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 bool ReferenceProcessor::_pending_list_uses_discovered_field = false;
40
41 // List of discovered references.
42 class DiscoveredList {
43 public:
44 DiscoveredList() : _len(0), _compressed_head(0), _oop_head(NULL) { }
45 oop head() const {
46 return UseCompressedOops ? oopDesc::decode_heap_oop(_compressed_head) :
47 _oop_head;
48 }
49 HeapWord* adr_head() {
50 return UseCompressedOops ? (HeapWord*)&_compressed_head :
51 (HeapWord*)&_oop_head;
52 }
53 void set_head(oop o) {
54 if (UseCompressedOops) {
55 // Must compress the head ptr.
56 _compressed_head = oopDesc::encode_heap_oop(o);
57 } else {
58 _oop_head = o;
59 }
60 }
61 bool empty() const { return head() == NULL; }
62 size_t length() { return _len; }
63 void set_length(size_t len) { _len = len; }
64 void inc_length(size_t inc) { _len += inc; assert(_len > 0, "Error"); }
65 void dec_length(size_t dec) { _len -= dec; }
66 private:
67 // Set value depending on UseCompressedOops. This could be a template class
68 // but then we have to fix all the instantiations and declarations that use this class.
69 oop _oop_head;
70 narrowOop _compressed_head;
71 size_t _len;
72 };
73
74 void referenceProcessor_init() {
75 ReferenceProcessor::init_statics();
76 }
77
78 void ReferenceProcessor::init_statics() {
79 // Initialize the master soft ref clock.
80 java_lang_ref_SoftReference::set_clock(os::javaTimeMillis());
81
82 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
83 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
84 NOT_COMPILER2(LRUCurrentHeapPolicy());
85 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
86 vm_exit_during_initialization("Could not allocate reference policy object");
87 }
88 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
89 RefDiscoveryPolicy == ReferentBasedDiscovery,
90 "Unrecongnized RefDiscoveryPolicy");
91 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
92 }
93
94 ReferenceProcessor::ReferenceProcessor(MemRegion span,
95 bool mt_processing,
96 int mt_processing_degree,
97 bool mt_discovery,
98 int mt_discovery_degree,
99 bool atomic_discovery,
100 BoolObjectClosure* is_alive_non_header,
101 bool discovered_list_needs_barrier) :
102 _discovering_refs(false),
103 _enqueuing_is_done(false),
104 _is_alive_non_header(is_alive_non_header),
105 _discovered_list_needs_barrier(discovered_list_needs_barrier),
106 _bs(NULL),
107 _processing_is_mt(mt_processing),
108 _next_id(0)
109 {
110 _span = span;
111 _discovery_is_atomic = atomic_discovery;
112 _discovery_is_mt = mt_discovery;
113 _num_q = MAX2(1, mt_processing_degree);
114 _max_num_q = MAX2(_num_q, mt_discovery_degree);
115 _discoveredSoftRefs = NEW_C_HEAP_ARRAY(DiscoveredList, _max_num_q * subclasses_of_ref);
116 if (_discoveredSoftRefs == NULL) {
117 vm_exit_during_initialization("Could not allocated RefProc Array");
118 }
119 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
120 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
121 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
122 // Initialized all entries to NULL
123 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
124 _discoveredSoftRefs[i].set_head(NULL);
125 _discoveredSoftRefs[i].set_length(0);
126 }
127 // If we do barreirs, cache a copy of the barrier set.
128 if (discovered_list_needs_barrier) {
129 _bs = Universe::heap()->barrier_set();
130 }
131 setup_policy(false /* default soft ref policy */);
132 }
133
134 #ifndef PRODUCT
135 void ReferenceProcessor::verify_no_references_recorded() {
136 guarantee(!_discovering_refs, "Discovering refs?");
137 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
138 guarantee(_discoveredSoftRefs[i].empty(),
139 "Found non-empty discovered list");
140 }
141 }
142 #endif
143
144 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
145 // Should this instead be
146 // for (int i = 0; i < subclasses_of_ref; i++_ {
147 // for (int j = 0; j < _num_q; j++) {
148 // int index = i * _max_num_q + j;
149 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
150 if (UseCompressedOops) {
151 f->do_oop((narrowOop*)_discoveredSoftRefs[i].adr_head());
152 } else {
153 f->do_oop((oop*)_discoveredSoftRefs[i].adr_head());
154 }
155 }
156 }
157
158 void ReferenceProcessor::update_soft_ref_master_clock() {
159 // Update (advance) the soft ref master clock field. This must be done
160 // after processing the soft ref list.
161 jlong now = os::javaTimeMillis();
162 jlong clock = java_lang_ref_SoftReference::clock();
163 NOT_PRODUCT(
164 if (now < clock) {
165 warning("time warp: %d to %d", clock, now);
166 }
167 )
168 // In product mode, protect ourselves from system time being adjusted
169 // externally and going backward; see note in the implementation of
170 // GenCollectedHeap::time_since_last_gc() for the right way to fix
171 // this uniformly throughout the VM; see bug-id 4741166. XXX
172 if (now > clock) {
173 java_lang_ref_SoftReference::set_clock(now);
174 }
175 // Else leave clock stalled at its old value until time progresses
176 // past clock value.
177 }
178
179 void ReferenceProcessor::process_discovered_references(
180 BoolObjectClosure* is_alive,
181 OopClosure* keep_alive,
182 VoidClosure* complete_gc,
183 AbstractRefProcTaskExecutor* task_executor) {
184 NOT_PRODUCT(verify_ok_to_handle_reflists());
185
186 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
187 // Stop treating discovered references specially.
188 disable_discovery();
189
190 bool trace_time = PrintGCDetails && PrintReferenceGC;
191 // Soft references
192 {
193 TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);
194 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
195 is_alive, keep_alive, complete_gc, task_executor);
196 }
197
198 update_soft_ref_master_clock();
199
200 // Weak references
201 {
202 TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);
203 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
204 is_alive, keep_alive, complete_gc, task_executor);
205 }
206
207 // Final references
208 {
209 TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);
210 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
211 is_alive, keep_alive, complete_gc, task_executor);
212 }
213
214 // Phantom references
215 {
216 TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);
217 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
218 is_alive, keep_alive, complete_gc, task_executor);
219 }
220
221 // Weak global JNI references. It would make more sense (semantically) to
222 // traverse these simultaneously with the regular weak references above, but
223 // that is not how the JDK1.2 specification is. See #4126360. Native code can
224 // thus use JNI weak references to circumvent the phantom references and
225 // resurrect a "post-mortem" object.
