1 /*
   2  * Copyright (c) 2001, 2014, 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 #if INCLUDE_JFR
  38 #include "jfr/jfr.hpp"
  39 #endif // INCLUDE_JFR
  40 
  41 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
  42 
  43 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  44 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  45 bool             ReferenceProcessor::_pending_list_uses_discovered_field = false;
  46 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;
  47 
  48 void referenceProcessor_init() {
  49   ReferenceProcessor::init_statics();
  50 }
  51 
  52 void ReferenceProcessor::init_statics() {
  53   // We need a monotonically non-deccreasing time in ms but
  54   // os::javaTimeMillis() does not guarantee monotonicity.
  55   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  56 
  57   // Initialize the soft ref timestamp clock.
  58   _soft_ref_timestamp_clock = now;
  59   // Also update the soft ref clock in j.l.r.SoftReference
  60   java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
  61 
  62   _always_clear_soft_ref_policy = new AlwaysClearPolicy();
  63   _default_soft_ref_policy      = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
  64                                       NOT_COMPILER2(LRUCurrentHeapPolicy());
  65   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  66     vm_exit_during_initialization("Could not allocate reference policy object");
  67   }
  68   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
  69             RefDiscoveryPolicy == ReferentBasedDiscovery,
  70             "Unrecongnized RefDiscoveryPolicy");
  71   _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
  72 }
  73 
  74 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
  75 #ifdef ASSERT
  76   // Verify that we're not currently discovering refs
  77   assert(!verify_disabled || !_discovering_refs, "nested call?");
  78 
  79   if (check_no_refs) {
  80     // Verify that the discovered lists are empty
  81     verify_no_references_recorded();
  82   }
  83 #endif // ASSERT
  84 
  85   // Someone could have modified the value of the static
  86   // field in the j.l.r.SoftReference class that holds the
  87   // soft reference timestamp clock using reflection or
  88   // Unsafe between GCs. Unconditionally update the static
  89   // field in ReferenceProcessor here so that we use the new
  90   // value during reference discovery.
  91 
  92   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
  93   _discovering_refs = true;
  94 }
  95 
  96 ReferenceProcessor::ReferenceProcessor(MemRegion span,
  97                                        bool      mt_processing,
  98                                        uint      mt_processing_degree,
  99                                        bool      mt_discovery,
 100                                        uint      mt_discovery_degree,
 101                                        bool      atomic_discovery,
 102                                        BoolObjectClosure* is_alive_non_header)  :
 103   _discovering_refs(false),
 104   _enqueuing_is_done(false),
 105   _is_alive_non_header(is_alive_non_header),
 106   _processing_is_mt(mt_processing),
 107   _next_id(0)
 108 {
 109   _span = span;
 110   _discovery_is_atomic = atomic_discovery;
 111   _discovery_is_mt     = mt_discovery;
 112   _num_q               = MAX2(1U, mt_processing_degree);
 113   _max_num_q           = MAX2(_num_q, mt_discovery_degree);
 114   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,
 115             _max_num_q * number_of_subclasses_of_ref(), mtGC);
 116 
 117   if (_discovered_refs == NULL) {
 118     vm_exit_during_initialization("Could not allocated RefProc Array");
 119   }
 120   _discoveredSoftRefs    = &_discovered_refs[0];
 121   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];
 122   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];
 123   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
 124   _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q];
 125 
 126   // Initialize all entries to NULL
 127   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 128     _discovered_refs[i].set_head(NULL);
 129     _discovered_refs[i].set_length(0);
 130   }
 131 
 132   setup_policy(false /* default soft ref policy */);
 133 }
 134 
 135 #ifndef PRODUCT
 136 void ReferenceProcessor::verify_no_references_recorded() {
 137   guarantee(!_discovering_refs, "Discovering refs?");
 138   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 139     guarantee(_discovered_refs[i].is_empty(),
 140               "Found non-empty discovered list");
 141   }
 142 }
 143 #endif
 144 
 145 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 146   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 147     if (UseCompressedOops) {
 148       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
 149     } else {
 150       f->do_oop((oop*)_discovered_refs[i].adr_head());
 151     }
 152   }
 153 }
 154 
 155 void ReferenceProcessor::update_soft_ref_master_clock() {
 156   // Update (advance) the soft ref master clock field. This must be done
 157   // after processing the soft ref list.
 158 
 159   // We need a monotonically non-deccreasing time in ms but
 160   // os::javaTimeMillis() does not guarantee monotonicity.
 161   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 162   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
 163   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
 164 
 165   NOT_PRODUCT(
 166   if (now < _soft_ref_timestamp_clock) {
 167     warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
 168             _soft_ref_timestamp_clock, now);
 169   }
 170   )
 171   // The values of now and _soft_ref_timestamp_clock are set using
 172   // javaTimeNanos(), which is guaranteed to be monotonically
 173   // non-decreasing provided the underlying platform provides such
 174   // a time source (and it is bug free).
 175   // In product mode, however, protect ourselves from non-monotonicty.
 176   if (now > _soft_ref_timestamp_clock) {
 177     _soft_ref_timestamp_clock = now;
 178     java_lang_ref_SoftReference::set_clock(now);
 179   }
 180   // Else leave clock stalled at its old value until time progresses
 181   // past clock value.
 182 }
 183 
 184 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
 185   size_t total = 0;
 186   for (uint i = 0; i < _max_num_q; ++i) {
 187     total += lists[i].length();
 188   }
 189   return total;
 190 }
 191 
 192 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 193   BoolObjectClosure*           is_alive,
 194   OopClosure*                  keep_alive,
 195   VoidClosure*                 complete_gc,
 196   AbstractRefProcTaskExecutor* task_executor,
 197   GCTimer*                     gc_timer,
 198   GCId                         gc_id) {
 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, gc_id);
 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, gc_id);
 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, gc_id);
 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, gc_id);
 250     phantom_count =
 251       process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 252                                  is_alive, keep_alive, complete_gc, task_executor);
 253 
 254     // Process cleaners, but include them in phantom statistics.  We expect
 255     // Cleaner references to be temporary, and don't want to deal with
 256     // possible incompatibilities arising from making it more visible.
