1 /* 2 * Copyright (c) 2001, 2013, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #include "precompiled.hpp" 26 #include "classfile/javaClasses.hpp" 27 #include "classfile/systemDictionary.hpp" 28 #include "gc_implementation/shared/gcTimer.hpp" 29 #include "gc_implementation/shared/gcTraceTime.hpp" 30 #include "gc_interface/collectedHeap.hpp" 31 #include "gc_interface/collectedHeap.inline.hpp" 32 #include "memory/referencePolicy.hpp" 33 #include "memory/referenceProcessor.hpp" 34 #include "oops/oop.inline.hpp" 35 #include "runtime/java.hpp" 36 #include "runtime/jniHandles.hpp" 37 38 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL; 39 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy = NULL; 40 bool ReferenceProcessor::_pending_list_uses_discovered_field = false; 41 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0; 42 43 void referenceProcessor_init() { 44 ReferenceProcessor::init_statics(); 45 } 46 47 void ReferenceProcessor::init_statics() { 48 // We need a monotonically non-decreasing time in ms but 49 // os::javaTimeMillis() does not guarantee monotonicity. 50 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; 51 52 // Initialize the soft ref timestamp clock. 53 _soft_ref_timestamp_clock = now; 54 // Also update the soft ref clock in j.l.r.SoftReference 55 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock); 56 57 _always_clear_soft_ref_policy = new AlwaysClearPolicy(); 58 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy()) 59 NOT_COMPILER2(LRUCurrentHeapPolicy()); 60 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) { 61 vm_exit_during_initialization("Could not allocate reference policy object"); 62 } 63 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery || 64 RefDiscoveryPolicy == ReferentBasedDiscovery, 65 "Unrecongnized RefDiscoveryPolicy"); 66 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field(); 67 } 68 69 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) { 70 #ifdef ASSERT 71 // Verify that we're not currently discovering refs 72 assert(!verify_disabled || !_discovering_refs, "nested call?"); 73 74 if (check_no_refs) { 75 // Verify that the discovered lists are empty 76 verify_no_references_recorded(); 77 } 78 #endif // ASSERT 79 80 // Someone could have modified the value of the static 81 // field in the j.l.r.SoftReference class that holds the 82 // soft reference timestamp clock using reflection or 83 // Unsafe between GCs. Unconditionally update the static 84 // field in ReferenceProcessor here so that we use the new 85 // value during reference discovery. 86 87 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 88 _discovering_refs = true; 89 } 90 91 ReferenceProcessor::ReferenceProcessor(MemRegion span, 92 bool mt_processing, 93 uint mt_processing_degree, 94 bool mt_discovery, 95 uint mt_discovery_degree, 96 bool atomic_discovery, 97 BoolObjectClosure* is_alive_non_header, 98 bool discovered_list_needs_barrier) : 99 _discovering_refs(false), 100 _enqueuing_is_done(false), 101 _is_alive_non_header(is_alive_non_header), 102 _discovered_list_needs_barrier(discovered_list_needs_barrier), 103 _bs(NULL), 104 _processing_is_mt(mt_processing), 105 _next_id(0) 106 { 107 _span = span; 108 _discovery_is_atomic = atomic_discovery; 109 _discovery_is_mt = mt_discovery; 110 _num_q = MAX2(1U, mt_processing_degree); 111 _max_num_q = MAX2(_num_q, mt_discovery_degree); 112 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList, 113 _max_num_q * number_of_subclasses_of_ref(), mtGC); 114 115 if (_discovered_refs == NULL) { 116 vm_exit_during_initialization("Could not allocated RefProc Array"); 117 } 118 _discoveredSoftRefs = &_discovered_refs[0]; 119 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q]; 120 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q]; 121 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q]; 122 123 // Initialize all entries to NULL 124 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 125 _discovered_refs[i].set_head(NULL); 126 _discovered_refs[i].set_length(0); 127 } 128 129 // If we do barriers, cache a copy of the barrier set. 130 if (discovered_list_needs_barrier) { 131 _bs = Universe::heap()->barrier_set(); 132 } 133 setup_policy(false /* default soft ref policy */); 134 } 135 136 #ifndef PRODUCT 137 void ReferenceProcessor::verify_no_references_recorded() { 138 guarantee(!_discovering_refs, "Discovering refs?"); 139 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 140 guarantee(_discovered_refs[i].is_empty(), 141 "Found non-empty discovered list"); 142 } 143 } 144 #endif 145 146 void ReferenceProcessor::weak_oops_do(OopClosure* f) { 147 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 148 if (UseCompressedOops) { 149 f->do_oop((narrowOop*)_discovered_refs[i].adr_head()); 150 } else { 151 f->do_oop((oop*)_discovered_refs[i].adr_head()); 152 } 153 } 154 } 155 156 void ReferenceProcessor::update_soft_ref_master_clock() { 157 // Update (advance) the soft ref master clock field. This must be done 158 // after processing the soft ref list. 159 160 // We need a monotonically non-decreasing time in ms but 161 // os::javaTimeMillis() does not guarantee monotonicity. 162 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; 163 jlong soft_ref_clock = java_lang_ref_SoftReference::clock(); 164 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync"); 165 166 NOT_PRODUCT( 167 if (now < _soft_ref_timestamp_clock) { 168 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT, 169 _soft_ref_timestamp_clock, now); 170 } 171 ) 172 // The values of now and _soft_ref_timestamp_clock are set using 173 // javaTimeNanos(), which is guaranteed to be monotonically 174 // non-decreasing provided the underlying platform provides such 175 // a time source (and it is bug free). 176 // In product mode, however, protect ourselves from non-monotonicity. 177 if (now > _soft_ref_timestamp_clock) { 178 _soft_ref_timestamp_clock = now; 179 java_lang_ref_SoftReference::set_clock(now); 180 } 181 // Else leave clock stalled at its old value until time progresses 182 // past clock value. 