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 jlong ReferenceProcessor::_soft_ref_timestamp_clock = 0; 40 41 void referenceProcessor_init() { 42 ReferenceProcessor::init_statics(); 43 } 44 45 void ReferenceProcessor::init_statics() { 46 // os::javaTimeMillis() does not guarantee monotonicity. 47 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; 48 49 // Initialize the soft ref timestamp clock. 50 _soft_ref_timestamp_clock = now; 51 // Also update the soft ref clock in j.l.r.SoftReference 52 java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock); 53 54 _always_clear_soft_ref_policy = new AlwaysClearPolicy(); 55 _default_soft_ref_policy = new COMPILER2_PRESENT(LRUMaxHeapPolicy()) 56 NOT_COMPILER2(LRUCurrentHeapPolicy()); 57 if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) { 58 vm_exit_during_initialization("Could not allocate reference policy object"); 59 } 60 guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery || 61 RefDiscoveryPolicy == ReferentBasedDiscovery, 62 "Unrecongnized RefDiscoveryPolicy"); 63 _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field(); 64 } 65 66 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) { 67 #ifdef ASSERT 68 // Verify that we're not currently discovering refs 69 assert(!verify_disabled || !_discovering_refs, "nested call?"); 70 71 if (check_no_refs) { 72 // Verify that the discovered lists are empty 73 verify_no_references_recorded(); 74 } 75 #endif // ASSERT 76 77 // Someone could have modified the value of the static 78 // field in the j.l.r.SoftReference class that holds the 79 // soft reference timestamp clock using reflection or 80 // Unsafe between GCs. Unconditionally update the static 81 // field in ReferenceProcessor here so that we use the new 82 // value during reference discovery. 83 84 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 85 _discovering_refs = true; 86 } 87 88 ReferenceProcessor::ReferenceProcessor(MemRegion span, 89 bool mt_processing, 90 int mt_processing_degree, 91 bool mt_discovery, 92 int mt_discovery_degree, 93 bool atomic_discovery, 94 BoolObjectClosure* is_alive_non_header, 95 bool discovered_list_needs_barrier) : 96 _discovering_refs(false), 97 _enqueuing_is_done(false), 98 _is_alive_non_header(is_alive_non_header), 99 _discovered_list_needs_barrier(discovered_list_needs_barrier), 100 _bs(NULL), 101 _processing_is_mt(mt_processing), 102 _next_id(0) 103 { 104 _span = span; 105 _discovery_is_atomic = atomic_discovery; 106 _discovery_is_mt = mt_discovery; 107 _num_q = MAX2(1, mt_processing_degree); 108 _max_num_q = MAX2(_num_q, mt_discovery_degree); 109 _discovered_refs = NEW_C_HEAP_ARRAY(DiscoveredList, 110 _max_num_q * number_of_subclasses_of_ref()); 111 if (_discovered_refs == NULL) { 112 vm_exit_during_initialization("Could not allocated RefProc Array"); 113 } 114 _discoveredSoftRefs = &_discovered_refs[0]; 115 _discoveredWeakRefs = &_discoveredSoftRefs[_max_num_q]; 116 _discoveredFinalRefs = &_discoveredWeakRefs[_max_num_q]; 117 _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q]; 118 119 // Initialize all entries to NULL 120 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 121 _discovered_refs[i].set_head(NULL); 122 _discovered_refs[i].set_length(0); 123 } 124 125 // If we do barriers, cache a copy of the barrier set. 126 if (discovered_list_needs_barrier) { 127 _bs = Universe::heap()->barrier_set(); 128 } 129 setup_policy(false /* default soft ref policy */); 130 } 131 132 #ifndef PRODUCT 133 void ReferenceProcessor::verify_no_references_recorded() { 134 guarantee(!_discovering_refs, "Discovering refs?"); 135 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 136 guarantee(_discovered_refs[i].is_empty(), 137 "Found non-empty discovered list"); 138 } 139 } 140 #endif 141 142 void ReferenceProcessor::weak_oops_do(OopClosure* f) { 143 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 144 if (UseCompressedOops) { 145 f->do_oop((narrowOop*)_discovered_refs[i].adr_head()); 146 } else { 147 f->do_oop((oop*)_discovered_refs[i].adr_head()); 148 } 149 } 150 } 151 152 void ReferenceProcessor::update_soft_ref_master_clock() { 153 // Update (advance) the soft ref master clock field. This must be done 154 // after processing the soft ref list. 155 156 // os::javaTimeMillis() does not guarantee monotonicity. 157 jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC; 158 jlong soft_ref_clock = java_lang_ref_SoftReference::clock(); 159 assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync"); 160 161 NOT_PRODUCT( 162 if (now < _soft_ref_timestamp_clock) { 163 warning("time warp: "INT64_FORMAT" to "INT64_FORMAT, 164 _soft_ref_timestamp_clock, now); 165 } 166 ) 167 // In product mode, protect ourselves from system time being adjusted 168 // externally and going backward; see note in the implementation of 169 // GenCollectedHeap::time_since_last_gc() for the right way to fix 170 // this uniformly throughout the VM; see bug-id 4741166. XXX 171 if (now > _soft_ref_timestamp_clock) { 172 _soft_ref_timestamp_clock = now; 173 java_lang_ref_SoftReference::set_clock(now); 174 } 175 // Else leave clock stalled at its old value until time progresses 176 // past clock value. 177 } 178 179 void ReferenceProcessor::process_discovered_references( 180 BoolObjectClosure* is_alive, 181 OopClosure* keep_alive, 182 VoidClosure* complete_gc, 183 AbstractRefProcTaskExecutor* task_executor) { 184 NOT_PRODUCT(verify_ok_to_handle_reflists()); 185 186 assert(!enqueuing_is_done(), "If here enqueuing should not be complete"); 187 // Stop treating discovered references specially. 