 257     phantom_count +=
 258       process_discovered_reflist(_discoveredCleanerRefs, NULL, true,
 259                                  is_alive, keep_alive, complete_gc, task_executor);
 260   }
 261 
 262   // Weak global JNI references. It would make more sense (semantically) to
 263   // traverse these simultaneously with the regular weak references above, but
 264   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 265   // thus use JNI weak references to circumvent the phantom references and
 266   // resurrect a "post-mortem" object.
 267   {
 268     GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer, gc_id);
 269     if (task_executor != NULL) {
 270       task_executor->set_single_threaded_mode();
 271     }
 272     process_phaseJNI(is_alive, keep_alive, complete_gc);
 273   }
 274 
 275   return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
 276 }
 277 
 278 #ifndef PRODUCT
 279 // Calculate the number of jni handles.
 280 uint ReferenceProcessor::count_jni_refs() {
 281   class CountHandleClosure: public OopClosure {
 282   private:
 283     int _count;
 284   public:
 285     CountHandleClosure(): _count(0) {}
 286     void do_oop(oop* unused)       { _count++; }
 287     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 288     int count() { return _count; }
 289   };
 290   CountHandleClosure global_handle_count;
 291   JNIHandles::weak_oops_do(&global_handle_count);
 292   return global_handle_count.count();
 293 }
 294 #endif
 295 
 296 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 297                                           OopClosure*        keep_alive,
 298                                           VoidClosure*       complete_gc) {
 299 #ifndef PRODUCT
 300   if (PrintGCDetails && PrintReferenceGC) {
 301     unsigned int count = count_jni_refs();
 302     gclog_or_tty->print(", %u refs", count);
 303   }
 304 #endif
 305   JNIHandles::weak_oops_do(is_alive, keep_alive);
 306   JFR_ONLY(Jfr::weak_oops_do(is_alive, keep_alive));
 307   complete_gc->do_void();
 308 }
 309 
 310 
 311 template <class T>
 312 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 313                                    AbstractRefProcTaskExecutor* task_executor) {
 314 
 315   // Remember old value of pending references list
 316   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
 317   T old_pending_list_value = *pending_list_addr;
 318 
 319   // Enqueue references that are not made active again, and
 320   // clear the decks for the next collection (cycle).
 321   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
 322   // Do the post-barrier on pending_list_addr missed in
 323   // enqueue_discovered_reflist.
 324   oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
 325 
 326   // Stop treating discovered references specially.
 327   ref->disable_discovery();
 328 
 329   // Return true if new pending references were added
 330   return old_pending_list_value != *pending_list_addr;
 331 }
 332 
 333 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
 334   NOT_PRODUCT(verify_ok_to_handle_reflists());
 335   if (UseCompressedOops) {
 336     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 337   } else {
 338     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 339   }
 340 }
 341 
 342 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 343                                                     HeapWord* pending_list_addr) {
 344   // Given a list of refs linked through the "discovered" field
 345   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 346   // thus distinguishing them from active References, then
 347   // prepend them to the pending list.
 348   //
 349   // The Java threads will see the Reference objects linked together through
 350   // the discovered field. Instead of trying to do the write barrier updates
 351   // in all places in the reference processor where we manipulate the discovered
 352   // field we make sure to do the barrier here where we anyway iterate through
 353   // all linked Reference objects. Note that it is important to not dirty any
 354   // cards during reference processing since this will cause card table
 355   // verification to fail for G1.
 356   //
 357   // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
 358   // the "next" field is used to chain the pending list, not the discovered
 359   // field.
 360   if (TraceReferenceGC && PrintGCDetails) {
 361     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
 362                            INTPTR_FORMAT, (address)refs_list.head());
 363   }
 364 
 365   oop obj = NULL;
 366   oop next_d = refs_list.head();
 367   if (pending_list_uses_discovered_field()) { // New behavior
 368     // Walk down the list, self-looping the next field
 369     // so that the References are not considered active.
 370     while (obj != next_d) {
 371       obj = next_d;
 372       assert(obj->is_instanceRef(), "should be reference object");
 373       next_d = java_lang_ref_Reference::discovered(obj);
 374       if (TraceReferenceGC && PrintGCDetails) {
 375         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 376                                (void *)obj, (void *)next_d);
 377       }
 378       assert(java_lang_ref_Reference::next(obj) == NULL,
 379              "Reference not active; should not be discovered");
 380       // Self-loop next, so as to make Ref not active.
 381       java_lang_ref_Reference::set_next_raw(obj, obj);
 382       if (next_d != obj) {
 383         oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
 384       } else {
 385         // This is the last object.
 386         // Swap refs_list into pending_list_addr and
 387         // set obj's discovered to what we read from pending_list_addr.
 388         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 389         // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
 390         java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
 391         oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
 392       }
 393     }
 394   } else { // Old behaviour
 395     // Walk down the list, copying the discovered field into
 396     // the next field and clearing the discovered field.
 397     while (obj != next_d) {
 398       obj = next_d;
 399       assert(obj->is_instanceRef(), "should be reference object");
 400       next_d = java_lang_ref_Reference::discovered(obj);
 401       if (TraceReferenceGC && PrintGCDetails) {
 402         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 403                                (void *)obj, (void *)next_d);
 404       }
 405       assert(java_lang_ref_Reference::next(obj) == NULL,
 406              "The reference should not be enqueued");
 407       if (next_d == obj) {  // obj is last
 408         // Swap refs_list into pendling_list_addr and
 409         // set obj's next to what we read from pending_list_addr.
