1 /*
2 * Copyright (c) 2001, 2011, 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_interface/collectedHeap.hpp"
29 #include "gc_interface/collectedHeap.inline.hpp"
30 #include "memory/referencePolicy.hpp"
31 #include "memory/referenceProcessor.hpp"
32 #include "oops/oop.inline.hpp"
33 #include "runtime/java.hpp"
34 #include "runtime/jniHandles.hpp"
35
36 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
37 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL;
38 const int subclasses_of_ref = REF_PHANTOM - REF_OTHER;
39
40 // List of discovered references.
41 class DiscoveredList {
42 public:
43 DiscoveredList() : _len(0), _compressed_head(0), _oop_head(NULL) { }
44 oop head() const {
45 return UseCompressedOops ? oopDesc::decode_heap_oop(_compressed_head) :
46 _oop_head;
47 }
48 HeapWord* adr_head() {
49 return UseCompressedOops ? (HeapWord*)&_compressed_head :
50 (HeapWord*)&_oop_head;
51 }
52 void set_head(oop o) {
53 if (UseCompressedOops) {
54 // Must compress the head ptr.
55 _compressed_head = oopDesc::encode_heap_oop(o);
56 } else {
57 _oop_head = o;
58 }
59 }
60 bool empty() const { return head() == NULL; }
61 size_t length() { return _len; }
62 void set_length(size_t len) { _len = len; }
63 void inc_length(size_t inc) { _len += inc; assert(_len > 0, "Error"); }
64 void dec_length(size_t dec) { _len -= dec; }
65 private:
66 // Set value depending on UseCompressedOops. This could be a template class
67 // but then we have to fix all the instantiations and declarations that use this class.
68 oop _oop_head;
69 narrowOop _compressed_head;
70 size_t _len;
71 };
72
73 void referenceProcessor_init() {
74 ReferenceProcessor::init_statics();
75 }
76
77 void ReferenceProcessor::init_statics() {
78 // Initialize the master soft ref clock.
79 java_lang_ref_SoftReference::set_clock(os::javaTimeMillis());
80
81 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
82 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
83 NOT_COMPILER2(LRUCurrentHeapPolicy());
84 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
85 vm_exit_during_initialization("Could not allocate reference policy object");
86 }
87 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
88 RefDiscoveryPolicy == ReferentBasedDiscovery,
89 "Unrecongnized RefDiscoveryPolicy");
90 }
91
92 ReferenceProcessor::ReferenceProcessor(MemRegion span,
93 bool mt_processing,
94 int mt_processing_degree,
95 bool mt_discovery,
96 int mt_discovery_degree,
97 bool atomic_discovery,
98 BoolObjectClosure* is_alive_non_header,
99 bool discovered_list_needs_barrier) :
100 _discovering_refs(false),
101 _enqueuing_is_done(false),
102 _is_alive_non_header(is_alive_non_header),
103 _discovered_list_needs_barrier(discovered_list_needs_barrier),
104 _bs(NULL),
105 _processing_is_mt(mt_processing),
106 _next_id(0)
107 {
108 _span = span;
109 _discovery_is_atomic = atomic_discovery;
110 _discovery_is_mt = mt_discovery;
111 _num_q = MAX2(1, mt_processing_degree);
112 _max_num_q = MAX2(_num_q, mt_discovery_degree);
113 _discoveredSoftRefs = NEW_C_HEAP_ARRAY(DiscoveredList, _max_num_q * subclasses_of_ref);
114 if (_discoveredSoftRefs == NULL) {
115 vm_exit_during_initialization("Could not allocated RefProc Array");
116 }
117 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
118 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
119 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
120 // Initialized all entries to NULL
121 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
122 _discoveredSoftRefs[i].set_head(NULL);
123 _discoveredSoftRefs[i].set_length(0);
124 }
125 // If we do barreirs, cache a copy of the barrier set.
126 if (discovered_list_needs_barrier) {
127 _bs = Universe::heap()->barrier_set();
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 (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
136 guarantee(_discoveredSoftRefs[i].empty(),
137 "Found non-empty discovered list");
138 }
139 }
140 #endif
141
142 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
143 // Should this instead be
144 // for (int i = 0; i < subclasses_of_ref; i++_ {
145 // for (int j = 0; j < _num_q; j++) {
146 // int index = i * _max_num_q + j;
147 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
148 if (UseCompressedOops) {
149 f->do_oop((narrowOop*)_discoveredSoftRefs[i].adr_head());
150 } else {
151 f->do_oop((oop*)_discoveredSoftRefs[i].adr_head());
152 }
153 }
154 }
155
156 void ReferenceProcessor::update_soft_ref_master_clock() {
157 // Update (advance) the soft ref master clock field. This must be done
158 // after processing the soft ref list.
159 jlong now = os::javaTimeMillis();
160 jlong clock = java_lang_ref_SoftReference::clock();
161 NOT_PRODUCT(
162 if (now < clock) {
163 warning("time warp: %d to %d", clock, now);
164 }
165 )
166 // In product mode, protect ourselves from system time being adjusted
167 // externally and going backward; see note in the implementation of
168 // GenCollectedHeap::time_since_last_gc() for the right way to fix
169 // this uniformly throughout the VM; see bug-id 4741166. XXX
170 if (now > clock) {
171 java_lang_ref_SoftReference::set_clock(now);
172 }
173 // Else leave clock stalled at its old value until time progresses
174 // past clock value.
175 }
176
177 void ReferenceProcessor::process_discovered_references(
178 BoolObjectClosure* is_alive,
179 OopClosure* keep_alive,
180 VoidClosure* complete_gc,
181 AbstractRefProcTaskExecutor* task_executor) {
182 NOT_PRODUCT(verify_ok_to_handle_reflists());
183
184 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
185 // Stop treating discovered references specially.
