // First _prev_next ref actually points into DiscoveredList (gross).
514 oop new_next;
515 if (oopDesc::safe_equals(_next, _ref)) {
516 // At the end of the list, we should make _prev point to itself.
517 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
518 // and _prev will be NULL.
519 new_next = _prev;
520 } else {
521 new_next = _next;
522 }
523 // Remove Reference object from discovered list. Note that G1 does not need a
524 // pre-barrier here because we know the Reference has already been found/marked,
525 // that's how it ended up in the discovered list in the first place.
526 oop_store_raw(_prev_next, new_next);
527 NOT_PRODUCT(_removed++);
528 _refs_list.dec_length(1);
529 }
530
531 // Make the Reference object active again.
532 void DiscoveredListIterator::make_active() {
533 // The pre barrier for G1 is probably just needed for the old
534 // reference processing behavior. Should we guard this with
535 // ReferenceProcessor::pending_list_uses_discovered_field() ?
536 if (UseG1GC) {
537 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
538 if (UseCompressedOops) {
539 oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
540 } else {
541 oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
542 }
543 }
544 java_lang_ref_Reference::set_next_raw(_ref, NULL);
545 }
546
547 void DiscoveredListIterator::clear_referent() {
548 oop_store_raw(_referent_addr, NULL);
549 }
550
551 // NOTE: process_phase*() are largely similar, and at a high level
552 // merely iterate over the extant list applying a predicate to
553 // each of its elements and possibly removing that element from the
554 // list and applying some further closures to that element.
555 // We should consider the possibility of replacing these
556 // process_phase*() methods by abstracting them into
557 // a single general iterator invocation that receives appropriate
558 // closures that accomplish this work.
559
560 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
561 // referents are not alive, but that should be kept alive for policy reasons.
562 // Keep alive the transitive closure of all such referents.
563 void
564 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
565 ReferencePolicy* policy,
566 BoolObjectClosure* is_alive,
567 OopClosure* keep_alive,
568 VoidClosure* complete_gc) {
569 assert(policy != NULL, "Must have a non-NULL policy");
570 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
571 // Decide which softly reachable refs should be kept alive.
572 while (iter.has_next()) {
573 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
574 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
575 if (referent_is_dead &&
576 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
577 if (TraceReferenceGC) {
578 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
579 (void *)iter.obj(), iter.obj()->klass()->internal_name());
580 }
581 // Remove Reference object from list
582 iter.remove();
583 // Make the Reference object active again
584 iter.make_active();
585 // keep the referent around
586 iter.make_referent_alive();
587 iter.move_to_next();
588 } else {
589 iter.next();
590 }
591 }
592 // Close the reachable set
593 complete_gc->do_void();
594 NOT_PRODUCT(
595 if (PrintGCDetails && TraceReferenceGC) {
596 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
597 "discovered Refs by policy, from list " INTPTR_FORMAT,
598 iter.removed(), iter.processed(), (address)refs_list.head());
599 }
600 )
601 }
602
603 // Traverse the list and remove any Refs that are not active, or
604 // whose referents are either alive or NULL.
605 void
606 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
607 BoolObjectClosure* is_alive,
608 OopClosure* keep_alive) {
609 assert(discovery_is_atomic(), "Error");
610 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
611 while (iter.has_next()) {
612 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
613 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
614 assert(next == NULL, "Should not discover inactive Reference");
615 if (iter.is_referent_alive()) {
616 if (TraceReferenceGC) {
617 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
618 (void *)iter.obj(), iter.obj()->klass()->internal_name());
619 }
620 // The referent is reachable after all.
621 // Remove Reference object from list.
622 iter.remove();
623 // Update the referent pointer as necessary: Note that this
624 // should not entail any recursive marking because the
625 // referent must already have been traversed.