 410         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 411         // Need oop_check on pending_list_addr above;
 412         // see special oop-check code at the end of
 413         // enqueue_discovered_reflists() further below.
 414         if (old == NULL) {
 415           // obj should be made to point to itself, since
 416           // pending list was empty.
 417           java_lang_ref_Reference::set_next(obj, obj);
 418         } else {
 419           java_lang_ref_Reference::set_next(obj, old);
 420         }
 421       } else {
 422         java_lang_ref_Reference::set_next(obj, next_d);
 423       }
 424       java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
 425     }
 426   }
 427 }
 428 
 429 // Parallel enqueue task
 430 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 431 public:
 432   RefProcEnqueueTask(ReferenceProcessor& ref_processor,
 433                      DiscoveredList      discovered_refs[],
 434                      HeapWord*           pending_list_addr,
 435                      int                 n_queues)
 436     : EnqueueTask(ref_processor, discovered_refs,
 437                   pending_list_addr, n_queues)
 438   { }
 439 
 440   virtual void work(unsigned int work_id) {
 441     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 442     // Simplest first cut: static partitioning.
 443     int index = work_id;
 444     // The increment on "index" must correspond to the maximum number of queues
 445     // (n_queues) with which that ReferenceProcessor was created.  That
 446     // is because of the "clever" way the discovered references lists were
 447     // allocated and are indexed into.
 448     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 449     for (int j = 0;
 450          j < ReferenceProcessor::number_of_subclasses_of_ref();
 451          j++, index += _n_queues) {
 452       _ref_processor.enqueue_discovered_reflist(
 453         _refs_lists[index], _pending_list_addr);
 454       _refs_lists[index].set_head(NULL);
 455       _refs_lists[index].set_length(0);
 456     }
 457   }
 458 };
 459 
 460 // Enqueue references that are not made active again
 461 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
 462   AbstractRefProcTaskExecutor* task_executor) {
 463   if (_processing_is_mt && task_executor != NULL) {
 464     // Parallel code
 465     RefProcEnqueueTask tsk(*this, _discovered_refs,
 466                            pending_list_addr, _max_num_q);
 467     task_executor->execute(tsk);
 468   } else {
 469     // Serial code: call the parent class's implementation
 470     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 471       enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
 472       _discovered_refs[i].set_head(NULL);
 473       _discovered_refs[i].set_length(0);
 474     }
 475   }
 476 }
 477 
 478 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 479   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 480   oop discovered = java_lang_ref_Reference::discovered(_ref);
 481   assert(_discovered_addr && discovered->is_oop_or_null(),
 482          "discovered field is bad");
 483   _next = discovered;
 484   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 485   _referent = java_lang_ref_Reference::referent(_ref);
 486   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 487          "Wrong oop found in java.lang.Reference object");
 488   assert(allow_null_referent ?
 489              _referent->is_oop_or_null()
 490            : _referent->is_oop(),
 491          "bad referent");
 492 }
 493 
 494 void DiscoveredListIterator::remove() {
 495   assert(_ref->is_oop(), "Dropping a bad reference");
 496   oop_store_raw(_discovered_addr, NULL);
 497 
 498   // First _prev_next ref actually points into DiscoveredList (gross).
 499   oop new_next;
 500   if (_next == _ref) {
 501     // At the end of the list, we should make _prev point to itself.
 502     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 503     // and _prev will be NULL.
 504     new_next = _prev;
 505   } else {
 506     new_next = _next;
 507   }
 508   // Remove Reference object from discovered list. Note that G1 does not need a
 509   // pre-barrier here because we know the Reference has already been found/marked,
 510   // that's how it ended up in the discovered list in the first place.
 511   oop_store_raw(_prev_next, new_next);
 512   NOT_PRODUCT(_removed++);
 513   _refs_list.dec_length(1);
 514 }
 515 
 516 // Make the Reference object active again.
 517 void DiscoveredListIterator::make_active() {
 518   // The pre barrier for G1 is probably just needed for the old
 519   // reference processing behavior. Should we guard this with
 520   // ReferenceProcessor::pending_list_uses_discovered_field() ?
 521   if (UseG1GC) {
 522     HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
 523     if (UseCompressedOops) {
 524       oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
 525     } else {
 526       oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
 527     }
 528   }
 529   java_lang_ref_Reference::set_next_raw(_ref, NULL);
 530 }
 531 
 532 void DiscoveredListIterator::clear_referent() {
 533   oop_store_raw(_referent_addr, NULL);
 534 }
 535 
 536 // NOTE: process_phase*() are largely similar, and at a high level
 537 // merely iterate over the extant list applying a predicate to
 538 // each of its elements and possibly removing that element from the
 539 // list and applying some further closures to that element.
 540 // We should consider the possibility of replacing these
 541 // process_phase*() methods by abstracting them into
 542 // a single general iterator invocation that receives appropriate
 543 // closures that accomplish this work.
 544 
 545 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 546 // referents are not alive, but that should be kept alive for policy reasons.
 547 // Keep alive the transitive closure of all such referents.
 548 void
 549 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 550                                    ReferencePolicy*   policy,
 551                                    BoolObjectClosure* is_alive,
 552                                    OopClosure*        keep_alive,
 553                                    VoidClosure*       complete_gc) {
 554   assert(policy != NULL, "Must have a non-NULL policy");
 555   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 556   // Decide which softly reachable refs should be kept alive.
