Fri, 10 Jan 2014 09:53:53 +0100
8029255: G1: Reference processing should not enqueue references on the shared SATB queue
Reviewed-by: brutisso, tschatzl
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.
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23 */
25 #include "precompiled.hpp"
26 #include "classfile/javaClasses.hpp"
27 #include "classfile/systemDictionary.hpp"
28 #include "gc_implementation/shared/gcTimer.hpp"
29 #include "gc_implementation/shared/gcTraceTime.hpp"
30 #include "gc_interface/collectedHeap.hpp"
31 #include "gc_interface/collectedHeap.inline.hpp"
32 #include "memory/referencePolicy.hpp"
33 #include "memory/referenceProcessor.hpp"
34 #include "oops/oop.inline.hpp"
35 #include "runtime/java.hpp"
36 #include "runtime/jniHandles.hpp"
38 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;
43 void referenceProcessor_init() {
44 ReferenceProcessor::init_statics();
45 }
47 void ReferenceProcessor::init_statics() {
48 // We need a monotonically non-deccreasing time in ms but
49 // os::javaTimeMillis() does not guarantee monotonicity.
50 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
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);
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 }
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?");
74 if (check_no_refs) {
75 // Verify that the discovered lists are empty
76 verify_no_references_recorded();
77 }
78 #endif // ASSERT
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.
87 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
88 _discovering_refs = true;
89 }
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);
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];
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 }
128 setup_policy(false /* default soft ref policy */);
129 }
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
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 }
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.
155 // We need a monotonically non-deccreasing 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");
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-monotonicty.
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 }
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 }
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());
196 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
197 // Stop treating discovered references specially.
198 disable_discovery();
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.
208 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
210 bool trace_time = PrintGCDetails && PrintReferenceGC;
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 }
221 update_soft_ref_master_clock();
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 }
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 }
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 }
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 }
263 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
264 }
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 };
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
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 }
304 template <class T>
305 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
306 AbstractRefProcTaskExecutor* task_executor) {
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;
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));
319 // Stop treating discovered references specially.
320 ref->disable_discovery();
322 // Return true if new pending references were added
323 return old_pending_list_value != *pending_list_addr;
324 }
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 }
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.
345 if (TraceReferenceGC && PrintGCDetails) {
346 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
347 INTPTR_FORMAT, (address)refs_list.head());
348 }
350 oop obj = NULL;
351 oop next_d = refs_list.head();
352 if (pending_list_uses_discovered_field()) { // New behaviour
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 pendling_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 behaviour
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 pendling_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 }
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 { }
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 };
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 }
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 }
479 void DiscoveredListIterator::remove() {
480 assert(_ref->is_oop(), "Dropping a bad reference");
481 oop_store_raw(_discovered_addr, NULL);
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 }
494 if (UseCompressedOops) {
495 // Remove Reference object from list.
496 oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next);
497 } else {
498 // Remove Reference object from list.
499 oopDesc::store_heap_oop((oop*)_prev_next, new_next);
500 }
501 NOT_PRODUCT(_removed++);
502 _refs_list.dec_length(1);
503 }
505 // Make the Reference object active again.
506 void DiscoveredListIterator::make_active() {
507 // For G1 we don't want to use set_next - it
508 // will dirty the card for the next field of
509 // the reference object and will fail
510 // CT verification.
511 if (UseG1GC) {
512 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
513 if (UseCompressedOops) {
514 oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
515 } else {
516 oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
517 }
518 java_lang_ref_Reference::set_next_raw(_ref, NULL);
519 } else {
520 java_lang_ref_Reference::set_next(_ref, NULL);
521 }
522 }
524 void DiscoveredListIterator::clear_referent() {
525 oop_store_raw(_referent_addr, NULL);
526 }
528 // NOTE: process_phase*() are largely similar, and at a high level
529 // merely iterate over the extant list applying a predicate to
530 // each of its elements and possibly removing that element from the
531 // list and applying some further closures to that element.
532 // We should consider the possibility of replacing these
533 // process_phase*() methods by abstracting them into
534 // a single general iterator invocation that receives appropriate
535 // closures that accomplish this work.