188 disable_discovery(); 189 190 // If discovery was concurrent, someone could have modified 191 // the value of the static field in the j.l.r.SoftReference 192 // class that holds the soft reference timestamp clock using 193 // reflection or Unsafe between when discovery was enabled and 194 // now. Unconditionally update the static field in ReferenceProcessor 195 // here so that we use the new value during processing of the 196 // discovered soft refs. 197 198 _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock(); 199 200 bool trace_time = PrintGCDetails && PrintReferenceGC; 201 // Soft references 202 { 203 TraceTime tt("SoftReference", trace_time, false, gclog_or_tty); 204 process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true, 205 is_alive, keep_alive, complete_gc, task_executor); 206 } 207 208 update_soft_ref_master_clock(); 209 210 // Weak references 211 { 212 TraceTime tt("WeakReference", trace_time, false, gclog_or_tty); 213 process_discovered_reflist(_discoveredWeakRefs, NULL, true, 214 is_alive, keep_alive, complete_gc, task_executor); 215 } 216 217 // Final references 218 { 219 TraceTime tt("FinalReference", trace_time, false, gclog_or_tty); 220 process_discovered_reflist(_discoveredFinalRefs, NULL, false, 221 is_alive, keep_alive, complete_gc, task_executor); 222 } 223 224 // Phantom references 225 { 226 TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty); 227 process_discovered_reflist(_discoveredPhantomRefs, NULL, false, 228 is_alive, keep_alive, complete_gc, task_executor); 229 } 230 231 // Weak global JNI references. It would make more sense (semantically) to 232 // traverse these simultaneously with the regular weak references above, but 233 // that is not how the JDK1.2 specification is. See #4126360. Native code can 234 // thus use JNI weak references to circumvent the phantom references and 235 // resurrect a "post-mortem" object. 236 { 237 TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty); 238 if (task_executor != NULL) { 239 task_executor->set_single_threaded_mode(); 240 } 241 process_phaseJNI(is_alive, keep_alive, complete_gc); 242 } 243 } 244 245 #ifndef PRODUCT 246 // Calculate the number of jni handles. 247 uint ReferenceProcessor::count_jni_refs() { 248 class AlwaysAliveClosure: public BoolObjectClosure { 249 public: 250 virtual bool do_object_b(oop obj) { return true; } 251 virtual void do_object(oop obj) { assert(false, "Don't call"); } 252 }; 253 254 class CountHandleClosure: public OopClosure { 255 private: 256 int _count; 257 public: 258 CountHandleClosure(): _count(0) {} 259 void do_oop(oop* unused) { _count++; } 260 void do_oop(narrowOop* unused) { ShouldNotReachHere(); } 261 int count() { return _count; } 262 }; 263 CountHandleClosure global_handle_count; 264 AlwaysAliveClosure always_alive; 265 JNIHandles::weak_oops_do(&always_alive, &global_handle_count); 266 return global_handle_count.count(); 267 } 268 #endif 269 270 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive, 271 OopClosure* keep_alive, 272 VoidClosure* complete_gc) { 273 #ifndef PRODUCT 274 if (PrintGCDetails && PrintReferenceGC) { 275 unsigned int count = count_jni_refs(); 276 gclog_or_tty->print(", %u refs", count); 277 } 278 #endif 279 JNIHandles::weak_oops_do(is_alive, keep_alive); 280 complete_gc->do_void(); 281 } 282 283 284 template <class T> 285 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref, 286 AbstractRefProcTaskExecutor* task_executor) { 287 288 // Remember old value of pending references list 289 T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr(); 290 T old_pending_list_value = *pending_list_addr; 291 292 // Enqueue references that are not made active again, and 293 // clear the decks for the next collection (cycle). 294 ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor); 295 // Do the oop-check on pending_list_addr missed in 296 // enqueue_discovered_reflist. We should probably 297 // do a raw oop_check so that future such idempotent 298 // oop_stores relying on the oop-check side-effect 299 // may be elided automatically and safely without 300 // affecting correctness. 301 oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr)); 302 303 // Stop treating discovered references specially. 304 ref->disable_discovery(); 305 306 // Return true if new pending references were added 307 return old_pending_list_value != *pending_list_addr; 308 } 309 310 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) { 311 NOT_PRODUCT(verify_ok_to_handle_reflists()); 312 if (UseCompressedOops) { 313 return enqueue_discovered_ref_helper<narrowOop>(this, task_executor); 314 } else { 315 return enqueue_discovered_ref_helper<oop>(this, task_executor); 316 } 317 } 318 319 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list, 320 HeapWord* pending_list_addr) { 321 // Given a list of refs linked through the "discovered" field 322 // (java.lang.ref.Reference.discovered), self-loop their "next" field 323 // thus distinguishing them from active References, then 324 // prepend them to the pending list. 325 // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777), 326 // the "next" field is used to chain the pending list, not the discovered 327 // field. 328 329 if (TraceReferenceGC && PrintGCDetails) { 330 gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list " 331 INTPTR_FORMAT, (address)refs_list.head()); 332 } 333 334 oop obj = NULL; 335 oop next_d = refs_list.head(); 336 if (pending_list_uses_discovered_field()) { // New behaviour 337 // Walk down the list, self-looping the next field 338 // so that the References are not considered active. 339 while (obj != next_d) { 340 obj = next_d; 341 assert(obj->is_instanceRef(), "should be reference object"); 342 next_d = java_lang_ref_Reference::discovered(obj); 343 if (TraceReferenceGC && PrintGCDetails) { 344 gclog_or_tty->print_cr(" obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, 345 obj, next_d); 346 } 347 assert(java_lang_ref_Reference::next(obj) == NULL, 348 "Reference not active; should not be discovered"); 349 // Self-loop next, so as to make Ref not active. 350 java_lang_ref_Reference::set_next(obj, obj); 351 if (next_d == obj) { // obj is last 352 // Swap refs_list into pendling_list_addr and 353 // set obj's discovered to what we read from pending_list_addr. 354 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr); 355 // Need oop_check on pending_list_addr above; 356 // see special oop-check code at the end of 357 // enqueue_discovered_reflists() further below. 358 java_lang_ref_Reference::set_discovered(obj, old); // old may be NULL 359 } 360 } 361 } else { // Old behaviour 362 // Walk down the list, copying the discovered field into 363 // the next field and clearing the discovered field. 