 557   while (iter.has_next()) {
 558     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 559     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 560     if (referent_is_dead &&
 561         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 562       if (TraceReferenceGC) {
 563         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 564                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 565       }
 566       // Remove Reference object from list
 567       iter.remove();
 568       // Make the Reference object active again
 569       iter.make_active();
 570       // keep the referent around
 571       iter.make_referent_alive();
 572       iter.move_to_next();
 573     } else {
 574       iter.next();
 575     }
 576   }
 577   // Close the reachable set
 578   complete_gc->do_void();
 579   NOT_PRODUCT(
 580     if (PrintGCDetails && TraceReferenceGC) {
 581       gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
 582         "discovered Refs by policy, from list " INTPTR_FORMAT,
 583         iter.removed(), iter.processed(), (address)refs_list.head());
 584     }
 585   )
 586 }
 587 
 588 // Traverse the list and remove any Refs that are not active, or
 589 // whose referents are either alive or NULL.
 590 void
 591 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 592                              BoolObjectClosure* is_alive,
 593                              OopClosure*        keep_alive) {
 594   assert(discovery_is_atomic(), "Error");
 595   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 596   while (iter.has_next()) {
 597     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 598     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 599     assert(next == NULL, "Should not discover inactive Reference");
 600     if (iter.is_referent_alive()) {
 601       if (TraceReferenceGC) {
 602         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 603                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 604       }
 605       // The referent is reachable after all.
 606       // Remove Reference object from list.
 607       iter.remove();
 608       // Update the referent pointer as necessary: Note that this
 609       // should not entail any recursive marking because the
 610       // referent must already have been traversed.
 611       iter.make_referent_alive();
 612       iter.move_to_next();
 613     } else {
 614       iter.next();
 615     }
 616   }
 617   NOT_PRODUCT(
 618     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 619       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 620         "Refs in discovered list " INTPTR_FORMAT,
 621         iter.removed(), iter.processed(), (address)refs_list.head());
 622     }
 623   )
 624 }
 625 
 626 void
 627 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 628                                                   BoolObjectClosure* is_alive,
 629                                                   OopClosure*        keep_alive,
 630                                                   VoidClosure*       complete_gc) {
 631   assert(!discovery_is_atomic(), "Error");
 632   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 633   while (iter.has_next()) {
 634     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 635     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 636     oop next = java_lang_ref_Reference::next(iter.obj());
 637     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 638          next != NULL)) {
 639       assert(next->is_oop_or_null(), "bad next field");
 640       // Remove Reference object from list
 641       iter.remove();
 642       // Trace the cohorts
 643       iter.make_referent_alive();
 644       if (UseCompressedOops) {
 645         keep_alive->do_oop((narrowOop*)next_addr);
 646       } else {
 647         keep_alive->do_oop((oop*)next_addr);
 648       }
 649       iter.move_to_next();
 650     } else {
 651       iter.next();
 652     }
 653   }
 654   // Now close the newly reachable set
 655   complete_gc->do_void();
 656   NOT_PRODUCT(
 657     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 658       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 659         "Refs in discovered list " INTPTR_FORMAT,
 660         iter.removed(), iter.processed(), (address)refs_list.head());
 661     }
 662   )
 663 }
 664 
 665 // Traverse the list and process the referents, by either
 666 // clearing them or keeping them (and their reachable
 667 // closure) alive.
 668 void
 669 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 670                                    bool               clear_referent,
 671                                    BoolObjectClosure* is_alive,
 672                                    OopClosure*        keep_alive,
 673                                    VoidClosure*       complete_gc) {
 674   ResourceMark rm;
 675   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 676   while (iter.has_next()) {
 677     iter.update_discovered();
 678     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 679     if (clear_referent) {
 680       // NULL out referent pointer
 681       iter.clear_referent();
 682     } else {
 683       // keep the referent around
 684       iter.make_referent_alive();
 685     }
 686     if (TraceReferenceGC) {
 687       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 688                              clear_referent ? "cleared " : "",
 689                              (void *)iter.obj(), iter.obj()->klass()->internal_name());
 690     }
 691     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 692     iter.next();
 693   }
 694   // Remember to update the next pointer of the last ref.
 695   iter.update_discovered();
 696   // Close the reachable set
 697   complete_gc->do_void();
 698 }
 699 
 700 void
 701 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 702   oop obj = NULL;
 703   oop next = refs_list.head();
 704   while (next != obj) {
 705     obj = next;
 706     next = java_lang_ref_Reference::discovered(obj);
 707     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 708   }
 709   refs_list.set_head(NULL);
 710   refs_list.set_length(0);
 711 }
 712 
 713 void
 714 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
 715   clear_discovered_references(refs_list);
 716 }
 717 
 718 void ReferenceProcessor::abandon_partial_discovery() {
 719   // loop over the lists
 720   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 721     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 722       gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
 723     }
 724     abandon_partial_discovered_list(_discovered_refs[i]);
 725   }
 726 }
 727 
 728 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 729 public:
 730   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 731                     DiscoveredList      refs_lists[],
 732                     ReferencePolicy*    policy,
 733                     bool                marks_oops_alive)
 734     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 735       _policy(policy)
 736   { }
 737   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 738                     OopClosure& keep_alive,
 739                     VoidClosure& complete_gc)
 740   {
 741     Thread* thr = Thread::current();
 742     int refs_list_index = ((WorkerThread*)thr)->id();
 743     _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
 744                                   &is_alive, &keep_alive, &complete_gc);
 745   }
 746 private:
 747   ReferencePolicy* _policy;
 748 };
 749 
 750 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 751 public:
 752   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 753                     DiscoveredList      refs_lists[],
 754                     bool                marks_oops_alive)
 755     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
 756   { }
 757   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 758                     OopClosure& keep_alive,
 759                     VoidClosure& complete_gc)
 760   {
 761     _ref_processor.process_phase2(_refs_lists[i],
 762                                   &is_alive, &keep_alive, &complete_gc);
 763   }
 764 };
 765 
 766 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 767 public:
 768   RefProcPhase3Task(ReferenceProcessor& ref_processor,
 769                     DiscoveredList      refs_lists[],
 770                     bool                clear_referent,
 771                     bool                marks_oops_alive)
 772     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 773       _clear_referent(clear_referent)
 774   { }
 775   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 776                     OopClosure& keep_alive,
 777                     VoidClosure& complete_gc)
 778   {
 779     // Don't use "refs_list_index" calculated in this way because
 780     // balance_queues() has moved the Ref's into the first n queues.
