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 bool ReferenceProcessor::_pending_list_uses_discovered_field = false;
39
40 void referenceProcessor_init() {
41 ReferenceProcessor::init_statics();
42 }
43
44 void ReferenceProcessor::init_statics() {
45 // Initialize the master soft ref clock.
46 java_lang_ref_SoftReference::set_clock(os::javaTimeMillis());
47
48 _always_clear_soft_ref_policy = new AlwaysClearPolicy();
49 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
50 NOT_COMPILER2(LRUCurrentHeapPolicy());
51 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
52 vm_exit_during_initialization("Could not allocate reference policy object");
53 }
54 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
55 RefDiscoveryPolicy == ReferentBasedDiscovery,
56 "Unrecongnized RefDiscoveryPolicy");
57 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
58 }
59
60 ReferenceProcessor::ReferenceProcessor(MemRegion span,
61 bool mt_processing,
62 int mt_processing_degree,
63 bool mt_discovery,
64 int mt_discovery_degree,
65 bool atomic_discovery,
66 BoolObjectClosure* is_alive_non_header,
67 bool discovered_list_needs_barrier) :
68 _discovering_refs(false),
69 _enqueuing_is_done(false),
70 _is_alive_non_header(is_alive_non_header),
71 _discovered_list_needs_barrier(discovered_list_needs_barrier),
72 _bs(NULL),
73 _processing_is_mt(mt_processing),
74 _next_id(0)
75 {
76 _span = span;
77 _discovery_is_atomic = atomic_discovery;
78 _discovery_is_mt = mt_discovery;
79 _num_q = MAX2(1, mt_processing_degree);
80 _max_num_q = MAX2(_num_q, mt_discovery_degree);
81 _discoveredSoftRefs = NEW_C_HEAP_ARRAY(DiscoveredList,
82 _max_num_q * number_of_subclasses_of_ref());
83 if (_discoveredSoftRefs == NULL) {
84 vm_exit_during_initialization("Could not allocated RefProc Array");
85 }
86 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q];
87 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q];
88 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
89 // Initialized all entries to NULL
90 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
91 _discoveredSoftRefs[i].set_head(NULL);
92 _discoveredSoftRefs[i].set_length(0);
93 }
94 // If we do barriers, cache a copy of the barrier set.
95 if (discovered_list_needs_barrier) {
96 _bs = Universe::heap()->barrier_set();
97 }
98 setup_policy(false /* default soft ref policy */);
99 }
100
101 #ifndef PRODUCT
102 void ReferenceProcessor::verify_no_references_recorded() {
103 guarantee(!_discovering_refs, "Discovering refs?");
104 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
105 guarantee(_discoveredSoftRefs[i].is_empty(),
106 "Found non-empty discovered list");
107 }
108 }
109 #endif
110
111 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
112 // An alternative implementation of this routine
113 // could use the following nested loop:
114 //
115 // for (int i = 0; i < number_of_subclasses_of_ref(); i++_ {
116 // for (int j = 0; j < _num_q; j++) {
117 // int index = i * _max_num_q + j;
118
119 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
120 if (UseCompressedOops) {
121 f->do_oop((narrowOop*)_discoveredSoftRefs[i].adr_head());
122 } else {
123 f->do_oop((oop*)_discoveredSoftRefs[i].adr_head());
124 }
125 }
126 }
127
128 void ReferenceProcessor::update_soft_ref_master_clock() {
129 // Update (advance) the soft ref master clock field. This must be done
130 // after processing the soft ref list.
131 jlong now = os::javaTimeMillis();
132 jlong clock = java_lang_ref_SoftReference::clock();
133 NOT_PRODUCT(
134 if (now < clock) {
135 warning("time warp: %d to %d", clock, now);
136 }
137 )
138 // In product mode, protect ourselves from system time being adjusted
139 // externally and going backward; see note in the implementation of
140 // GenCollectedHeap::time_since_last_gc() for the right way to fix
141 // this uniformly throughout the VM; see bug-id 4741166. XXX
142 if (now > clock) {
143 java_lang_ref_SoftReference::set_clock(now);
144 }
145 // Else leave clock stalled at its old value until time progresses
146 // past clock value.
147 }
148
149 void ReferenceProcessor::process_discovered_references(
150 BoolObjectClosure* is_alive,
151 OopClosure* keep_alive,
152 VoidClosure* complete_gc,
153 AbstractRefProcTaskExecutor* task_executor) {
154 NOT_PRODUCT(verify_ok_to_handle_reflists());
155
156 assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
157 // Stop treating discovered references specially.
158 disable_discovery();
159
160 bool trace_time = PrintGCDetails && PrintReferenceGC;
161 // Soft references
162 {
163 TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);
164 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
165 is_alive, keep_alive, complete_gc, task_executor);
166 }
167
168 update_soft_ref_master_clock();
169
170 // Weak references
171 {
172 TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);
173 process_discovered_reflist(_discoveredWeakRefs, NULL, true,
174 is_alive, keep_alive, complete_gc, task_executor);
175 }
176
177 // Final references
178 {
179 TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);
180 process_discovered_reflist(_discoveredFinalRefs, NULL, false,
181 is_alive, keep_alive, complete_gc, task_executor);
182 }
183
184 // Phantom references
185 {
186 TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);
187 process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
188 is_alive, keep_alive, complete_gc, task_executor);
189 }
190
191 // Weak global JNI references. It would make more sense (semantically) to
192 // traverse these simultaneously with the regular weak references above, but
193 // that is not how the JDK1.2 specification is. See #4126360. Native code can
194 // thus use JNI weak references to circumvent the phantom references and
195 // resurrect a "post-mortem" object.