186 disable_discovery();
187
188 bool trace_time = PrintGCDetails && PrintReferenceGC;
189 // Soft references
190 {
191 TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);
192 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
193 is_alive, keep_alive, complete_gc, task_executor);
194 }
195
196 update_soft_ref_master_clock();
197
198 // Weak references
199 {
200 TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);
201 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
202 is_alive, keep_alive, complete_gc, task_executor);
203 }
204
205 // Final references
206 {
207 TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);
208 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
209 is_alive, keep_alive, complete_gc, task_executor);
210 }
211
212 // Phantom references
213 {
214 TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);
215 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
216 is_alive, keep_alive, complete_gc, task_executor);
217 }
218
219 // Weak global JNI references. It would make more sense (semantically) to
220 // traverse these simultaneously with the regular weak references above, but
221 // that is not how the JDK1.2 specification is. See #4126360. Native code can
222 // thus use JNI weak references to circumvent the phantom references and
223 // resurrect a "post-mortem" object.
224 {
225 TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);
226 if (task_executor != NULL) {
227 task_executor->set_single_threaded_mode();
228 }
229 process_phaseJNI(is_alive, keep_alive, complete_gc);
230 }
231 }
232
233 #ifndef PRODUCT
234 // Calculate the number of jni handles.
235 uint ReferenceProcessor::count_jni_refs() {
236 class AlwaysAliveClosure: public BoolObjectClosure {
237 public:
238 virtual bool do_object_b(oop obj) { return true; }
239 virtual void do_object(oop obj) { assert(false, "Don't call"); }
240 };
241
242 class CountHandleClosure: public OopClosure {
243 private:
244 int _count;
245 public:
246 CountHandleClosure(): _count(0) {}
247 void do_oop(oop* unused) { _count++; }
248 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
249 int count() { return _count; }
250 };
251 CountHandleClosure global_handle_count;
252 AlwaysAliveClosure always_alive;
253 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
254 return global_handle_count.count();
255 }
256 #endif
257
258 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
259 OopClosure* keep_alive,
260 VoidClosure* complete_gc) {
261 #ifndef PRODUCT
262 if (PrintGCDetails && PrintReferenceGC) {
263 unsigned int count = count_jni_refs();
264 gclog_or_tty->print(", %u refs", count);
265 }
266 #endif
267 JNIHandles::weak_oops_do(is_alive, keep_alive);
268 complete_gc->do_void();
269 }
270
271
272 template <class T>
273 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
274 AbstractRefProcTaskExecutor* task_executor) {
275
276 // Remember old value of pending references list
277 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
278 T old_pending_list_value = *pending_list_addr;
279
280 // Enqueue references that are not made active again, and
281 // clear the decks for the next collection (cycle).
282 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
283 // Do the oop-check on pending_list_addr missed in
284 // enqueue_discovered_reflist. We should probably
285 // do a raw oop_check so that future such idempotent
286 // oop_stores relying on the oop-check side-effect
287 // may be elided automatically and safely without
288 // affecting correctness.
289 oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
290
291 // Stop treating discovered references specially.
292 ref->disable_discovery();
293
294 // Return true if new pending references were added
295 return old_pending_list_value != *pending_list_addr;
296 }
297
298 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
299 NOT_PRODUCT(verify_ok_to_handle_reflists());
300 if (UseCompressedOops) {
301 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
302 } else {
303 return enqueue_discovered_ref_helper<oop>(this, task_executor);
304 }
305 }
306
307 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
308 HeapWord* pending_list_addr) {
309 // Given a list of refs linked through the "discovered" field
310 // (java.lang.ref.Reference.discovered) chain them through the
311 // "next" field (java.lang.ref.Reference.next) and prepend
312 // to the pending list.
313 if (TraceReferenceGC && PrintGCDetails) {
314 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
315 INTPTR_FORMAT, (address)refs_list.head());
316 }
317
318 oop obj = NULL;
319 oop next = refs_list.head();
320 // Walk down the list, copying the discovered field into
321 // the next field and clearing it.
322 while (obj != next) {
323 obj = next;
324 assert(obj->is_instanceRef(), "should be reference object");
325 next = java_lang_ref_Reference::discovered(obj);
326 if (TraceReferenceGC && PrintGCDetails) {
327 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next " INTPTR_FORMAT,
328 obj, next);
329 }
330 assert(java_lang_ref_Reference::next(obj) == NULL,
331 "The reference should not be enqueued");
332 if (next == obj) { // obj is last
333 // Swap refs_list into pendling_list_addr and
334 // set obj's next to what we read from pending_list_addr.
335 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
336 // Need oop_check on pending_list_addr above;
337 // see special oop-check code at the end of
338 // enqueue_discovered_reflists() further below.
339 if (old == NULL) {
340 // obj should be made to point to itself, since
341 // pending list was empty.
342 java_lang_ref_Reference::set_next(obj, obj);
343 } else {
344 java_lang_ref_Reference::set_next(obj, old);
345 }
346 } else {
347 java_lang_ref_Reference::set_next(obj, next);
348 }
349 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
350 }
351 }
352
353 // Parallel enqueue task
354 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
355 public:
356 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
357 DiscoveredList discovered_refs[],
358 HeapWord* pending_list_addr,
359 int n_queues)
360 : EnqueueTask(ref_processor, discovered_refs,
361 pending_list_addr, n_queues)
362 { }
363
364 virtual void work(unsigned int work_id) {
365 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
366 // Simplest first cut: static partitioning.
367 int index = work_id;
368 // The increment on "index" must correspond to the maximum number of queues
369 // (n_queues) with which that ReferenceProcessor was created. That
370 // is because of the "clever" way the discovered references lists were
371 // allocated and are indexed into.
372 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
373 for (int j = 0;
374 j < subclasses_of_ref;
375 j++, index += _n_queues) {
376 _ref_processor.enqueue_discovered_reflist(
377 _refs_lists[index], _pending_list_addr);
378 _refs_lists[index].set_head(NULL);
379 _refs_lists[index].set_length(0);
380 }
381 }
382 };
383
384 // Enqueue references that are not made active again
385 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
386 AbstractRefProcTaskExecutor* task_executor) {
387 if (_processing_is_mt && task_executor != NULL) {
388 // Parallel code
389 RefProcEnqueueTask tsk(*this, _discoveredSoftRefs,
390 pending_list_addr, _max_num_q);
391 task_executor->execute(tsk);
392 } else {
393 // Serial code: call the parent class's implementation
394 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
395 enqueue_discovered_reflist(_discoveredSoftRefs[i], pending_list_addr);
396 _discoveredSoftRefs[i].set_head(NULL);
397 _discoveredSoftRefs[i].set_length(0);
398 }
399 }
400 }
401
402 // Iterator for the list of discovered references.