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 obj, next_d); 371 } 372 assert(java_lang_ref_Reference::next(obj) == NULL, 373 "The reference should not be enqueued"); 374 if (next_d == obj) { // obj is last 375 // Swap refs_list into pendling_list_addr and 376 // set obj's next to what we read from pending_list_addr. 377 oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr); 378 // Need oop_check on pending_list_addr above; 379 // see special oop-check code at the end of 380 // enqueue_discovered_reflists() further below. 381 if (old == NULL) { 382 // obj should be made to point to itself, since 383 // pending list was empty. 384 java_lang_ref_Reference::set_next(obj, obj); 385 } else { 386 java_lang_ref_Reference::set_next(obj, old); 387 } 388 } else { 389 java_lang_ref_Reference::set_next(obj, next_d); 390 } 391 java_lang_ref_Reference::set_discovered(obj, (oop) NULL); 392 } 393 } 394 } 395 396 // Parallel enqueue task 397 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask { 398 public: 399 RefProcEnqueueTask(ReferenceProcessor& ref_processor, 400 DiscoveredList discovered_refs[], 401 HeapWord* pending_list_addr, 402 int n_queues) 403 : EnqueueTask(ref_processor, discovered_refs, 404 pending_list_addr, n_queues) 405 { } 406 407 virtual void work(unsigned int work_id) { 408 assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds"); 409 // Simplest first cut: static partitioning. 410 int index = work_id; 411 // The increment on "index" must correspond to the maximum number of queues 412 // (n_queues) with which that ReferenceProcessor was created. That 413 // is because of the "clever" way the discovered references lists were 414 // allocated and are indexed into. 415 assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected"); 416 for (int j = 0; 417 j < ReferenceProcessor::number_of_subclasses_of_ref(); 418 j++, index += _n_queues) { 419 _ref_processor.enqueue_discovered_reflist( 420 _refs_lists[index], _pending_list_addr); 421 _refs_lists[index].set_head(NULL); 422 _refs_lists[index].set_length(0); 423 } 424 } 425 }; 426 427 // Enqueue references that are not made active again 428 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr, 429 AbstractRefProcTaskExecutor* task_executor) { 430 if (_processing_is_mt && task_executor != NULL) { 431 // Parallel code 432 RefProcEnqueueTask tsk(*this, _discovered_refs, 433 pending_list_addr, _max_num_q); 434 task_executor->execute(tsk); 435 } else { 436 // Serial code: call the parent class's implementation 437 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 438 enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr); 439 _discovered_refs[i].set_head(NULL); 440 _discovered_refs[i].set_length(0); 441 } 442 } 443 } 444 445 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) { 446 _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref); 447 oop discovered = java_lang_ref_Reference::discovered(_ref); 448 assert(_discovered_addr && discovered->is_oop_or_null(), 449 "discovered field is bad"); 450 _next = discovered; 451 _referent_addr = java_lang_ref_Reference::referent_addr(_ref); 452 _referent = java_lang_ref_Reference::referent(_ref); 453 assert(Universe::heap()->is_in_reserved_or_null(_referent), 454 "Wrong oop found in java.lang.Reference object"); 455 assert(allow_null_referent ? 456 _referent->is_oop_or_null() 457 : _referent->is_oop(), 458 "bad referent"); 459 } 460 461 void DiscoveredListIterator::remove() { 462 assert(_ref->is_oop(), "Dropping a bad reference"); 463 oop_store_raw(_discovered_addr, NULL); 464 465 // First _prev_next ref actually points into DiscoveredList (gross). 466 oop new_next; 467 if (_next == _ref) { 468 // At the end of the list, we should make _prev point to itself. 469 // If _ref is the first ref, then _prev_next will be in the DiscoveredList, 470 // and _prev will be NULL. 471 new_next = _prev; 472 } else { 473 new_next = _next; 474 } 475 476 if (UseCompressedOops) { 477 // Remove Reference object from list. 478 oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next); 479 } else { 480 // Remove Reference object from list. 481 oopDesc::store_heap_oop((oop*)_prev_next, new_next); 482 } 483 NOT_PRODUCT(_removed++); 484 _refs_list.dec_length(1); 485 } 486 487 // Make the Reference object active again. 488 void DiscoveredListIterator::make_active() { 489 // For G1 we don't want to use set_next - it 490 // will dirty the card for the next field of 491 // the reference object and will fail 492 // CT verification. 493 if (UseG1GC) { 494 BarrierSet* bs = oopDesc::bs(); 495 HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref); 496 497 if (UseCompressedOops) { 498 bs->write_ref_field_pre((narrowOop*)next_addr, NULL); 499 } else { 500 bs->write_ref_field_pre((oop*)next_addr, NULL); 501 } 502 java_lang_ref_Reference::set_next_raw(_ref, NULL); 503 } else { 504 java_lang_ref_Reference::set_next(_ref, NULL); 505 } 506 } 507 508 void DiscoveredListIterator::clear_referent() { 509 oop_store_raw(_referent_addr, NULL); 510 } 511 512 // NOTE: process_phase*() are largely similar, and at a high level 513 // merely iterate over the extant list applying a predicate to 514 // each of its elements and possibly removing that element from the 515 // list and applying some further closures to that element. 516 // We should consider the possibility of replacing these 517 // process_phase*() methods by abstracting them into 518 // a single general iterator invocation that receives appropriate 519 // closures that accomplish this work. 520 521 // (SoftReferences only) Traverse the list and remove any SoftReferences whose 522 // referents are not alive, but that should be kept alive for policy reasons. 523 // Keep alive the transitive closure of all such referents. 524 void 525 ReferenceProcessor::process_phase1(DiscoveredList& refs_list, 526 ReferencePolicy* policy, 527 BoolObjectClosure* is_alive, 528 OopClosure* keep_alive, 529 VoidClosure* complete_gc) { 530 assert(policy != NULL, "Must have a non-NULL policy"); 531 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 532 // Decide which softly reachable refs should be kept alive. 533 while (iter.has_next()) { 534 iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */)); 535 bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive(); 536 if (referent_is_dead && 537 !