196 {
197 TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);
198 if (task_executor != NULL) {
199 task_executor->set_single_threaded_mode();
200 }
201 process_phaseJNI(is_alive, keep_alive, complete_gc);
202 }
203 }
204
205 #ifndef PRODUCT
206 // Calculate the number of jni handles.
207 uint ReferenceProcessor::count_jni_refs() {
208 class AlwaysAliveClosure: public BoolObjectClosure {
209 public:
210 virtual bool do_object_b(oop obj) { return true; }
211 virtual void do_object(oop obj) { assert(false, "Don't call"); }
212 };
213
214 class CountHandleClosure: public OopClosure {
215 private:
216 int _count;
217 public:
218 CountHandleClosure(): _count(0) {}
219 void do_oop(oop* unused) { _count++; }
220 void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
221 int count() { return _count; }
222 };
223 CountHandleClosure global_handle_count;
224 AlwaysAliveClosure always_alive;
225 JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
226 return global_handle_count.count();
227 }
228 #endif
229
230 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
231 OopClosure* keep_alive,
232 VoidClosure* complete_gc) {
233 #ifndef PRODUCT
234 if (PrintGCDetails && PrintReferenceGC) {
235 unsigned int count = count_jni_refs();
236 gclog_or_tty->print(", %u refs", count);
237 }
238 #endif
239 JNIHandles::weak_oops_do(is_alive, keep_alive);
240 complete_gc->do_void();
241 }
242
243
244 template <class T>
245 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
246 AbstractRefProcTaskExecutor* task_executor) {
247
248 // Remember old value of pending references list
249 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
250 T old_pending_list_value = *pending_list_addr;
251
252 // Enqueue references that are not made active again, and
253 // clear the decks for the next collection (cycle).
254 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
255 // Do the oop-check on pending_list_addr missed in
256 // enqueue_discovered_reflist. We should probably
257 // do a raw oop_check so that future such idempotent
258 // oop_stores relying on the oop-check side-effect
259 // may be elided automatically and safely without
260 // affecting correctness.
261 oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
262
263 // Stop treating discovered references specially.
264 ref->disable_discovery();
265
266 // Return true if new pending references were added
267 return old_pending_list_value != *pending_list_addr;
268 }
269
270 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
271 NOT_PRODUCT(verify_ok_to_handle_reflists());
272 if (UseCompressedOops) {
273 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
274 } else {
275 return enqueue_discovered_ref_helper<oop>(this, task_executor);
276 }
277 }
278
279 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
280 HeapWord* pending_list_addr) {
281 // Given a list of refs linked through the "discovered" field
282 // (java.lang.ref.Reference.discovered), self-loop their "next" field
283 // thus distinguishing them from active References, then
284 // prepend them to the pending list.
285 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
286 // the "next" field is used to chain the pending list, not the discovered
287 // field.
288
289 if (TraceReferenceGC && PrintGCDetails) {
290 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
291 INTPTR_FORMAT, (address)refs_list.head());
292 }
293
294 oop obj = NULL;
295 oop next_d = refs_list.head();
296 if (pending_list_uses_discovered_field()) { // New behaviour
297 // Walk down the list, self-looping the next field
298 // so that the References are not considered active.
299 while (obj != next_d) {
300 obj = next_d;
301 assert(obj->is_instanceRef(), "should be reference object");
302 next_d = java_lang_ref_Reference::discovered(obj);
303 if (TraceReferenceGC && PrintGCDetails) {
304 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
305 obj, next_d);
306 }
307 assert(java_lang_ref_Reference::next(obj) == NULL,
308 "Reference not active; should not be discovered");
309 // Self-loop next, so as to make Ref not active.
310 java_lang_ref_Reference::set_next(obj, obj);
311 if (next_d == obj) { // obj is last
312 // Swap refs_list into pendling_list_addr and
313 // set obj's discovered to what we read from pending_list_addr.
314 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
315 // Need oop_check on pending_list_addr above;
316 // see special oop-check code at the end of
317 // enqueue_discovered_reflists() further below.
318 java_lang_ref_Reference::set_discovered(obj, old); // old may be NULL
319 }
320 }
321 } else { // Old behaviour
322 // Walk down the list, copying the discovered field into
323 // the next field and clearing the discovered field.
324 while (obj != next_d) {
325 obj = next_d;
326 assert(obj->is_instanceRef(), "should be reference object");
327 next_d = java_lang_ref_Reference::discovered(obj);
328 if (TraceReferenceGC && PrintGCDetails) {
329 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
330 obj, next_d);
331 }
332 assert(java_lang_ref_Reference::next(obj) == NULL,
333 "The reference should not be enqueued");
334 if (next_d == obj) { // obj is last
335 // Swap refs_list into pendling_list_addr and
336 // set obj's next to what we read from pending_list_addr.
337 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
338 // Need oop_check on pending_list_addr above;
339 // see special oop-check code at the end of
340 // enqueue_discovered_reflists() further below.
341 if (old == NULL) {
342 // obj should be made to point to itself, since
343 // pending list was empty.
344 java_lang_ref_Reference::set_next(obj, obj);
345 } else {
346 java_lang_ref_Reference::set_next(obj, old);
347 }
348 } else {
349 java_lang_ref_Reference::set_next(obj, next_d);
350 }
351 java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
352 }
353 }
354 }
355
356 // Parallel enqueue task
357 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
358 public:
359 RefProcEnqueueTask(ReferenceProcessor& ref_processor,
360 DiscoveredList discovered_refs[],
361 HeapWord* pending_list_addr,
362 int n_queues)
363 : EnqueueTask(ref_processor, discovered_refs,
364 pending_list_addr, n_queues)
365 { }
366
367 virtual void work(unsigned int work_id) {
368 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
369 // Simplest first cut: static partitioning.
