rev 6446 : [mq]: ref-write-new-fix

   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 
  38 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
  39 
  40 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  41 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  42 bool             ReferenceProcessor::_pending_list_uses_discovered_field = false;
  43 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;
  44 
  45 void referenceProcessor_init() {
  46   ReferenceProcessor::init_statics();
  47 }
  48 
  49 void ReferenceProcessor::init_statics() {
  50   // We need a monotonically non-decreasing time in ms but
  51   // os::javaTimeMillis() does not guarantee monotonicity.
  52   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  53 
  54   // Initialize the soft ref timestamp clock.
  55   _soft_ref_timestamp_clock = now;
  56   // Also update the soft ref clock in j.l.r.SoftReference
  57   java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
  58 
  59   _always_clear_soft_ref_policy = new AlwaysClearPolicy();
  60   _default_soft_ref_policy      = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
  61                                       NOT_COMPILER2(LRUCurrentHeapPolicy());
  62   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  63     vm_exit_during_initialization("Could not allocate reference policy object");
  64   }
  65   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
  66             RefDiscoveryPolicy == ReferentBasedDiscovery,
  67             "Unrecognized RefDiscoveryPolicy");
  68   _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
  69 }
  70 
  71 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
  72 #ifdef ASSERT
  73   // Verify that we're not currently discovering refs
  74   assert(!verify_disabled || !_discovering_refs, "nested call?");
  75 
  76   if (check_no_refs) {
  77     // Verify that the discovered lists are empty
  78     verify_no_references_recorded();
  79   }
  80 #endif // ASSERT
  81 
  82   // Someone could have modified the value of the static
  83   // field in the j.l.r.SoftReference class that holds the
  84   // soft reference timestamp clock using reflection or
  85   // Unsafe between GCs. Unconditionally update the static
  86   // field in ReferenceProcessor here so that we use the new
  87   // value during reference discovery.
  88 
  89   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
  90   _discovering_refs = true;
  91 }
  92 
  93 ReferenceProcessor::ReferenceProcessor(MemRegion span,
  94                                        bool      mt_processing,
  95                                        uint      mt_processing_degree,
  96                                        bool      mt_discovery,
  97                                        uint      mt_discovery_degree,
  98                                        bool      atomic_discovery,
  99                                        BoolObjectClosure* is_alive_non_header)  :

 100   _discovering_refs(false),
 101   _enqueuing_is_done(false),
 102   _is_alive_non_header(is_alive_non_header),