403 class DiscoveredListIterator {
404 public:
405 inline DiscoveredListIterator(DiscoveredList& refs_list,
406 OopClosure* keep_alive,
407 BoolObjectClosure* is_alive);
408
409 // End Of List.
410 inline bool has_next() const { return _ref != NULL; }
411
412 // Get oop to the Reference object.
413 inline oop obj() const { return _ref; }
414
415 // Get oop to the referent object.
416 inline oop referent() const { return _referent; }
417
418 // Returns true if referent is alive.
419 inline bool is_referent_alive() const;
420
421 // Loads data for the current reference.
422 // The "allow_null_referent" argument tells us to allow for the possibility
423 // of a NULL referent in the discovered Reference object. This typically
424 // happens in the case of concurrent collectors that may have done the
425 // discovery concurrently, or interleaved, with mutator execution.
426 inline void load_ptrs(DEBUG_ONLY(bool allow_null_referent));
427
428 // Move to the next discovered reference.
429 inline void next();
430
431 // Remove the current reference from the list
432 inline void remove();
433
434 // Make the Reference object active again.
435 inline void make_active() { java_lang_ref_Reference::set_next(_ref, NULL); }
436
437 // Make the referent alive.
438 inline void make_referent_alive() {
439 if (UseCompressedOops) {
440 _keep_alive->do_oop((narrowOop*)_referent_addr);
441 } else {
442 _keep_alive->do_oop((oop*)_referent_addr);
443 }
444 }
445
446 // Update the discovered field.
447 inline void update_discovered() {
448 // First _prev_next ref actually points into DiscoveredList (gross).
449 if (UseCompressedOops) {
450 if (!oopDesc::is_null(*(narrowOop*)_prev_next)) {
451 _keep_alive->do_oop((narrowOop*)_prev_next);
452 }
453 } else {
454 if (!oopDesc::is_null(*(oop*)_prev_next)) {
455 _keep_alive->do_oop((oop*)_prev_next);
456 }
457 }
458 }
459
460 // NULL out referent pointer.
461 inline void clear_referent() { oop_store_raw(_referent_addr, NULL); }
462
463 // Statistics
464 NOT_PRODUCT(
465 inline size_t processed() const { return _processed; }
466 inline size_t removed() const { return _removed; }
467 )
468
469 inline void move_to_next();
470
471 private:
472 DiscoveredList& _refs_list;
473 HeapWord* _prev_next;
474 oop _prev;
475 oop _ref;
476 HeapWord* _discovered_addr;
477 oop _next;
478 HeapWord* _referent_addr;
479 oop _referent;
480 OopClosure* _keep_alive;
481 BoolObjectClosure* _is_alive;
482 DEBUG_ONLY(
483 oop _first_seen; // cyclic linked list check
484 )
485 NOT_PRODUCT(
486 size_t _processed;
487 size_t _removed;
488 )
489 };
490
491 inline DiscoveredListIterator::DiscoveredListIterator(DiscoveredList& refs_list,
492 OopClosure* keep_alive,
493 BoolObjectClosure* is_alive)
494 : _refs_list(refs_list),
495 _prev_next(refs_list.adr_head()),
496 _prev(NULL),
497 _ref(refs_list.head()),
498 #ifdef ASSERT
499 _first_seen(refs_list.head()),
500 #endif
501 #ifndef PRODUCT
502 _processed(0),
503 _removed(0),
504 #endif
505 _next(NULL),
506 _keep_alive(keep_alive),
507 _is_alive(is_alive)
508 { }
509
510 inline bool DiscoveredListIterator::is_referent_alive() const {
511 return _is_alive->do_object_b(_referent);
512 }
513
514 inline void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
515 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
516 oop discovered = java_lang_ref_Reference::discovered(_ref);
517 assert(_discovered_addr && discovered->is_oop_or_null(),
518 "discovered field is bad");
519 _next = discovered;
520 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
521 _referent = java_lang_ref_Reference::referent(_ref);
522 assert(Universe::heap()->is_in_reserved_or_null(_referent),
523 "Wrong oop found in java.lang.Reference object");
524 assert(allow_null_referent ?
525 _referent->is_oop_or_null()
526 : _referent->is_oop(),
527 "bad referent");
528 }
529
530 inline void DiscoveredListIterator::next() {
531 _prev_next = _discovered_addr;
532 _prev = _ref;
533 move_to_next();
534 }
535
536 inline void DiscoveredListIterator::remove() {
537 assert(_ref->is_oop(), "Dropping a bad reference");
538 oop_store_raw(_discovered_addr, NULL);
539
540 // First _prev_next ref actually points into DiscoveredList (gross).
541 oop new_next;
542 if (_next == _ref) {
543 // At the end of the list, we should make _prev point to itself.
544 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
545 // and _prev will be NULL.
546 new_next = _prev;
547 } else {
548 new_next = _next;
549 }
550
551 if (UseCompressedOops) {
552 // Remove Reference object from list.
553 oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next);
554 } else {
555 // Remove Reference object from list.
556 oopDesc::store_heap_oop((oop*)_prev_next, new_next);
557 }
558 NOT_PRODUCT(_removed++);
559 _refs_list.dec_length(1);
560 }
561
562 inline void DiscoveredListIterator::move_to_next() {
563 if (_ref == _next) {
564 // End of the list.
565 _ref = NULL;
566 } else {
567 _ref = _next;
568 }
569 assert(_ref != _first_seen, "cyclic ref_list found");
570 NOT_PRODUCT(_processed++);
571 }
572
573 // NOTE: process_phase*() are largely similar, and at a high level
574 // merely iterate over the extant list applying a predicate to
575 // each of its elements and possibly removing that element from the
576 // list and applying some further closures to that element.