370 int index = work_id;
371 // The increment on "index" must correspond to the maximum number of queues
372 // (n_queues) with which that ReferenceProcessor was created. That
373 // is because of the "clever" way the discovered references lists were
374 // allocated and are indexed into.
375 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
376 for (int j = 0;
377 j < ReferenceProcessor::number_of_subclasses_of_ref();
378 j++, index += _n_queues) {
379 _ref_processor.enqueue_discovered_reflist(
380 _refs_lists[index], _pending_list_addr);
381 _refs_lists[index].set_head(NULL);
382 _refs_lists[index].set_length(0);
383 }
384 }
385 };
386
387 // Enqueue references that are not made active again
388 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
389 AbstractRefProcTaskExecutor* task_executor) {
390 if (_processing_is_mt && task_executor != NULL) {
391 // Parallel code
392 RefProcEnqueueTask tsk(*this, _discoveredSoftRefs,
393 pending_list_addr, _max_num_q);
394 task_executor->execute(tsk);
395 } else {
396 // Serial code: call the parent class's implementation
397 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
398 enqueue_discovered_reflist(_discoveredSoftRefs[i], pending_list_addr);
399 _discoveredSoftRefs[i].set_head(NULL);
400 _discoveredSoftRefs[i].set_length(0);
401 }
402 }
403 }
404
405 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
406 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
407 oop discovered = java_lang_ref_Reference::discovered(_ref);
408 assert(_discovered_addr && discovered->is_oop_or_null(),
409 "discovered field is bad");
410 _next = discovered;
411 _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
412 _referent = java_lang_ref_Reference::referent(_ref);
413 assert(Universe::heap()->is_in_reserved_or_null(_referent),
414 "Wrong oop found in java.lang.Reference object");
415 assert(allow_null_referent ?
416 _referent->is_oop_or_null()
417 : _referent->is_oop(),
418 "bad referent");
419 }
420
421 void DiscoveredListIterator::remove() {
422 assert(_ref->is_oop(), "Dropping a bad reference");
423 oop_store_raw(_discovered_addr, NULL);
424
425 // First _prev_next ref actually points into DiscoveredList (gross).
426 oop new_next;
427 if (_next == _ref) {
428 // At the end of the list, we should make _prev point to itself.
429 // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
430 // and _prev will be NULL.
431 new_next = _prev;
432 } else {
433 new_next = _next;
434 }
435
436 if (UseCompressedOops) {
437 // Remove Reference object from list.
438 oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next);
439 } else {
440 // Remove Reference object from list.
441 oopDesc::store_heap_oop((oop*)_prev_next, new_next);
442 }
443 NOT_PRODUCT(_removed++);
444 _refs_list.dec_length(1);
445 }
446
447 // Make the Reference object active again.
448 void DiscoveredListIterator::make_active() {
449 // For G1 we don't want to use set_next - it
450 // will dirty the card for the next field of
451 // the reference object and will fail
452 // CT verification.
453 if (UseG1GC) {
454 BarrierSet* bs = oopDesc::bs();
455 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
456
457 if (UseCompressedOops) {
458 bs->write_ref_field_pre((narrowOop*)next_addr, NULL);
459 } else {
460 bs->write_ref_field_pre((oop*)next_addr, NULL);
461 }
462 java_lang_ref_Reference::set_next_raw(_ref, NULL);
463 } else {
464 java_lang_ref_Reference::set_next(_ref, NULL);
465 }
466 }
467
468 void DiscoveredListIterator::clear_referent() {
469 oop_store_raw(_referent_addr, NULL);
470 }
471
472 // NOTE: process_phase*() are largely similar, and at a high level
473 // merely iterate over the extant list applying a predicate to
474 // each of its elements and possibly removing that element from the
475 // list and applying some further closures to that element.
476 // We should consider the possibility of replacing these
477 // process_phase*() methods by abstracting them into
478 // a single general iterator invocation that receives appropriate
479 // closures that accomplish this work.
480
481 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
482 // referents are not alive, but that should be kept alive for policy reasons.
483 // Keep alive the transitive closure of all such referents.
484 void
485 ReferenceProcessor::process_phase1(DiscoveredList& refs_list,
486 ReferencePolicy* policy,
487 BoolObjectClosure* is_alive,
488 OopClosure* keep_alive,
489 VoidClosure* complete_gc) {
490 assert(policy != NULL, "Must have a non-NULL policy");
491 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
492 // Decide which softly reachable refs should be kept alive.
493 while (iter.has_next()) {
494 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
495 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
496 if (referent_is_dead && !policy->should_clear_reference(iter.obj())) {
497 if (TraceReferenceGC) {
498 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy",
499 iter.obj(), iter.obj()->blueprint()->internal_name());
500 }
501 // Remove Reference object from list
502 iter.remove();
503 // Make the Reference object active again
504 iter.make_active();
505 // keep the referent around
506 iter.make_referent_alive();
507 iter.move_to_next();
508 } else {
509 iter.next();
510 }
511 }
512 // Close the reachable set
513 complete_gc->do_void();
514 NOT_PRODUCT(
515 if (PrintGCDetails && TraceReferenceGC) {
516 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
517 "discovered Refs by policy, from list " INTPTR_FORMAT,
518 iter.removed(), iter.processed(), (address)refs_list.head());
519 }
520 )
521 }
522
523 // Traverse the list and remove any Refs that are not active, or
524 // whose referents are either alive or NULL.