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) { 538 if (TraceReferenceGC) { 539 gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s" ") by policy", 540 iter.obj(), iter.obj()->blueprint()->internal_name()); 541 } 542 // Remove Reference object from list 543 iter.remove(); 544 // Make the Reference object active again 545 iter.make_active(); 546 // keep the referent around 547 iter.make_referent_alive(); 548 iter.move_to_next(); 549 } else { 550 iter.next(); 551 } 552 } 553 // Close the reachable set 554 complete_gc->do_void(); 555 NOT_PRODUCT( 556 if (PrintGCDetails && TraceReferenceGC) { 557 gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d " 558 "discovered Refs by policy, from list " INTPTR_FORMAT, 559 iter.removed(), iter.processed(), (address)refs_list.head()); 560 } 561 ) 562 } 563 564 // Traverse the list and remove any Refs that are not active, or 565 // whose referents are either alive or NULL. 566 void 567 ReferenceProcessor::pp2_work(DiscoveredList& refs_list, 568 BoolObjectClosure* is_alive, 569 OopClosure* keep_alive) { 570 assert(discovery_is_atomic(), "Error"); 571 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 572 while (iter.has_next()) { 573 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 574 DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());) 575 assert(next == NULL, "Should not discover inactive Reference"); 576 if (iter.is_referent_alive()) { 577 if (TraceReferenceGC) { 578 gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)", 579 iter.obj(), iter.obj()->blueprint()->internal_name()); 580 } 581 // The referent is reachable after all. 582 // Remove Reference object from list. 583 iter.remove(); 584 // Update the referent pointer as necessary: Note that this 585 // should not entail any recursive marking because the 586 // referent must already have been traversed. 587 iter.make_referent_alive(); 588 iter.move_to_next(); 589 } else { 590 iter.next(); 591 } 592 } 593 NOT_PRODUCT( 594 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) { 595 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d " 596 "Refs in discovered list " INTPTR_FORMAT, 597 iter.removed(), iter.processed(), (address)refs_list.head()); 598 } 599 ) 600 } 601 602 void 603 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList& refs_list, 604 BoolObjectClosure* is_alive, 605 OopClosure* keep_alive, 606 VoidClosure* complete_gc) { 607 assert(!discovery_is_atomic(), "Error"); 608 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 609 while (iter.has_next()) { 610 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 611 HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj()); 612 oop next = java_lang_ref_Reference::next(iter.obj()); 613 if ((iter.referent() == NULL || iter.is_referent_alive() || 614 next != NULL)) { 615 assert(next->is_oop_or_null(), "bad next field"); 616 // Remove Reference object from list 617 iter.remove(); 618 // Trace the cohorts 619 iter.make_referent_alive(); 620 if (UseCompressedOops) { 621 keep_alive->do_oop((narrowOop*)next_addr); 622 } else { 623 keep_alive->do_oop((oop*)next_addr); 624 } 625 iter.move_to_next(); 626 } else { 627 iter.next(); 628 } 629 } 630 // Now close the newly reachable set 631 complete_gc->do_void(); 632 NOT_PRODUCT( 633 if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) { 634 gclog_or_tty->print_cr(" Dropped %d active Refs out of %d " 635 "Refs in discovered list " INTPTR_FORMAT, 636 iter.removed(), iter.processed(), (address)refs_list.head()); 637 } 638 ) 639 } 640 641 // Traverse the list and process the referents, by either 642 // clearing them or keeping them (and their reachable 643 // closure) alive. 644 void 645 ReferenceProcessor::process_phase3(DiscoveredList& refs_list, 646 bool clear_referent, 647 BoolObjectClosure* is_alive, 648 OopClosure* keep_alive, 649 VoidClosure* complete_gc) { 650 ResourceMark rm; 651 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 652 while (iter.has_next()) { 653 iter.update_discovered(); 654 iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */)); 655 if (clear_referent) { 656 // NULL out referent pointer 657 iter.clear_referent(); 658 } else { 659 // keep the referent around 660 iter.make_referent_alive(); 661 } 662 if (TraceReferenceGC) { 663 gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending", 664 clear_referent ? "cleared " : "", 665 iter.obj(), iter.obj()->blueprint()->internal_name()); 666 } 667 assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference"); 668 iter.next(); 669 } 670 // Remember to update the next pointer of the last ref. 671 iter.update_discovered(); 672 // Close the reachable set 673 complete_gc->do_void(); 674 } 675 676 void 677 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) { 678 oop obj = NULL; 679 oop next = refs_list.head(); 680 while (next != obj) { 681 obj = next; 682 next = java_lang_ref_Reference::discovered(obj); 683 java_lang_ref_Reference::set_discovered_raw(obj, NULL); 684 } 685 refs_list.set_head(NULL); 686 refs_list.set_length(0); 687 } 688 689 void 690 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) { 691 clear_discovered_references(refs_list); 692 } 693 694 void ReferenceProcessor::abandon_partial_discovery() { 695 // loop over the lists 696 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 697 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) { 698 gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i)); 699 } 700 abandon_partial_discovered_list(_discovered_refs[i]); 701 } 702 } 703 704 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask { 705 public: 706 RefProcPhase1Task(ReferenceProcessor& ref_processor, 707 DiscoveredList refs_lists[], 708 ReferencePolicy* policy, 709 bool marks_oops_alive) 710 : ProcessTask(ref_processor, refs_lists, marks_oops_alive), 711 _policy(policy) 712 { } 713 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 714 OopClosure& keep_alive, 715 VoidClosure& complete_gc) 716 { 717 Thread* thr = Thread::current(); 718 int refs_list_index = ((WorkerThread*)thr)->id(); 719 _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy, 720 &is_alive, &keep_alive, &complete_gc); 721 } 722 private: 723 ReferencePolicy* _policy; 724 }; 725 726 