183 } 184 185 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) { 186 size_t total = 0; 187 for (uint i = 0; i < _max_num_q; ++i) { 188 total += lists[i].length(); 189 } 190 return total; 191 } 192 193 ReferenceProcessorStats ReferenceProcessor::process_discovered_references( 194 BoolObjectClosure* is_alive, 195 OopClosure* keep_alive, 196 VoidClosure* complete_gc, 197 AbstractRefProcTaskExecutor* task_executor, 198 GCTimer* gc_timer) { 199 NOT_PRODUCT(verify_ok_to_handle_reflists()); 200 201 assert(!enqueuing_is_done(), "If here enqueuing should not be complete"); 202 // Stop treating discovered references specially. 203 disable_discovery(); 204 205 // If discovery was concurrent, someone could have modified 206 // the value of the static field in the j.l.r.SoftReference 207 // class that holds the soft reference timestamp clock using 208 // reflection or Unsafe between when discovery was enabled and 209 // now. Unconditionally update the static field in ReferenceProcessor 210 // here so that we use the new value during processing of the 211 // discovered soft refs. 212 213 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 214 215 bool trace_time = PrintGCDetails && PrintReferenceGC; 216 217 // Soft references 218 size_t soft_count = 0; 219 { 220 GCTraceTime tt("SoftReference", trace_time, false, gc_timer); 221 soft_count = 222 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true, 223 is_alive, keep_alive, complete_gc, task_executor); 224 } 225 226 update_soft_ref_master_clock(); 227 228 // Weak references 229 size_t weak_count = 0; 230 { 231 GCTraceTime tt("WeakReference", trace_time, false, gc_timer); 232 weak_count = 233 process_discovered_reflist(_discoveredWeakRefs, NULL, true, 234 is_alive, keep_alive, complete_gc, task_executor); 235 } 236 237 // Final references 238 size_t final_count = 0; 239 { 240 GCTraceTime tt("FinalReference", trace_time, false, gc_timer); 241 final_count = 242 process_discovered_reflist(_discoveredFinalRefs, NULL, false, 243 is_alive, keep_alive, complete_gc, task_executor); 244 } 245 246 // Phantom references 247 size_t phantom_count = 0; 248 { 249 GCTraceTime tt("PhantomReference", trace_time, false, gc_timer); 250 phantom_count = 251 process_discovered_reflist(_discoveredPhantomRefs, NULL, false, 252 is_alive, keep_alive, complete_gc, task_executor); 253 } 254 255 // Weak global JNI references. It would make more sense (semantically) to 256 // traverse these simultaneously with the regular weak references above, but 257 // that is not how the JDK1.2 specification is. See #4126360. Native code can 258 // thus use JNI weak references to circumvent the phantom references and 259 // resurrect a "post-mortem" object. 260 { 261 GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer); 262 if (task_executor != NULL) { 263 task_executor->set_single_threaded_mode(); 264 } 265 process_phaseJNI(is_alive, keep_alive, complete_gc); 266 } 267 268 return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count); 269 } 270 271 #ifndef PRODUCT 272 // Calculate the number of jni handles. 273 uint ReferenceProcessor::count_jni_refs() { 274 class AlwaysAliveClosure: public BoolObjectClosure { 275 public: 276 virtual bool do_object_b(oop obj) { return true; } 277 }; 278 279 class CountHandleClosure: public OopClosure { 280 private: 281 int _count; 282 public: 283 CountHandleClosure(): _count(0) {} 284 void do_oop(oop* unused) { _count++; } 285 void do_oop(narrowOop* unused) { ShouldNotReachHere(); } 286 int count() { return _count; } 287 }; 288 CountHandleClosure global_handle_count; 289 AlwaysAliveClosure always_alive; 290 JNIHandles::weak_oops_do(&always_alive, &global_handle_count); 291 return global_handle_count.count(); 292 } 293 #endif 294 295 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive, 296 OopClosure* keep_alive, 297 VoidClosure* complete_gc) { 298 #ifndef PRODUCT 299 if (PrintGCDetails && PrintReferenceGC) { 300 unsigned int count = count_jni_refs(); 301 gclog_or_tty->print(", %u refs", count); 302 } 303 #endif 304 JNIHandles::weak_oops_do(is_alive, keep_alive); 305 complete_gc->do_void(); 306 } 307 308 309 template <class T> 310 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref, 311 AbstractRefProcTaskExecutor* task_executor) { 312 313 // Remember old value of pending references list 314 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr(); 315 T old_pending_list_value = *pending_list_addr; 316 317 // Enqueue references that are not made active again, and 318 // clear the decks for the next collection (cycle). 319 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor); 320 // Do the oop-check on pending_list_addr missed in 321 // enqueue_discovered_reflist. We should probably 322 // do a raw oop_check so that future such idempotent 323 // oop_stores relying on the oop-check side-effect 324 // may be elided automatically and safely without 325 // affecting correctness. 326 oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr)); 327 328 // Stop treating discovered references specially. 329 ref->disable_discovery(); 330 331 // Return true if new pending references were added 332 return old_pending_list_value != *pending_list_addr; 333 } 334 335 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) { 336 NOT_PRODUCT(verify_ok_to_handle_reflists()); 337 if (UseCompressedOops) { 338 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor); 339 } else { 340 return enqueue_discovered_ref_helper<oop>(this, task_executor); 341 } 342 } 343 344 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list, 345 HeapWord* pending_list_addr) { 346 // Given a list of refs linked through the "discovered" field 347 // (java.lang.ref.Reference.discovered), self-loop their "next" field 348 // thus distinguishing them from active References, then 349 // prepend them to the pending list. 350 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777), 351 // the "next" field is used to chain the pending list, not the discovered 352 // field. 353 354 if (TraceReferenceGC && PrintGCDetails) { 355 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list " 356 INTPTR_FORMAT, (address)refs_list.head()); 357 } 358 359 oop obj = NULL; 360 oop next_d = refs_list.head(); 361 if (pending_list_uses_discovered_field()) { // New behavior 362 // Walk down the list, self-looping the next field 363 // so that the References are not considered active. 