525 void
526 ReferenceProcessor::pp2_work(DiscoveredList& refs_list,
527 BoolObjectClosure* is_alive,
528 OopClosure* keep_alive) {
529 assert(discovery_is_atomic(), "Error");
530 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
531 while (iter.has_next()) {
532 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
533 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
534 assert(next == NULL, "Should not discover inactive Reference");
535 if (iter.is_referent_alive()) {
536 if (TraceReferenceGC) {
537 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
538 iter.obj(), iter.obj()->blueprint()->internal_name());
539 }
540 // The referent is reachable after all.
541 // Remove Reference object from list.
542 iter.remove();
543 // Update the referent pointer as necessary: Note that this
544 // should not entail any recursive marking because the
545 // referent must already have been traversed.
546 iter.make_referent_alive();
547 iter.move_to_next();
548 } else {
549 iter.next();
550 }
551 }
552 NOT_PRODUCT(
553 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
554 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
555 "Refs in discovered list " INTPTR_FORMAT,
556 iter.removed(), iter.processed(), (address)refs_list.head());
557 }
558 )
559 }
560
561 void
562 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list,
563 BoolObjectClosure* is_alive,
564 OopClosure* keep_alive,
565 VoidClosure* complete_gc) {
566 assert(!discovery_is_atomic(), "Error");
567 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
568 while (iter.has_next()) {
569 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
570 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
571 oop next = java_lang_ref_Reference::next(iter.obj());
572 if ((iter.referent() == NULL || iter.is_referent_alive() ||
573 next != NULL)) {
574 assert(next->is_oop_or_null(), "bad next field");
575 // Remove Reference object from list
576 iter.remove();
577 // Trace the cohorts
578 iter.make_referent_alive();
579 if (UseCompressedOops) {
580 keep_alive->do_oop((narrowOop*)next_addr);
581 } else {
582 keep_alive->do_oop((oop*)next_addr);
583 }
584 iter.move_to_next();
585 } else {
586 iter.next();
587 }
588 }
589 // Now close the newly reachable set
590 complete_gc->do_void();
591 NOT_PRODUCT(
592 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
593 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
594 "Refs in discovered list " INTPTR_FORMAT,
595 iter.removed(), iter.processed(), (address)refs_list.head());
596 }
597 )
598 }
599
600 // Traverse the list and process the referents, by either
601 // clearing them or keeping them (and their reachable
602 // closure) alive.
603 void
604 ReferenceProcessor::process_phase3(DiscoveredList& refs_list,
605 bool clear_referent,
606 BoolObjectClosure* is_alive,
607 OopClosure* keep_alive,
608 VoidClosure* complete_gc) {
609 ResourceMark rm;
610 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
611 while (iter.has_next()) {
612 iter.update_discovered();
613 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
614 if (clear_referent) {
615 // NULL out referent pointer
616 iter.clear_referent();
617 } else {
618 // keep the referent around
619 iter.make_referent_alive();
620 }
621 if (TraceReferenceGC) {
622 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
623 clear_referent ? "cleared " : "",
624 iter.obj(), iter.obj()->blueprint()->internal_name());
625 }
626 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
627 iter.next();
628 }
629 // Remember to update the next pointer of the last ref.
630 iter.update_discovered();
631 // Close the reachable set
632 complete_gc->do_void();
633 }
634
635 void
636 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
637 oop obj = NULL;
638 oop next = refs_list.head();
639 while (next != obj) {
640 obj = next;
641 next = java_lang_ref_Reference::discovered(obj);
642 java_lang_ref_Reference::set_discovered_raw(obj, NULL);
643 }
644 refs_list.set_head(NULL);
645 refs_list.set_length(0);
646 }
647
648 void
649 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
650 clear_discovered_references(refs_list);
651 }
652
653 void ReferenceProcessor::abandon_partial_discovery() {
654 // loop over the lists
655 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
656 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
657 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
658 }
659 abandon_partial_discovered_list(_discoveredSoftRefs[i]);
660 }
661 }
662
663 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
664 public:
665 RefProcPhase1Task(ReferenceProcessor& ref_processor,
666 DiscoveredList refs_lists[],
667 ReferencePolicy* policy,
668 bool marks_oops_alive)
669 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
670 _policy(policy)
671 { }
672 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
673 OopClosure& keep_alive,
674 VoidClosure& complete_gc)
675 {
676 Thread* thr = Thread::current();
677 int refs_list_index = ((WorkerThread*)thr)->id();
678 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
679 &is_alive, &keep_alive, &complete_gc);
680 }
681 private:
682 ReferencePolicy* _policy;
683 };
684
685 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
686 public:
687 RefProcPhase2Task(ReferenceProcessor& ref_processor,
688 DiscoveredList refs_lists[],
689 bool marks_oops_alive)
690 : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
691 { }
692 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
693 OopClosure& keep_alive,
694 VoidClosure& complete_gc)
695 {
696 _ref_processor.process_phase2(_refs_lists[i],
697 &is_alive, &keep_alive, &complete_gc);
698 }
699 };
700
701 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
702 public:
703 RefProcPhase3Task(ReferenceProcessor& ref_processor,
704 DiscoveredList refs_lists[],
705 bool clear_referent,
706 bool marks_oops_alive)
707 : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
708 _clear_referent(clear_referent)
709 { }
710 virtual void work(unsigned int i, BoolObjectClosure& is_alive,
711 OopClosure& keep_alive,
712 VoidClosure& complete_gc)
713 {
714 // Don't use "refs_list_index" calculated in this way because
715 // balance_queues() has moved the Ref's into the first n queues.
716 // Thread* thr = Thread::current();
717 // int refs_list_index = ((WorkerThread*)thr)->id();
718 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
719 _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
720 &is_alive, &keep_alive, &complete_gc);
721 }
722 private:
723 bool _clear_referent;
724 };
725
726 void ReferenceProcessor::set_discovered(oop ref, oop value) {
727 if (_discovered_list_needs_barrier) {
728 java_lang_ref_Reference::set_discovered(ref, value);
729 } else {
730 java_lang_ref_Reference::set_discovered_raw(ref, value);
731 }
732 }
733
734 // Balances reference queues.