226 {
227 TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);
228 if (task_executor != NULL) {
229 task_executor->set_single_threaded_mode();
230 }
231 process_phaseJNI(is_alive, keep_alive, complete_gc);
232 }
233 }
234
235 #ifndef PRODUCT
236 // Calculate the number of jni handles.
237 uint ReferenceProcessor::count_jni_refs() {
238 class AlwaysAliveClosure: public BoolObjectClosure {
239 public:
240 virtual bool do_object_b(oop obj) { return true; }
241 virtual void do_object(oop obj) { assert(false, "Don't call"); }
242 };
243
244 class CountHandleClosure: public OopClosure {
245 private:
246 int _count;
247 public:
248 CountHandleClosure(): _count(0) {}
249 void do_oop(oop* unused) { _count++; }
250 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
251 int count() { return _count; }
252 };
253 CountHandleClosure global_handle_count;
254 AlwaysAliveClosure always_alive;
255 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
256 return global_handle_count.count();
257 }
258 #endif
259
260 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
261 OopClosure* keep_alive,
262 VoidClosure* complete_gc) {
263 #ifndef PRODUCT
264 if (PrintGCDetails && PrintReferenceGC) {
265 unsigned int count = count_jni_refs();
266 gclog_or_tty->print(", %u refs", count);
267 }
268 #endif
269 JNIHandles::weak_oops_do(is_alive, keep_alive);
270 complete_gc->do_void();
271 }
272
273
274 template <class T>
275 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
276 AbstractRefProcTaskExecutor* task_executor) {
277
278 // Remember old value of pending references list
279 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
280 T old_pending_list_value = *pending_list_addr;
281
282 // Enqueue references that are not made active again, and
283 // clear the decks for the next collection (cycle).
284 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
285 // Do the oop-check on pending_list_addr missed in
286 // enqueue_discovered_reflist. We should probably
287 // do a raw oop_check so that future such idempotent
288 // oop_stores relying on the oop-check side-effect
289 // may be elided automatically and safely without
290 // affecting correctness.
291 oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
292
293 // Stop treating discovered references specially.
294 ref->disable_discovery();
295
296 // Return true if new pending references were added
297 return old_pending_list_value != *pending_list_addr;
298 }
299
300 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
301 NOT_PRODUCT(verify_ok_to_handle_reflists());
302 if (UseCompressedOops) {
303 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
304 } else {
305 return enqueue_discovered_ref_helper<oop>(this, task_executor);
306 }
307 }
308
309 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
310 HeapWord* pending_list_addr) {
311 // Given a list of refs linked through the "discovered" field
312 // (java.lang.ref.Reference.discovered), self-loop their "next" field
313 // thus distinguishing them from active References, then
314 // prepend them to the pending list.
315 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
316 // the "next" field is used to chain the pending list, not the discovered
317 // field.
318
319 if (TraceReferenceGC && PrintGCDetails) {
320 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
321 INTPTR_FORMAT, (address)refs_list.head());
322 }
323
324 oop obj = NULL;
325 oop next_d = refs_list.head();
326 if (pending_list_uses_discovered_field()) { // New behaviour
327 // Walk down the list, self-looping the next field
328 // so that the References are not considered active.
329 while (obj != next_d) {
330 obj = next_d;
331 assert(obj->is_instanceRef(), "should be reference object");
332 next_d = java_lang_ref_Reference::discovered(obj);
333 if (TraceReferenceGC && PrintGCDetails) {
334 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
335 obj, next_d);
336 }
337 assert(java_lang_ref_Reference::next(obj) == NULL,
338 "Reference not active; should not be discovered");
339 // Self-loop next, so as to make Ref not active.
340 java_lang_ref_Reference::set_next(obj, obj);
341 if (next_d == obj) { // obj is last
342 // Swap refs_list into pendling_list_addr and
343 // set obj's discovered to what we read from pending_list_addr.
344 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
345 // Need oop_check on pending_list_addr above;
346 // see special oop-check code at the end of
347 // enqueue_discovered_reflists() further below.
348 java_lang_ref_Reference::set_discovered(obj, old); // old may be NULL
349 }
350 }
351 } else { // Old behaviour
352 // Walk down the list, copying the discovered field into
353 // the next field and clearing the discovered field.
354 while (obj != next_d) {
355 obj = next_d;
356 assert(obj->is_instanceRef(), "should be reference object");
357 next_d = java_lang_ref_Reference::discovered(obj);
358 if (TraceReferenceGC && PrintGCDetails) {
359 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
360 obj, next_d);
361 }
362 assert(java_lang_ref_Reference::next(obj) == NULL,
363 "The reference should not be enqueued");
364 if (next_d == obj) { // obj is last
365 // Swap refs_list into pendling_list_addr and
366 // set obj's next to what we read from pending_list_addr.
367 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
368 // Need oop_check on pending_list_addr above;
369 // see special oop-check code at the end of
370 // enqueue_discovered_reflists() further below.
371 if (old == NULL) {
372 // obj should be made to point to itself, since
373 // pending list was empty.
374 java_lang_ref_Reference::set_next(obj, obj);
375 } else {
376 java_lang_ref_Reference::set_next(obj, old);
377 }
378 } else {
379 java_lang_ref_Reference::set_next(obj, next_d);
380 }
381 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
382 }
383 }
384 }
385
386 // Parallel enqueue task
387 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
388 public:
389 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
390 DiscoveredList discovered_refs[],
391 HeapWord* pending_list_addr,
392 int n_queues)
393 : EnqueueTask(ref_processor, discovered_refs,
394 pending_list_addr, n_queues)
395 { }
396
397 virtual void work(unsigned int work_id) {
398 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
399 // Simplest first cut: static partitioning.