626 iter.make_referent_alive();
627 iter.move_to_next();
628 } else {
629 iter.next();
630 }
631 }
632 NOT_PRODUCT(
633 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
634 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
635 "Refs in discovered list " INTPTR_FORMAT,
636 iter.removed(), iter.processed(), (address)refs_list.head());
637 }
638 )
639 }
640
641 void
642 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
643 BoolObjectClosure* is_alive,
644 OopClosure* keep_alive,
645 VoidClosure* complete_gc) {
646 assert(!discovery_is_atomic(), "Error");
647 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
648 while (iter.has_next()) {
649 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
650 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
651 oop next = java_lang_ref_Reference::next(iter.obj());
652 if ((iter.referent() == NULL || iter.is_referent_alive() ||
653 next != NULL)) {
654 assert(next->is_oop_or_null(), "bad next field");
655 // Remove Reference object from list
656 iter.remove();
657 // Trace the cohorts
658 iter.make_referent_alive();
659 if (UseCompressedOops) {
660 keep_alive->do_oop((narrowOop*)next_addr);
661 } else {
662 keep_alive->do_oop((oop*)next_addr);
663 }
664 iter.move_to_next();
665 } else {
666 iter.next();
667 }
668 }
669 // Now close the newly reachable set
670 complete_gc->do_void();
671 NOT_PRODUCT(
672 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
673 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
674 "Refs in discovered list " INTPTR_FORMAT,
675 iter.removed(), iter.processed(), (address)refs_list.head());
676 }
677 )
678 }
679
680 // Traverse the list and process the referents, by either
681 // clearing them or keeping them (and their reachable
682 // closure) alive.
683 void
684 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
685 bool clear_referent,
686 BoolObjectClosure* is_alive,
687 OopClosure* keep_alive,
688 VoidClosure* complete_gc) {
689 ResourceMark rm;
690 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
691 while (iter.has_next()) {
692 iter.update_discovered();
693 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
694 if (clear_referent) {
695 // NULL out referent pointer
696 iter.clear_referent();
697 } else {
698 // keep the referent around
699 iter.make_referent_alive();
700 }
701 if (TraceReferenceGC) {
702 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
703 clear_referent ? "cleared " : "",
704 (void *)iter.obj(), iter.obj()->klass()->internal_name());
705 }
706 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
707 iter.next();
708 }
709 // Remember to update the next pointer of the last ref.
710 iter.update_discovered();
711 // Close the reachable set
712 complete_gc->do_void();
713 }
714
715 void
716 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
717 oop obj = NULL;
718 oop next = refs_list.head();
719 while (! oopDesc::safe_equals(next, obj)) {
720 obj = next;
721 next = java_lang_ref_Reference::discovered(obj);
722 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
723 }
724 refs_list.set_head(NULL);
725 refs_list.set_length(0);
726 }
727
728 void
729 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
730 clear_discovered_references(refs_list);
731 }
732
733 void ReferenceProcessor::abandon_partial_discovery() {
734 // loop over the lists
735 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
736 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
737 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
738 }
739 abandon_partial_discovered_list(_discovered_refs[i]);
740 }
741 }
742
743 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
744 public:
745 RefProcPhase1Task(ReferenceProcessor& ref_processor,
746 DiscoveredList refs_lists[],
747 ReferencePolicy* policy,
748 bool marks_oops_alive)
749 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
750 _policy(policy)
751 { }
752 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
753 OopClosure& keep_alive,
754 VoidClosure& complete_gc)
755 {
756 Thread* thr = Thread::current();
757 int refs_list_index = ((WorkerThread*)thr)->id();
758 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
759 &is_alive, &keep_alive, &complete_gc);
760 }
761 private:
762 ReferencePolicy* _policy;
763 };
764
765 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
766 public:
767 RefProcPhase2Task(ReferenceProcessor& ref_processor,
768 DiscoveredList refs_lists[],
769 bool marks_oops_alive)
770 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
771 { }
772 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
773 OopClosure& keep_alive,
774 VoidClosure& complete_gc)
775 {
776 _ref_processor.process_phase2(_refs_lists[i],
777 &is_alive, &keep_alive, &complete_gc);
778 }
779 };
780
781 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
782 public:
783 RefProcPhase3Task(ReferenceProcessor& ref_processor,
784 DiscoveredList refs_lists[],
785 bool clear_referent,
786 bool marks_oops_alive)
787 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
788 _clear_referent(clear_referent)
789 { }
790 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
791 OopClosure& keep_alive,
792 VoidClosure& complete_gc)
793 {
794 // Don't use "refs_list_index" calculated in this way because
795 // balance_queues() has moved the Ref's into the first n queues.