735 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
736 // queues[0, 1, ..., _num_q-1] because only the first _num_q
737 // corresponding to the active workers will be processed.
738 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
739 {
740 // calculate total length
741 size_t total_refs = 0;
742 if (TraceReferenceGC && PrintGCDetails) {
743 gclog_or_tty->print_cr("\nBalance ref_lists ");
744 }
745
746 for (int i = 0; i < _max_num_q; ++i) {
747 total_refs += ref_lists[i].length();
748 if (TraceReferenceGC && PrintGCDetails) {
749 gclog_or_tty->print("%d ", ref_lists[i].length());
750 }
751 }
752 if (TraceReferenceGC && PrintGCDetails) {
753 gclog_or_tty->print_cr(" = %d", total_refs);
754 }
755 size_t avg_refs = total_refs / _num_q + 1;
756 int to_idx = 0;
757 for (int from_idx = 0; from_idx < _max_num_q; from_idx++) {
758 bool move_all = false;
759 if (from_idx >= _num_q) {
760 move_all = ref_lists[from_idx].length() > 0;
761 }
762 while ((ref_lists[from_idx].length() > avg_refs) ||
763 move_all) {
764 assert(to_idx < _num_q, "Sanity Check!");
765 if (ref_lists[to_idx].length() < avg_refs) {
766 // move superfluous refs
767 size_t refs_to_move;
768 // Move all the Ref's if the from queue will not be processed.
769 if (move_all) {
770 refs_to_move = MIN2(ref_lists[from_idx].length(),
771 avg_refs - ref_lists[to_idx].length());
772 } else {
773 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
774 avg_refs - ref_lists[to_idx].length());
775 }
776
777 assert(refs_to_move > 0, "otherwise the code below will fail");
778
779 oop move_head = ref_lists[from_idx].head();
780 oop move_tail = move_head;
781 oop new_head = move_head;
782 // find an element to split the list on
783 for (size_t j = 0; j < refs_to_move; ++j) {
784 move_tail = new_head;
785 new_head = java_lang_ref_Reference::discovered(new_head);
786 }
787
788 // Add the chain to the to list.
789 if (ref_lists[to_idx].head() == NULL) {
790 // to list is empty. Make a loop at the end.
791 set_discovered(move_tail, move_tail);
792 } else {
793 set_discovered(move_tail, ref_lists[to_idx].head());
794 }
795 ref_lists[to_idx].set_head(move_head);
796 ref_lists[to_idx].inc_length(refs_to_move);
797
798 // Remove the chain from the from list.
799 if (move_tail == new_head) {
800 // We found the end of the from list.
801 ref_lists[from_idx].set_head(NULL);
802 } else {
803 ref_lists[from_idx].set_head(new_head);
804 }
805 ref_lists[from_idx].dec_length(refs_to_move);
806 if (ref_lists[from_idx].length() == 0) {
807 break;
808 }
809 } else {
810 to_idx = (to_idx + 1) % _num_q;
811 }
812 }
813 }
814 #ifdef ASSERT
815 size_t balanced_total_refs = 0;
816 for (int i = 0; i < _max_num_q; ++i) {
817 balanced_total_refs += ref_lists[i].length();
818 if (TraceReferenceGC && PrintGCDetails) {
819 gclog_or_tty->print("%d ", ref_lists[i].length());
820 }
821 }
822 if (TraceReferenceGC && PrintGCDetails) {
823 gclog_or_tty->print_cr(" = %d", balanced_total_refs);
824 gclog_or_tty->flush();
825 }
826 assert(total_refs == balanced_total_refs, "Balancing was incomplete");
827 #endif
828 }
829
830 void ReferenceProcessor::balance_all_queues() {
831 balance_queues(_discoveredSoftRefs);
832 balance_queues(_discoveredWeakRefs);
833 balance_queues(_discoveredFinalRefs);
834 balance_queues(_discoveredPhantomRefs);
835 }
836
837 void
838 ReferenceProcessor::process_discovered_reflist(
839 DiscoveredList refs_lists[],
840 ReferencePolicy* policy,
841 bool clear_referent,
842 BoolObjectClosure* is_alive,
843 OopClosure* keep_alive,
844 VoidClosure* complete_gc,
845 AbstractRefProcTaskExecutor* task_executor)
846 {
847 bool mt_processing = task_executor != NULL && _processing_is_mt;
848 // If discovery used MT and a dynamic number of GC threads, then
849 // the queues must be balanced for correctness if fewer than the
850 // maximum number of queues were used. The number of queue used
851 // during discovery may be different than the number to be used
852 // for processing so don't depend of _num_q < _max_num_q as part
853 // of the test.
854 bool must_balance = _discovery_is_mt;
855
856 if ((mt_processing && ParallelRefProcBalancingEnabled) ||
857 must_balance) {
858 balance_queues(refs_lists);
859 }
860 if (PrintReferenceGC && PrintGCDetails) {
861 size_t total = 0;
862 for (int i = 0; i < _max_num_q; ++i) {
863 total += refs_lists[i].length();
864 }
865 gclog_or_tty->print(", %u refs", total);
866 }
867
868 // Phase 1 (soft refs only):
869 // . Traverse the list and remove any SoftReferences whose
870 // referents are not alive, but that should be kept alive for
871 // policy reasons. Keep alive the transitive closure of all
872 // such referents.