400 int index = work_id;
401 // The increment on "index" must correspond to the maximum number of queues
402 // (n_queues) with which that ReferenceProcessor was created. That
403 // is because of the "clever" way the discovered references lists were
404 // allocated and are indexed into.
405 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
406 for (int j = 0;
407 j < subclasses_of_ref;
408 j++, index += _n_queues) {
409 _ref_processor.enqueue_discovered_reflist(
410 _refs_lists[index], _pending_list_addr);
411 _refs_lists[index].set_head(NULL);
412 _refs_lists[index].set_length(0);
413 }
414 }
415 };
416
417 // Enqueue references that are not made active again
418 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
419 AbstractRefProcTaskExecutor* task_executor) {
420 if (_processing_is_mt && task_executor != NULL) {
421 // Parallel code
422 RefProcEnqueueTask tsk(*this, _discoveredSoftRefs,
423 pending_list_addr, _max_num_q);
424 task_executor->execute(tsk);
425 } else {
426 // Serial code: call the parent class's implementation
427 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
428 enqueue_discovered_reflist(_discoveredSoftRefs[i], pending_list_addr);
429 _discoveredSoftRefs[i].set_head(NULL);
430 _discoveredSoftRefs[i].set_length(0);
431 }
432 }
433 }
434
435 // Iterator for the list of discovered references.
436 class DiscoveredListIterator {
437 public:
438 inline DiscoveredListIterator(DiscoveredList& refs_list,
439 OopClosure* keep_alive,
440 BoolObjectClosure* is_alive);
441
442 // End Of List.
443 inline bool has_next() const { return _ref != NULL; }
444
445 // Get oop to the Reference object.
446 inline oop obj() const { return _ref; }
447
448 // Get oop to the referent object.
449 inline oop referent() const { return _referent; }
450
451 // Returns true if referent is alive.
452 inline bool is_referent_alive() const;
453
454 // Loads data for the current reference.
455 // The "allow_null_referent" argument tells us to allow for the possibility
456 // of a NULL referent in the discovered Reference object. This typically
457 // happens in the case of concurrent collectors that may have done the
458 // discovery concurrently, or interleaved, with mutator execution.
459 inline void load_ptrs(DEBUG_ONLY(bool allow_null_referent));
460
461 // Move to the next discovered reference.
462 inline void next();
463
464 // Remove the current reference from the list
465 inline void remove();
466
467 // Make the Reference object active again.
468 inline void make_active() { java_lang_ref_Reference::set_next(_ref, NULL); }
469
470 // Make the referent alive.
471 inline void make_referent_alive() {
472 if (UseCompressedOops) {
473 _keep_alive->do_oop((narrowOop*)_referent_addr);
474 } else {
475 _keep_alive->do_oop((oop*)_referent_addr);
476 }
477 }
478
479 // Update the discovered field.
480 inline void update_discovered() {
481 // First _prev_next ref actually points into DiscoveredList (gross).
482 if (UseCompressedOops) {
483 if (!oopDesc::is_null(*(narrowOop*)_prev_next)) {
484 _keep_alive->do_oop((narrowOop*)_prev_next);
485 }
486 } else {
487 if (!oopDesc::is_null(*(oop*)_prev_next)) {
488 _keep_alive->do_oop((oop*)_prev_next);
489 }
490 }
491 }
492
493 // NULL out referent pointer.
494 inline void clear_referent() { oop_store_raw(_referent_addr, NULL); }
495
496 // Statistics
497 NOT_PRODUCT(
498 inline size_t processed() const { return _processed; }
499 inline size_t removed() const { return _removed; }
500 )
501
502 inline void move_to_next();
503
504 private:
505 DiscoveredList& _refs_list;
506 HeapWord* _prev_next;
507 oop _prev;
508 oop _ref;
509 HeapWord* _discovered_addr;
510 oop _next;
511 HeapWord* _referent_addr;
512 oop _referent;
513 OopClosure* _keep_alive;
514 BoolObjectClosure* _is_alive;
515 DEBUG_ONLY(
516 oop _first_seen; // cyclic linked list check
517 )
518 NOT_PRODUCT(
519 size_t _processed;
520 size_t _removed;
521 )
522 };
523
524 inline DiscoveredListIterator::DiscoveredListIterator(DiscoveredList& refs_list,
525 OopClosure* keep_alive,
526 BoolObjectClosure* is_alive)
527 : _refs_list(refs_list),
528 _prev_next(refs_list.adr_head()),
529 _prev(NULL),
530 _ref(refs_list.head()),
531 #ifdef ASSERT
532 _first_seen(refs_list.head()),
533 #endif
534 #ifndef PRODUCT
535 _processed(0),
536 _removed(0),
537 #endif
538 _next(NULL),
539 _keep_alive(keep_alive),
540 _is_alive(is_alive)
541 { }
542
543 inline bool DiscoveredListIterator::is_referent_alive() const {
544 return _is_alive->do_object_b(_referent);
545 }
546
547 inline void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
548 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
549 oop discovered = java_lang_ref_Reference::discovered(_ref);
550 assert(_discovered_addr && discovered->is_oop_or_null(),
551 "discovered field is bad");
552 _next = discovered;
553 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
554 _referent = java_lang_ref_Reference::referent(_ref);
555 assert(Universe::heap()->is_in_reserved_or_null(_referent),
556 "Wrong oop found in java.lang.Reference object");
557 assert(allow_null_referent ?
558 _referent->is_oop_or_null()
559 : _referent->is_oop(),
560 "bad referent");
561 }
562
563 inline void DiscoveredListIterator::next() {
564 _prev_next = _discovered_addr;
565 _prev = _ref;
566 move_to_next();
567 }
568
569 inline void DiscoveredListIterator::remove() {
570 assert(_ref->is_oop(), "Dropping a bad reference");
571 oop_store_raw(_discovered_addr, NULL);
572
573 // First _prev_next ref actually points into DiscoveredList (gross).
574 oop new_next;
575 if (_next == _ref) {
576 // At the end of the list, we should make _prev point to itself.
577 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
578 // and _prev will be NULL.