537 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
538 // referents are not alive, but that should be kept alive for policy reasons.
539 // Keep alive the transitive closure of all such referents.
540 void
541 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
542 ReferencePolicy* policy,
543 BoolObjectClosure* is_alive,
544 OopClosure* keep_alive,
545 VoidClosure* complete_gc) {
546 assert(policy != NULL, "Must have a non-NULL policy");
547 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
548 // Decide which softly reachable refs should be kept alive.
549 while (iter.has_next()) {
550 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
551 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
552 if (referent_is_dead &&
553 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
554 if (TraceReferenceGC) {
555 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
556 (void *)iter.obj(), iter.obj()->klass()->internal_name());
557 }
558 // Remove Reference object from list
559 iter.remove();
560 // Make the Reference object active again
561 iter.make_active();
562 // keep the referent around
563 iter.make_referent_alive();
564 iter.move_to_next();
565 } else {
566 iter.next();
567 }
568 }
569 // Close the reachable set
570 complete_gc->do_void();
571 NOT_PRODUCT(
572 if (PrintGCDetails && TraceReferenceGC) {
573 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
574 "discovered Refs by policy, from list " INTPTR_FORMAT,
575 iter.removed(), iter.processed(), (address)refs_list.head());
576 }
577 )
578 }
580 // Traverse the list and remove any Refs that are not active, or
581 // whose referents are either alive or NULL.
582 void
583 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
584 BoolObjectClosure* is_alive,
585 OopClosure* keep_alive) {
586 assert(discovery_is_atomic(), "Error");
587 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
588 while (iter.has_next()) {
589 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
590 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
591 assert(next == NULL, "Should not discover inactive Reference");
592 if (iter.is_referent_alive()) {
593 if (TraceReferenceGC) {
594 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
595 (void *)iter.obj(), iter.obj()->klass()->internal_name());
596 }
597 // The referent is reachable after all.
598 // Remove Reference object from list.
599 iter.remove();
600 // Update the referent pointer as necessary: Note that this
601 // should not entail any recursive marking because the
602 // referent must already have been traversed.
603 iter.make_referent_alive();
604 iter.move_to_next();
605 } else {
606 iter.next();
607 }
608 }
609 NOT_PRODUCT(
610 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
611 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
612 "Refs in discovered list " INTPTR_FORMAT,
613 iter.removed(), iter.processed(), (address)refs_list.head());
614 }
615 )
616 }
618 void
619 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
620 BoolObjectClosure* is_alive,
621 OopClosure* keep_alive,
622 VoidClosure* complete_gc) {
623 assert(!discovery_is_atomic(), "Error");
624 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
625 while (iter.has_next()) {
626 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
627 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
628 oop next = java_lang_ref_Reference::next(iter.obj());
629 if ((iter.referent() == NULL || iter.is_referent_alive() ||
630 next != NULL)) {
631 assert(next->is_oop_or_null(), "bad next field");
632 // Remove Reference object from list
633 iter.remove();
634 // Trace the cohorts
635 iter.make_referent_alive();
636 if (UseCompressedOops) {
637 keep_alive->do_oop((narrowOop*)next_addr);
638 } else {
639 keep_alive->do_oop((oop*)next_addr);
640 }
641 iter.move_to_next();
642 } else {
643 iter.next();
644 }
645 }
646 // Now close the newly reachable set
647 complete_gc->do_void();
648 NOT_PRODUCT(
649 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
650 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
651 "Refs in discovered list " INTPTR_FORMAT,
652 iter.removed(), iter.processed(), (address)refs_list.head());
653 }
654 )
655 }
657 // Traverse the list and process the referents, by either
658 // clearing them or keeping them (and their reachable
659 // closure) alive.
660 void
661 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
662 bool clear_referent,
663 BoolObjectClosure* is_alive,
664 OopClosure* keep_alive,
665 VoidClosure* complete_gc) {
666 ResourceMark rm;
667 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
668 while (iter.has_next()) {
669 iter.update_discovered();
670 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
671 if (clear_referent) {
672 // NULL out referent pointer
673 iter.clear_referent();
674 } else {
675 // keep the referent around
676 iter.make_referent_alive();
677 }
678 if (TraceReferenceGC) {
679 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
680 clear_referent ? "cleared " : "",
681 (void *)iter.obj(), iter.obj()->klass()->internal_name());
682 }
683 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
684 iter.next();
685 }
686 // Remember to update the next pointer of the last ref.