873 if (policy != NULL) {
874 if (mt_processing) {
875 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
876 task_executor->execute(phase1);
877 } else {
878 for (int i = 0; i < _max_num_q; i++) {
879 process_phase1(refs_lists[i], policy,
880 is_alive, keep_alive, complete_gc);
881 }
882 }
883 } else { // policy == NULL
884 assert(refs_lists != _discoveredSoftRefs,
885 "Policy must be specified for soft references.");
886 }
887
888 // Phase 2:
889 // . Traverse the list and remove any refs whose referents are alive.
890 if (mt_processing) {
891 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
892 task_executor->execute(phase2);
893 } else {
894 for (int i = 0; i < _max_num_q; i++) {
895 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
896 }
897 }
898
899 // Phase 3:
900 // . Traverse the list and process referents as appropriate.
901 if (mt_processing) {
902 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
903 task_executor->execute(phase3);
904 } else {
905 for (int i = 0; i < _max_num_q; i++) {
906 process_phase3(refs_lists[i], clear_referent,
907 is_alive, keep_alive, complete_gc);
908 }
909 }
910 }
911
912 void ReferenceProcessor::clean_up_discovered_references() {
913 // loop over the lists
914
915 // An alternative implementation of this routine could
916 // use the following nested loop:
917 //
918 // for (int i = 0; i < number_of_subclasses_of_ref(); i++) {
919 // for (int j = 0; j < _num_q; j++) {
920 // int index = i * _max_num_q + j;
921
922 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
923 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
924 gclog_or_tty->print_cr(
925 "\nScrubbing %s discovered list of Null referents",
926 list_name(i));
927 }
928 clean_up_discovered_reflist(_discoveredSoftRefs[i]);
929 }
930 }
931
932 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
933 assert(!discovery_is_atomic(), "Else why call this method?");
934 DiscoveredListIterator iter(refs_list, NULL, NULL);
935 while (iter.has_next()) {
936 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
937 oop next = java_lang_ref_Reference::next(iter.obj());
938 assert(next->is_oop_or_null(), "bad next field");
939 // If referent has been cleared or Reference is not active,
940 // drop it.
941 if (iter.referent() == NULL || next != NULL) {
942 debug_only(
943 if (PrintGCDetails && TraceReferenceGC) {
944 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
945 INTPTR_FORMAT " with next field: " INTPTR_FORMAT
946 " and referent: " INTPTR_FORMAT,
947 iter.obj(), next, iter.referent());
948 }
949 )
950 // Remove Reference object from list
951 iter.remove();
952 iter.move_to_next();
953 } else {
954 iter.next();
955 }
956 }
957 NOT_PRODUCT(
958 if (PrintGCDetails && TraceReferenceGC) {
959 gclog_or_tty->print(
960 " Removed %d Refs with NULL referents out of %d discovered Refs",
961 iter.removed(), iter.processed());
962 }
963 )
964 }
965
966 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
967 int id = 0;
968 // Determine the queue index to use for this object.
969 if (_discovery_is_mt) {
970 // During a multi-threaded discovery phase,
971 // each thread saves to its "own" list.
972 Thread* thr = Thread::current();
973 id = thr->as_Worker_thread()->id();
974 } else {
975 // single-threaded discovery, we save in round-robin
976 // fashion to each of the lists.
977 if (_processing_is_mt) {
978 id = next_id();
979 }
980 }
981 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
982
983 // Get the discovered queue to which we will add
984 DiscoveredList* list = NULL;
985 switch (rt) {
986 case REF_OTHER:
987 // Unknown reference type, no special treatment
988 break;
989 case REF_SOFT:
990 list = &_discoveredSoftRefs[id];
991 break;
992 case REF_WEAK:
993 list = &_discoveredWeakRefs[id];
994 break;
995 case REF_FINAL:
996 list = &_discoveredFinalRefs[id];
997 break;
998 case REF_PHANTOM:
999 list = &_discoveredPhantomRefs[id];
1000 break;
1001 case REF_NONE:
1002 // we should not reach here if we are an instanceRefKlass
1003 default:
1004 ShouldNotReachHere();
1005 }
1006 if (TraceReferenceGC && PrintGCDetails) {
1007 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1008 }
1009 return list;
1010 }
1011
1012 inline void
1013 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1014 oop obj,
1015 HeapWord* discovered_addr) {
1016 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1017 // First we must make sure this object is only enqueued once. CAS in a non null
1018 // discovered_addr.
1019 oop current_head = refs_list.head();
1020 // The last ref must have its discovered field pointing to itself.
1021 oop next_discovered = (current_head != NULL) ? current_head : obj;
1022
1023 // Note: In the case of G1, this specific pre-barrier is strictly
1024 // not necessary because the only case we are interested in
1025 // here is when *discovered_addr is NULL (see the CAS further below),
1026 // so this will expand to nothing. As a result, we have manually
1027 // elided this out for G1, but left in the test for some future
1028 // collector that might have need for a pre-barrier here, e.g.:-
1029 // _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1030 assert(!_discovered_list_needs_barrier || UseG1GC,
1031 "Need to check non-G1 collector: "
1032 "may need a pre-write-barrier for CAS from NULL below");
1033 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1034 NULL);
1035 if (retest == NULL) {
1036 // This thread just won the right to enqueue the object.
1037 // We have separate lists for enqueueing, so no synchronization
1038 // is necessary.
1039 refs_list.set_head(obj);
1040 refs_list.inc_length(1);
1041 if (_discovered_list_needs_barrier) {
1042 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1043 }
1044
1045 if (TraceReferenceGC) {
1046 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1047 obj, obj->blueprint()->internal_name());
1048 }
1049 } else {
1050 // If retest was non NULL, another thread beat us to it:
1051 // The reference has already been discovered...