579 new_next = _prev;
580 } else {
581 new_next = _next;
582 }
583
584 if (UseCompressedOops) {
585 // Remove Reference object from list.
586 oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next);
587 } else {
588 // Remove Reference object from list.
589 oopDesc::store_heap_oop((oop*)_prev_next, new_next);
590 }
591 NOT_PRODUCT(_removed++);
592 _refs_list.dec_length(1);
593 }
594
595 inline void DiscoveredListIterator::move_to_next() {
596 if (_ref == _next) {
597 // End of the list.
598 _ref = NULL;
599 } else {
600 _ref = _next;
601 }
602 assert(_ref != _first_seen, "cyclic ref_list found");
603 NOT_PRODUCT(_processed++);
604 }
605
606 // NOTE: process_phase*() are largely similar, and at a high level
607 // merely iterate over the extant list applying a predicate to
608 // each of its elements and possibly removing that element from the
609 // list and applying some further closures to that element.
610 // We should consider the possibility of replacing these
611 // process_phase*() methods by abstracting them into
612 // a single general iterator invocation that receives appropriate
613 // closures that accomplish this work.
614
615 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
616 // referents are not alive, but that should be kept alive for policy reasons.
617 // Keep alive the transitive closure of all such referents.
618 void
619 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
620 ReferencePolicy* policy,
621 BoolObjectClosure* is_alive,
622 OopClosure* keep_alive,
623 VoidClosure* complete_gc) {
624 assert(policy != NULL, "Must have a non-NULL policy");
625 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
626 // Decide which softly reachable refs should be kept alive.
627 while (iter.has_next()) {
628 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
629 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
630 if (referent_is_dead && !policy->should_clear_reference(iter.obj())) {
631 if (TraceReferenceGC) {
632 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
633 iter.obj(), iter.obj()->blueprint()->internal_name());
634 }
635 // Remove Reference object from list
636 iter.remove();
637 // Make the Reference object active again
638 iter.make_active();
639 // keep the referent around
640 iter.make_referent_alive();
641 iter.move_to_next();
642 } else {
643 iter.next();
644 }
645 }
646 // Close the reachable set
647 complete_gc->do_void();
648 NOT_PRODUCT(
649 if (PrintGCDetails && TraceReferenceGC) {
650 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
651 "discovered Refs by policy, from list " INTPTR_FORMAT,
652 iter.removed(), iter.processed(), (address)refs_list.head());
653 }
654 )
655 }
656
657 // Traverse the list and remove any Refs that are not active, or
658 // whose referents are either alive or NULL.
659 void
660 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
661 BoolObjectClosure* is_alive,
662 OopClosure* keep_alive) {
663 assert(discovery_is_atomic(), "Error");
664 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
665 while (iter.has_next()) {
666 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
667 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
668 assert(next == NULL, "Should not discover inactive Reference");
669 if (iter.is_referent_alive()) {
670 if (TraceReferenceGC) {
671 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
672 iter.obj(), iter.obj()->blueprint()->internal_name());
673 }
674 // The referent is reachable after all.
675 // Remove Reference object from list.
676 iter.remove();
677 // Update the referent pointer as necessary: Note that this
678 // should not entail any recursive marking because the
679 // referent must already have been traversed.
680 iter.make_referent_alive();
681 iter.move_to_next();
682 } else {
683 iter.next();
684 }
685 }
686 NOT_PRODUCT(
687 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
688 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
689 "Refs in discovered list " INTPTR_FORMAT,
690 iter.removed(), iter.processed(), (address)refs_list.head());
691 }
692 )
693 }
694
695 void
696 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
697 BoolObjectClosure* is_alive,
698 OopClosure* keep_alive,
699 VoidClosure* complete_gc) {
700 assert(!discovery_is_atomic(), "Error");
701 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
702 while (iter.has_next()) {
703 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
704 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
705 oop next = java_lang_ref_Reference::next(iter.obj());
706 if ((iter.referent() == NULL || iter.is_referent_alive() ||
707 next != NULL)) {
708 assert(next->is_oop_or_null(), "bad next field");
709 // Remove Reference object from list
710 iter.remove();
711 // Trace the cohorts
712 iter.make_referent_alive();
713 if (UseCompressedOops) {
714 keep_alive->do_oop((narrowOop*)next_addr);
715 } else {
716 keep_alive->do_oop((oop*)next_addr);
717 }
718 iter.move_to_next();
719 } else {
720 iter.next();
721 }
722 }
723 // Now close the newly reachable set
724 complete_gc->do_void();
725 NOT_PRODUCT(
726 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
727 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
728 "Refs in discovered list " INTPTR_FORMAT,
729 iter.removed(), iter.processed(), (address)refs_list.head());
730 }
731 )
732 }
733
734 // Traverse the list and process the referents, by either
735 // clearing them or keeping them (and their reachable
736 // closure) alive.
737 void
738 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
739 bool clear_referent,
740 BoolObjectClosure* is_alive,
741 OopClosure* keep_alive,
742 VoidClosure* complete_gc) {
743 ResourceMark rm;
744 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
745 while (iter.has_next()) {
746 iter.update_discovered();
747 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
748 if (clear_referent) {
749 // NULL out referent pointer
750 iter.clear_referent();
751 } else {
752 // keep the referent around
753 iter.make_referent_alive();
754 }
755 if (TraceReferenceGC) {
756 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
757 clear_referent ? "cleared " : "",
758 iter.obj(), iter.obj()->blueprint()->internal_name());
759 }
760 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
761 iter.next();
762 }
763 // Remember to update the next pointer of the last ref.