364 while (obj != next_d) { 365 obj = next_d; 366 assert(obj->is_instanceRef(), "should be reference object"); 367 next_d = java_lang_ref_Reference::discovered(obj); 368 if (TraceReferenceGC && PrintGCDetails) { 369 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, 370 (void *)obj, (void *)next_d); 371 } 372 assert(java_lang_ref_Reference::next(obj) == NULL, 373 "Reference not active; should not be discovered"); 374 // Self-loop next, so as to make Ref not active. 375 java_lang_ref_Reference::set_next(obj, obj); 376 if (next_d == obj) { // obj is last 377 // Swap refs_list into pending_list_addr and 378 // set obj's discovered to what we read from pending_list_addr. 379 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr); 380 // Need oop_check on pending_list_addr above; 381 // see special oop-check code at the end of 382 // enqueue_discovered_reflists() further below. 383 java_lang_ref_Reference::set_discovered(obj, old); // old may be NULL 384 } 385 } 386 } else { // Old behavior 387 // Walk down the list, copying the discovered field into 388 // the next field and clearing the discovered field. 389 while (obj != next_d) { 390 obj = next_d; 391 assert(obj->is_instanceRef(), "should be reference object"); 392 next_d = java_lang_ref_Reference::discovered(obj); 393 if (TraceReferenceGC && PrintGCDetails) { 394 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, 395 (void *)obj, (void *)next_d); 396 } 397 assert(java_lang_ref_Reference::next(obj) == NULL, 398 "The reference should not be enqueued"); 399 if (next_d == obj) { // obj is last 400 // Swap refs_list into pending_list_addr and 401 // set obj's next to what we read from pending_list_addr. 402 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr); 403 // Need oop_check on pending_list_addr above; 404 // see special oop-check code at the end of 405 // enqueue_discovered_reflists() further below. 406 if (old == NULL) { 407 // obj should be made to point to itself, since 408 // pending list was empty. 409 java_lang_ref_Reference::set_next(obj, obj); 410 } else { 411 java_lang_ref_Reference::set_next(obj, old); 412 } 413 } else { 414 java_lang_ref_Reference::set_next(obj, next_d); 415 } 416 java_lang_ref_Reference::set_discovered(obj, (oop) NULL); 417 } 418 } 419 } 420 421 // Parallel enqueue task 422 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask { 423 public: 424 RefProcEnqueueTask(ReferenceProcessor& ref_processor, 425 DiscoveredList discovered_refs[], 426 HeapWord* pending_list_addr, 427 int n_queues) 428 : EnqueueTask(ref_processor, discovered_refs, 429 pending_list_addr, n_queues) 430 { } 431 432 virtual void work(unsigned int work_id) { 433 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds"); 434 // Simplest first cut: static partitioning. 435 int index = work_id; 436 // The increment on "index" must correspond to the maximum number of queues 437 // (n_queues) with which that ReferenceProcessor was created. That 438 // is because of the "clever" way the discovered references lists were 439 // allocated and are indexed into. 440 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected"); 441 for (int j = 0; 442 j < ReferenceProcessor::number_of_subclasses_of_ref(); 443 j++, index += _n_queues) { 444 _ref_processor.enqueue_discovered_reflist( 445 _refs_lists[index], _pending_list_addr); 446 _refs_lists[index].set_head(NULL); 447 _refs_lists[index].set_length(0); 448 } 449 } 450 }; 451 452 // Enqueue references that are not made active again 453 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr, 454 AbstractRefProcTaskExecutor* task_executor) { 455 if (_processing_is_mt && task_executor != NULL) { 456 // Parallel code 457 RefProcEnqueueTask tsk(*this, _discovered_refs, 458 pending_list_addr, _max_num_q); 459 task_executor->execute(tsk); 460 } else { 461 // Serial code: call the parent class's implementation 462 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 463 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr); 464 _discovered_refs[i].set_head(NULL); 465 _discovered_refs[i].set_length(0); 466 } 467 } 468 } 469 470 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) { 471 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref); 472 oop discovered = java_lang_ref_Reference::discovered(_ref); 473 assert(_discovered_addr && discovered->is_oop_or_null(), 474 "discovered field is bad"); 475 _next = discovered; 476 _referent_addr = java_lang_ref_Reference::referent_addr(_ref); 477 _referent = java_lang_ref_Reference::referent(_ref); 478 assert(Universe::heap()->is_in_reserved_or_null(_referent), 479 "Wrong oop found in java.lang.Reference object"); 480 assert(allow_null_referent ? 481 _referent->is_oop_or_null() 482 : _referent->is_oop(), 483 "bad referent"); 484 } 485 486 void DiscoveredListIterator::remove() { 487 assert(_ref->is_oop(), "Dropping a bad reference"); 488 oop_store_raw(_discovered_addr, NULL); 489 490 // First _prev_next ref actually points into DiscoveredList (gross). 491 oop new_next; 492 if (_next == _ref) { 493 // At the end of the list, we should make _prev point to itself. 494 // If _ref is the first ref, then _prev_next will be in the DiscoveredList, 495 // and _prev will be NULL. 496 new_next = _prev; 497 } else { 498 new_next = _next; 499 } 500 501 if (UseCompressedOops) { 502 // Remove Reference object from list. 503 oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next); 504 } else { 505 // Remove Reference object from list. 506 oopDesc::store_heap_oop((oop*)_prev_next, new_next); 507 } 508 NOT_PRODUCT(_removed++); 509 _refs_list.dec_length(1); 510 } 511 512 // Make the Reference object active again. 513 void DiscoveredListIterator::make_active() { 514 // For G1 we don't want to use set_next - it 515 // will dirty the card for the next field of 516 // the reference object and will fail 517 // CT verification. 518 if (UseG1GC) { 519 BarrierSet* bs = oopDesc::bs(); 520 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref); 521 522 if (UseCompressedOops) { 523 bs->write_ref_field_pre((narrowOop*)next_addr, NULL); 524 } else { 525 bs->write_ref_field_pre((oop*)next_addr, NULL); 526 } 527 java_lang_ref_Reference::set_next_raw(_ref, NULL); 528 } else { 529 java_lang_ref_Reference::set_next(_ref, NULL); 530 } 531 } 532 533 void DiscoveredListIterator::clear_referent() { 534 oop_store_raw(_referent_addr, NULL); 535 } 536 537 // NOTE: process_phase*() are largely similar, and at a high level 538 // merely iterate over the extant list applying a predicate to 539 // each of its elements and possibly removing that element from the 540 // list and applying some further closures to that element. 