577 // We should consider the possibility of replacing these
578 // process_phase*() methods by abstracting them into
579 // a single general iterator invocation that receives appropriate
580 // closures that accomplish this work.
581
582 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
583 // referents are not alive, but that should be kept alive for policy reasons.
584 // Keep alive the transitive closure of all such referents.
585 void
586 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
587 ReferencePolicy* policy,
588 BoolObjectClosure* is_alive,
589 OopClosure* keep_alive,
590 VoidClosure* complete_gc) {
591 assert(policy != NULL, "Must have a non-NULL policy");
592 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
593 // Decide which softly reachable refs should be kept alive.
594 while (iter.has_next()) {
595 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
596 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
597 if (referent_is_dead && !policy->should_clear_reference(iter.obj())) {
598 if (TraceReferenceGC) {
599 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
600 iter.obj(), iter.obj()->blueprint()->internal_name());
601 }
602 // Remove Reference object from list
603 iter.remove();
604 // Make the Reference object active again
605 iter.make_active();
606 // keep the referent around
607 iter.make_referent_alive();
608 iter.move_to_next();
609 } else {
610 iter.next();
611 }
612 }
613 // Close the reachable set
614 complete_gc->do_void();
615 NOT_PRODUCT(
616 if (PrintGCDetails && TraceReferenceGC) {
617 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
618 "discovered Refs by policy list " INTPTR_FORMAT,
619 iter.removed(), iter.processed(), (address)refs_list.head());
620 }
621 )
622 }
623
624 // Traverse the list and remove any Refs that are not active, or
625 // whose referents are either alive or NULL.
626 void
627 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
628 BoolObjectClosure* is_alive,
629 OopClosure* keep_alive) {
630 assert(discovery_is_atomic(), "Error");
631 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
632 while (iter.has_next()) {
633 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
634 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
635 assert(next == NULL, "Should not discover inactive Reference");
636 if (iter.is_referent_alive()) {
637 if (TraceReferenceGC) {
638 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
639 iter.obj(), iter.obj()->blueprint()->internal_name());
640 }
641 // The referent is reachable after all.
642 // Remove Reference object from list.
643 iter.remove();
644 // Update the referent pointer as necessary: Note that this
645 // should not entail any recursive marking because the
646 // referent must already have been traversed.
647 iter.make_referent_alive();
648 iter.move_to_next();
649 } else {
650 iter.next();
651 }
652 }
653 NOT_PRODUCT(
654 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
655 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
656 "Refs in discovered list " INTPTR_FORMAT,
657 iter.removed(), iter.processed(), (address)refs_list.head());
658 }
659 )
660 }
661
662 void
663 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
664 BoolObjectClosure* is_alive,
665 OopClosure* keep_alive,
666 VoidClosure* complete_gc) {
667 assert(!discovery_is_atomic(), "Error");
668 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
669 while (iter.has_next()) {
670 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
671 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
672 oop next = java_lang_ref_Reference::next(iter.obj());
673 if ((iter.referent() == NULL || iter.is_referent_alive() ||
674 next != NULL)) {
675 assert(next->is_oop_or_null(), "bad next field");
676 // Remove Reference object from list
677 iter.remove();
678 // Trace the cohorts
679 iter.make_referent_alive();
680 if (UseCompressedOops) {
681 keep_alive->do_oop((narrowOop*)next_addr);
682 } else {
683 keep_alive->do_oop((oop*)next_addr);
684 }
685 iter.move_to_next();
686 } else {
687 iter.next();
688 }
689 }
690 // Now close the newly reachable set
691 complete_gc->do_void();
692 NOT_PRODUCT(
693 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
694 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
695 "Refs in discovered list " INTPTR_FORMAT,
696 iter.removed(), iter.processed(), (address)refs_list.head());
697 }
698 )
699 }
700
701 // Traverse the list and process the referents, by either
702 // clearing them or keeping them (and their reachable
703 // closure) alive.
704 void
705 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
706 bool clear_referent,
707 BoolObjectClosure* is_alive,
708 OopClosure* keep_alive,
709 VoidClosure* complete_gc) {
710 ResourceMark rm;
711 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
712 while (iter.has_next()) {
713 iter.update_discovered();
714 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
715 if (clear_referent) {
716 // NULL out referent pointer
717 iter.clear_referent();
718 } else {
719 // keep the referent around
720 iter.make_referent_alive();
721 }
722 if (TraceReferenceGC) {
723 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
724 clear_referent ? "cleared " : "",
725 iter.obj(), iter.obj()->blueprint()->internal_name());
726 }
727 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
728 iter.next();
729 }
730 // Remember to update the next pointer of the last ref.
731 iter.update_discovered();
732 // Close the reachable set
733 complete_gc->do_void();
734 }
735
736 void
737 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
738 oop obj = NULL;
739 oop next = refs_list.head();
740 while (next != obj) {
741 obj = next;
742 next = java_lang_ref_Reference::discovered(obj);
743 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
744 }
745 refs_list.set_head(NULL);
746 refs_list.set_length(0);
747 }
748
749 void
750 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
751 clear_discovered_references(refs_list);
752 }
753
754 void ReferenceProcessor::abandon_partial_discovery() {
755 // loop over the lists
756 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
757 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
758 gclog_or_tty->print_cr("\nAbandoning %s discovered list",
759 list_name(i));
760 }
761 abandon_partial_discovered_list(_discoveredSoftRefs[i]);
762 }
763 }
764
765 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
766 public:
767 RefProcPhase1Task(ReferenceProcessor& ref_processor,
768 DiscoveredList refs_lists[],
769 ReferencePolicy* policy,
770 bool marks_oops_alive)
771 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
772 _policy(policy)
773 { }
774 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
775 OopClosure& keep_alive,
776 VoidClosure& complete_gc)
777 {
778 Thread* thr = Thread::current();
779 int refs_list_index = ((WorkerThread*)thr)->id();
780 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
781 &is_alive, &keep_alive, &complete_gc);
782 }
783 private:
784 ReferencePolicy* _policy;
785 };
786
787 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
788 public:
789 RefProcPhase2Task(ReferenceProcessor& ref_processor,
790 DiscoveredList refs_lists[],
791 bool marks_oops_alive)
792 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
793 { }
794 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
795 OopClosure& keep_alive,
796 VoidClosure& complete_gc)
797 {
798 _ref_processor.process_phase2(_refs_lists[i],
799 &is_alive, &keep_alive, &complete_gc);
800 }
801 };
802
803 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
804 public:
805 RefProcPhase3Task(ReferenceProcessor& ref_processor,
806 DiscoveredList refs_lists[],
807 bool clear_referent,
808 bool marks_oops_alive)
809 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
810 _clear_referent(clear_referent)
811 { }
812 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
813 OopClosure& keep_alive,
814 VoidClosure& complete_gc)
815 {
816 // Don't use "refs_list_index" calculated in this way because
817 // balance_queues() has moved the Ref's into the first n queues.