764 iter.update_discovered();
765 // Close the reachable set
766 complete_gc->do_void();
767 }
768
769 void
770 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
771 oop obj = NULL;
772 oop next = refs_list.head();
773 while (next != obj) {
774 obj = next;
775 next = java_lang_ref_Reference::discovered(obj);
776 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
777 }
778 refs_list.set_head(NULL);
779 refs_list.set_length(0);
780 }
781
782 void
783 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
784 clear_discovered_references(refs_list);
785 }
786
787 void ReferenceProcessor::abandon_partial_discovery() {
788 // loop over the lists
789 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
790 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
791 gclog_or_tty->print_cr("\nAbandoning %s discovered list",
792 list_name(i));
793 }
794 abandon_partial_discovered_list(_discoveredSoftRefs[i]);
795 }
796 }
797
798 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
799 public:
800 RefProcPhase1Task(ReferenceProcessor& ref_processor,
801 DiscoveredList refs_lists[],
802 ReferencePolicy* policy,
803 bool marks_oops_alive)
804 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
805 _policy(policy)
806 { }
807 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
808 OopClosure& keep_alive,
809 VoidClosure& complete_gc)
810 {
811 Thread* thr = Thread::current();
812 int refs_list_index = ((WorkerThread*)thr)->id();
813 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
814 &is_alive, &keep_alive, &complete_gc);
815 }
816 private:
817 ReferencePolicy* _policy;
818 };
819
820 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
821 public:
822 RefProcPhase2Task(ReferenceProcessor& ref_processor,
823 DiscoveredList refs_lists[],
824 bool marks_oops_alive)
825 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
826 { }
827 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
828 OopClosure& keep_alive,
829 VoidClosure& complete_gc)
830 {
831 _ref_processor.process_phase2(_refs_lists[i],
832 &is_alive, &keep_alive, &complete_gc);
833 }
834 };
835
836 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
837 public:
838 RefProcPhase3Task(ReferenceProcessor& ref_processor,
839 DiscoveredList refs_lists[],
840 bool clear_referent,
841 bool marks_oops_alive)
842 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
843 _clear_referent(clear_referent)
844 { }
845 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
846 OopClosure& keep_alive,
847 VoidClosure& complete_gc)
848 {
849 // Don't use "refs_list_index" calculated in this way because
850 // balance_queues() has moved the Ref's into the first n queues.
851 // Thread* thr = Thread::current();
852 // int refs_list_index = ((WorkerThread*)thr)->id();
853 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
854 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
855 &is_alive, &keep_alive, &complete_gc);
856 }
857 private:
858 bool _clear_referent;
859 };
860
861 // Balances reference queues.
862 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
863 // queues[0, 1, ..., _num_q-1] because only the first _num_q
864 // corresponding to the active workers will be processed.
865 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
866 {
867 // calculate total length
868 size_t total_refs = 0;
869 if (TraceReferenceGC && PrintGCDetails) {
870 gclog_or_tty->print_cr("\nBalance ref_lists ");
871 }
872
873 for (int i = 0; i < _max_num_q; ++i) {
874 total_refs += ref_lists[i].length();
875 if (TraceReferenceGC && PrintGCDetails) {
876 gclog_or_tty->print("%d ", ref_lists[i].length());
877 }
878 }
879 if (TraceReferenceGC && PrintGCDetails) {
880 gclog_or_tty->print_cr(" = %d", total_refs);
881 }
882 size_t avg_refs = total_refs / _num_q + 1;
883 int to_idx = 0;
884 for (int from_idx = 0; from_idx < _max_num_q; from_idx++) {
885 bool move_all = false;
886 if (from_idx >= _num_q) {
887 move_all = ref_lists[from_idx].length() > 0;
888 }
889 while ((ref_lists[from_idx].length() > avg_refs) ||
890 move_all) {
891 assert(to_idx < _num_q, "Sanity Check!");
892 if (ref_lists[to_idx].length() < avg_refs) {
893 // move superfluous refs
894 size_t refs_to_move;
895 // Move all the Ref's if the from queue will not be processed.
896 if (move_all) {
897 refs_to_move = MIN2(ref_lists[from_idx].length(),
898 avg_refs - ref_lists[to_idx].length());
899 } else {
900 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
901 avg_refs - ref_lists[to_idx].length());
902 }
903
904 assert(refs_to_move > 0, "otherwise the code below will fail");
905
906 oop move_head = ref_lists[from_idx].head();
907 oop move_tail = move_head;
908 oop new_head = move_head;
909 // find an element to split the list on
910 for (size_t j = 0; j < refs_to_move; ++j) {
911 move_tail = new_head;
912 new_head = java_lang_ref_Reference::discovered(new_head);
913 }
914
915 // Add the chain to the to list.
916 if (ref_lists[to_idx].head() == NULL) {
917 // to list is empty. Make a loop at the end.
918 java_lang_ref_Reference::set_discovered(move_tail, move_tail);
919 } else {
920 java_lang_ref_Reference::set_discovered(move_tail, ref_lists[to_idx].head());
921 }
922 ref_lists[to_idx].set_head(move_head);
923 ref_lists[to_idx].inc_length(refs_to_move);
924
925 // Remove the chain from the from list.
926 if (move_tail == new_head) {
927 // We found the end of the from list.
928 ref_lists[from_idx].set_head(NULL);
929 } else {
930 ref_lists[from_idx].set_head(new_head);
931 }
932 ref_lists[from_idx].dec_length(refs_to_move);
933 if (ref_lists[from_idx].length() == 0) {
934 break;
935 }
936 } else {
937 to_idx = (to_idx + 1) % _num_q;
938 }
939 }
940 }
941 #ifdef ASSERT
942 size_t balanced_total_refs = 0;
943 for (int i = 0; i < _max_num_q; ++i) {
944 balanced_total_refs += ref_lists[i].length();
945 if (TraceReferenceGC && PrintGCDetails) {
946 gclog_or_tty->print("%d ", ref_lists[i].length());
947 }
948 }
949 if (TraceReferenceGC && PrintGCDetails) {
950 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
951 gclog_or_tty->flush();
952 }
953 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
954 #endif
955 }
956
957 void ReferenceProcessor::balance_all_queues() {
958 balance_queues(_discoveredSoftRefs);
959 balance_queues(_discoveredWeakRefs);
960 balance_queues(_discoveredFinalRefs);
961 balance_queues(_discoveredPhantomRefs);
962 }
963
964 void
965 ReferenceProcessor::process_discovered_reflist(
966 DiscoveredList refs_lists[],
967 ReferencePolicy* policy,
968 bool clear_referent,
969 BoolObjectClosure* is_alive,
970 OopClosure* keep_alive,
971 VoidClosure* complete_gc,
972 AbstractRefProcTaskExecutor* task_executor)
973 {
974 bool mt_processing = task_executor != NULL && _processing_is_mt;
975 // If discovery used MT and a dynamic number of GC threads, then
976 // the queues must be balanced for correctness if fewer than the
977 // maximum number of queues were used. The number of queue used
978 // during discovery may be different than the number to be used
979 // for processing so don't depend of _num_q < _max_num_q as part
980 // of the test.