541 // We should consider the possibility of replacing these 542 // process_phase*() methods by abstracting them into 543 // a single general iterator invocation that receives appropriate 544 // closures that accomplish this work. 545 546 // (SoftReferences only) Traverse the list and remove any SoftReferences whose 547 // referents are not alive, but that should be kept alive for policy reasons. 548 // Keep alive the transitive closure of all such referents. 549 void 550 ReferenceProcessor::process_phase1(DiscoveredList& refs_list, 551 ReferencePolicy* policy, 552 BoolObjectClosure* is_alive, 553 OopClosure* keep_alive, 554 VoidClosure* complete_gc) { 555 assert(policy != NULL, "Must have a non-NULL policy"); 556 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 557 // Decide which softly reachable refs should be kept alive. 558 while (iter.has_next()) { 559 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */)); 560 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive(); 561 if (referent_is_dead && 562 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) { 563 if (TraceReferenceGC) { 564 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy", 565 (void *)iter.obj(), iter.obj()->klass()->internal_name()); 566 } 567 // Remove Reference object from list 568 iter.remove(); 569 // Make the Reference object active again 570 iter.make_active(); 571 // keep the referent around 572 iter.make_referent_alive(); 573 iter.move_to_next(); 574 } else { 575 iter.next(); 576 } 577 } 578 // Close the reachable set 579 complete_gc->do_void(); 580 NOT_PRODUCT( 581 if (PrintGCDetails && TraceReferenceGC) { 582 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d " 583 "discovered Refs by policy, from list " INTPTR_FORMAT, 584 iter.removed(), iter.processed(), (address)refs_list.head()); 585 } 586 ) 587 } 588 589 // Traverse the list and remove any Refs that are not active, or 590 // whose referents are either alive or NULL. 591 void 592 ReferenceProcessor::pp2_work(DiscoveredList& refs_list, 593 BoolObjectClosure* is_alive, 594 OopClosure* keep_alive) { 595 assert(discovery_is_atomic(), "Error"); 596 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 597 while (iter.has_next()) { 598 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 599 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());) 600 assert(next == NULL, "Should not discover inactive Reference"); 601 if (iter.is_referent_alive()) { 602 if (TraceReferenceGC) { 603 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)", 604 (void *)iter.obj(), iter.obj()->klass()->internal_name()); 605 } 606 // The referent is reachable after all. 607 // Remove Reference object from list. 608 iter.remove(); 609 // Update the referent pointer as necessary: Note that this 610 // should not entail any recursive marking because the 611 // referent must already have been traversed. 612 iter.make_referent_alive(); 613 iter.move_to_next(); 614 } else { 615 iter.next(); 616 } 617 } 618 NOT_PRODUCT( 619 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) { 620 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d " 621 "Refs in discovered list " INTPTR_FORMAT, 622 iter.removed(), iter.processed(), (address)refs_list.head()); 623 } 624 ) 625 } 626 627 void 628 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list, 629 BoolObjectClosure* is_alive, 630 OopClosure* keep_alive, 631 VoidClosure* complete_gc) { 632 assert(!discovery_is_atomic(), "Error"); 633 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 634 while (iter.has_next()) { 635 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 636 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj()); 637 oop next = java_lang_ref_Reference::next(iter.obj()); 638 if ((iter.referent() == NULL || iter.is_referent_alive() || 639 next != NULL)) { 640 assert(next->is_oop_or_null(), "bad next field"); 641 // Remove Reference object from list 642 iter.remove(); 643 // Trace the cohorts 644 iter.make_referent_alive(); 645 if (UseCompressedOops) { 646 keep_alive->do_oop((narrowOop*)next_addr); 647 } else { 648 keep_alive->do_oop((oop*)next_addr); 649 } 650 iter.move_to_next(); 651 } else { 652 iter.next(); 653 } 654 } 655 // Now close the newly reachable set 656 complete_gc->do_void(); 657 NOT_PRODUCT( 658 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) { 659 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d " 660 "Refs in discovered list " INTPTR_FORMAT, 661 iter.removed(), iter.processed(), (address)refs_list.head()); 662 } 663 ) 664 } 665 666 // Traverse the list and process the referents, by either 667 // clearing them or keeping them (and their reachable 668 // closure) alive. 669 void 670 ReferenceProcessor::process_phase3(DiscoveredList& refs_list, 671 bool clear_referent, 672 BoolObjectClosure* is_alive, 673 OopClosure* keep_alive, 674 VoidClosure* complete_gc) { 675 ResourceMark rm; 676 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 677 while (iter.has_next()) { 678 iter.update_discovered(); 679 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 680 if (clear_referent) { 681 // NULL out referent pointer 682 iter.clear_referent(); 683 } else { 684 // keep the referent around 685 iter.make_referent_alive(); 686 } 687 if (TraceReferenceGC) { 688 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending", 689 clear_referent ? "cleared " : "", 690 (void *)iter.obj(), iter.obj()->klass()->internal_name()); 691 } 692 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference"); 693 iter.next(); 694 } 695 // Remember to update the next pointer of the last ref. 696 iter.update_discovered(); 697 // Close the reachable set 698 complete_gc->do_void(); 699 } 700 701 void 702 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) { 703 oop obj = NULL; 704 oop next = refs_list.head(); 705 while (next != obj) { 706 obj = next; 707 next = java_lang_ref_Reference::discovered(obj); 708 java_lang_ref_Reference::set_discovered_raw(obj, NULL); 709 } 710 refs_list.set_head(NULL); 711 refs_list.set_length(0); 712 } 713 714 void 715 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) { 716 clear_discovered_references(refs_list); 717 } 718 719 void ReferenceProcessor::abandon_partial_discovery() { 720 // loop over the lists 721 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 