 781     // Thread* thr = Thread::current();
 782     // int refs_list_index = ((WorkerThread*)thr)->id();
 783     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 784     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 785                                   &is_alive, &keep_alive, &complete_gc);
 786   }
 787 private:
 788   bool _clear_referent;
 789 };
 790 
 791 // Balances reference queues.
 792 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 793 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 794 // corresponding to the active workers will be processed.
 795 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 796 {
 797   // calculate total length
 798   size_t total_refs = 0;
 799   if (TraceReferenceGC && PrintGCDetails) {
 800     gclog_or_tty->print_cr("\nBalance ref_lists ");
 801   }
 802 
 803   for (uint i = 0; i < _max_num_q; ++i) {
 804     total_refs += ref_lists[i].length();
 805     if (TraceReferenceGC && PrintGCDetails) {
 806       gclog_or_tty->print("%d ", ref_lists[i].length());
 807     }
 808   }
 809   if (TraceReferenceGC && PrintGCDetails) {
 810     gclog_or_tty->print_cr(" = %d", total_refs);
 811   }
 812   size_t avg_refs = total_refs / _num_q + 1;
 813   uint to_idx = 0;
 814   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 815     bool move_all = false;
 816     if (from_idx >= _num_q) {
 817       move_all = ref_lists[from_idx].length() > 0;
 818     }
 819     while ((ref_lists[from_idx].length() > avg_refs) ||
 820            move_all) {
 821       assert(to_idx < _num_q, "Sanity Check!");
 822       if (ref_lists[to_idx].length() < avg_refs) {
 823         // move superfluous refs
 824         size_t refs_to_move;
 825         // Move all the Ref's if the from queue will not be processed.
 826         if (move_all) {
 827           refs_to_move = MIN2(ref_lists[from_idx].length(),
 828                               avg_refs - ref_lists[to_idx].length());
 829         } else {
 830           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 831                               avg_refs - ref_lists[to_idx].length());
 832         }
 833 
 834         assert(refs_to_move > 0, "otherwise the code below will fail");
 835 
 836         oop move_head = ref_lists[from_idx].head();
 837         oop move_tail = move_head;
 838         oop new_head  = move_head;
 839         // find an element to split the list on
 840         for (size_t j = 0; j < refs_to_move; ++j) {
 841           move_tail = new_head;
 842           new_head = java_lang_ref_Reference::discovered(new_head);
 843         }
 844 
 845         // Add the chain to the to list.
 846         if (ref_lists[to_idx].head() == NULL) {
 847           // to list is empty. Make a loop at the end.
 848           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
 849         } else {
 850           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
 851         }
 852         ref_lists[to_idx].set_head(move_head);
 853         ref_lists[to_idx].inc_length(refs_to_move);
 854 
 855         // Remove the chain from the from list.
 856         if (move_tail == new_head) {
 857           // We found the end of the from list.
 858           ref_lists[from_idx].set_head(NULL);
 859         } else {
 860           ref_lists[from_idx].set_head(new_head);
 861         }
 862         ref_lists[from_idx].dec_length(refs_to_move);
 863         if (ref_lists[from_idx].length() == 0) {
 864           break;
 865         }
 866       } else {
 867         to_idx = (to_idx + 1) % _num_q;
 868       }
 869     }
 870   }
 871 #ifdef ASSERT
 872   size_t balanced_total_refs = 0;
 873   for (uint i = 0; i < _max_num_q; ++i) {
 874     balanced_total_refs += ref_lists[i].length();
 875     if (TraceReferenceGC && PrintGCDetails) {
 876       gclog_or_tty->print("%d ", ref_lists[i].length());
 877     }
 878   }
 879   if (TraceReferenceGC && PrintGCDetails) {
 880     gclog_or_tty->print_cr(" = %d", balanced_total_refs);
 881     gclog_or_tty->flush();
 882   }
 883   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 884 #endif
 885 }
 886 
 887 void ReferenceProcessor::balance_all_queues() {
 888   balance_queues(_discoveredSoftRefs);
 889   balance_queues(_discoveredWeakRefs);
 890   balance_queues(_discoveredFinalRefs);
 891   balance_queues(_discoveredPhantomRefs);
 892   balance_queues(_discoveredCleanerRefs);
 893 }
 894 
 895 size_t
 896 ReferenceProcessor::process_discovered_reflist(
 897   DiscoveredList               refs_lists[],
 898   ReferencePolicy*             policy,
 899   bool                         clear_referent,
 900   BoolObjectClosure*           is_alive,
 901   OopClosure*                  keep_alive,
 902   VoidClosure*                 complete_gc,
 903   AbstractRefProcTaskExecutor* task_executor)
 904 {
 905   bool mt_processing = task_executor != NULL && _processing_is_mt;
 906   // If discovery used MT and a dynamic number of GC threads, then
 907   // the queues must be balanced for correctness if fewer than the
 908   // maximum number of queues were used.  The number of queue used
 909   // during discovery may be different than the number to be used
 910   // for processing so don't depend of _num_q < _max_num_q as part
 911   // of the test.