981 bool must_balance = _discovery_is_mt;
982
983 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
984 must_balance) {
985 balance_queues(refs_lists);
986 }
987 if (PrintReferenceGC && PrintGCDetails) {
988 size_t total = 0;
989 for (int i = 0; i < _max_num_q; ++i) {
990 total += refs_lists[i].length();
991 }
992 gclog_or_tty->print(", %u refs", total);
993 }
994
995 // Phase 1 (soft refs only):
996 // . Traverse the list and remove any SoftReferences whose
997 // referents are not alive, but that should be kept alive for
998 // policy reasons. Keep alive the transitive closure of all
999 // such referents.
1000 if (policy != NULL) {
1001 if (mt_processing) {
1002 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
1003 task_executor->execute(phase1);
1004 } else {
1005 for (int i = 0; i < _max_num_q; i++) {
1006 process_phase1(refs_lists[i], policy,
1007 is_alive, keep_alive, complete_gc);
1008 }
1009 }
1010 } else { // policy == NULL
1011 assert(refs_lists != _discoveredSoftRefs,
1012 "Policy must be specified for soft references.");
1013 }
1014
1015 // Phase 2:
1016 // . Traverse the list and remove any refs whose referents are alive.
1017 if (mt_processing) {
1018 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
1019 task_executor->execute(phase2);
1020 } else {
1021 for (int i = 0; i < _max_num_q; i++) {
1022 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
1023 }
1024 }
1025
1026 // Phase 3:
1027 // . Traverse the list and process referents as appropriate.
1028 if (mt_processing) {
1029 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
1030 task_executor->execute(phase3);
1031 } else {
1032 for (int i = 0; i < _max_num_q; i++) {
1033 process_phase3(refs_lists[i], clear_referent,
1034 is_alive, keep_alive, complete_gc);
1035 }
1036 }
1037 }
1038
1039 void ReferenceProcessor::clean_up_discovered_references() {
1040 // loop over the lists
1041 // Should this instead be
1042 // for (int i = 0; i < subclasses_of_ref; i++_ {
1043 // for (int j = 0; j < _num_q; j++) {
1044 // int index = i * _max_num_q + j;
1045 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
1046 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
1047 gclog_or_tty->print_cr(
1048 "\nScrubbing %s discovered list of Null referents",
1049 list_name(i));
1050 }
1051 clean_up_discovered_reflist(_discoveredSoftRefs[i]);
1052 }
1053 }
1054
1055 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
1056 assert(!discovery_is_atomic(), "Else why call this method?");
1057 DiscoveredListIterator iter(refs_list, NULL, NULL);
1058 while (iter.has_next()) {
1059 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1060 oop next = java_lang_ref_Reference::next(iter.obj());
1061 assert(next->is_oop_or_null(), "bad next field");
1062 // If referent has been cleared or Reference is not active,
1063 // drop it.
1064 if (iter.referent() == NULL || next != NULL) {
1065 debug_only(
1066 if (PrintGCDetails && TraceReferenceGC) {
1067 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
1068 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
1069 " and referent: " INTPTR_FORMAT,
1070 iter.obj(), next, iter.referent());
1071 }
1072 )
1073 // Remove Reference object from list
1074 iter.remove();
1075 iter.move_to_next();
1076 } else {
1077 iter.next();
1078 }
1079 }
1080 NOT_PRODUCT(
1081 if (PrintGCDetails && TraceReferenceGC) {
1082 gclog_or_tty->print(
1083 " Removed %d Refs with NULL referents out of %d discovered Refs",
1084 iter.removed(), iter.processed());
1085 }
1086 )
1087 }
1088
1089 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1090 int id = 0;
1091 // Determine the queue index to use for this object.
1092 if (_discovery_is_mt) {
1093 // During a multi-threaded discovery phase,
1094 // each thread saves to its "own" list.
1095 Thread* thr = Thread::current();
1096 id = thr->as_Worker_thread()->id();
1097 } else {
1098 // single-threaded discovery, we save in round-robin
1099 // fashion to each of the lists.
1100 if (_processing_is_mt) {
1101 id = next_id();
1102 }
1103 }
1104 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1105
1106 // Get the discovered queue to which we will add
1107 DiscoveredList* list = NULL;
1108 switch (rt) {
1109 case REF_OTHER:
1110 // Unknown reference type, no special treatment
1111 break;
1112 case REF_SOFT:
1113 list = &_discoveredSoftRefs[id];
1114 break;
1115 case REF_WEAK:
1116 list = &_discoveredWeakRefs[id];
1117 break;
1118 case REF_FINAL:
1119 list = &_discoveredFinalRefs[id];
1120 break;
1121 case REF_PHANTOM:
1122 list = &_discoveredPhantomRefs[id];
1123 break;
1124 case REF_NONE:
1125 // we should not reach here if we are an instanceRefKlass
1126 default:
1127 ShouldNotReachHere();
1128 }
1129 if (TraceReferenceGC && PrintGCDetails) {
1130 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1131 }
1132 return list;
1133 }
1134
1135 inline void
1136 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1137 oop obj,
1138 HeapWord* discovered_addr) {
1139 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1140 // First we must make sure this object is only enqueued once. CAS in a non null
1141 // discovered_addr.
1142 oop current_head = refs_list.head();
1143 // The last ref must have its discovered field pointing to itself.