722 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) { 723 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i)); 724 } 725 abandon_partial_discovered_list(_discovered_refs[i]); 726 } 727 } 728 729 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask { 730 public: 731 RefProcPhase1Task(ReferenceProcessor& ref_processor, 732 DiscoveredList refs_lists[], 733 ReferencePolicy* policy, 734 bool marks_oops_alive) 735 : ProcessTask(ref_processor, refs_lists, marks_oops_alive), 736 _policy(policy) 737 { } 738 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 739 OopClosure& keep_alive, 740 VoidClosure& complete_gc) 741 { 742 Thread* thr = Thread::current(); 743 int refs_list_index = ((WorkerThread*)thr)->id(); 744 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy, 745 &is_alive, &keep_alive, &complete_gc); 746 } 747 private: 748 ReferencePolicy* _policy; 749 }; 750 751 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask { 752 public: 753 RefProcPhase2Task(ReferenceProcessor& ref_processor, 754 DiscoveredList refs_lists[], 755 bool marks_oops_alive) 756 : ProcessTask(ref_processor, refs_lists, marks_oops_alive) 757 { } 758 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 759 OopClosure& keep_alive, 760 VoidClosure& complete_gc) 761 { 762 _ref_processor.process_phase2(_refs_lists[i], 763 &is_alive, &keep_alive, &complete_gc); 764 } 765 }; 766 767 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask { 768 public: 769 RefProcPhase3Task(ReferenceProcessor& ref_processor, 770 DiscoveredList refs_lists[], 771 bool clear_referent, 772 bool marks_oops_alive) 773 : ProcessTask(ref_processor, refs_lists, marks_oops_alive), 774 _clear_referent(clear_referent) 775 { } 776 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 777 OopClosure& keep_alive, 778 VoidClosure& complete_gc) 779 { 780 // Don't use "refs_list_index" calculated in this way because 781 // balance_queues() has moved the Ref's into the first n queues. 782 // Thread* thr = Thread::current(); 783 // int refs_list_index = ((WorkerThread*)thr)->id(); 784 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent, 785 _ref_processor.process_phase3(_refs_lists[i], _clear_referent, 786 &is_alive, &keep_alive, &complete_gc); 787 } 788 private: 789 bool _clear_referent; 790 }; 791 792 void ReferenceProcessor::set_discovered(oop ref, oop value) { 793 if (_discovered_list_needs_barrier) { 794 java_lang_ref_Reference::set_discovered(ref, value); 795 } else { 796 java_lang_ref_Reference::set_discovered_raw(ref, value); 797 } 798 } 799 800 // Balances reference queues. 801 // Move entries from all queues[0, 1, ..., _max_num_q-1] to 802 // queues[0, 1, ..., _num_q-1] because only the first _num_q 803 // corresponding to the active workers will be processed. 804 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[]) 805 { 806 // calculate total length 807 size_t total_refs = 0; 808 if (TraceReferenceGC && PrintGCDetails) { 809 gclog_or_tty->print_cr("\nBalance ref_lists "); 810 } 811 812 for (uint i = 0; i < _max_num_q; ++i) { 813 total_refs += ref_lists[i].length(); 814 if (TraceReferenceGC && PrintGCDetails) { 815 gclog_or_tty->print("%d ", ref_lists[i].length()); 816 } 817 } 818 if (TraceReferenceGC && PrintGCDetails) { 819 gclog_or_tty->print_cr(" = %d", total_refs); 820 } 821 size_t avg_refs = total_refs / _num_q + 1; 822 uint to_idx = 0; 823 for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) { 824 bool move_all = false; 825 if (from_idx >= _num_q) { 826 move_all = ref_lists[from_idx].length() > 0; 827 } 828 while ((ref_lists[from_idx].length() > avg_refs) || 829 move_all) { 830 assert(to_idx < _num_q, "Sanity Check!"); 831 if (ref_lists[to_idx].length() < avg_refs) { 832 // move superfluous refs 833 size_t refs_to_move; 834 // Move all the Ref's if the from queue will not be processed. 835 if (move_all) { 836 refs_to_move = MIN2(ref_lists[from_idx].length(), 837 avg_refs - ref_lists[to_idx].length()); 838 } else { 839 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs, 840 avg_refs - ref_lists[to_idx].length()); 841 } 842 843 assert(refs_to_move > 0, "otherwise the code below will fail"); 844 845 oop move_head = ref_lists[from_idx].head(); 846 oop move_tail = move_head; 847 oop new_head = move_head; 848 // find an element to split the list on 849 for (size_t j = 0; j < refs_to_move; ++j) { 850 move_tail = new_head; 851 new_head = java_lang_ref_Reference::discovered(new_head); 852 } 853 854 // Add the chain to the to list. 855 if (ref_lists[to_idx].head() == NULL) { 856 // to list is empty. Make a loop at the end. 857 set_discovered(move_tail, move_tail); 858 } else { 859 set_discovered(move_tail, ref_lists[to_idx].head()); 860 } 861 ref_lists[to_idx].set_head(move_head); 862 ref_lists[to_idx].inc_length(refs_to_move); 863 864 // Remove the chain from the from list. 865 if (move_tail == new_head) { 866 // We found the end of the from list. 867 ref_lists[from_idx].set_head(NULL); 868 } else { 869 ref_lists[from_idx].set_head(new_head); 870 } 871 ref_lists[from_idx].dec_length(refs_to_move); 872 if (ref_lists[from_idx].length() == 0) { 873 break; 874 } 875 } else { 876 to_idx = (to_idx + 1) % _num_q; 877 } 878 } 879 } 880 #ifdef ASSERT 881 size_t balanced_total_refs = 0; 882 for (uint i = 0; i < _max_num_q; ++i) { 883 balanced_total_refs += ref_lists[i].length(); 884 if (TraceReferenceGC && PrintGCDetails) { 885 gclog_or_tty->print("%d ", ref_lists[i].length()); 886 } 887 } 888 if (TraceReferenceGC && PrintGCDetails) { 889 gclog_or_tty->print_cr(" = %d", balanced_total_refs); 890 gclog_or_tty->flush(); 891 } 892 assert(total_refs == balanced_total_refs, "Balancing was incomplete"); 893 #endif 894 } 895 896 void ReferenceProcessor::balance_all_queues() { 897 balance_queues(_discoveredSoftRefs); 898 balance_queues(_discoveredWeakRefs); 899 balance_queues(_discoveredFinalRefs); 900 balance_queues(_discoveredPhantomRefs); 901 } 902 903 size_t 904 ReferenceProcessor::process_discovered_reflist( 905 DiscoveredList refs_lists[], 906 ReferencePolicy* policy, 907 bool clear_referent, 908 BoolObjectClosure* is_alive, 909 OopClosure* keep_alive, 910 VoidClosure* complete_gc, 911 AbstractRefProcTaskExecutor* task_executor) 912 { 913 bool mt_processing = task_executor != NULL && _processing_is_mt; 914 // If discovery used MT and a dynamic number of GC threads, then 915 // the queues must be balanced for correctness if fewer than the 916 // maximum number of queues were used. The number of queue used 917 // during discovery may be different than the number to be used 918 // for processing so don't depend of _num_q < _max_num_q as part 919 // of the test. 920 bool must_balance = _discovery_is_mt; 921 922 if ((mt_processing && ParallelRefProcBalancingEnabled) || 923 must_balance) { 924 balance_queues(refs_lists); 925 } 926 927 size_t total_list_count = total_count(refs_lists); 928 929 if (PrintReferenceGC && PrintGCDetails) { 930 gclog_or_tty->print(", %u refs", total_list_count); 931 } 932 933 // Phase 1 (soft refs only): 934 // . Traverse the list and remove any SoftReferences whose 935 // referents are not alive, but that should be kept alive for 936 // policy reasons. Keep alive the transitive closure of all 937 // such referents. 