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask { 727 public: 728 RefProcPhase2Task(ReferenceProcessor& ref_processor, 729 DiscoveredList refs_lists[], 730 bool marks_oops_alive) 731 : ProcessTask(ref_processor, refs_lists, marks_oops_alive) 732 { } 733 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 734 OopClosure& keep_alive, 735 VoidClosure& complete_gc) 736 { 737 _ref_processor.process_phase2(_refs_lists[i], 738 &is_alive, &keep_alive, &complete_gc); 739 } 740 }; 741 742 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask { 743 public: 744 RefProcPhase3Task(ReferenceProcessor& ref_processor, 745 DiscoveredList refs_lists[], 746 bool clear_referent, 747 bool marks_oops_alive) 748 : ProcessTask(ref_processor, refs_lists, marks_oops_alive), 749 _clear_referent(clear_referent) 750 { } 751 virtual void work(unsigned int i, BoolObjectClosure& is_alive, 752 OopClosure& keep_alive, 753 VoidClosure& complete_gc) 754 { 755 // Don't use "refs_list_index" calculated in this way because 756 // balance_queues() has moved the Ref's into the first n queues. 757 // Thread* thr = Thread::current(); 758 // int refs_list_index = ((WorkerThread*)thr)->id(); 759 // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent, 760 _ref_processor.process_phase3(_refs_lists[i], _clear_referent, 761 &is_alive, &keep_alive, &complete_gc); 762 } 763 private: 764 bool _clear_referent; 765 }; 766 767 void ReferenceProcessor::set_discovered(oop ref, oop value) { 768 if (_discovered_list_needs_barrier) { 769 java_lang_ref_Reference::set_discovered(ref, value); 770 } else { 771 java_lang_ref_Reference::set_discovered_raw(ref, value); 772 } 773 } 774 775 // Balances reference queues. 776 // Move entries from all queues[0, 1, ..., _max_num_q-1] to 777 // queues[0, 1, ..., _num_q-1] because only the first _num_q 778 // corresponding to the active workers will be processed. 779 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[]) 780 { 781 // calculate total length 782 size_t total_refs = 0; 783 if (TraceReferenceGC && PrintGCDetails) { 784 gclog_or_tty->print_cr("\nBalance ref_lists "); 785 } 786 787 for (int i = 0; i < _max_num_q; ++i) { 788 total_refs += ref_lists[i].length(); 789 if (TraceReferenceGC && PrintGCDetails) { 790 gclog_or_tty->print("%d ", ref_lists[i].length()); 791 } 792 } 793 if (TraceReferenceGC && PrintGCDetails) { 794 gclog_or_tty->print_cr(" = %d", total_refs); 795 } 796 size_t avg_refs = total_refs / _num_q + 1; 797 int to_idx = 0; 798 for (int from_idx = 0; from_idx < _max_num_q; from_idx++) { 799 bool move_all = false; 800 if (from_idx >= _num_q) { 801 move_all = ref_lists[from_idx].length() > 0; 802 } 803 while ((ref_lists[from_idx].length() > avg_refs) || 804 move_all) { 805 assert(to_idx < _num_q, "Sanity Check!"); 806 if (ref_lists[to_idx].length() < avg_refs) { 807 // move superfluous refs 808 size_t refs_to_move; 809 // Move all the Ref's if the from queue will not be processed. 810 if (move_all) { 811 refs_to_move = MIN2(ref_lists[from_idx].length(), 812 avg_refs - ref_lists[to_idx].length()); 813 } else { 814 refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs, 815 avg_refs - ref_lists[to_idx].length()); 816 } 817 818 assert(refs_to_move > 0, "otherwise the code below will fail"); 819 820 oop move_head = ref_lists[from_idx].head(); 821 oop move_tail = move_head; 822 oop new_head = move_head; 823 // find an element to split the list on 824 for (size_t j = 0; j < refs_to_move; ++j) { 825 move_tail = new_head; 826 new_head = java_lang_ref_Reference::discovered(new_head); 827 } 828 829 // Add the chain to the to list. 830 if (ref_lists[to_idx].head() == NULL) { 831 // to list is empty. Make a loop at the end. 832 set_discovered(move_tail, move_tail); 833 } else { 834 set_discovered(move_tail, ref_lists[to_idx].head()); 835 } 836 ref_lists[to_idx].set_head(move_head); 837 ref_lists[to_idx].inc_length(refs_to_move); 838 839 // Remove the chain from the from list. 840 if (move_tail == new_head) { 841 // We found the end of the from list. 842 ref_lists[from_idx].set_head(NULL); 843 } else { 844 ref_lists[from_idx].set_head(new_head); 845 } 846 ref_lists[from_idx].dec_length(refs_to_move); 847 if (ref_lists[from_idx].length() == 0) { 848 break; 849 } 850 } else { 851 to_idx = (to_idx + 1) % _num_q; 852 } 853 } 854 } 855 #ifdef ASSERT 856 size_t balanced_total_refs = 0; 857 for (int i = 0; i < _max_num_q; ++i) { 858 balanced_total_refs += ref_lists[i].length(); 859 if (TraceReferenceGC && PrintGCDetails) { 860 gclog_or_tty->print("%d ", ref_lists[i].length()); 861 } 862 } 863 if (TraceReferenceGC && PrintGCDetails) { 864 gclog_or_tty->print_cr(" = %d", balanced_total_refs); 865 gclog_or_tty->flush(); 866 } 867 assert(total_refs == balanced_total_refs, "Balancing was incomplete"); 868 #endif 869 } 870 871 void ReferenceProcessor::balance_all_queues() { 872 balance_queues(_discoveredSoftRefs); 873 balance_queues(_discoveredWeakRefs); 874 balance_queues(_discoveredFinalRefs); 875 balance_queues(_discoveredPhantomRefs); 876 } 877 878 void 879 ReferenceProcessor::process_discovered_reflist( 880 DiscoveredList refs_lists[], 881 ReferencePolicy* policy, 882 bool clear_referent, 883 BoolObjectClosure* is_alive, 884 OopClosure* keep_alive, 885 VoidClosure* complete_gc, 886 AbstractRefProcTaskExecutor* task_executor) 887 { 888 bool mt_processing = task_executor != NULL && _processing_is_mt; 889 // If discovery used MT and a dynamic number of GC threads, then 890 // the queues must be balanced for correctness if fewer than the 891 // maximum number of queues were used. The number of queue used 892 // during discovery may be different than the number to be used 893 // for processing so don't depend of _num_q < _max_num_q as part 894 // of the test. 895 bool must_balance = _discovery_is_mt; 896 897 if ((mt_processing && ParallelRefProcBalancingEnabled) || 898 must_balance) { 899 balance_queues(refs_lists); 900 } 901 if (PrintReferenceGC && PrintGCDetails) { 902 size_t total = 0; 903 for (int i = 0; i < _max_num_q; ++i) { 904 total += refs_lists[i].length(); 905 } 906 gclog_or_tty->print(", %u refs", total); 907 } 908 909 // Phase 1 (soft refs only): 910 // . Traverse the list and remove any SoftReferences whose 911 // referents are not alive, but that should be kept alive for 912 // policy reasons. Keep alive the transitive closure of all 913 // such referents. 