818 // Thread* thr = Thread::current();
819 // int refs_list_index = ((WorkerThread*)thr)->id();
820 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
821 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
822 &is_alive, &keep_alive, &complete_gc);
823 }
824 private:
825 bool _clear_referent;
826 };
827
828 // Balances reference queues.
829 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
830 // queues[0, 1, ..., _num_q-1] because only the first _num_q
831 // corresponding to the active workers will be processed.
832 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
833 {
834 // calculate total length
835 size_t total_refs = 0;
836 if (TraceReferenceGC && PrintGCDetails) {
837 gclog_or_tty->print_cr("\nBalance ref_lists ");
838 }
839
840 for (int i = 0; i < _max_num_q; ++i) {
841 total_refs += ref_lists[i].length();
842 if (TraceReferenceGC && PrintGCDetails) {
843 gclog_or_tty->print("%d ", ref_lists[i].length());
844 }
845 }
846 if (TraceReferenceGC && PrintGCDetails) {
847 gclog_or_tty->print_cr(" = %d", total_refs);
848 }
849 size_t avg_refs = total_refs / _num_q + 1;
850 int to_idx = 0;
851 for (int from_idx = 0; from_idx < _max_num_q; from_idx++) {
852 bool move_all = false;
853 if (from_idx >= _num_q) {
854 move_all = ref_lists[from_idx].length() > 0;
855 }
856 while ((ref_lists[from_idx].length() > avg_refs) ||
857 move_all) {
858 assert(to_idx < _num_q, "Sanity Check!");
859 if (ref_lists[to_idx].length() < avg_refs) {
860 // move superfluous refs
861 size_t refs_to_move;
862 // Move all the Ref's if the from queue will not be processed.
863 if (move_all) {
864 refs_to_move = MIN2(ref_lists[from_idx].length(),
865 avg_refs - ref_lists[to_idx].length());
866 } else {
867 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
868 avg_refs - ref_lists[to_idx].length());
869 }
870
871 assert(refs_to_move > 0, "otherwise the code below will fail");
872
873 oop move_head = ref_lists[from_idx].head();
874 oop move_tail = move_head;
875 oop new_head = move_head;
876 // find an element to split the list on
877 for (size_t j = 0; j < refs_to_move; ++j) {
878 move_tail = new_head;
879 new_head = java_lang_ref_Reference::discovered(new_head);
880 }
881
882 // Add the chain to the to list.
883 if (ref_lists[to_idx].head() == NULL) {
884 // to list is empty. Make a loop at the end.
885 java_lang_ref_Reference::set_discovered(move_tail, move_tail);
886 } else {
887 java_lang_ref_Reference::set_discovered(move_tail, ref_lists[to_idx].head());
888 }
889 ref_lists[to_idx].set_head(move_head);
890 ref_lists[to_idx].inc_length(refs_to_move);
891
892 // Remove the chain from the from list.
893 if (move_tail == new_head) {
894 // We found the end of the from list.
895 ref_lists[from_idx].set_head(NULL);
896 } else {
897 ref_lists[from_idx].set_head(new_head);
898 }
899 ref_lists[from_idx].dec_length(refs_to_move);
900 if (ref_lists[from_idx].length() == 0) {
901 break;
902 }
903 } else {
904 to_idx = (to_idx + 1) % _num_q;
905 }
906 }
907 }
908 #ifdef ASSERT
909 size_t balanced_total_refs = 0;
910 for (int i = 0; i < _max_num_q; ++i) {
911 balanced_total_refs += ref_lists[i].length();
912 if (TraceReferenceGC && PrintGCDetails) {
913 gclog_or_tty->print("%d ", ref_lists[i].length());
914 }
915 }
916 if (TraceReferenceGC && PrintGCDetails) {
917 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
918 gclog_or_tty->flush();
919 }
920 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
921 #endif
922 }
923
924 void ReferenceProcessor::balance_all_queues() {
925 balance_queues(_discoveredSoftRefs);
926 balance_queues(_discoveredWeakRefs);
927 balance_queues(_discoveredFinalRefs);
928 balance_queues(_discoveredPhantomRefs);
929 }
930
931 void
932 ReferenceProcessor::process_discovered_reflist(
933 DiscoveredList refs_lists[],
934 ReferencePolicy* policy,
935 bool clear_referent,
936 BoolObjectClosure* is_alive,
937 OopClosure* keep_alive,
938 VoidClosure* complete_gc,
939 AbstractRefProcTaskExecutor* task_executor)
940 {
941 bool mt_processing = task_executor != NULL && _processing_is_mt;
942 // If discovery used MT and a dynamic number of GC threads, then
943 // the queues must be balanced for correctness if fewer than the
944 // maximum number of queues were used. The number of queue used
945 // during discovery may be different than the number to be used
946 // for processing so don't depend of _num_q < _max_num_q as part
947 // of the test.