796 // Thread* thr = Thread::current();
797 // int refs_list_index = ((WorkerThread*)thr)->id();
798 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
799 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
800 &is_alive, &keep_alive, &complete_gc);
801 }
802 private:
803 bool _clear_referent;
804 };
805
806 // Balances reference queues.
807 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
808 // queues[0, 1, ..., _num_q-1] because only the first _num_q
809 // corresponding to the active workers will be processed.
810 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
811 {
812 // calculate total length
813 size_t total_refs = 0;
814 if (TraceReferenceGC && PrintGCDetails) {
815 gclog_or_tty->print_cr("\nBalance ref_lists ");
816 }
817
818 for (uint i = 0; i < _max_num_q; ++i) {
819 total_refs += ref_lists[i].length();
820 if (TraceReferenceGC && PrintGCDetails) {
821 gclog_or_tty->print("%d ", ref_lists[i].length());
822 }
823 }
824 if (TraceReferenceGC && PrintGCDetails) {
825 gclog_or_tty->print_cr(" = %d", total_refs);
826 }
827 size_t avg_refs = total_refs / _num_q + 1;
828 uint to_idx = 0;
829 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
830 bool move_all = false;
831 if (from_idx >= _num_q) {
832 move_all = ref_lists[from_idx].length() > 0;
833 }
834 while ((ref_lists[from_idx].length() > avg_refs) ||
835 move_all) {
836 assert(to_idx < _num_q, "Sanity Check!");
837 if (ref_lists[to_idx].length() < avg_refs) {
838 // move superfluous refs
839 size_t refs_to_move;
840 // Move all the Ref's if the from queue will not be processed.
841 if (move_all) {
842 refs_to_move = MIN2(ref_lists[from_idx].length(),
843 avg_refs - ref_lists[to_idx].length());
844 } else {
845 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
846 avg_refs - ref_lists[to_idx].length());
847 }
848
849 assert(refs_to_move > 0, "otherwise the code below will fail");
850
851 oop move_head = ref_lists[from_idx].head();
852 oop move_tail = move_head;
853 oop new_head = move_head;
854 // find an element to split the list on
855 for (size_t j = 0; j < refs_to_move; ++j) {
856 move_tail = new_head;
857 new_head = java_lang_ref_Reference::discovered(new_head);
858 }
859
860 // Add the chain to the to list.
861 if (ref_lists[to_idx].head() == NULL) {
862 // to list is empty. Make a loop at the end.
863 java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
864 } else {
865 java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
866 }
867 ref_lists[to_idx].set_head(move_head);
868 ref_lists[to_idx].inc_length(refs_to_move);
869
870 // Remove the chain from the from list.
871 if (oopDesc::safe_equals(move_tail, new_head)) {
872 // We found the end of the from list.
873 ref_lists[from_idx].set_head(NULL);
874 } else {
875 ref_lists[from_idx].set_head(new_head);
876 }
877 ref_lists[from_idx].dec_length(refs_to_move);
878 if (ref_lists[from_idx].length() == 0) {
879 break;
880 }
881 } else {
882 to_idx = (to_idx + 1) % _num_q;
883 }
884 }
885 }
886 #ifdef ASSERT
887 size_t balanced_total_refs = 0;
888 for (uint i = 0; i < _max_num_q; ++i) {
889 balanced_total_refs += ref_lists[i].length();
890 if (TraceReferenceGC && PrintGCDetails) {
891 gclog_or_tty->print("%d ", ref_lists[i].length());
892 }
893 }
894 if (TraceReferenceGC && PrintGCDetails) {
895 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
896 gclog_or_tty->flush();
897 }
898 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
899 #endif
900 }
901
902 void ReferenceProcessor::balance_all_queues() {
903 balance_queues(_discoveredSoftRefs);
904 balance_queues(_discoveredWeakRefs);
905 balance_queues(_discoveredFinalRefs);
906 balance_queues(_discoveredPhantomRefs);
907 balance_queues(_discoveredCleanerRefs);
908 }
909
910 size_t
911 ReferenceProcessor::process_discovered_reflist(
912 DiscoveredList refs_lists[],
913 ReferencePolicy* policy,
914 bool clear_referent,
915 BoolObjectClosure* is_alive,
916 OopClosure* keep_alive,
917 VoidClosure* complete_gc,
918 AbstractRefProcTaskExecutor* task_executor)
919 {
920 bool mt_processing = task_executor != NULL && _processing_is_mt;
921 // If discovery used MT and a dynamic number of GC threads, then
922 // the queues must be balanced for correctness if fewer than the
923 // maximum number of queues were used. The number of queue used
924 // during discovery may be different than the number to be used
925 // for processing so don't depend of _num_q < _max_num_q as part
926 // of the test.