687 iter.update_discovered();
688 // Close the reachable set
689 complete_gc->do_void();
690 }
692 void
693 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
694 oop obj = NULL;
695 oop next = refs_list.head();
696 while (next != obj) {
697 obj = next;
698 next = java_lang_ref_Reference::discovered(obj);
699 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
700 }
701 refs_list.set_head(NULL);
702 refs_list.set_length(0);
703 }
705 void
706 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
707 clear_discovered_references(refs_list);
708 }
710 void ReferenceProcessor::abandon_partial_discovery() {
711 // loop over the lists
712 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
713 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
714 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
715 }
716 abandon_partial_discovered_list(_discovered_refs[i]);
717 }
718 }
720 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
721 public:
722 RefProcPhase1Task(ReferenceProcessor& ref_processor,
723 DiscoveredList refs_lists[],
724 ReferencePolicy* policy,
725 bool marks_oops_alive)
726 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
727 _policy(policy)
728 { }
729 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
730 OopClosure& keep_alive,
731 VoidClosure& complete_gc)
732 {
733 Thread* thr = Thread::current();
734 int refs_list_index = ((WorkerThread*)thr)->id();
735 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
736 &is_alive, &keep_alive, &complete_gc);
737 }
738 private:
739 ReferencePolicy* _policy;
740 };
742 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
743 public:
744 RefProcPhase2Task(ReferenceProcessor& ref_processor,
745 DiscoveredList refs_lists[],
746 bool marks_oops_alive)
747 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
748 { }
749 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
750 OopClosure& keep_alive,
751 VoidClosure& complete_gc)
752 {
753 _ref_processor.process_phase2(_refs_lists[i],
754 &is_alive, &keep_alive, &complete_gc);
755 }
756 };
758 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
759 public:
760 RefProcPhase3Task(ReferenceProcessor& ref_processor,
761 DiscoveredList refs_lists[],
762 bool clear_referent,
763 bool marks_oops_alive)
764 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
765 _clear_referent(clear_referent)
766 { }
767 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
768 OopClosure& keep_alive,
769 VoidClosure& complete_gc)
770 {
771 // Don't use "refs_list_index" calculated in this way because
772 // balance_queues() has moved the Ref's into the first n queues.
773 // Thread* thr = Thread::current();
774 // int refs_list_index = ((WorkerThread*)thr)->id();
775 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
776 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
777 &is_alive, &keep_alive, &complete_gc);
778 }
779 private:
780 bool _clear_referent;
781 };
783 void ReferenceProcessor::set_discovered(oop ref, oop value) {
784 java_lang_ref_Reference::set_discovered_raw(ref, value);
785 if (_discovered_list_needs_barrier) {
786 oopDesc::bs()->write_ref_field(ref, value);
787 }
788 }
790 // Balances reference queues.
791 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
792 // queues[0, 1, ..., _num_q-1] because only the first _num_q
793 // corresponding to the active workers will be processed.
794 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
795 {
796 // calculate total length
797 size_t total_refs = 0;
798 if (TraceReferenceGC && PrintGCDetails) {
799 gclog_or_tty->print_cr("\nBalance ref_lists ");
800 }
802 for (uint i = 0; i < _max_num_q; ++i) {
803 total_refs += ref_lists[i].length();
804 if (TraceReferenceGC && PrintGCDetails) {
805 gclog_or_tty->print("%d ", ref_lists[i].length());
806 }
807 }
808 if (TraceReferenceGC && PrintGCDetails) {
809 gclog_or_tty->print_cr(" = %d", total_refs);
810 }
811 size_t avg_refs = total_refs / _num_q + 1;
812 uint to_idx = 0;
813 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
814 bool move_all = false;
815 if (from_idx >= _num_q) {
816 move_all = ref_lists[from_idx].length() > 0;
817 }
818 while ((ref_lists[from_idx].length() > avg_refs) ||
819 move_all) {
820 assert(to_idx < _num_q, "Sanity Check!");
821 if (ref_lists[to_idx].length() < avg_refs) {
822 // move superfluous refs
823 size_t refs_to_move;
824 // Move all the Ref's if the from queue will not be processed.