 912   bool must_balance = _discovery_is_mt;
 913 
 914   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 915       must_balance) {
 916     balance_queues(refs_lists);
 917   }
 918 
 919   size_t total_list_count = total_count(refs_lists);
 920 
 921   if (PrintReferenceGC && PrintGCDetails) {
 922     gclog_or_tty->print(", %u refs", total_list_count);
 923   }
 924 
 925   // Phase 1 (soft refs only):
 926   // . Traverse the list and remove any SoftReferences whose
 927   //   referents are not alive, but that should be kept alive for
 928   //   policy reasons. Keep alive the transitive closure of all
 929   //   such referents.
 930   if (policy != NULL) {
 931     if (mt_processing) {
 932       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
 933       task_executor->execute(phase1);
 934     } else {
 935       for (uint i = 0; i < _max_num_q; i++) {
 936         process_phase1(refs_lists[i], policy,
 937                        is_alive, keep_alive, complete_gc);
 938       }
 939     }
 940   } else { // policy == NULL
 941     assert(refs_lists != _discoveredSoftRefs,
 942            "Policy must be specified for soft references.");
 943   }
 944 
 945   // Phase 2:
 946   // . Traverse the list and remove any refs whose referents are alive.
 947   if (mt_processing) {
 948     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
 949     task_executor->execute(phase2);
 950   } else {
 951     for (uint i = 0; i < _max_num_q; i++) {
 952       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 953     }
 954   }
 955 
 956   // Phase 3:
 957   // . Traverse the list and process referents as appropriate.
 958   if (mt_processing) {
 959     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
 960     task_executor->execute(phase3);
 961   } else {
 962     for (uint i = 0; i < _max_num_q; i++) {
 963       process_phase3(refs_lists[i], clear_referent,
 964                      is_alive, keep_alive, complete_gc);
 965     }
 966   }
 967 
 968   return total_list_count;
 969 }
 970 
 971 void ReferenceProcessor::clean_up_discovered_references() {
 972   // loop over the lists
 973   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 974     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 975       gclog_or_tty->print_cr(
 976         "\nScrubbing %s discovered list of Null referents",
 977         list_name(i));
 978     }
 979     clean_up_discovered_reflist(_discovered_refs[i]);
 980   }
 981 }
 982 
 983 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
 984   assert(!discovery_is_atomic(), "Else why call this method?");
 985   DiscoveredListIterator iter(refs_list, NULL, NULL);
 986   while (iter.has_next()) {
 987     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 988     oop next = java_lang_ref_Reference::next(iter.obj());
 989     assert(next->is_oop_or_null(), "bad next field");
 990     // If referent has been cleared or Reference is not active,
 991     // drop it.
 992     if (iter.referent() == NULL || next != NULL) {
 993       debug_only(
 994         if (PrintGCDetails && TraceReferenceGC) {
 995           gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
 996             INTPTR_FORMAT " with next field: " INTPTR_FORMAT
 997             " and referent: " INTPTR_FORMAT,
 998             (void *)iter.obj(), (void *)next, (void *)iter.referent());
 999         }
1000       )
1001       // Remove Reference object from list
1002       iter.remove();
1003       iter.move_to_next();
1004     } else {
1005       iter.next();
1006     }
1007   }
1008   NOT_PRODUCT(
1009     if (PrintGCDetails && TraceReferenceGC) {
1010       gclog_or_tty->print(
1011         " Removed %d Refs with NULL referents out of %d discovered Refs",
1012         iter.removed(), iter.processed());
1013     }
1014   )
1015 }
1016 
1017 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1018   uint id = 0;
1019   // Determine the queue index to use for this object.
1020   if (_discovery_is_mt) {
1021     // During a multi-threaded discovery phase,
1022     // each thread saves to its "own" list.
1023     Thread* thr = Thread::current();
1024     id = thr->as_Worker_thread()->id();
1025   } else {
1026     // single-threaded discovery, we save in round-robin
1027     // fashion to each of the lists.
1028     if (_processing_is_mt) {
1029       id = next_id();
1030     }
1031   }
1032   assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1033 
1034   // Get the discovered queue to which we will add
1035   DiscoveredList* list = NULL;
1036   switch (rt) {
1037     case REF_OTHER:
1038       // Unknown reference type, no special treatment
1039       break;
1040     case REF_SOFT:
1041       list = &_discoveredSoftRefs[id];
1042       break;
1043     case REF_WEAK:
1044       list = &_discoveredWeakRefs[id];
1045       break;
1046     case REF_FINAL:
1047       list = &_discoveredFinalRefs[id];
1048       break;
1049     case REF_PHANTOM:
1050       list = &_discoveredPhantomRefs[id];
1051       break;
1052     case REF_CLEANER:
1053       list = &_discoveredCleanerRefs[id];
1054       break;
1055     case REF_NONE:
1056       // we should not reach here if we are an InstanceRefKlass
1057     default:
1058       ShouldNotReachHere();
1059   }
1060   if (TraceReferenceGC && PrintGCDetails) {
1061     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1062   }
1063   return list;
1064 }
1065 
1066 inline void
1067 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1068                                               oop             obj,
1069                                               HeapWord*       discovered_addr) {
1070   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1071   // First we must make sure this object is only enqueued once. CAS in a non null
1072   // discovered_addr.
1073   oop current_head = refs_list.head();
1074   // The last ref must have its discovered field pointing to itself.
1075   oop next_discovered = (current_head != NULL) ? current_head : obj;
1076 
1077   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1078                                                     NULL);
1079   if (retest == NULL) {
1080     // This thread just won the right to enqueue the object.
1081     // We have separate lists for enqueueing, so no synchronization
1082     // is necessary.