927 bool must_balance = _discovery_is_mt;
928
929 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
930 must_balance) {
931 balance_queues(refs_lists);
932 }
933
934 size_t total_list_count = total_count(refs_lists);
935
936 if (PrintReferenceGC && PrintGCDetails) {
937 gclog_or_tty->print(", %u refs", total_list_count);
938 }
939
940 // Phase 1 (soft refs only):
941 // . Traverse the list and remove any SoftReferences whose
942 // referents are not alive, but that should be kept alive for
943 // policy reasons. Keep alive the transitive closure of all
944 // such referents.
945 if (policy != NULL) {
946 if (mt_processing) {
947 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
948 task_executor->execute(phase1);
949 } else {
950 for (uint i = 0; i < _max_num_q; i++) {
951 process_phase1(refs_lists[i], policy,
952 is_alive, keep_alive, complete_gc);
953 }
954 }
955 } else { // policy == NULL
956 assert(refs_lists != _discoveredSoftRefs,
957 "Policy must be specified for soft references.");
958 }
959
960 // Phase 2:
961 // . Traverse the list and remove any refs whose referents are alive.
962 if (mt_processing) {
963 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
964 task_executor->execute(phase2);
965 } else {
966 for (uint i = 0; i < _max_num_q; i++) {
967 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
968 }
969 }
970
971 // Phase 3:
972 // . Traverse the list and process referents as appropriate.
973 if (mt_processing) {
974 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
975 task_executor->execute(phase3);
976 } else {
977 for (uint i = 0; i < _max_num_q; i++) {
978 process_phase3(refs_lists[i], clear_referent,
979 is_alive, keep_alive, complete_gc);
980 }
981 }
982
983 return total_list_count;
984 }
985
986 void ReferenceProcessor::clean_up_discovered_references() {
987 // loop over the lists
988 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
989 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
990 gclog_or_tty->print_cr(
991 "\nScrubbing %s discovered list of Null referents",
992 list_name(i));
993 }
994 clean_up_discovered_reflist(_discovered_refs[i]);
995 }
996 }
997
998 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
999 assert(!discovery_is_atomic(), "Else why call this method?");
1000 DiscoveredListIterator iter(refs_list, NULL, NULL);
1001 while (iter.has_next()) {
1002 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1003 oop next = java_lang_ref_Reference::next(iter.obj());
1004 assert(next->is_oop_or_null(), "bad next field");
1005 // If referent has been cleared or Reference is not active,
1006 // drop it.
1007 if (iter.referent() == NULL || next != NULL) {
1008 debug_only(
1009 if (PrintGCDetails && TraceReferenceGC) {
1010 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
1011 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
1012 " and referent: " INTPTR_FORMAT,
1013 (void *)iter.obj(), (void *)next, (void *)iter.referent());
1014 }
1015 )
1016 // Remove Reference object from list
1017 iter.remove();
1018 iter.move_to_next();
1019 } else {
1020 iter.next();
1021 }
1022 }
1023 NOT_PRODUCT(
1024 if (PrintGCDetails && TraceReferenceGC) {
1025 gclog_or_tty->print(
1026 " Removed %d Refs with NULL referents out of %d discovered Refs",
1027 iter.removed(), iter.processed());
1028 }
1029 )
1030 }
1031
1032 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1033 uint id = 0;
1034 // Determine the queue index to use for this object.
1035 if (_discovery_is_mt) {
1036 // During a multi-threaded discovery phase,
1037 // each thread saves to its "own" list.