938 if (policy != NULL) { 939 if (mt_processing) { 940 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/); 941 task_executor->execute(phase1); 942 } else { 943 for (uint i = 0; i < _max_num_q; i++) { 944 process_phase1(refs_lists[i], policy, 945 is_alive, keep_alive, complete_gc); 946 } 947 } 948 } else { // policy == NULL 949 assert(refs_lists != _discoveredSoftRefs, 950 "Policy must be specified for soft references."); 951 } 952 953 // Phase 2: 954 // . Traverse the list and remove any refs whose referents are alive. 955 if (mt_processing) { 956 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/); 957 task_executor->execute(phase2); 958 } else { 959 for (uint i = 0; i < _max_num_q; i++) { 960 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc); 961 } 962 } 963 964 // Phase 3: 965 // . Traverse the list and process referents as appropriate. 966 if (mt_processing) { 967 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/); 968 task_executor->execute(phase3); 969 } else { 970 for (uint i = 0; i < _max_num_q; i++) { 971 process_phase3(refs_lists[i], clear_referent, 972 is_alive, keep_alive, complete_gc); 973 } 974 } 975 976 return total_list_count; 977 } 978 979 void ReferenceProcessor::clean_up_discovered_references() { 980 // loop over the lists 981 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 982 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) { 983 gclog_or_tty->print_cr( 984 "\nScrubbing %s discovered list of Null referents", 985 list_name(i)); 986 } 987 clean_up_discovered_reflist(_discovered_refs[i]); 988 } 989 } 990 991 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) { 992 assert(!discovery_is_atomic(), "Else why call this method?"); 993 DiscoveredListIterator iter(refs_list, NULL, NULL); 994 while (iter.has_next()) { 995 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 996 oop next = java_lang_ref_Reference::next(iter.obj()); 997 assert(next->is_oop_or_null(), "bad next field"); 998 // If referent has been cleared or Reference is not active, 999 // drop it. 1000 if (iter.referent() == NULL || next != NULL) { 1001 debug_only( 1002 if (PrintGCDetails && TraceReferenceGC) { 1003 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: " 1004 INTPTR_FORMAT " with next field: " INTPTR_FORMAT 1005 " and referent: " INTPTR_FORMAT, 1006 (void *)iter.obj(), (void *)next, (void *)iter.referent()); 1007 } 1008 ) 1009 // Remove Reference object from list 1010 iter.remove(); 1011 iter.move_to_next(); 1012 } else { 1013 iter.next(); 1014 } 1015 } 1016 NOT_PRODUCT( 1017 if (PrintGCDetails && TraceReferenceGC) { 1018 gclog_or_tty->print( 1019 " Removed %d Refs with NULL referents out of %d discovered Refs", 1020 iter.removed(), iter.processed()); 1021 } 1022 ) 1023 } 1024 1025 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) { 1026 uint id = 0; 1027 // Determine the queue index to use for this object. 1028 if (_discovery_is_mt) { 1029 // During a multi-threaded discovery phase, 1030 // each thread saves to its "own" list. 1031 Thread* thr = Thread::current(); 1032 id = thr->as_Worker_thread()->id(); 1033 } else { 1034 // single-threaded discovery, we save in round-robin 1035 // fashion to each of the lists. 1036 if (_processing_is_mt) { 1037 id = next_id(); 1038 } 1039 } 1040 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)"); 1041 1042 // Get the discovered queue to which we will add 1043 DiscoveredList* list = NULL; 1044 switch (rt) { 1045 case REF_OTHER: 1046 // Unknown reference type, no special treatment 1047 break; 1048 case REF_SOFT: 1049 list = &_discoveredSoftRefs[id]; 1050 break; 1051 case REF_WEAK: 1052 list = &_discoveredWeakRefs[id]; 1053 break; 1054 case REF_FINAL: 1055 list = &_discoveredFinalRefs[id]; 1056 break; 1057 case REF_PHANTOM: 1058 list = &_discoveredPhantomRefs[id]; 1059 break; 1060 case REF_NONE: 1061 // we should not reach here if we are an InstanceRefKlass 1062 default: 1063 ShouldNotReachHere(); 1064 } 1065 if (TraceReferenceGC && PrintGCDetails) { 1066 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list); 1067 } 1068 return list; 1069 } 1070 1071 inline void 1072 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list, 1073 oop obj, 1074 HeapWord* discovered_addr) { 1075 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller"); 1076 // First we must make sure this object is only enqueued once. CAS in a non null 1077 // discovered_addr. 1078 oop current_head = refs_list.head(); 1079 // The last ref must have its discovered field pointing to itself. 1080 oop next_discovered = (current_head != NULL) ? current_head : obj; 1081 1082 // Note: In the case of G1, this specific pre-barrier is strictly 1083 // not necessary because the only case we are interested in 1084 // here is when *discovered_addr is NULL (see the CAS further below), 1085 // so this will expand to nothing. As a result, we have manually 1086 // elided this out for G1, but left in the test for some future 1087 // collector that might have need for a pre-barrier here, e.g.:- 1088 // _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered); 1089 assert(!_discovered_list_needs_barrier || UseG1GC, 1090 "Need to check non-G1 collector: " 1091 "may need a pre-write-barrier for CAS from NULL below"); 1092 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr, 1093 NULL); 1094 if (retest == NULL) { 1095 // This thread just won the right to enqueue the object. 1096 // We have separate lists for enqueueing, so no synchronization 1097 // is necessary. 