1052 if (TraceReferenceGC) {
1053 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1054 obj, obj->blueprint()->internal_name());
1055 }
1056 }
1057 }
1058
1059 #ifndef PRODUCT
1060 // Non-atomic (i.e. concurrent) discovery might allow us
1061 // to observe j.l.References with NULL referents, being those
1062 // cleared concurrently by mutators during (or after) discovery.
1063 void ReferenceProcessor::verify_referent(oop obj) {
1064 bool da = discovery_is_atomic();
1065 oop referent = java_lang_ref_Reference::referent(obj);
1066 assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1067 err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1068 INTPTR_FORMAT " during %satomic discovery ",
1069 (intptr_t)referent, (intptr_t)obj, da ? "" : "non-"));
1070 }
1071 #endif
1072
1073 // We mention two of several possible choices here:
1074 // #0: if the reference object is not in the "originating generation"
1075 // (or part of the heap being collected, indicated by our "span"
1076 // we don't treat it specially (i.e. we scan it as we would
1077 // a normal oop, treating its references as strong references).
1078 // This means that references can't be discovered unless their
1079 // referent is also in the same span. This is the simplest,
1080 // most "local" and most conservative approach, albeit one
1081 // that may cause weak references to be enqueued least promptly.
1082 // We call this choice the "ReferenceBasedDiscovery" policy.
1083 // #1: the reference object may be in any generation (span), but if
1084 // the referent is in the generation (span) being currently collected
1085 // then we can discover the reference object, provided
1086 // the object has not already been discovered by
1087 // a different concurrently running collector (as may be the
1088 // case, for instance, if the reference object is in CMS and
1089 // the referent in DefNewGeneration), and provided the processing
1090 // of this reference object by the current collector will
1091 // appear atomic to every other collector in the system.
1092 // (Thus, for instance, a concurrent collector may not
1093 // discover references in other generations even if the
1094 // referent is in its own generation). This policy may,
1095 // in certain cases, enqueue references somewhat sooner than
1096 // might Policy #0 above, but at marginally increased cost
1097 // and complexity in processing these references.
1098 // We call this choice the "RefeferentBasedDiscovery" policy.
1099 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1100 // Make sure we are discovering refs (rather than processing discovered refs).
1101 if (!_discovering_refs || !RegisterReferences) {
1102 return false;
1103 }
1104 // We only discover active references.
1105 oop next = java_lang_ref_Reference::next(obj);
1106 if (next != NULL) { // Ref is no longer active
1107 return false;
1108 }
1109
1110 HeapWord* obj_addr = (HeapWord*)obj;
1111 if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1112 !_span.contains(obj_addr)) {
1113 // Reference is not in the originating generation;
1114 // don't treat it specially (i.e. we want to scan it as a normal
1115 // object with strong references).
1116 return false;
1117 }
1118
1119 // We only discover references whose referents are not (yet)
1120 // known to be strongly reachable.
1121 if (is_alive_non_header() != NULL) {
1122 verify_referent(obj);
1123 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1124 return false; // referent is reachable
1125 }
1126 }
1127 if (rt == REF_SOFT) {
1128 // For soft refs we can decide now if these are not
1129 // current candidates for clearing, in which case we
1130 // can mark through them now, rather than delaying that
1131 // to the reference-processing phase. Since all current
1132 // time-stamp policies advance the soft-ref clock only
1133 // at a major collection cycle, this is always currently
1134 // accurate.
1135 if (!_current_soft_ref_policy->should_clear_reference(obj)) {
1136 return false;
1137 }
1138 }
1139
1140 ResourceMark rm; // Needed for tracing.
1141
1142 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1143 const oop discovered = java_lang_ref_Reference::discovered(obj);
1144 assert(discovered->is_oop_or_null(), "bad discovered field");
1145 if (discovered != NULL) {
1146 // The reference has already been discovered...
1147 if (TraceReferenceGC) {
1148 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1149 obj, obj->blueprint()->internal_name());
1150 }
1151 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1152 // assumes that an object is not processed twice;
1153 // if it's been already discovered it must be on another
1154 // generation's discovered list; so we won't discover it.
1155 return false;
1156 } else {
1157 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1158 "Unrecognized policy");
1159 // Check assumption that an object is not potentially
1160 // discovered twice except by concurrent collectors that potentially
1161 // trace the same Reference object twice.
1162 assert(UseConcMarkSweepGC || UseG1GC,
1163 "Only possible with a concurrent marking collector");
1164 return true;
1165 }
1166 }
1167
1168 if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1169 verify_referent(obj);
1170 // Discover if and only if EITHER:
1171 // .. reference is in our span, OR
1172 // .. we are an atomic collector and referent is in our span
1173 if (_span.contains(obj_addr) ||
1174 (discovery_is_atomic() &&
1175 _span.contains(java_lang_ref_Reference::referent(obj)))) {
1176 // should_enqueue = true;
1177 } else {
1178 return false;
1179 }
1180 } else {
1181 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1182 _span.contains(obj_addr), "code inconsistency");
1183 }
1184
1185 // Get the right type of discovered queue head.
1186 DiscoveredList* list = get_discovered_list(rt);
1187 if (list == NULL) {
1188 return false; // nothing special needs to be done
1189 }
1190
1191 if (_discovery_is_mt) {
1192 add_to_discovered_list_mt(*list, obj, discovered_addr);
1193 } else {
1194 // If "_discovered_list_needs_barrier", we do write barriers when
1195 // updating the discovered reference list. Otherwise, we do a raw store
1196 // here: the field will be visited later when processing the discovered
1197 // references.
1198 oop current_head = list->head();
1199 // The last ref must have its discovered field pointing to itself.