1144 oop next_discovered = (current_head != NULL) ? current_head : obj;
1145
1146 // Note: In the case of G1, this specific pre-barrier is strictly
1147 // not necessary because the only case we are interested in
1148 // here is when *discovered_addr is NULL (see the CAS further below),
1149 // so this will expand to nothing. As a result, we have manually
1150 // elided this out for G1, but left in the test for some future
1151 // collector that might have need for a pre-barrier here, e.g.:-
1152 // _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1153 assert(!_discovered_list_needs_barrier || UseG1GC,
1154 "Need to check non-G1 collector: "
1155 "may need a pre-write-barrier for CAS from NULL below");
1156 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1157 NULL);
1158 if (retest == NULL) {
1159 // This thread just won the right to enqueue the object.
1160 // We have separate lists for enqueueing, so no synchronization
1161 // is necessary.
1162 refs_list.set_head(obj);
1163 refs_list.inc_length(1);
1164 if (_discovered_list_needs_barrier) {
1165 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1166 }
1167
1168 if (TraceReferenceGC) {
1169 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1170 obj, obj->blueprint()->internal_name());
1171 }
1172 } else {
1173 // If retest was non NULL, another thread beat us to it:
1174 // The reference has already been discovered...
1175 if (TraceReferenceGC) {
1176 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1177 obj, obj->blueprint()->internal_name());
1178 }
1179 }
1180 }
1181
1182 #ifndef PRODUCT
1183 // Non-atomic (i.e. concurrent) discovery might allow us
1184 // to observe j.l.References with NULL referents, being those
1185 // cleared concurrently by mutators during (or after) discovery.
1186 void ReferenceProcessor::verify_referent(oop obj) {
1187 bool da = discovery_is_atomic();
1188 oop referent = java_lang_ref_Reference::referent(obj);
1189 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1190 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1191 INTPTR_FORMAT " during %satomic discovery ",
1192 (intptr_t)referent, (intptr_t)obj, da ? "" : "non-"));
1193 }
1194 #endif
1195
1196 // We mention two of several possible choices here:
1197 // #0: if the reference object is not in the "originating generation"
1198 // (or part of the heap being collected, indicated by our "span"
1199 // we don't treat it specially (i.e. we scan it as we would
1200 // a normal oop, treating its references as strong references).
1201 // This means that references can't be discovered unless their
1202 // referent is also in the same span. This is the simplest,
1203 // most "local" and most conservative approach, albeit one
1204 // that may cause weak references to be enqueued least promptly.
1205 // We call this choice the "ReferenceBasedDiscovery" policy.
1206 // #1: the reference object may be in any generation (span), but if
1207 // the referent is in the generation (span) being currently collected
1208 // then we can discover the reference object, provided
1209 // the object has not already been discovered by
1210 // a different concurrently running collector (as may be the
1211 // case, for instance, if the reference object is in CMS and
1212 // the referent in DefNewGeneration), and provided the processing
1213 // of this reference object by the current collector will
1214 // appear atomic to every other collector in the system.
1215 // (Thus, for instance, a concurrent collector may not
1216 // discover references in other generations even if the
1217 // referent is in its own generation). This policy may,
1218 // in certain cases, enqueue references somewhat sooner than
1219 // might Policy #0 above, but at marginally increased cost
1220 // and complexity in processing these references.
1221 // We call this choice the "RefeferentBasedDiscovery" policy.
1222 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1223 // Make sure we are discovering refs (rather than processing discovered refs).
1224 if (!_discovering_refs || !RegisterReferences) {
1225 return false;
1226 }
1227 // We only discover active references.
1228 oop next = java_lang_ref_Reference::next(obj);
1229 if (next != NULL) { // Ref is no longer active
1230 return false;
1231 }
1232
1233 HeapWord* obj_addr = (HeapWord*)obj;
1234 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1235 !_span.contains(obj_addr)) {
1236 // Reference is not in the originating generation;
1237 // don't treat it specially (i.e. we want to scan it as a normal
1238 // object with strong references).
1239 return false;
1240 }
1241
1242 // We only discover references whose referents are not (yet)
1243 // known to be strongly reachable.
1244 if (is_alive_non_header() != NULL) {
1245 verify_referent(obj);
1246 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1247 return false; // referent is reachable
1248 }
1249 }
1250 if (rt == REF_SOFT) {
1251 // For soft refs we can decide now if these are not
1252 // current candidates for clearing, in which case we
1253 // can mark through them now, rather than delaying that
1254 // to the reference-processing phase. Since all current
1255 // time-stamp policies advance the soft-ref clock only
1256 // at a major collection cycle, this is always currently
1257 // accurate.
1258 if (!_current_soft_ref_policy->should_clear_reference(obj)) {
1259 return false;
1260 }
1261 }
1262
1263 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1264 const oop discovered = java_lang_ref_Reference::discovered(obj);
1265 assert(discovered->is_oop_or_null(), "bad discovered field");
1266 if (discovered != NULL) {
1267 // The reference has already been discovered...
1268 if (TraceReferenceGC) {
1269 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1270 obj, obj->blueprint()->internal_name());
1271 }
1272 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1273 // assumes that an object is not processed twice;
1274 // if it's been already discovered it must be on another
1275 // generation's discovered list; so we won't discover it.
1276 return false;
1277 } else {
1278 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1279 "Unrecognized policy");
1280 // Check assumption that an object is not potentially
1281 // discovered twice except by concurrent collectors that potentially
1282 // trace the same Reference object twice.
1283 assert(UseConcMarkSweepGC || UseG1GC,
1284 "Only possible with a concurrent marking collector");
1285 return true;
1286 }
1287 }
1288
1289 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1290 verify_referent(obj);
1291 // Discover if and only if EITHER:
1292 // .. reference is in our span, OR
1293 // .. we are an atomic collector and referent is in our span
1294 if (_span.contains(obj_addr) ||
1295 (discovery_is_atomic() &&
1296 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1297 // should_enqueue = true;
1298 } else {
1299 return false;
1300 }
1301 } else {
1302 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1303 _span.contains(obj_addr), "code inconsistency");
1304 }
1305
1306 // Get the right type of discovered queue head.
1307 DiscoveredList* list = get_discovered_list(rt);
1308 if (list == NULL) {
1309 return false; // nothing special needs to be done
1310 }
1311
1312 if (_discovery_is_mt) {
1313 add_to_discovered_list_mt(*list, obj, discovered_addr);
1314 } else {
1315 // If "_discovered_list_needs_barrier", we do write barriers when
1316 // updating the discovered reference list. Otherwise, we do a raw store
1317 // here: the field will be visited later when processing the discovered
1318 // references.