914 if (policy != NULL) { 915 if (mt_processing) { 916 RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/); 917 task_executor->execute(phase1); 918 } else { 919 for (int i = 0; i < _max_num_q; i++) { 920 process_phase1(refs_lists[i], policy, 921 is_alive, keep_alive, complete_gc); 922 } 923 } 924 } else { // policy == NULL 925 assert(refs_lists != _discoveredSoftRefs, 926 "Policy must be specified for soft references."); 927 } 928 929 // Phase 2: 930 // . Traverse the list and remove any refs whose referents are alive. 931 if (mt_processing) { 932 RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/); 933 task_executor->execute(phase2); 934 } else { 935 for (int i = 0; i < _max_num_q; i++) { 936 process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc); 937 } 938 } 939 940 // Phase 3: 941 // . Traverse the list and process referents as appropriate. 942 if (mt_processing) { 943 RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/); 944 task_executor->execute(phase3); 945 } else { 946 for (int i = 0; i < _max_num_q; i++) { 947 process_phase3(refs_lists[i], clear_referent, 948 is_alive, keep_alive, complete_gc); 949 } 950 } 951 } 952 953 void ReferenceProcessor::clean_up_discovered_references() { 954 // loop over the lists 955 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 956 if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) { 957 gclog_or_tty->print_cr( 958 "\nScrubbing %s discovered list of Null referents", 959 list_name(i)); 960 } 961 clean_up_discovered_reflist(_discovered_refs[i]); 962 } 963 } 964 965 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) { 966 assert(!discovery_is_atomic(), "Else why call this method?"); 967 DiscoveredListIterator iter(refs_list, NULL, NULL); 968 while (iter.has_next()) { 969 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 970 oop next = java_lang_ref_Reference::next(iter.obj()); 971 assert(next->is_oop_or_null(), "bad next field"); 972 // If referent has been cleared or Reference is not active, 973 // drop it. 974 if (iter.referent() == NULL || next != NULL) { 975 debug_only( 976 if (PrintGCDetails && TraceReferenceGC) { 977 gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: " 978 INTPTR_FORMAT " with next field: " INTPTR_FORMAT 979 " and referent: " INTPTR_FORMAT, 980 iter.obj(), next, iter.referent()); 981 } 982 ) 983 // Remove Reference object from list 984 iter.remove(); 985 iter.move_to_next(); 986 } else { 987 iter.next(); 988 } 989 } 990 NOT_PRODUCT( 991 if (PrintGCDetails && TraceReferenceGC) { 992 gclog_or_tty->print( 993 " Removed %d Refs with NULL referents out of %d discovered Refs", 994 iter.removed(), iter.processed()); 995 } 996 ) 997 } 998 999 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) { 1000 int id = 0; 1001 // Determine the queue index to use for this object. 1002 if (_discovery_is_mt) { 1003 // During a multi-threaded discovery phase, 1004 // each thread saves to its "own" list. 1005 Thread* thr = Thread::current(); 1006 id = thr->as_Worker_thread()->id(); 1007 } else { 1008 // single-threaded discovery, we save in round-robin 1009 // fashion to each of the lists. 1010 if (_processing_is_mt) { 1011 id = next_id(); 1012 } 1013 } 1014 assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)"); 1015 1016 // Get the discovered queue to which we will add 1017 DiscoveredList* list = NULL; 1018 switch (rt) { 1019 case REF_OTHER: 1020 // Unknown reference type, no special treatment 1021 break; 1022 case REF_SOFT: 1023 list = &_discoveredSoftRefs[id]; 1024 break; 1025 case REF_WEAK: 1026 list = &_discoveredWeakRefs[id]; 1027 break; 1028 case REF_FINAL: 1029 list = &_discoveredFinalRefs[id]; 1030 break; 1031 case REF_PHANTOM: 1032 list = &_discoveredPhantomRefs[id]; 1033 break; 1034 case REF_NONE: 1035 // we should not reach here if we are an instanceRefKlass 1036 default: 1037 ShouldNotReachHere(); 1038 } 1039 if (TraceReferenceGC && PrintGCDetails) { 1040 gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list); 1041 } 1042 return list; 1043 } 1044 1045 inline void 1046 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list, 1047 oop obj, 1048 HeapWord* discovered_addr) { 1049 assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller"); 1050 // First we must make sure this object is only enqueued once. CAS in a non null 1051 // discovered_addr. 1052 oop current_head = refs_list.head(); 1053 // The last ref must have its discovered field pointing to itself. 1054 oop next_discovered = (current_head != NULL) ? current_head : obj; 1055 1056 // Note: In the case of G1, this specific pre-barrier is strictly 1057 // not necessary because the only case we are interested in 1058 // here is when *discovered_addr is NULL (see the CAS further below), 1059 // so this will expand to nothing. As a result, we have manually 1060 // elided this out for G1, but left in the test for some future 1061 // collector that might have need for a pre-barrier here, e.g.:- 1062 // _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered); 1063 assert(!_discovered_list_needs_barrier || UseG1GC, 1064 "Need to check non-G1 collector: " 1065 "may need a pre-write-barrier for CAS from NULL below"); 1066 oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr, 1067 NULL); 1068 if (retest == NULL) { 1069 // This thread just won the right to enqueue the object. 1070 // We have separate lists for enqueueing, so no synchronization 1071 // is necessary. 1072 refs_list.set_head(obj); 1073 refs_list.inc_length(1); 1074 if (_discovered_list_needs_barrier) { 1075 _bs->write_ref_field((void*)discovered_addr, next_discovered); 1076 } 1077 1078 if (TraceReferenceGC) { 1079 gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)", 1080 obj, obj->blueprint()->internal_name()); 1081 } 1082 } else { 1083 // If retest was non NULL, another thread beat us to it: 1084 // The reference has already been discovered... 