1038 Thread* thr = Thread::current();
1039 id = thr->as_Worker_thread()->id();
1040 } else {
1041 // single-threaded discovery, we save in round-robin
1042 // fashion to each of the lists.
1043 if (_processing_is_mt) {
1044 id = next_id();
1045 }
1046 }
1047 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1048
1049 // Get the discovered queue to which we will add
1050 DiscoveredList* list = NULL;
1051 switch (rt) {
1052 case REF_OTHER:
1053 // Unknown reference type, no special treatment
1054 break;
1055 case REF_SOFT:
1056 list = &_discoveredSoftRefs[id];
1057 break;
1058 case REF_WEAK:
1059 list = &_discoveredWeakRefs[id];
1060 break;
1061 case REF_FINAL:
1062 list = &_discoveredFinalRefs[id];
1063 break;
1064 case REF_PHANTOM:
1065 list = &_discoveredPhantomRefs[id];
1066 break;
1067 case REF_CLEANER:
1068 list = &_discoveredCleanerRefs[id];
1069 break;
1070 case REF_NONE:
1071 // we should not reach here if we are an InstanceRefKlass
1072 default:
1073 ShouldNotReachHere();
1074 }
1075 if (TraceReferenceGC && PrintGCDetails) {
1076 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1077 }
1078 return list;
1079 }
1080
1081 inline void
1082 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1083 oop obj,
1084 HeapWord* discovered_addr) {
1085 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1086 // First we must make sure this object is only enqueued once. CAS in a non null
1087 // discovered_addr.
1088 oop current_head = refs_list.head();
1089 // The last ref must have its discovered field pointing to itself.
1090 oop next_discovered = (current_head != NULL) ? current_head : obj;
1091
1092 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1093 NULL);
1094 if (retest == NULL) {
1095 // This thread just won the right to enqueue the object.
1096 // We have separate lists for enqueueing, so no synchronization
1097 // is necessary.
1098 refs_list.set_head(obj);
1099 refs_list.inc_length(1);
1100
1101 if (TraceReferenceGC) {
1102 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1103 (void *)obj, obj->klass()->internal_name());
1104 }
1105 } else {
1106 // If retest was non NULL, another thread beat us to it:
1107 // The reference has already been discovered...
1108 if (TraceReferenceGC) {
1109 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1110 (void *)obj, obj->klass()->internal_name());
1111 }
1112 }
1113 }
1114
1115 #ifndef PRODUCT
1116 // Non-atomic (i.e. concurrent) discovery might allow us
1117 // to observe j.l.References with NULL referents, being those
1118 // cleared concurrently by mutators during (or after) discovery.
1119 void ReferenceProcessor::verify_referent(oop obj) {
1120 bool da = discovery_is_atomic();
1121 oop referent = java_lang_ref_Reference::referent(obj);
1122 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1123 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1124 INTPTR_FORMAT " during %satomic discovery ",
1125 (void *)referent, (void *)obj, da ? "" : "non-"));
1126 }
1127 #endif
1128
1129 // We mention two of several possible choices here:
1130 // #0: if the reference object is not in the "originating generation"
1131 // (or part of the heap being collected, indicated by our "span"
1132 // we don't treat it specially (i.e. we scan it as we would
1133 // a normal oop, treating its references as strong references).
1134 // This means that references can't be discovered unless their
1135 // referent is also in the same span. This is the simplest,
1136 // most "local" and most conservative approach, albeit one
1137 // that may cause weak references to be enqueued least promptly.
1138 // We call this choice the "ReferenceBasedDiscovery" policy.
1139 // #1: the reference object may be in any generation (span), but if
1140 // the referent is in the generation (span) being currently collected
1141 // then we can discover the reference object, provided
1142 // the object has not already been discovered by
1143 // a different concurrently running collector (as may be the
1144 // case, for instance, if the reference object is in CMS and
1145 // the referent in DefNewGeneration), and provided the processing
1146 // of this reference object by the current collector will
1147 // appear atomic to every other collector in the system.
1148 // (Thus, for instance, a concurrent collector may not
1149 // discover references in other generations even if the
1150 // referent is in its own generation). This policy may,
1151 // in certain cases, enqueue references somewhat sooner than
1152 // might Policy #0 above, but at marginally increased cost
1153 // and complexity in processing these references.