1098 refs_list.set_head(obj); 1099 refs_list.inc_length(1); 1100 if (_discovered_list_needs_barrier) { 1101 _bs->write_ref_field((void*)discovered_addr, next_discovered); 1102 } 1103 1104 if (TraceReferenceGC) { 1105 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)", 1106 (void *)obj, obj->klass()->internal_name()); 1107 } 1108 } else { 1109 // If retest was non NULL, another thread beat us to it: 1110 // The reference has already been discovered... 1111 if (TraceReferenceGC) { 1112 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)", 1113 (void *)obj, obj->klass()->internal_name()); 1114 } 1115 } 1116 } 1117 1118 #ifndef PRODUCT 1119 // Non-atomic (i.e. concurrent) discovery might allow us 1120 // to observe j.l.References with NULL referents, being those 1121 // cleared concurrently by mutators during (or after) discovery. 1122 void ReferenceProcessor::verify_referent(oop obj) { 1123 bool da = discovery_is_atomic(); 1124 oop referent = java_lang_ref_Reference::referent(obj); 1125 assert(da ? referent->is_oop() : referent->is_oop_or_null(), 1126 err_msg("Bad referent " INTPTR_FORMAT " found in Reference " 1127 INTPTR_FORMAT " during %satomic discovery ", 1128 (void *)referent, (void *)obj, da ? "" : "non-")); 1129 } 1130 #endif 1131 1132 // We mention two of several possible choices here: 1133 // #0: if the reference object is not in the "originating generation" 1134 // (or part of the heap being collected, indicated by our "span" 1135 // we don't treat it specially (i.e. we scan it as we would 1136 // a normal oop, treating its references as strong references). 1137 // This means that references can't be discovered unless their 1138 // referent is also in the same span. This is the simplest, 1139 // most "local" and most conservative approach, albeit one 1140 // that may cause weak references to be enqueued least promptly. 1141 // We call this choice the "ReferenceBasedDiscovery" policy. 1142 // #1: the reference object may be in any generation (span), but if 1143 // the referent is in the generation (span) being currently collected 1144 // then we can discover the reference object, provided 1145 // the object has not already been discovered by 1146 // a different concurrently running collector (as may be the 1147 // case, for instance, if the reference object is in CMS and 1148 // the referent in DefNewGeneration), and provided the processing 1149 // of this reference object by the current collector will 1150 // appear atomic to every other collector in the system. 1151 // (Thus, for instance, a concurrent collector may not 1152 // discover references in other generations even if the 1153 // referent is in its own generation). This policy may, 1154 // in certain cases, enqueue references somewhat sooner than 1155 // might Policy #0 above, but at marginally increased cost 1156 // and complexity in processing these references. 1157 // We call this choice the "RefeferentBasedDiscovery" policy. 1158 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) { 1159 // Make sure we are discovering refs (rather than processing discovered refs). 1160 if (!_discovering_refs || !RegisterReferences) { 1161 return false; 1162 } 1163 // We only discover active references. 1164 oop next = java_lang_ref_Reference::next(obj); 1165 if (next != NULL) { // Ref is no longer active 1166 return false; 1167 } 1168 1169 HeapWord* obj_addr = (HeapWord*)obj; 1170 if (RefDiscoveryPolicy == ReferenceBasedDiscovery && 1171 !_span.contains(obj_addr)) { 1172 // Reference is not in the originating generation; 1173 // don't treat it specially (i.e. we want to scan it as a normal 1174 // object with strong references). 1175 return false; 1176 } 1177 1178 // We only discover references whose referents are not (yet) 1179 // known to be strongly reachable. 1180 if (is_alive_non_header() != NULL) { 1181 verify_referent(obj); 1182 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) { 1183 return false; // referent is reachable 1184 } 1185 } 1186 if (rt == REF_SOFT) { 1187 // For soft refs we can decide now if these are not 1188 // current candidates for clearing, in which case we 1189 // can mark through them now, rather than delaying that 1190 // to the reference-processing phase. Since all current 1191 // time-stamp policies advance the soft-ref clock only 1192 // at a major collection cycle, this is always currently 1193 // accurate. 1194 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) { 1195 return false; 1196 } 1197 } 1198 1199 ResourceMark rm; // Needed for tracing. 1200 1201 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj); 1202 const oop discovered = java_lang_ref_Reference::discovered(obj); 1203 assert(discovered->is_oop_or_null(), "bad discovered field"); 1204 if (discovered != NULL) { 1205 // The reference has already been discovered... 1206 if (TraceReferenceGC) { 1207 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)", 1208 (void *)obj, obj->klass()->internal_name()); 1209 } 1210 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1211 // assumes that an object is not processed twice; 1212 // if it's been already discovered it must be on another 1213 // generation's discovered list; so we won't discover it. 1214 return false; 1215 } else { 1216 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery, 1217 "Unrecognized policy"); 1218 // Check assumption that an object is not potentially 1219 // discovered twice except by concurrent collectors that potentially 1220 // trace the same Reference object twice. 1221 assert(UseConcMarkSweepGC || UseG1GC, 1222 "Only possible with a concurrent marking collector"); 1223 return true; 1224 } 1225 } 1226 1227 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1228 verify_referent(obj); 1229 // Discover if and only if EITHER: 1230 // .. reference is in our span, OR 1231 // .. we are an atomic collector and referent is in our span 1232 if (_span.contains(obj_addr) || 1233 (discovery_is_atomic() && 1234 _span.contains(java_lang_ref_Reference::referent(obj)))) { 1235 // should_enqueue = true; 1236 } else { 1237 return false; 1238 } 1239 } else { 1240 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery && 1241 _span.contains(obj_addr), "code inconsistency"); 1242 } 1243 1244 // Get the right type of discovered queue head. 1245 DiscoveredList* list = get_discovered_list(rt); 1246 if (list == NULL) { 1247 return false; // nothing special needs to be done 1248 } 1249 1250 if (_discovery_is_mt) { 1251 add_to_discovered_list_mt(*list, obj, discovered_addr); 1252 } else { 1253 // If "_discovered_list_needs_barrier", we do write barriers when 1254 // updating the discovered reference list. Otherwise, we do a raw store 1255 // here: the field will be visited later when processing the discovered 1256 // references. 