825 if (move_all) {
826 refs_to_move = MIN2(ref_lists[from_idx].length(),
827 avg_refs - ref_lists[to_idx].length());
828 } else {
829 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
830 avg_refs - ref_lists[to_idx].length());
831 }
833 assert(refs_to_move > 0, "otherwise the code below will fail");
835 oop move_head = ref_lists[from_idx].head();
836 oop move_tail = move_head;
837 oop new_head = move_head;
838 // find an element to split the list on
839 for (size_t j = 0; j < refs_to_move; ++j) {
840 move_tail = new_head;
841 new_head = java_lang_ref_Reference::discovered(new_head);
842 }
844 // Add the chain to the to list.
845 if (ref_lists[to_idx].head() == NULL) {
846 // to list is empty. Make a loop at the end.
847 set_discovered(move_tail, move_tail);
848 } else {
849 set_discovered(move_tail, ref_lists[to_idx].head());
850 }
851 ref_lists[to_idx].set_head(move_head);
852 ref_lists[to_idx].inc_length(refs_to_move);
854 // Remove the chain from the from list.
855 if (move_tail == new_head) {
856 // We found the end of the from list.
857 ref_lists[from_idx].set_head(NULL);
858 } else {
859 ref_lists[from_idx].set_head(new_head);
860 }
861 ref_lists[from_idx].dec_length(refs_to_move);
862 if (ref_lists[from_idx].length() == 0) {
863 break;
864 }
865 } else {
866 to_idx = (to_idx + 1) % _num_q;
867 }
868 }
869 }
870 #ifdef ASSERT
871 size_t balanced_total_refs = 0;
872 for (uint i = 0; i < _max_num_q; ++i) {
873 balanced_total_refs += ref_lists[i].length();
874 if (TraceReferenceGC && PrintGCDetails) {
875 gclog_or_tty->print("%d ", ref_lists[i].length());
876 }
877 }
878 if (TraceReferenceGC && PrintGCDetails) {
879 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
880 gclog_or_tty->flush();
881 }
882 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
883 #endif
884 }
886 void ReferenceProcessor::balance_all_queues() {
887 balance_queues(_discoveredSoftRefs);
888 balance_queues(_discoveredWeakRefs);
889 balance_queues(_discoveredFinalRefs);
890 balance_queues(_discoveredPhantomRefs);
891 }
893 size_t
894 ReferenceProcessor::process_discovered_reflist(
895 DiscoveredList refs_lists[],
896 ReferencePolicy* policy,
897 bool clear_referent,
898 BoolObjectClosure* is_alive,
899 OopClosure* keep_alive,
900 VoidClosure* complete_gc,
901 AbstractRefProcTaskExecutor* task_executor)
902 {
903 bool mt_processing = task_executor != NULL && _processing_is_mt;
904 // If discovery used MT and a dynamic number of GC threads, then
905 // the queues must be balanced for correctness if fewer than the
906 // maximum number of queues were used. The number of queue used
907 // during discovery may be different than the number to be used
908 // for processing so don't depend of _num_q < _max_num_q as part
909 // of the test.
910 bool must_balance = _discovery_is_mt;
912 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
913 must_balance) {
914 balance_queues(refs_lists);
915 }
917 size_t total_list_count = total_count(refs_lists);
919 if (PrintReferenceGC && PrintGCDetails) {
920 gclog_or_tty->print(", %u refs", total_list_count);
921 }
923 // Phase 1 (soft refs only):
924 // . Traverse the list and remove any SoftReferences whose
925 // referents are not alive, but that should be kept alive for
926 // policy reasons. Keep alive the transitive closure of all
927 // such referents.