1200 oop next_discovered = (current_head != NULL) ? current_head : obj;
1201
1202 // As in the case further above, since we are over-writing a NULL
1203 // pre-value, we can safely elide the pre-barrier here for the case of G1.
1204 // e.g.:- _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1205 assert(discovered == NULL, "control point invariant");
1206 assert(!_discovered_list_needs_barrier || UseG1GC,
1207 "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
1208 oop_store_raw(discovered_addr, next_discovered);
1209 if (_discovered_list_needs_barrier) {
1210 _bs->write_ref_field((void*)discovered_addr, next_discovered);
1211 }
1212 list->set_head(obj);
1213 list->inc_length(1);
1214
1215 if (TraceReferenceGC) {
1216 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1217 obj, obj->blueprint()->internal_name());
1218 }
1219 }
1220 assert(obj->is_oop(), "Discovered a bad reference");
1221 verify_referent(obj);
1222 return true;
1223 }
1224
1225 // Preclean the discovered references by removing those
1226 // whose referents are alive, and by marking from those that
1227 // are not active. These lists can be handled here
1228 // in any order and, indeed, concurrently.
1229 void ReferenceProcessor::preclean_discovered_references(
1230 BoolObjectClosure* is_alive,
1231 OopClosure* keep_alive,
1232 VoidClosure* complete_gc,
1233 YieldClosure* yield,
1234 bool should_unload_classes) {
1235
1236 NOT_PRODUCT(verify_ok_to_handle_reflists());
1237
1238 #ifdef ASSERT
1239 bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC ||
1240 CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
1241 ExplicitGCInvokesConcurrentAndUnloadsClasses &&
1242 UseConcMarkSweepGC && should_unload_classes;
1243 RememberKlassesChecker mx(must_remember_klasses);
1244 #endif
1245 // Soft references
1246 {
1247 TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1248 false, gclog_or_tty);
1249 for (int i = 0; i < _max_num_q; i++) {
1250 if (yield->should_return()) {
1251 return;
1252 }
1253 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1254 keep_alive, complete_gc, yield);
1255 }
1256 }
1257
1258 // Weak references
1259 {
1260 TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1261 false, gclog_or_tty);
1262 for (int i = 0; i < _max_num_q; i++) {
1263 if (yield->should_return()) {
1264 return;
1265 }
1266 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1267 keep_alive, complete_gc, yield);
1268 }
1269 }
1270
1271 // Final references
1272 {
1273 TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1274 false, gclog_or_tty);
1275 for (int i = 0; i < _max_num_q; i++) {
1276 if (yield->should_return()) {
1277 return;
1278 }
1279 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1280 keep_alive, complete_gc, yield);
1281 }
1282 }
1283
1284 // Phantom references
1285 {
1286 TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1287 false, gclog_or_tty);
1288 for (int i = 0; i < _max_num_q; i++) {
1289 if (yield->should_return()) {
1290 return;
1291 }
1292 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1293 keep_alive, complete_gc, yield);
1294 }
1295 }
1296 }
1297
1298 // Walk the given discovered ref list, and remove all reference objects
1299 // whose referents are still alive, whose referents are NULL or which
1300 // are not active (have a non-NULL next field). NOTE: When we are
1301 // thus precleaning the ref lists (which happens single-threaded today),
1302 // we do not disable refs discovery to honour the correct semantics of
1303 // java.lang.Reference. As a result, we need to be careful below
1304 // that ref removal steps interleave safely with ref discovery steps
1305 // (in this thread).
1306 void
1307 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list,
1308 BoolObjectClosure* is_alive,
1309 OopClosure* keep_alive,
1310 VoidClosure* complete_gc,
1311 YieldClosure* yield) {
1312 DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1313 while (iter.has_next()) {
1314 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1315 oop obj = iter.obj();
1316 oop next = java_lang_ref_Reference::next(obj);
1317 if (iter.referent() == NULL || iter.is_referent_alive() ||
1318 next != NULL) {
1319 // The referent has been cleared, or is alive, or the Reference is not
1320 // active; we need to trace and mark its cohort.
1321 if (TraceReferenceGC) {
1322 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1323 iter.obj(), iter.obj()->blueprint()->internal_name());
1324 }
1325 // Remove Reference object from list
1326 iter.remove();
1327 // Keep alive its cohort.
1328 iter.make_referent_alive();
1329 if (UseCompressedOops) {
1330 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1331 keep_alive->do_oop(next_addr);
1332 } else {
1333 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1334 keep_alive->do_oop(next_addr);
1335 }
1336 iter.move_to_next();
1337 } else {
1338 iter.next();
1339 }
1340 }
1341 // Close the reachable set
1342 complete_gc->do_void();
1343
1344 NOT_PRODUCT(
1345 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1346 gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1347 "Refs in discovered list " INTPTR_FORMAT,
1348 iter.removed(), iter.processed(), (address)refs_list.head());
1349 }
1350 )
1351 }
1352
1353 const char* ReferenceProcessor::list_name(int i) {
1354 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1355 "Out of bounds index");
1356
1357 int j = i / _max_num_q;
1358 switch (j) {
1359 case 0: return "SoftRef";
1360 case 1: return "WeakRef";
1361 case 2: return "FinalRef";
1362 case 3: return "PhantomRef";
1363 }
1364 ShouldNotReachHere();
1365 return NULL;
1366 }
1367
1368 #ifndef PRODUCT
1369 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1370 // empty for now
1371 }
1372 #endif
1373
1374 #ifndef PRODUCT
1375 void ReferenceProcessor::clear_discovered_references() {
1376 guarantee(!_discovering_refs, "Discovering refs?");
1377 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1378 clear_discovered_references(_discoveredSoftRefs[i]);
1379 }
1380 }
1381
1382 #endif // PRODUCT