1083     refs_list.set_head(obj);
1084     refs_list.inc_length(1);
1085 
1086     if (TraceReferenceGC) {
1087       gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1088                              (void *)obj, obj->klass()->internal_name());
1089     }
1090   } else {
1091     // If retest was non NULL, another thread beat us to it:
1092     // The reference has already been discovered...
1093     if (TraceReferenceGC) {
1094       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1095                              (void *)obj, obj->klass()->internal_name());
1096     }
1097   }
1098 }
1099 
1100 #ifndef PRODUCT
1101 // Non-atomic (i.e. concurrent) discovery might allow us
1102 // to observe j.l.References with NULL referents, being those
1103 // cleared concurrently by mutators during (or after) discovery.
1104 void ReferenceProcessor::verify_referent(oop obj) {
1105   bool da = discovery_is_atomic();
1106   oop referent = java_lang_ref_Reference::referent(obj);
1107   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1108          err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1109                  INTPTR_FORMAT " during %satomic discovery ",
1110                  (void *)referent, (void *)obj, da ? "" : "non-"));
1111 }
1112 #endif
1113 
1114 // We mention two of several possible choices here:
1115 // #0: if the reference object is not in the "originating generation"
1116 //     (or part of the heap being collected, indicated by our "span"
1117 //     we don't treat it specially (i.e. we scan it as we would
1118 //     a normal oop, treating its references as strong references).
1119 //     This means that references can't be discovered unless their
1120 //     referent is also in the same span. This is the simplest,
1121 //     most "local" and most conservative approach, albeit one
1122 //     that may cause weak references to be enqueued least promptly.
1123 //     We call this choice the "ReferenceBasedDiscovery" policy.
1124 // #1: the reference object may be in any generation (span), but if
1125 //     the referent is in the generation (span) being currently collected
1126 //     then we can discover the reference object, provided
1127 //     the object has not already been discovered by
1128 //     a different concurrently running collector (as may be the
1129 //     case, for instance, if the reference object is in CMS and
1130 //     the referent in DefNewGeneration), and provided the processing
1131 //     of this reference object by the current collector will
1132 //     appear atomic to every other collector in the system.
1133 //     (Thus, for instance, a concurrent collector may not
1134 //     discover references in other generations even if the
1135 //     referent is in its own generation). This policy may,
1136 //     in certain cases, enqueue references somewhat sooner than
1137 //     might Policy #0 above, but at marginally increased cost
1138 //     and complexity in processing these references.
1139 //     We call this choice the "RefeferentBasedDiscovery" policy.
1140 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1141   // Make sure we are discovering refs (rather than processing discovered refs).
1142   if (!_discovering_refs || !RegisterReferences) {
1143     return false;
1144   }
1145   // We only discover active references.
1146   oop next = java_lang_ref_Reference::next(obj);
1147   if (next != NULL) {   // Ref is no longer active
1148     return false;
1149   }
1150 
1151   HeapWord* obj_addr = (HeapWord*)obj;
1152   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1153       !_span.contains(obj_addr)) {
1154     // Reference is not in the originating generation;
1155     // don't treat it specially (i.e. we want to scan it as a normal
1156     // object with strong references).
1157     return false;
1158   }
1159 
1160   // We only discover references whose referents are not (yet)
1161   // known to be strongly reachable.
1162   if (is_alive_non_header() != NULL) {
1163     verify_referent(obj);
1164     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1165       return false;  // referent is reachable
1166     }
1167   }
1168   if (rt == REF_SOFT) {
1169     // For soft refs we can decide now if these are not
1170     // current candidates for clearing, in which case we
1171     // can mark through them now, rather than delaying that
1172     // to the reference-processing phase. Since all current
1173     // time-stamp policies advance the soft-ref clock only
1174     // at a major collection cycle, this is always currently
1175     // accurate.
1176     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1177       return false;
1178     }
1179   }
1180 
1181   ResourceMark rm;      // Needed for tracing.
1182 
1183   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1184   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1185   assert(discovered->is_oop_or_null(), "bad discovered field");
1186   if (discovered != NULL) {
1187     // The reference has already been discovered...
1188     if (TraceReferenceGC) {
1189       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1190                              (void *)obj, obj->klass()->internal_name());
1191     }
1192     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1193       // assumes that an object is not processed twice;
1194       // if it's been already discovered it must be on another
1195       // generation's discovered list; so we won't discover it.
1196       return false;
1197     } else {
1198       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1199              "Unrecognized policy");
1200       // Check assumption that an object is not potentially
1201       // discovered twice except by concurrent collectors that potentially
1202       // trace the same Reference object twice.
1203       assert(UseConcMarkSweepGC || UseG1GC,
1204              "Only possible with a concurrent marking collector");
1205       return true;
1206     }
1207   }
1208 
1209   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1210     verify_referent(obj);
1211     // Discover if and only if EITHER:
1212     // .. reference is in our span, OR
1213     // .. we are an atomic collector and referent is in our span
1214     if (_span.contains(obj_addr) ||
1215         (discovery_is_atomic() &&
1216          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1217       // should_enqueue = true;
1218     } else {
1219       return false;
1220     }
1221   } else {
1222     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1223            _span.contains(obj_addr), "code inconsistency");
1224   }
1225 
1226   // Get the right type of discovered queue head.
1227   DiscoveredList* list = get_discovered_list(rt);
1228   if (list == NULL) {
1229     return false;   // nothing special needs to be done
1230   }
1231 
1232   if (_discovery_is_mt) {
1233     add_to_discovered_list_mt(*list, obj, discovered_addr);
1234   } else {
1235     // We do a raw store here: the field will be visited later when processing
1236     // the discovered references.
1237     oop current_head = list->head();
1238     // The last ref must have its discovered field pointing to itself.