928 if (policy != NULL) {
929 if (mt_processing) {
930 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
931 task_executor->execute(phase1);
932 } else {
933 for (uint i = 0; i < _max_num_q; i++) {
934 process_phase1(refs_lists[i], policy,
935 is_alive, keep_alive, complete_gc);
936 }
937 }
938 } else { // policy == NULL
939 assert(refs_lists != _discoveredSoftRefs,
940 "Policy must be specified for soft references.");
941 }
943 // Phase 2:
944 // . Traverse the list and remove any refs whose referents are alive.
945 if (mt_processing) {
946 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
947 task_executor->execute(phase2);
948 } else {
949 for (uint i = 0; i < _max_num_q; i++) {
950 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
951 }
952 }
954 // Phase 3:
955 // . Traverse the list and process referents as appropriate.
956 if (mt_processing) {
957 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
958 task_executor->execute(phase3);
959 } else {
960 for (uint i = 0; i < _max_num_q; i++) {
961 process_phase3(refs_lists[i], clear_referent,
962 is_alive, keep_alive, complete_gc);
963 }
964 }
966 return total_list_count;
967 }
969 void ReferenceProcessor::clean_up_discovered_references() {
970 // loop over the lists
971 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
972 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
973 gclog_or_tty->print_cr(
974 "\nScrubbing %s discovered list of Null referents",
975 list_name(i));
976 }
977 clean_up_discovered_reflist(_discovered_refs[i]);
978 }
979 }
981 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
982 assert(!discovery_is_atomic(), "Else why call this method?");
983 DiscoveredListIterator iter(refs_list, NULL, NULL);
984 while (iter.has_next()) {
985 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
986 oop next = java_lang_ref_Reference::next(iter.obj());
987 assert(next->is_oop_or_null(), "bad next field");
988 // If referent has been cleared or Reference is not active,
989 // drop it.
990 if (iter.referent() == NULL || next != NULL) {
991 debug_only(
992 if (PrintGCDetails && TraceReferenceGC) {
993 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
994 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
995 " and referent: " INTPTR_FORMAT,
996 (void *)iter.obj(), (void *)next, (void *)iter.referent());
997 }
998 )
999 // Remove Reference object from list
1000 iter.remove();
1001 iter.move_to_next();
1002 } else {
1003 iter.next();
1004 }
1005 }
1006 NOT_PRODUCT(
1007 if (PrintGCDetails && TraceReferenceGC) {
1008 gclog_or_tty->print(
1009 " Removed %d Refs with NULL referents out of %d discovered Refs",
1010 iter.removed(), iter.processed());
1011 }
1012 )
1013 }
1015 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1016 uint id = 0;
1017 // Determine the queue index to use for this object.
1018 if (_discovery_is_mt) {
1019 // During a multi-threaded discovery phase,
1020 // each thread saves to its "own" list.
1021 Thread* thr = Thread::current();
1022 id = thr->as_Worker_thread()->id();
1023 } else {
1024 // single-threaded discovery, we save in round-robin
1025 // fashion to each of the lists.
1026 if (_processing_is_mt) {
1027 id = next_id();
1028 }
1029 }
1030 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1032 // Get the discovered queue to which we will add
1033 DiscoveredList* list = NULL;
1034 switch (rt) {
1035 case REF_OTHER:
1036 // Unknown reference type, no special treatment
1037 break;
1038 case REF_SOFT:
1039 list = &_discoveredSoftRefs[id];
1040 break;
1041 case REF_WEAK:
1042 list = &_discoveredWeakRefs[id];
1043 break;
1044 case REF_FINAL:
1045 list = &_discoveredFinalRefs[id];
1046 break;
1047 case REF_PHANTOM:
1048 list = &_discoveredPhantomRefs[id];
1049 break;
1050 case REF_NONE:
1051 // we should not reach here if we are an InstanceRefKlass
1052 default:
1053 ShouldNotReachHere();
1054 }
1055 if (TraceReferenceGC && PrintGCDetails) {
1056 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1057 }
1058 return list;
1059 }
1061 inline void
1062 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1063 oop obj,
1064 HeapWord* discovered_addr) {
1065 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1066 // First we must make sure this object is only enqueued once. CAS in a non null
1067 // discovered_addr.