1085 if (TraceReferenceGC) { 1086 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)", 1087 obj, obj->blueprint()->internal_name()); 1088 } 1089 } 1090 } 1091 1092 #ifndef PRODUCT 1093 // Non-atomic (i.e. concurrent) discovery might allow us 1094 // to observe j.l.References with NULL referents, being those 1095 // cleared concurrently by mutators during (or after) discovery. 1096 void ReferenceProcessor::verify_referent(oop obj) { 1097 bool da = discovery_is_atomic(); 1098 oop referent = java_lang_ref_Reference::referent(obj); 1099 assert(da ? referent->is_oop() : referent->is_oop_or_null(), 1100 err_msg("Bad referent " INTPTR_FORMAT " found in Reference " 1101 INTPTR_FORMAT " during %satomic discovery ", 1102 (intptr_t)referent, (intptr_t)obj, da ? "" : "non-")); 1103 } 1104 #endif 1105 1106 // We mention two of several possible choices here: 1107 // #0: if the reference object is not in the "originating generation" 1108 // (or part of the heap being collected, indicated by our "span" 1109 // we don't treat it specially (i.e. we scan it as we would 1110 // a normal oop, treating its references as strong references). 1111 // This means that references can't be discovered unless their 1112 // referent is also in the same span. This is the simplest, 1113 // most "local" and most conservative approach, albeit one 1114 // that may cause weak references to be enqueued least promptly. 1115 // We call this choice the "ReferenceBasedDiscovery" policy. 1116 // #1: the reference object may be in any generation (span), but if 1117 // the referent is in the generation (span) being currently collected 1118 // then we can discover the reference object, provided 1119 // the object has not already been discovered by 1120 // a different concurrently running collector (as may be the 1121 // case, for instance, if the reference object is in CMS and 1122 // the referent in DefNewGeneration), and provided the processing 1123 // of this reference object by the current collector will 1124 // appear atomic to every other collector in the system. 1125 // (Thus, for instance, a concurrent collector may not 1126 // discover references in other generations even if the 1127 // referent is in its own generation). This policy may, 1128 // in certain cases, enqueue references somewhat sooner than 1129 // might Policy #0 above, but at marginally increased cost 1130 // and complexity in processing these references. 1131 // We call this choice the "RefeferentBasedDiscovery" policy. 1132 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) { 1133 // Make sure we are discovering refs (rather than processing discovered refs). 1134 if (!_discovering_refs || !RegisterReferences) { 1135 return false; 1136 } 1137 // We only discover active references. 1138 oop next = java_lang_ref_Reference::next(obj); 1139 if (next != NULL) { // Ref is no longer active 1140 return false; 1141 } 1142 1143 HeapWord* obj_addr = (HeapWord*)obj; 1144 if (RefDiscoveryPolicy == ReferenceBasedDiscovery && 1145 !_span.contains(obj_addr)) { 1146 // Reference is not in the originating generation; 1147 // don't treat it specially (i.e. we want to scan it as a normal 1148 // object with strong references). 1149 return false; 1150 } 1151 1152 // We only discover references whose referents are not (yet) 1153 // known to be strongly reachable. 1154 if (is_alive_non_header() != NULL) { 1155 verify_referent(obj); 1156 if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) { 1157 return false; // referent is reachable 1158 } 1159 } 1160 if (rt == REF_SOFT) { 1161 // For soft refs we can decide now if these are not 1162 // current candidates for clearing, in which case we 1163 // can mark through them now, rather than delaying that 1164 // to the reference-processing phase. Since all current 1165 // time-stamp policies advance the soft-ref clock only 1166 // at a major collection cycle, this is always currently 1167 // accurate. 1168 if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) { 1169 return false; 1170 } 1171 } 1172 1173 ResourceMark rm; // Needed for tracing. 1174 1175 HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj); 1176 const oop discovered = java_lang_ref_Reference::discovered(obj); 1177 assert(discovered->is_oop_or_null(), "bad discovered field"); 1178 if (discovered != NULL) { 1179 // The reference has already been discovered... 1180 if (TraceReferenceGC) { 1181 gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)", 1182 obj, obj->blueprint()->internal_name()); 1183 } 1184 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1185 // assumes that an object is not processed twice; 1186 // if it's been already discovered it must be on another 1187 // generation's discovered list; so we won't discover it. 1188 return false; 1189 } else { 1190 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery, 1191 "Unrecognized policy"); 1192 // Check assumption that an object is not potentially 1193 // discovered twice except by concurrent collectors that potentially 1194 // trace the same Reference object twice. 1195 assert(UseConcMarkSweepGC || UseG1GC, 1196 "Only possible with a concurrent marking collector"); 1197 return true; 1198 } 1199 } 1200 1201 if (RefDiscoveryPolicy == ReferentBasedDiscovery) { 1202 verify_referent(obj); 1203 // Discover if and only if EITHER: 1204 // .. reference is in our span, OR 1205 // .. we are an atomic collector and referent is in our span 1206 if (_span.contains(obj_addr) || 1207 (discovery_is_atomic() && 1208 _span.contains(java_lang_ref_Reference::referent(obj)))) { 1209 // should_enqueue = true; 1210 } else { 1211 return false; 1212 } 1213 } else { 1214 assert(RefDiscoveryPolicy == ReferenceBasedDiscovery && 1215 _span.contains(obj_addr), "code inconsistency"); 1216 } 1217 1218 // Get the right type of discovered queue head. 1219 DiscoveredList* list = get_discovered_list(rt); 1220 if (list == NULL) { 1221 return false; // nothing special needs to be done 1222 } 1223 1224 if (_discovery_is_mt) { 1225 add_to_discovered_list_mt(*list, obj, discovered_addr); 1226 } else { 1227 // If "_discovered_list_needs_barrier", we do write barriers when 1228 // updating the discovered reference list. Otherwise, we do a raw store 1229 // here: the field will be visited later when processing the discovered 1230 // references. 1231 oop current_head = list->head(); 1232 // The last ref must have its discovered field pointing to itself. 