948 bool must_balance = _discovery_is_mt;
949
950 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
951 must_balance) {
952 balance_queues(refs_lists);
953 }
954 if (PrintReferenceGC && PrintGCDetails) {
955 size_t total = 0;
956 for (int i = 0; i < _max_num_q; ++i) {
957 total += refs_lists[i].length();
958 }
959 gclog_or_tty->print(", %u refs", total);
960 }
961
962 // Phase 1 (soft refs only):
963 // . Traverse the list and remove any SoftReferences whose
964 // referents are not alive, but that should be kept alive for
965 // policy reasons. Keep alive the transitive closure of all
966 // such referents.
967 if (policy != NULL) {
968 if (mt_processing) {
969 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
970 task_executor->execute(phase1);
971 } else {
972 for (int i = 0; i < _max_num_q; i++) {
973 process_phase1(refs_lists[i], policy,
974 is_alive, keep_alive, complete_gc);
975 }
976 }
977 } else { // policy == NULL
978 assert(refs_lists != _discoveredSoftRefs,
979 "Policy must be specified for soft references.");
980 }
981
982 // Phase 2:
983 // . Traverse the list and remove any refs whose referents are alive.
984 if (mt_processing) {
985 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
986 task_executor->execute(phase2);
987 } else {
988 for (int i = 0; i < _max_num_q; i++) {
989 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
990 }
991 }
992
993 // Phase 3:
994 // . Traverse the list and process referents as appropriate.
995 if (mt_processing) {
996 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
997 task_executor->execute(phase3);
998 } else {
999 for (int i = 0; i < _max_num_q; i++) {
1000 process_phase3(refs_lists[i], clear_referent,
1001 is_alive, keep_alive, complete_gc);
1002 }
1003 }
1004 }
1005
1006 void ReferenceProcessor::clean_up_discovered_references() {
1007 // loop over the lists
1008 // Should this instead be
1009 // for (int i = 0; i < subclasses_of_ref; i++_ {
1010 // for (int j = 0; j < _num_q; j++) {
1011 // int index = i * _max_num_q + j;
1012 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
1013 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
1014 gclog_or_tty->print_cr(
1015 "\nScrubbing %s discovered list of Null referents",
1016 list_name(i));
1017 }
1018 clean_up_discovered_reflist(_discoveredSoftRefs[i]);
1019 }
1020 }
1021
1022 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
1023 assert(!discovery_is_atomic(), "Else why call this method?");
1024 DiscoveredListIterator iter(refs_list, NULL, NULL);
1025 while (iter.has_next()) {
1026 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1027 oop next = java_lang_ref_Reference::next(iter.obj());
1028 assert(next->is_oop_or_null(), "bad next field");
1029 // If referent has been cleared or Reference is not active,
1030 // drop it.
1031 if (iter.referent() == NULL || next != NULL) {
1032 debug_only(
1033 if (PrintGCDetails && TraceReferenceGC) {
1034 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
1035 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
1036 " and referent: " INTPTR_FORMAT,
1037 iter.obj(), next, iter.referent());
1038 }
1039 )
1040 // Remove Reference object from list
1041 iter.remove();
1042 iter.move_to_next();
1043 } else {
1044 iter.next();
1045 }
1046 }
1047 NOT_PRODUCT(
1048 if (PrintGCDetails && TraceReferenceGC) {
1049 gclog_or_tty->print(
1050 " Removed %d Refs with NULL referents out of %d discovered Refs",
1051 iter.removed(), iter.processed());
1052 }
1053 )
1054 }
1055
1056 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1057 int id = 0;
1058 // Determine the queue index to use for this object.
1059 if (_discovery_is_mt) {
1060 // During a multi-threaded discovery phase,
1061 // each thread saves to its "own" list.
1062 Thread* thr = Thread::current();
1063 id = thr->as_Worker_thread()->id();
1064 } else {
1065 // single-threaded discovery, we save in round-robin
1066 // fashion to each of the lists.
1067 if (_processing_is_mt) {
1068 id = next_id();
1069 }
1070 }
1071 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1072
1073 // Get the discovered queue to which we will add
1074 DiscoveredList* list = NULL;
1075 switch (rt) {
1076 case REF_OTHER:
1077 // Unknown reference type, no special treatment
1078 break;
1079 case REF_SOFT:
1080 list = &_discoveredSoftRefs[id];
1081 break;
1082 case REF_WEAK:
1083 list = &_discoveredWeakRefs[id];
1084 break;
1085 case REF_FINAL:
1086 list = &_discoveredFinalRefs[id];
1087 break;
1088 case REF_PHANTOM:
1089 list = &_discoveredPhantomRefs[id];
1090 break;
1091 case REF_NONE:
1092 // we should not reach here if we are an instanceRefKlass
1093 default:
1094 ShouldNotReachHere();
1095 }
1096 if (TraceReferenceGC && PrintGCDetails) {
1097 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1098 }
1099 return list;
1100 }
1101
1102 inline void
1103 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1104 oop obj,
1105 HeapWord* discovered_addr) {
1106 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1107 // First we must make sure this object is only enqueued once. CAS in a non null
1108 // discovered_addr.
1109 oop current_head = refs_list.head();
1110 // The last ref must have its discovered field pointing to itself.
1111 oop next_discovered = (current_head != NULL) ? current_head : obj;
1112
1113 // Note: In the case of G1, this specific pre-barrier is strictly
1114 // not necessary because the only case we are interested in
1115 // here is when *discovered_addr is NULL (see the CAS further below),
1116 // so this will expand to nothing. As a result, we have manually
1117 // elided this out for G1, but left in the test for some future
1118 // collector that might have need for a pre-barrier here.
1119 if (_discovered_list_needs_barrier && !UseG1GC) {
1120 if (UseCompressedOops) {
1121 _bs->write_ref_field_pre((narrowOop*)discovered_addr, next_discovered);
1122 } else {
1123 _bs->write_ref_field_pre((oop*)discovered_addr, next_discovered);
1124 }
1125 guarantee(false, "Need to check non-G1 collector");
1126 }
1127 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1128 NULL);
1129 if (retest == NULL) {
1130 // This thread just won the right to enqueue the object.
1131 // We have separate lists for enqueueing so no synchronization
1132 // is necessary.