1068 oop current_head = refs_list.head();
1069 // The last ref must have its discovered field pointing to itself.
1070 oop next_discovered = (current_head != NULL) ? current_head : obj;
1072 // Note: In the case of G1, this specific pre-barrier is strictly
1073 // not necessary because the only case we are interested in
1074 // here is when *discovered_addr is NULL (see the CAS further below),
1075 // so this will expand to nothing. As a result, we have manually
1076 // elided this out for G1, but left in the test for some future
1077 // collector that might have need for a pre-barrier here, e.g.:-
1078 // oopDesc::bs()->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1079 assert(!_discovered_list_needs_barrier || UseG1GC,
1080 "Need to check non-G1 collector: "
1081 "may need a pre-write-barrier for CAS from NULL below");
1082 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1083 NULL);
1084 if (retest == NULL) {
1085 // This thread just won the right to enqueue the object.
1086 // We have separate lists for enqueueing, so no synchronization
1087 // is necessary.
1088 refs_list.set_head(obj);
1089 refs_list.inc_length(1);
1090 if (_discovered_list_needs_barrier) {
1091 oopDesc::bs()->write_ref_field((void*)discovered_addr, next_discovered);
1092 }
1094 if (TraceReferenceGC) {
1095 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1096 (void *)obj, obj->klass()->internal_name());
1097 }
1098 } else {
1099 // If retest was non NULL, another thread beat us to it:
1100 // The reference has already been discovered...
1101 if (TraceReferenceGC) {
1102 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1103 (void *)obj, obj->klass()->internal_name());
1104 }
1105 }
1106 }
1108 #ifndef PRODUCT
1109 // Non-atomic (i.e. concurrent) discovery might allow us
1110 // to observe j.l.References with NULL referents, being those
1111 // cleared concurrently by mutators during (or after) discovery.
1112 void ReferenceProcessor::verify_referent(oop obj) {
1113 bool da = discovery_is_atomic();
1114 oop referent = java_lang_ref_Reference::referent(obj);
1115 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1116 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1117 INTPTR_FORMAT " during %satomic discovery ",
1118 (void *)referent, (void *)obj, da ? "" : "non-"));
1119 }
1120 #endif
1122 // We mention two of several possible choices here:
1123 // #0: if the reference object is not in the "originating generation"
1124 // (or part of the heap being collected, indicated by our "span"
1125 // we don't treat it specially (i.e. we scan it as we would
1126 // a normal oop, treating its references as strong references).
1127 // This means that references can't be discovered unless their
1128 // referent is also in the same span. This is the simplest,
1129 // most "local" and most conservative approach, albeit one
1130 // that may cause weak references to be enqueued least promptly.
1131 // We call this choice the "ReferenceBasedDiscovery" policy.
1132 // #1: the reference object may be in any generation (span), but if
1133 // the referent is in the generation (span) being currently collected
1134 // then we can discover the reference object, provided
1135 // the object has not already been discovered by
1136 // a different concurrently running collector (as may be the
1137 // case, for instance, if the reference object is in CMS and
1138 // the referent in DefNewGeneration), and provided the processing
1139 // of this reference object by the current collector will
1140 // appear atomic to every other collector in the system.
1141 // (Thus, for instance, a concurrent collector may not
1142 // discover references in other generations even if the
1143 // referent is in its own generation). This policy may,
1144 // in certain cases, enqueue references somewhat sooner than
1145 // might Policy #0 above, but at marginally increased cost
1146 // and complexity in processing these references.
1147 // We call this choice the "RefeferentBasedDiscovery" policy.
1148 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1149 // Make sure we are discovering refs (rather than processing discovered refs).
1150 if (!_discovering_refs || !RegisterReferences) {
1151 return false;
1152 }
1153 // We only discover active references.
1154 oop next = java_lang_ref_Reference::next(obj);
1155 if (next != NULL) { // Ref is no longer active
1156 return false;
1157 }
1159 HeapWord* obj_addr = (HeapWord*)obj;
1160 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1161 !_span.contains(obj_addr)) {
1162 // Reference is not in the originating generation;
1163 // don't treat it specially (i.e. we want to scan it as a normal
1164 // object with strong references).