1257 oop current_head = list->head(); 1258 // The last ref must have its discovered field pointing to itself. 1259 oop next_discovered = (current_head != NULL) ? current_head : obj; 1260 1261 // As in the case further above, since we are over-writing a NULL 1262 // pre-value, we can safely elide the pre-barrier here for the case of G1. 1263 // e.g.:- _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered); 1264 assert(discovered == NULL, "control point invariant"); 1265 assert(!_discovered_list_needs_barrier || UseG1GC, 1266 "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below"); 1267 oop_store_raw(discovered_addr, next_discovered); 1268 if (_discovered_list_needs_barrier) { 1269 _bs->write_ref_field((void*)discovered_addr, next_discovered); 1270 } 1271 list->set_head(obj); 1272 list->inc_length(1); 1273 1274 if (TraceReferenceGC) { 1275 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)", 1276 (void *)obj, obj->klass()->internal_name()); 1277 } 1278 } 1279 assert(obj->is_oop(), "Discovered a bad reference"); 1280 verify_referent(obj); 1281 return true; 1282 } 1283 1284 // Preclean the discovered references by removing those 1285 // whose referents are alive, and by marking from those that 1286 // are not active. These lists can be handled here 1287 // in any order and, indeed, concurrently. 1288 void ReferenceProcessor::preclean_discovered_references( 1289 BoolObjectClosure* is_alive, 1290 OopClosure* keep_alive, 1291 VoidClosure* complete_gc, 1292 YieldClosure* yield, 1293 GCTimer* gc_timer) { 1294 1295 NOT_PRODUCT(verify_ok_to_handle_reflists()); 1296 1297 // Soft references 1298 { 1299 GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC, 1300 false, gc_timer); 1301 for (uint i = 0; i < _max_num_q; i++) { 1302 if (yield->should_return()) { 1303 return; 1304 } 1305 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive, 1306 keep_alive, complete_gc, yield); 1307 } 1308 } 1309 1310 // Weak references 1311 { 1312 GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC, 1313 false, gc_timer); 1314 for (uint i = 0; i < _max_num_q; i++) { 1315 if (yield->should_return()) { 1316 return; 1317 } 1318 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive, 1319 keep_alive, complete_gc, yield); 1320 } 1321 } 1322 1323 // Final references 1324 { 1325 GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC, 1326 false, gc_timer); 1327 for (uint i = 0; i < _max_num_q; i++) { 1328 if (yield->should_return()) { 1329 return; 1330 } 1331 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive, 1332 keep_alive, complete_gc, yield); 1333 } 1334 } 1335 1336 // Phantom references 1337 { 1338 GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC, 1339 false, gc_timer); 1340 for (uint i = 0; i < _max_num_q; i++) { 1341 if (yield->should_return()) { 1342 return; 1343 } 1344 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive, 1345 keep_alive, complete_gc, yield); 1346 } 1347 } 1348 } 1349 1350 // Walk the given discovered ref list, and remove all reference objects 1351 // whose referents are still alive, whose referents are NULL or which 1352 // are not active (have a non-NULL next field). NOTE: When we are 1353 // thus precleaning the ref lists (which happens single-threaded today), 1354 // we do not disable refs discovery to honor the correct semantics of 1355 // java.lang.Reference. As a result, we need to be careful below 1356 // that ref removal steps interleave safely with ref discovery steps 1357 // (in this thread). 1358 void 1359 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list, 1360 BoolObjectClosure* is_alive, 1361 OopClosure* keep_alive, 1362 VoidClosure* complete_gc, 1363 YieldClosure* yield) { 1364 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 1365 while (iter.has_next()) { 1366 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 1367 oop obj = iter.obj(); 1368 oop next = java_lang_ref_Reference::next(obj); 1369 if (iter.referent() == NULL || iter.is_referent_alive() || 1370 next != NULL) { 1371 // The referent has been cleared, or is alive, or the Reference is not 1372 // active; we need to trace and mark its cohort. 1373 if (TraceReferenceGC) { 1374 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)", 1375 (void *)iter.obj(), iter.obj()->klass()->internal_name()); 1376 } 1377 // Remove Reference object from list 1378 iter.remove(); 1379 // Keep alive its cohort. 1380 iter.make_referent_alive(); 1381 if (UseCompressedOops) { 1382 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj); 1383 keep_alive->do_oop(next_addr); 1384 } else { 1385 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj); 1386 keep_alive->do_oop(next_addr); 1387 } 1388 iter.move_to_next(); 1389 } else { 1390 iter.next(); 1391 } 1392 } 1393 // Close the reachable set 1394 complete_gc->do_void(); 1395 1396 NOT_PRODUCT( 1397 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) { 1398 gclog_or_tty->print_cr(" Dropped %d Refs out of %d " 1399 "Refs in discovered list " INTPTR_FORMAT, 1400 iter.removed(), iter.processed(), (address)refs_list.head()); 1401 } 1402 ) 1403 } 1404 1405 const char* ReferenceProcessor::list_name(uint i) { 1406 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(), 1407 "Out of bounds index"); 1408 1409 int j = i / _max_num_q; 1410 switch (j) { 1411 case 0: return "SoftRef"; 1412 case 1: return "WeakRef"; 1413 case 2: return "FinalRef"; 1414 case 3: return "PhantomRef"; 1415 } 1416 ShouldNotReachHere(); 1417 return NULL; 1418 } 1419 1420 #ifndef PRODUCT 1421 void ReferenceProcessor::verify_ok_to_handle_reflists() { 1422 // empty for now 1423 } 1424 #endif 1425 1426 #ifndef PRODUCT 1427 void ReferenceProcessor::clear_discovered_references() { 1428 guarantee(!_discovering_refs, "Discovering refs?"); 1429 for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 1430 clear_discovered_references(_discovered_refs[i]); 1431 } 1432 } 1433 1434 #endif // PRODUCT