1233 oop next_discovered = (current_head != NULL) ? current_head : obj; 1234 1235 // As in the case further above, since we are over-writing a NULL 1236 // pre-value, we can safely elide the pre-barrier here for the case of G1. 1237 // e.g.:- _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered); 1238 assert(discovered == NULL, "control point invariant"); 1239 assert(!_discovered_list_needs_barrier || UseG1GC, 1240 "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below"); 1241 oop_store_raw(discovered_addr, next_discovered); 1242 if (_discovered_list_needs_barrier) { 1243 _bs->write_ref_field((void*)discovered_addr, next_discovered); 1244 } 1245 list->set_head(obj); 1246 list->inc_length(1); 1247 1248 if (TraceReferenceGC) { 1249 gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)", 1250 obj, obj->blueprint()->internal_name()); 1251 } 1252 } 1253 assert(obj->is_oop(), "Discovered a bad reference"); 1254 verify_referent(obj); 1255 return true; 1256 } 1257 1258 // Preclean the discovered references by removing those 1259 // whose referents are alive, and by marking from those that 1260 // are not active. These lists can be handled here 1261 // in any order and, indeed, concurrently. 1262 void ReferenceProcessor::preclean_discovered_references( 1263 BoolObjectClosure* is_alive, 1264 OopClosure* keep_alive, 1265 VoidClosure* complete_gc, 1266 YieldClosure* yield, 1267 bool should_unload_classes) { 1268 1269 NOT_PRODUCT(verify_ok_to_handle_reflists()); 1270 1271 #ifdef ASSERT 1272 bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC || 1273 CMSClassUnloadingEnabled && UseConcMarkSweepGC || 1274 ExplicitGCInvokesConcurrentAndUnloadsClasses && 1275 UseConcMarkSweepGC && should_unload_classes; 1276 RememberKlassesChecker mx(must_remember_klasses); 1277 #endif 1278 // Soft references 1279 { 1280 TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC, 1281 false, gclog_or_tty); 1282 for (int i = 0; i < _max_num_q; i++) { 1283 if (yield->should_return()) { 1284 return; 1285 } 1286 preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive, 1287 keep_alive, complete_gc, yield); 1288 } 1289 } 1290 1291 // Weak references 1292 { 1293 TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC, 1294 false, gclog_or_tty); 1295 for (int i = 0; i < _max_num_q; i++) { 1296 if (yield->should_return()) { 1297 return; 1298 } 1299 preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive, 1300 keep_alive, complete_gc, yield); 1301 } 1302 } 1303 1304 // Final references 1305 { 1306 TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC, 1307 false, gclog_or_tty); 1308 for (int i = 0; i < _max_num_q; i++) { 1309 if (yield->should_return()) { 1310 return; 1311 } 1312 preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive, 1313 keep_alive, complete_gc, yield); 1314 } 1315 } 1316 1317 // Phantom references 1318 { 1319 TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC, 1320 false, gclog_or_tty); 1321 for (int i = 0; i < _max_num_q; i++) { 1322 if (yield->should_return()) { 1323 return; 1324 } 1325 preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive, 1326 keep_alive, complete_gc, yield); 1327 } 1328 } 1329 } 1330 1331 // Walk the given discovered ref list, and remove all reference objects 1332 // whose referents are still alive, whose referents are NULL or which 1333 // are not active (have a non-NULL next field). NOTE: When we are 1334 // thus precleaning the ref lists (which happens single-threaded today), 1335 // we do not disable refs discovery to honour the correct semantics of 1336 // java.lang.Reference. As a result, we need to be careful below 1337 // that ref removal steps interleave safely with ref discovery steps 1338 // (in this thread). 1339 void 1340 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList& refs_list, 1341 BoolObjectClosure* is_alive, 1342 OopClosure* keep_alive, 1343 VoidClosure* complete_gc, 1344 YieldClosure* yield) { 1345 DiscoveredListIterator iter(refs_list, keep_alive, is_alive); 1346 while (iter.has_next()) { 1347 iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */)); 1348 oop obj = iter.obj(); 1349 oop next = java_lang_ref_Reference::next(obj); 1350 if (iter.referent() == NULL || iter.is_referent_alive() || 1351 next != NULL) { 1352 // The referent has been cleared, or is alive, or the Reference is not 1353 // active; we need to trace and mark its cohort. 1354 if (TraceReferenceGC) { 1355 gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)", 1356 iter.obj(), iter.obj()->blueprint()->internal_name()); 1357 } 1358 // Remove Reference object from list 1359 iter.remove(); 1360 // Keep alive its cohort. 1361 iter.make_referent_alive(); 1362 if (UseCompressedOops) { 1363 narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj); 1364 keep_alive->do_oop(next_addr); 1365 } else { 1366 oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj); 1367 keep_alive->do_oop(next_addr); 1368 } 1369 iter.move_to_next(); 1370 } else { 1371 iter.next(); 1372 } 1373 } 1374 // Close the reachable set 1375 complete_gc->do_void(); 1376 1377 NOT_PRODUCT( 1378 if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) { 1379 gclog_or_tty->print_cr(" Dropped %d Refs out of %d " 1380 "Refs in discovered list " INTPTR_FORMAT, 1381 iter.removed(), iter.processed(), (address)refs_list.head()); 1382 } 1383 ) 1384 } 1385 1386 const char* ReferenceProcessor::list_name(int i) { 1387 assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(), 1388 "Out of bounds index"); 1389 1390 int j = i / _max_num_q; 1391 switch (j) { 1392 case 0: return "SoftRef"; 1393 case 1: return "WeakRef"; 1394 case 2: return "FinalRef"; 1395 case 3: return "PhantomRef"; 1396 } 1397 ShouldNotReachHere(); 1398 return NULL; 1399 } 1400 1401 #ifndef PRODUCT 1402 void ReferenceProcessor::verify_ok_to_handle_reflists() { 1403 // empty for now 1404 } 1405 #endif 1406 1407 #ifndef PRODUCT 1408 void ReferenceProcessor::clear_discovered_references() { 1409 guarantee(!_discovering_refs, "Discovering refs?"); 1410 for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) { 1411 clear_discovered_references(_discovered_refs[i]); 1412 } 1413 } 1414 1415 #endif // PRODUCT