1165 return false;
1166 }
1168 // We only discover references whose referents are not (yet)
1169 // known to be strongly reachable.
1170 if (is_alive_non_header() != NULL) {
1171 verify_referent(obj);
1172 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1173 return false; // referent is reachable
1174 }
1175 }
1176 if (rt == REF_SOFT) {
1177 // For soft refs we can decide now if these are not
1178 // current candidates for clearing, in which case we
1179 // can mark through them now, rather than delaying that
1180 // to the reference-processing phase. Since all current
1181 // time-stamp policies advance the soft-ref clock only
1182 // at a major collection cycle, this is always currently
1183 // accurate.
1184 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1185 return false;
1186 }
1187 }
1189 ResourceMark rm; // Needed for tracing.
1191 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1192 const oop discovered = java_lang_ref_Reference::discovered(obj);
1193 assert(discovered->is_oop_or_null(), "bad discovered field");
1194 if (discovered != NULL) {
1195 // The reference has already been discovered...
1196 if (TraceReferenceGC) {
1197 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1198 (void *)obj, obj->klass()->internal_name());
1199 }
1200 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1201 // assumes that an object is not processed twice;
1202 // if it's been already discovered it must be on another
1203 // generation's discovered list; so we won't discover it.
1204 return false;
1205 } else {
1206 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1207 "Unrecognized policy");
1208 // Check assumption that an object is not potentially
1209 // discovered twice except by concurrent collectors that potentially
1210 // trace the same Reference object twice.
1211 assert(UseConcMarkSweepGC || UseG1GC,
1212 "Only possible with a concurrent marking collector");
1213 return true;
1214 }
1215 }
1217 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1218 verify_referent(obj);
1219 // Discover if and only if EITHER:
1220 // .. reference is in our span, OR
1221 // .. we are an atomic collector and referent is in our span
1222 if (_span.contains(obj_addr) ||
1223 (discovery_is_atomic() &&
1224 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1225 // should_enqueue = true;
1226 } else {
1227 return false;
1228 }
1229 } else {
1230 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1231 _span.contains(obj_addr), "code inconsistency");
1232 }
1234 // Get the right type of discovered queue head.
1235 DiscoveredList* list = get_discovered_list(rt);
1236 if (list == NULL) {
1237 return false; // nothing special needs to be done
1238 }
1240 if (_discovery_is_mt) {
1241 add_to_discovered_list_mt(*list, obj, discovered_addr);
1242 } else {
1243 // If "_discovered_list_needs_barrier", we do write barriers when
1244 // updating the discovered reference list. Otherwise, we do a raw store
1245 // here: the field will be visited later when processing the discovered
1246 // references.
1247 oop current_head = list->head();
1248 // The last ref must have its discovered field pointing to itself.
1249 oop next_discovered = (current_head != NULL) ? current_head : obj;
1251 // As in the case further above, since we are over-writing a NULL
1252 // pre-value, we can safely elide the pre-barrier here for the case of G1.
1253 // e.g.:- oopDesc::bs()->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1254 assert(discovered == NULL, "control point invariant");
1255 assert(!_discovered_list_needs_barrier || UseG1GC,
1256 "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
1257 oop_store_raw(discovered_addr, next_discovered);
1258 if (_discovered_list_needs_barrier) {
1259 oopDesc::bs()->write_ref_field((void*)discovered_addr, next_discovered);
1260 }
1261 list->set_head(obj);
1262 list->inc_length(1);
1264 if (TraceReferenceGC) {
1265 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1266 (void *)obj, obj->klass()->internal_name());
1267 }
1268 }
1269 assert(obj->is_oop(), "Discovered a bad reference");
1270 verify_referent(obj);
1271 return true;
1272 }
1274 // Preclean the discovered references by removing those
1275 // whose referents are alive, and by marking from those that
1276 // are not active. These lists can be handled here
1277 // in any order and, indeed, concurrently.