1154 // We call this choice the "RefeferentBasedDiscovery" policy.
1155 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1156 // Make sure we are discovering refs (rather than processing discovered refs).
1157 if (!_discovering_refs || !RegisterReferences) {
1158 return false;
1159 }
1160 DEBUG_ONLY(oopDesc::bs()->verify_safe_oop(obj);)
1161 // We only discover active references.
1162 oop next = java_lang_ref_Reference::next(obj);
1163 if (next != NULL) { // Ref is no longer active
1164 return false;
1165 }
1166
1167 HeapWord* obj_addr = (HeapWord*)obj;
1168 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1169 !_span.contains(obj_addr)) {
1170 // Reference is not in the originating generation;
1171 // don't treat it specially (i.e. we want to scan it as a normal
1172 // object with strong references).
1173 return false;
1174 }
1175
1176 // We only discover references whose referents are not (yet)
1177 // known to be strongly reachable.
1178 if (is_alive_non_header() != NULL) {
1179 verify_referent(obj);
1180 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1181 return false; // referent is reachable
1182 }
1183 }
1184 if (rt == REF_SOFT) {
1185 // For soft refs we can decide now if these are not
1186 // current candidates for clearing, in which case we
1187 // can mark through them now, rather than delaying that
1188 // to the reference-processing phase. Since all current
1189 // time-stamp policies advance the soft-ref clock only
1190 // at a major collection cycle, this is always currently
1191 // accurate.
1192 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1193 return false;
1194 }
1195 }
1196
1197 ResourceMark rm; // Needed for tracing.
1198
1199 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1200 const oop discovered = java_lang_ref_Reference::discovered(obj);
1201 assert(discovered->is_oop_or_null(), "bad discovered field");
1202 if (discovered != NULL) {
1203 // The reference has already been discovered...
1204 if (TraceReferenceGC) {
1205 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1206 (void *)obj, obj->klass()->internal_name());
1207 }
1208 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1209 // assumes that an object is not processed twice;
1210 // if it's been already discovered it must be on another
1211 // generation's discovered list; so we won't discover it.
1212 return false;
1213 } else {
1214 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1215 "Unrecognized policy");
1216 // Check assumption that an object is not potentially
1217 // discovered twice except by concurrent collectors that potentially
1218 // trace the same Reference object twice.
1219 assert(UseConcMarkSweepGC || UseG1GC || UseShenandoahGC,
1220 "Only possible with a concurrent marking collector");
1221 return true;
1222 }
1223 }
1224
1225 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1226 verify_referent(obj);
1227 // Discover if and only if EITHER:
1228 // .. reference is in our span, OR
1229 // .. we are an atomic collector and referent is in our span
1230 if (_span.contains(obj_addr) ||
1231 (discovery_is_atomic() &&
1232 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1233 // should_enqueue = true;
1234 } else {
1235 return false;
1236 }
1237 } else {
1238 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1239 _span.contains(obj_addr), "code inconsistency");
1240 }
1241
1242 // Get the right type of discovered queue head.
1243 DiscoveredList* list = get_discovered_list(rt);
1244 if (list == NULL) {
1245 return false; // nothing special needs to be done
1246 }
1247
1248 if (_discovery_is_mt) {
1249 add_to_discovered_list_mt(*list, obj, discovered_addr);
1250 } else {
1251 // We do a raw store here: the field will be visited later when processing
1252 // the discovered references.
1253 oop current_head = list->head();
1254 // The last ref must have its discovered field pointing to itself.