1133 refs_list.set_head(obj);
1134 refs_list.inc_length(1);
1135 if (_discovered_list_needs_barrier) {
1136 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1137 }
1138
1139 if (TraceReferenceGC) {
1140 gclog_or_tty->print_cr("Enqueued reference (mt) (" INTPTR_FORMAT ": %s)",
1141 obj, obj->blueprint()->internal_name());
1142 }
1143 } else {
1144 // If retest was non NULL, another thread beat us to it:
1145 // The reference has already been discovered...
1146 if (TraceReferenceGC) {
1147 gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",
1148 obj, obj->blueprint()->internal_name());
1149 }
1150 }
1151 }
1152
1153 #ifndef PRODUCT
1154 // Non-atomic (i.e. concurrent) discovery might allow us
1155 // to observe j.l.References with NULL referents, being those
1156 // cleared concurrently by mutators during (or after) discovery.
1157 void ReferenceProcessor::verify_referent(oop obj) {
1158 bool da = discovery_is_atomic();
1159 oop referent = java_lang_ref_Reference::referent(obj);
1160 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1161 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1162 INTPTR_FORMAT " during %satomic discovery ",
1163 (intptr_t)referent, (intptr_t)obj, da ? "" : "non-"));
1164 }
1165 #endif
1166
1167 // We mention two of several possible choices here:
1168 // #0: if the reference object is not in the "originating generation"
1169 // (or part of the heap being collected, indicated by our "span"
1170 // we don't treat it specially (i.e. we scan it as we would
1171 // a normal oop, treating its references as strong references).
1172 // This means that references can't be enqueued unless their
1173 // referent is also in the same span. This is the simplest,
1174 // most "local" and most conservative approach, albeit one
1175 // that may cause weak references to be enqueued least promptly.
1176 // We call this choice the "ReferenceBasedDiscovery" policy.
1177 // #1: the reference object may be in any generation (span), but if
1178 // the referent is in the generation (span) being currently collected
1179 // then we can discover the reference object, provided
1180 // the object has not already been discovered by
1181 // a different concurrently running collector (as may be the
1182 // case, for instance, if the reference object is in CMS and
1183 // the referent in DefNewGeneration), and provided the processing
1184 // of this reference object by the current collector will
1185 // appear atomic to every other collector in the system.
1186 // (Thus, for instance, a concurrent collector may not
1187 // discover references in other generations even if the
1188 // referent is in its own generation). This policy may,
1189 // in certain cases, enqueue references somewhat sooner than
1190 // might Policy #0 above, but at marginally increased cost
1191 // and complexity in processing these references.
1192 // We call this choice the "RefeferentBasedDiscovery" policy.
1193 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1194 // We enqueue references only if we are discovering refs
1195 // (rather than processing discovered refs).
1196 if (!_discovering_refs || !RegisterReferences) {
1197 return false;
1198 }
1199 // We only enqueue active references.
1200 oop next = java_lang_ref_Reference::next(obj);
1201 if (next != NULL) {
1202 return false;
1203 }
1204
1205 HeapWord* obj_addr = (HeapWord*)obj;
1206 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1207 !_span.contains(obj_addr)) {
1208 // Reference is not in the originating generation;
1209 // don't treat it specially (i.e. we want to scan it as a normal
1210 // object with strong references).
1211 return false;
1212 }
1213
1214 // We only enqueue references whose referents are not (yet) strongly
1215 // reachable.
1216 if (is_alive_non_header() != NULL) {
1217 verify_referent(obj);
1218 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1219 return false; // referent is reachable
1220 }
1221 }
1222 if (rt == REF_SOFT) {
1223 // For soft refs we can decide now if these are not
1224 // current candidates for clearing, in which case we
1225 // can mark through them now, rather than delaying that
1226 // to the reference-processing phase. Since all current
1227 // time-stamp policies advance the soft-ref clock only
1228 // at a major collection cycle, this is always currently
1229 // accurate.
1230 if (!_current_soft_ref_policy->should_clear_reference(obj)) {
1231 return false;
1232 }
1233 }
1234
1235 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1236 const oop discovered = java_lang_ref_Reference::discovered(obj);
1237 assert(discovered->is_oop_or_null(), "bad discovered field");
1238 if (discovered != NULL) {
1239 // The reference has already been discovered...
1240 if (TraceReferenceGC) {
1241 gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",
1242 obj, obj->blueprint()->internal_name());
1243 }
1244 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1245 // assumes that an object is not processed twice;
1246 // if it's been already discovered it must be on another
1247 // generation's discovered list; so we won't discover it.
1248 return false;
1249 } else {
1250 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1251 "Unrecognized policy");
1252 // Check assumption that an object is not potentially
1253 // discovered twice except by concurrent collectors that potentially
1254 // trace the same Reference object twice.
1255 assert(UseConcMarkSweepGC || UseG1GC,
1256 "Only possible with a concurrent marking collector");
1257 return true;
1258 }
1259 }
1260
1261 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1262 verify_referent(obj);
1263 // enqueue if and only if either:
1264 // reference is in our span or
1265 // we are an atomic collector and referent is in our span
1266 if (_span.contains(obj_addr) ||
1267 (discovery_is_atomic() &&
1268 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1269 // should_enqueue = true;
1270 } else {
1271 return false;
1272 }
1273 } else {
1274 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1275 _span.contains(obj_addr), "code inconsistency");
1276 }
1277
1278 // Get the right type of discovered queue head.
1279 DiscoveredList* list = get_discovered_list(rt);
1280 if (list == NULL) {
1281 return false; // nothing special needs to be done
1282 }
1283
1284 if (_discovery_is_mt) {
1285 add_to_discovered_list_mt(*list, obj, discovered_addr);
1286 } else {
1287 // If "_discovered_list_needs_barrier", we do write barriers when
1288 // updating the discovered reference list. Otherwise, we do a raw store
1289 // here: the field will be visited later when processing the discovered
1290 // references.
1291 oop current_head = list->head();
1292 // The last ref must have its discovered field pointing to itself.