1239     oop next_discovered = (current_head != NULL) ? current_head : obj;
1240 
1241     assert(discovered == NULL, "control point invariant");
1242     oop_store_raw(discovered_addr, next_discovered);
1243     list->set_head(obj);
1244     list->inc_length(1);
1245 
1246     if (TraceReferenceGC) {
1247       gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1248                                 (void *)obj, obj->klass()->internal_name());
1249     }
1250   }
1251   assert(obj->is_oop(), "Discovered a bad reference");
1252   verify_referent(obj);
1253   return true;
1254 }
1255 
1256 // Preclean the discovered references by removing those
1257 // whose referents are alive, and by marking from those that
1258 // are not active. These lists can be handled here
1259 // in any order and, indeed, concurrently.
1260 void ReferenceProcessor::preclean_discovered_references(
1261   BoolObjectClosure* is_alive,
1262   OopClosure* keep_alive,
1263   VoidClosure* complete_gc,
1264   YieldClosure* yield,
1265   GCTimer* gc_timer,
1266   GCId     gc_id) {
1267 
1268   NOT_PRODUCT(verify_ok_to_handle_reflists());
1269 
1270   // Soft references
1271   {
1272     GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1273               false, gc_timer, gc_id);
1274     for (uint i = 0; i < _max_num_q; i++) {
1275       if (yield->should_return()) {
1276         return;
1277       }
1278       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1279                                   keep_alive, complete_gc, yield);
1280     }
1281   }
1282 
1283   // Weak references
1284   {
1285     GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1286               false, gc_timer, gc_id);
1287     for (uint i = 0; i < _max_num_q; i++) {
1288       if (yield->should_return()) {
1289         return;
1290       }
1291       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1292                                   keep_alive, complete_gc, yield);
1293     }
1294   }
1295 
1296   // Final references
1297   {
1298     GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1299               false, gc_timer, gc_id);
1300     for (uint i = 0; i < _max_num_q; i++) {
1301       if (yield->should_return()) {
1302         return;
1303       }
1304       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1305                                   keep_alive, complete_gc, yield);
1306     }
1307   }
1308 
1309   // Phantom references
1310   {
1311     GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1312               false, gc_timer, gc_id);
1313     for (uint i = 0; i < _max_num_q; i++) {
1314       if (yield->should_return()) {
1315         return;
1316       }
1317       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1318                                   keep_alive, complete_gc, yield);
1319     }
1320 
1321     // Cleaner references.  Included in timing for phantom references.  We
1322     // expect Cleaner references to be temporary, and don't want to deal with
1323     // possible incompatibilities arising from making it more visible.
1324     for (uint i = 0; i < _max_num_q; i++) {
1325       if (yield->should_return()) {
1326         return;
1327       }
1328       preclean_discovered_reflist(_discoveredCleanerRefs[i], is_alive,
1329                                   keep_alive, complete_gc, yield);
1330     }
1331   }
1332 }
1333 
1334 // Walk the given discovered ref list, and remove all reference objects
1335 // whose referents are still alive, whose referents are NULL or which
1336 // are not active (have a non-NULL next field). NOTE: When we are
1337 // thus precleaning the ref lists (which happens single-threaded today),
1338 // we do not disable refs discovery to honour the correct semantics of
1339 // java.lang.Reference. As a result, we need to be careful below
1340 // that ref removal steps interleave safely with ref discovery steps
1341 // (in this thread).
1342 void
1343 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1344                                                 BoolObjectClosure* is_alive,
1345                                                 OopClosure*        keep_alive,
1346                                                 VoidClosure*       complete_gc,
1347                                                 YieldClosure*      yield) {
1348   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1349   while (iter.has_next()) {
1350     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1351     oop obj = iter.obj();
1352     oop next = java_lang_ref_Reference::next(obj);
1353     if (iter.referent() == NULL || iter.is_referent_alive() ||
1354         next != NULL) {
1355       // The referent has been cleared, or is alive, or the Reference is not
1356       // active; we need to trace and mark its cohort.
1357       if (TraceReferenceGC) {
1358         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1359                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
1360       }
1361       // Remove Reference object from list
1362       iter.remove();
1363       // Keep alive its cohort.
1364       iter.make_referent_alive();
1365       if (UseCompressedOops) {
1366         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1367         keep_alive->do_oop(next_addr);
1368       } else {
1369         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1370         keep_alive->do_oop(next_addr);
1371       }
1372       iter.move_to_next();
1373     } else {
1374       iter.next();
1375     }
1376   }
1377   // Close the reachable set
1378   complete_gc->do_void();
1379 
1380   NOT_PRODUCT(
1381     if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1382       gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1383         "Refs in discovered list " INTPTR_FORMAT,
1384         iter.removed(), iter.processed(), (address)refs_list.head());
1385     }
1386   )
1387 }
1388 
1389 const char* ReferenceProcessor::list_name(uint i) {
1390    assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1391           "Out of bounds index");
1392 
1393    int j = i / _max_num_q;
1394    switch (j) {
1395      case 0: return "SoftRef";
1396      case 1: return "WeakRef";
1397      case 2: return "FinalRef";
1398      case 3: return "PhantomRef";
1399      case 4: return "CleanerRef";
1400    }
1401    ShouldNotReachHere();
1402    return NULL;
1403 }
1404 
1405 #ifndef PRODUCT
1406 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1407   // empty for now
1408 }
1409 #endif
1410 
1411 #ifndef PRODUCT
1412 void ReferenceProcessor::clear_discovered_references() {
1413   guarantee(!_discovering_refs, "Discovering refs?");
1414   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1415     clear_discovered_references(_discovered_refs[i]);
1416   }
1417 }
1418 
1419 #endif // PRODUCT
--- EOF ---