1255 oop next_discovered = (current_head != NULL) ? current_head : obj;
1256
1257 assert(discovered == NULL, "control point invariant");
1258 oop_store_raw(discovered_addr, next_discovered);
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 GCId gc_id) {
1283
1284 NOT_PRODUCT(verify_ok_to_handle_reflists());
1285
1286 // Soft references
1287 {
1288 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1289 false, gc_timer, gc_id);
1290 for (uint i = 0; i < _max_num_q; i++) {
1291 if (yield->should_return()) {
1292 return;
1293 }
1294 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1295 keep_alive, complete_gc, yield);
1296 }
1297 }
1298
1299 // Weak references
1300 {
1301 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1302 false, gc_timer, gc_id);
1303 for (uint i = 0; i < _max_num_q; i++) {
1304 if (yield->should_return()) {
1305 return;
1306 }
1307 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1308 keep_alive, complete_gc, yield);
1309 }
1310 }
1311
1312 // Final references
1313 {
1314 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1315 false, gc_timer, gc_id);
1316 for (uint i = 0; i < _max_num_q; i++) {
1317 if (yield->should_return()) {
1318 return;
1319 }
1320 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1321 keep_alive, complete_gc, yield);
1322 }
1323 }
1324
1325 // Phantom references
1326 {
1327 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1328 false, gc_timer, gc_id);
1329 for (uint i = 0; i < _max_num_q; i++) {
1330 if (yield->should_return()) {
1331 return;
1332 }
1333 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1334 keep_alive, complete_gc, yield);
1335 }
1336
1337 // Cleaner references. Included in timing for phantom references. We
1338 // expect Cleaner references to be temporary, and don't want to deal with
1339 // possible incompatibilities arising from making it more visible.
1340 for (uint i = 0; i < _max_num_q; i++) {
1341 if (yield->should_return()) {
1342 return;
1343 }
1344 preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1345 keep_alive, complete_gc, yield);
1346 }
1347 }
1348 }
1349
1350 // Walk the given discovered ref list, and remove all reference objects
1351 // whose referents are still alive, whose referents are NULL or which
1352 // are not active (have a non-NULL next field). NOTE: When we are
1353 // thus precleaning the ref lists (which happens single-threaded today),
1354 // we do not disable refs discovery to honour the correct semantics of
1355 // java.lang.Reference. As a result, we need to be careful below
1356 // that ref removal steps interleave safely with ref discovery steps
1357 // (in this thread).
1358 void
1359 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1360 BoolObjectClosure* is_alive,
1361 OopClosure* keep_alive,
1362 VoidClosure* complete_gc,
1363 YieldClosure* yield) {
1364 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1365 while (iter.has_next()) {
1366 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1367 oop obj = iter.obj();
1368 oop next = java_lang_ref_Reference::next(obj);
1369 if (iter.referent() == NULL || iter.is_referent_alive() ||
1370 next != NULL) {
1371 // The referent has been cleared, or is alive, or the Reference is not
1372 // active; we need to trace and mark its cohort.
1373 if (TraceReferenceGC) {
1374 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1375 (void *)iter.obj(), iter.obj()->klass()->internal_name());
1376 }
1377 // Remove Reference object from list
1378 iter.remove();
1379 // Keep alive its cohort.
1380 iter.make_referent_alive();
1381 if (UseCompressedOops) {
1382 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1383 keep_alive->do_oop(next_addr);
1384 } else {
1385 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1386 keep_alive->do_oop(next_addr);
1387 }
1388 iter.move_to_next();
1389 } else {
1390 iter.next();
1391 }
1392 }
1393 // Close the reachable set
1394 complete_gc->do_void();
1395
1396 NOT_PRODUCT(
1397 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1398 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1399 "Refs in discovered list " INTPTR_FORMAT,
1400 iter.removed(), iter.processed(), (address)refs_list.head());
1401 }
1402 )
1403 }
1404
1405 const char* ReferenceProcessor::list_name(uint i) {
1406 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1407 "Out of bounds index");
1408
1409 int j = i / _max_num_q;
1410 switch (j) {
1411 case 0: return "SoftRef";
1412 case 1: return "WeakRef";
1413 case 2: return "FinalRef";
1414 case 3: return "PhantomRef";
1415 case 4: return "CleanerRef";
1416 }
1417 ShouldNotReachHere();
1418 return NULL;
1419 }
1420
1421 #ifndef PRODUCT
1422 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1423 // empty for now
1424 }
1425 #endif
1426
1427 #ifndef PRODUCT
1428 void ReferenceProcessor::clear_discovered_references() {
1429 guarantee(!_discovering_refs, "Discovering refs?");
1430 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1431 clear_discovered_references(_discovered_refs[i]);
1432 }
1433 }
1434
1435 #endif // PRODUCT