1293 oop next_discovered = (current_head != NULL) ? current_head : obj;
1294
1295 // As in the case further above, since we are over-writing a NULL
1296 // pre-value, we can safely elide the pre-barrier here for the case of G1.
1297 assert(discovered == NULL, "control point invariant");
1298 if (_discovered_list_needs_barrier && !UseG1GC) { // safe to elide for G1
1299 if (UseCompressedOops) {
1300 _bs->write_ref_field_pre((narrowOop*)discovered_addr, next_discovered);
1301 } else {
1302 _bs->write_ref_field_pre((oop*)discovered_addr, next_discovered);
1303 }
1304 guarantee(false, "Need to check non-G1 collector");
1305 }
1306 oop_store_raw(discovered_addr, next_discovered);
1307 if (_discovered_list_needs_barrier) {
1308 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1309 }
1310 list->set_head(obj);
1311 list->inc_length(1);
1312
1313 if (TraceReferenceGC) {
1314 gclog_or_tty->print_cr("Enqueued reference (" INTPTR_FORMAT ": %s)",
1315 obj, obj->blueprint()->internal_name());
1316 }
1317 }
1318 assert(obj->is_oop(), "Enqueued a bad reference");
1319 verify_referent(obj);
1320 return true;
1321 }
1322
1323 // Preclean the discovered references by removing those
1324 // whose referents are alive, and by marking from those that
1325 // are not active. These lists can be handled here
1326 // in any order and, indeed, concurrently.
1327 void ReferenceProcessor::preclean_discovered_references(
1328 BoolObjectClosure* is_alive,
1329 OopClosure* keep_alive,
1330 VoidClosure* complete_gc,
1331 YieldClosure* yield,
1332 bool should_unload_classes) {
1333
1334 NOT_PRODUCT(verify_ok_to_handle_reflists());
1335
1336 #ifdef ASSERT
1337 bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC ||
1338 CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
1339 ExplicitGCInvokesConcurrentAndUnloadsClasses &&
1340 UseConcMarkSweepGC && should_unload_classes;
1341 RememberKlassesChecker mx(must_remember_klasses);
1342 #endif
1343 // Soft references
1344 {
1345 TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1346 false, gclog_or_tty);
1347 for (int i = 0; i < _max_num_q; i++) {
1348 if (yield->should_return()) {
1349 return;
1350 }
1351 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1352 keep_alive, complete_gc, yield);
1353 }
1354 }
1355
1356 // Weak references
1357 {
1358 TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1359 false, gclog_or_tty);
1360 for (int i = 0; i < _max_num_q; i++) {
1361 if (yield->should_return()) {
1362 return;
1363 }
1364 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1365 keep_alive, complete_gc, yield);
1366 }
1367 }
1368
1369 // Final references
1370 {
1371 TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1372 false, gclog_or_tty);
1373 for (int i = 0; i < _max_num_q; i++) {
1374 if (yield->should_return()) {
1375 return;
1376 }
1377 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1378 keep_alive, complete_gc, yield);
1379 }
1380 }
1381
1382 // Phantom references
1383 {
1384 TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1385 false, gclog_or_tty);
1386 for (int i = 0; i < _max_num_q; i++) {
1387 if (yield->should_return()) {
1388 return;
1389 }
1390 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1391 keep_alive, complete_gc, yield);
1392 }
1393 }
1394 }
1395
1396 // Walk the given discovered ref list, and remove all reference objects
1397 // whose referents are still alive, whose referents are NULL or which
1398 // are not active (have a non-NULL next field). NOTE: When we are
1399 // thus precleaning the ref lists (which happens single-threaded today),
1400 // we do not disable refs discovery to honour the correct semantics of
1401 // java.lang.Reference. As a result, we need to be careful below
1402 // that ref removal steps interleave safely with ref discovery steps
1403 // (in this thread).
1404 void
1405 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1406 BoolObjectClosure* is_alive,
1407 OopClosure* keep_alive,
1408 VoidClosure* complete_gc,
1409 YieldClosure* yield) {
1410 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1411 while (iter.has_next()) {
1412 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1413 oop obj = iter.obj();
1414 oop next = java_lang_ref_Reference::next(obj);
1415 if (iter.referent() == NULL || iter.is_referent_alive() ||
1416 next != NULL) {
1417 // The referent has been cleared, or is alive, or the Reference is not
1418 // active; we need to trace and mark its cohort.
1419 if (TraceReferenceGC) {
1420 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1421 iter.obj(), iter.obj()->blueprint()->internal_name());
1422 }
1423 // Remove Reference object from list
1424 iter.remove();
1425 // Keep alive its cohort.
1426 iter.make_referent_alive();
1427 if (UseCompressedOops) {
1428 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1429 keep_alive->do_oop(next_addr);
1430 } else {
1431 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1432 keep_alive->do_oop(next_addr);
1433 }
1434 iter.move_to_next();
1435 } else {
1436 iter.next();
1437 }
1438 }
1439 // Close the reachable set
1440 complete_gc->do_void();
1441
1442 NOT_PRODUCT(
1443 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1444 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1445 "Refs in discovered list " INTPTR_FORMAT,
1446 iter.removed(), iter.processed(), (address)refs_list.head());
1447 }
1448 )
1449 }
1450
1451 const char* ReferenceProcessor::list_name(int i) {
1452 assert(i >= 0 && i <= _max_num_q * subclasses_of_ref, "Out of bounds index");
1453 int j = i / _max_num_q;
1454 switch (j) {
1455 case 0: return "SoftRef";
1456 case 1: return "WeakRef";
1457 case 2: return "FinalRef";
1458 case 3: return "PhantomRef";
1459 }
1460 ShouldNotReachHere();
1461 return NULL;
1462 }
1463
1464 #ifndef PRODUCT
1465 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1466 // empty for now
1467 }
1468 #endif
1469
1470 #ifndef PRODUCT
1471 void ReferenceProcessor::clear_discovered_references() {
1472 guarantee(!_discovering_refs, "Discovering refs?");
1473 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
1474 clear_discovered_references(_discoveredSoftRefs[i]);
1475 }
1476 }
1477
1478 #endif // PRODUCT