1278 void ReferenceProcessor::preclean_discovered_references(
1279 BoolObjectClosure* is_alive,
1280 OopClosure* keep_alive,
1281 VoidClosure* complete_gc,
1282 YieldClosure* yield,
1283 GCTimer* gc_timer) {
1285 NOT_PRODUCT(verify_ok_to_handle_reflists());
1287 // Soft references
1288 {
1289 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1290 false, gc_timer);
1291 for (uint i = 0; i < _max_num_q; i++) {
1292 if (yield->should_return()) {
1293 return;
1294 }
1295 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1296 keep_alive, complete_gc, yield);
1297 }
1298 }
1300 // Weak references
1301 {
1302 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1303 false, gc_timer);
1304 for (uint i = 0; i < _max_num_q; i++) {
1305 if (yield->should_return()) {
1306 return;
1307 }
1308 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1309 keep_alive, complete_gc, yield);
1310 }
1311 }
1313 // Final references
1314 {
1315 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1316 false, gc_timer);
1317 for (uint i = 0; i < _max_num_q; i++) {
1318 if (yield->should_return()) {
1319 return;
1320 }
1321 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1322 keep_alive, complete_gc, yield);
1323 }
1324 }
1326 // Phantom references
1327 {
1328 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1329 false, gc_timer);
1330 for (uint i = 0; i < _max_num_q; i++) {
1331 if (yield->should_return()) {
1332 return;
1333 }
1334 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1335 keep_alive, complete_gc, yield);
1336 }
1337 }
1338 }
1340 // Walk the given discovered ref list, and remove all reference objects
1341 // whose referents are still alive, whose referents are NULL or which
1342 // are not active (have a non-NULL next field). NOTE: When we are
1343 // thus precleaning the ref lists (which happens single-threaded today),
1344 // we do not disable refs discovery to honour the correct semantics of
1345 // java.lang.Reference. As a result, we need to be careful below
1346 // that ref removal steps interleave safely with ref discovery steps
1347 // (in this thread).
1348 void
1349 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1350 BoolObjectClosure* is_alive,
1351 OopClosure* keep_alive,
1352 VoidClosure* complete_gc,
1353 YieldClosure* yield) {
1354 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1355 while (iter.has_next()) {
1356 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1357 oop obj = iter.obj();
1358 oop next = java_lang_ref_Reference::next(obj);
1359 if (iter.referent() == NULL || iter.is_referent_alive() ||
1360 next != NULL) {
1361 // The referent has been cleared, or is alive, or the Reference is not
1362 // active; we need to trace and mark its cohort.
1363 if (TraceReferenceGC) {
1364 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1365 (void *)iter.obj(), iter.obj()->klass()->internal_name());
1366 }
1367 // Remove Reference object from list
1368 iter.remove();
1369 // Keep alive its cohort.
1370 iter.make_referent_alive();
1371 if (UseCompressedOops) {
1372 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1373 keep_alive->do_oop(next_addr);
1374 } else {
1375 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1376 keep_alive->do_oop(next_addr);
1377 }
1378 iter.move_to_next();
1379 } else {
1380 iter.next();
1381 }
1382 }
1383 // Close the reachable set
1384 complete_gc->do_void();
1386 NOT_PRODUCT(
1387 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1388 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1389 "Refs in discovered list " INTPTR_FORMAT,
1390 iter.removed(), iter.processed(), (address)refs_list.head());
1391 }
1392 )
1393 }
1395 const char* ReferenceProcessor::list_name(uint i) {
1396 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1397 "Out of bounds index");
1399 int j = i / _max_num_q;
1400 switch (j) {
1401 case 0: return "SoftRef";
1402 case 1: return "WeakRef";
1403 case 2: return "FinalRef";
1404 case 3: return "PhantomRef";
1405 }
1406 ShouldNotReachHere();
1407 return NULL;
1408 }
1410 #ifndef PRODUCT
1411 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1412 // empty for now
1413 }
1414 #endif
1416 #ifndef PRODUCT
1417 void ReferenceProcessor::clear_discovered_references() {
1418 guarantee(!_discovering_refs, "Discovering refs?");
1419 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1420 clear_discovered_references(_discovered_refs[i]);
1421 }
1422 }
1424 #endif // PRODUCT