1319 oop current_head = list->head();
1320 // The last ref must have its discovered field pointing to itself.
1321 oop next_discovered = (current_head != NULL) ? current_head : obj;
1322
1323 // As in the case further above, since we are over-writing a NULL
1324 // pre-value, we can safely elide the pre-barrier here for the case of G1.
1325 // e.g.:- _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1326 assert(discovered == NULL, "control point invariant");
1327 assert(!_discovered_list_needs_barrier || UseG1GC,
1328 "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
1329 oop_store_raw(discovered_addr, next_discovered);
1330 if (_discovered_list_needs_barrier) {
1331 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1332 }
1333 list->set_head(obj);
1334 list->inc_length(1);
1335
1336 if (TraceReferenceGC) {
1337 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1338 obj, obj->blueprint()->internal_name());
1339 }
1340 }
1341 assert(obj->is_oop(), "Discovered a bad reference");
1342 verify_referent(obj);
1343 return true;
1344 }
1345
1346 // Preclean the discovered references by removing those
1347 // whose referents are alive, and by marking from those that
1348 // are not active. These lists can be handled here
1349 // in any order and, indeed, concurrently.
1350 void ReferenceProcessor::preclean_discovered_references(
1351 BoolObjectClosure* is_alive,
1352 OopClosure* keep_alive,
1353 VoidClosure* complete_gc,
1354 YieldClosure* yield,
1355 bool should_unload_classes) {
1356
1357 NOT_PRODUCT(verify_ok_to_handle_reflists());
1358
1359 #ifdef ASSERT
1360 bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC ||
1361 CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
1362 ExplicitGCInvokesConcurrentAndUnloadsClasses &&
1363 UseConcMarkSweepGC && should_unload_classes;
1364 RememberKlassesChecker mx(must_remember_klasses);
1365 #endif
1366 // Soft references
1367 {
1368 TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1369 false, gclog_or_tty);
1370 for (int i = 0; i < _max_num_q; i++) {
1371 if (yield->should_return()) {
1372 return;
1373 }
1374 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1375 keep_alive, complete_gc, yield);
1376 }
1377 }
1378
1379 // Weak references
1380 {
1381 TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1382 false, gclog_or_tty);
1383 for (int i = 0; i < _max_num_q; i++) {
1384 if (yield->should_return()) {
1385 return;
1386 }
1387 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1388 keep_alive, complete_gc, yield);
1389 }
1390 }
1391
1392 // Final references
1393 {
1394 TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1395 false, gclog_or_tty);
1396 for (int i = 0; i < _max_num_q; i++) {
1397 if (yield->should_return()) {
1398 return;
1399 }
1400 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1401 keep_alive, complete_gc, yield);
1402 }
1403 }
1404
1405 // Phantom references
1406 {
1407 TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1408 false, gclog_or_tty);
1409 for (int i = 0; i < _max_num_q; i++) {
1410 if (yield->should_return()) {
1411 return;
1412 }
1413 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1414 keep_alive, complete_gc, yield);
1415 }
1416 }
1417 }
1418
1419 // Walk the given discovered ref list, and remove all reference objects
1420 // whose referents are still alive, whose referents are NULL or which
1421 // are not active (have a non-NULL next field). NOTE: When we are
1422 // thus precleaning the ref lists (which happens single-threaded today),
1423 // we do not disable refs discovery to honour the correct semantics of
1424 // java.lang.Reference. As a result, we need to be careful below
1425 // that ref removal steps interleave safely with ref discovery steps
1426 // (in this thread).
1427 void
1428 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1429 BoolObjectClosure* is_alive,
1430 OopClosure* keep_alive,
1431 VoidClosure* complete_gc,
1432 YieldClosure* yield) {
1433 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1434 while (iter.has_next()) {
1435 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1436 oop obj = iter.obj();
1437 oop next = java_lang_ref_Reference::next(obj);
1438 if (iter.referent() == NULL || iter.is_referent_alive() ||
1439 next != NULL) {
1440 // The referent has been cleared, or is alive, or the Reference is not
1441 // active; we need to trace and mark its cohort.
1442 if (TraceReferenceGC) {
1443 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1444 iter.obj(), iter.obj()->blueprint()->internal_name());
1445 }
1446 // Remove Reference object from list
1447 iter.remove();
1448 // Keep alive its cohort.
1449 iter.make_referent_alive();
1450 if (UseCompressedOops) {
1451 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1452 keep_alive->do_oop(next_addr);
1453 } else {
1454 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1455 keep_alive->do_oop(next_addr);
1456 }
1457 iter.move_to_next();
1458 } else {
1459 iter.next();
1460 }
1461 }
1462 // Close the reachable set
1463 complete_gc->do_void();
1464
1465 NOT_PRODUCT(
1466 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1467 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1468 "Refs in discovered list " INTPTR_FORMAT,
1469 iter.removed(), iter.processed(), (address)refs_list.head());
1470 }
1471 )
1472 }
1473
1474 const char* ReferenceProcessor::list_name(int i) {
1475 assert(i >= 0 && i <= _max_num_q * subclasses_of_ref, "Out of bounds index");
1476 int j = i / _max_num_q;
1477 switch (j) {
1478 case 0: return "SoftRef";
1479 case 1: return "WeakRef";
1480 case 2: return "FinalRef";
1481 case 3: return "PhantomRef";
1482 }
1483 ShouldNotReachHere();
1484 return NULL;
1485 }
1486
1487 #ifndef PRODUCT
1488 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1489 // empty for now
1490 }
1491 #endif
1492
1493 #ifndef PRODUCT
1494 void ReferenceProcessor::clear_discovered_references() {
1495 guarantee(!_discovering_refs, "Discovering refs?");
1496 for (int i = 0; i < _max_num_q * subclasses_of_ref; i++) {
1497 clear_discovered_references(_discoveredSoftRefs[i]);
1498 }
1499 }
1500
1501 #endif // PRODUCT