 103   _processing_is_mt(mt_processing),
 104   _next_id(0)
 105 {
 106   _span = span;
 107   _discovery_is_atomic = atomic_discovery;
 108   _discovery_is_mt     = mt_discovery;
 109   _num_q               = MAX2(1U, mt_processing_degree);
 110   _max_num_q           = MAX2(_num_q, mt_discovery_degree);
 111   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,
 112             _max_num_q * number_of_subclasses_of_ref(), mtGC);
 113 
 114   if (_discovered_refs == NULL) {
 115     vm_exit_during_initialization("Could not allocated RefProc Array");
 116   }
 117   _discoveredSoftRefs    = &_discovered_refs[0];
 118   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];
 119   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];
 120   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
 121 
 122   // Initialize all entries to NULL
 123   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 124     _discovered_refs[i].set_head(NULL);
 125     _discovered_refs[i].set_length(0);
 126   }
 127 
 128   setup_policy(false /* default soft ref policy */);
 129 }
 130 
 131 #ifndef PRODUCT
 132 void ReferenceProcessor::verify_no_references_recorded() {
 133   guarantee(!_discovering_refs, "Discovering refs?");
 134   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 135     guarantee(_discovered_refs[i].is_empty(),
 136               "Found non-empty discovered list");
 137   }
 138 }
 139 #endif
 140 
 141 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 142   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 143     if (UseCompressedOops) {
 144       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
 145     } else {
 146       f->do_oop((oop*)_discovered_refs[i].adr_head());
 147     }
 148   }
 149 }
 150 
 151 void ReferenceProcessor::update_soft_ref_master_clock() {
 152   // Update (advance) the soft ref master clock field. This must be done
 153   // after processing the soft ref list.
 154 
 155   // We need a monotonically non-decreasing time in ms but
 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   // The values of now and _soft_ref_timestamp_clock are set using
 168   // javaTimeNanos(), which is guaranteed to be monotonically
 169   // non-decreasing provided the underlying platform provides such
 170   // a time source (and it is bug free).
 171   // In product mode, however, protect ourselves from non-monotonicity.
 172   if (now > _soft_ref_timestamp_clock) {
 173     _soft_ref_timestamp_clock = now;
 174     java_lang_ref_SoftReference::set_clock(now);
 175   }
 176   // Else leave clock stalled at its old value until time progresses
 177   // past clock value.
 178 }
 179 
 180 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
 181   size_t total = 0;
 182   for (uint i = 0; i < _max_num_q; ++i) {
 183     total += lists[i].length();
 184   }
 185   return total;
 186 }
 187 
 188 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 189   BoolObjectClosure*           is_alive,
 190   OopClosure*                  keep_alive,
 191   VoidClosure*                 complete_gc,
 192   AbstractRefProcTaskExecutor* task_executor,
 193   GCTimer*                     gc_timer) {
 194   NOT_PRODUCT(verify_ok_to_handle_reflists());
 195 
 196   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 197   // Stop treating discovered references specially.
 198   disable_discovery();
 199 
 200   // If discovery was concurrent, someone could have modified
 201   // the value of the static field in the j.l.r.SoftReference
 202   // class that holds the soft reference timestamp clock using
 203   // reflection or Unsafe between when discovery was enabled and
 204   // now. Unconditionally update the static field in ReferenceProcessor
 205   // here so that we use the new value during processing of the
 206   // discovered soft refs.
 207 
 208   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 209 
 210   bool trace_time = PrintGCDetails && PrintReferenceGC;
 211 
 212   // Soft references
 213   size_t soft_count = 0;
 214   {
 215     GCTraceTime tt("SoftReference", trace_time, false, gc_timer);
 216     soft_count =
 217       process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 218                                  is_alive, keep_alive, complete_gc, task_executor);
 219   }
 220 
 221   update_soft_ref_master_clock();
 222 
 223   // Weak references
 224   size_t weak_count = 0;
 225   {
 226     GCTraceTime tt("WeakReference", trace_time, false, gc_timer);
 227     weak_count =
 228       process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 229                                  is_alive, keep_alive, complete_gc, task_executor);
 230   }
 231 
 232   // Final references
 233   size_t final_count = 0;
 234   {
 235     GCTraceTime tt("FinalReference", trace_time, false, gc_timer);
 236     final_count =
 237       process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 238                                  is_alive, keep_alive, complete_gc, task_executor);
 239   }
 240 
 241   // Phantom references
 242   size_t phantom_count = 0;
 243   {
 244     GCTraceTime tt("PhantomReference", trace_time, false, gc_timer);
 245     phantom_count =
 246       process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 247                                  is_alive, keep_alive, complete_gc, task_executor);
 248   }
 249 
 250   // Weak global JNI references. It would make more sense (semantically) to
 251   // traverse these simultaneously with the regular weak references above, but
 252   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 253   // thus use JNI weak references to circumvent the phantom references and
 254   // resurrect a "post-mortem" object.
 255   {
 256     GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
 257     if (task_executor != NULL) {
 258       task_executor->set_single_threaded_mode();
 259     }
 260     process_phaseJNI(is_alive, keep_alive, complete_gc);
 261   }
 262 
 263   return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
 264 }
 265 
 266 #ifndef PRODUCT
 267 // Calculate the number of jni handles.
 268 uint ReferenceProcessor::count_jni_refs() {
 269   class AlwaysAliveClosure: public BoolObjectClosure {
 270   public:
 271     virtual bool do_object_b(oop obj) { return true; }
 272   };
 273 
 274   class CountHandleClosure: public OopClosure {
 275   private:
 276     int _count;
 277   public:
 278     CountHandleClosure(): _count(0) {}
 279     void do_oop(oop* unused)       { _count++; }
 280     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 281     int count() { return _count; }
 282   };
 283   CountHandleClosure global_handle_count;
 284   AlwaysAliveClosure always_alive;
 285   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
 286   return global_handle_count.count();
 287 }
 288 #endif
 289 
 290 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 291                                           OopClosure*        keep_alive,
 292                                           VoidClosure*       complete_gc) {
 293 #ifndef PRODUCT
 294   if (PrintGCDetails && PrintReferenceGC) {
 295     unsigned int count = count_jni_refs();
 296     gclog_or_tty->print(", %u refs", count);
 297   }
 298 #endif
 299   JNIHandles::weak_oops_do(is_alive, keep_alive);
 300   complete_gc->do_void();
 301 }
 302 
 303 
 304 template <class T>
 305 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 306                                    AbstractRefProcTaskExecutor* task_executor) {
 307 
 308   // Remember old value of pending references list
 309   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
 310   T old_pending_list_value = *pending_list_addr;
 311 
 312   // Enqueue references that are not made active again, and
 313   // clear the decks for the next collection (cycle).
 314   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
 315   // Do the post-barrier on pending_list_addr missed in
 316   // enqueue_discovered_reflist.
 317   oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
 318 
 319   // Stop treating discovered references specially.
 320   ref->disable_discovery();
 321 
 322   // Return true if new pending references were added
 323   return old_pending_list_value != *pending_list_addr;
 324 }
 325 
 326 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
 327   NOT_PRODUCT(verify_ok_to_handle_reflists());
 328   if (UseCompressedOops) {
 329     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 330   } else {
 331     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 332   }
 333 }
 334 
 335 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 336                                                     HeapWord* pending_list_addr) {
 337   // Given a list of refs linked through the "discovered" field
 338   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 339   // thus distinguishing them from active References, then
 340   // prepend them to the pending list.
 341   // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
 342   // the "next" field is used to chain the pending list, not the discovered
 343   // field.
 344 
 345   if (TraceReferenceGC && PrintGCDetails) {
 346     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
 347                            INTPTR_FORMAT, (address)refs_list.head());
 348   }
 349 
 350   oop obj = NULL;
 351   oop next_d = refs_list.head();
 352   if (pending_list_uses_discovered_field()) { // New behavior
 353     // Walk down the list, self-looping the next field
 354     // so that the References are not considered active.
 355     while (obj != next_d) {
 356       obj = next_d;
 357       assert(obj->is_instanceRef(), "should be reference object");
 358       next_d = java_lang_ref_Reference::discovered(obj);
 359       if (TraceReferenceGC && PrintGCDetails) {
 360         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 361                                (void *)obj, (void *)next_d);
 362       }
 363       assert(java_lang_ref_Reference::next(obj) == NULL,
 364              "Reference not active; should not be discovered");
 365       // Self-loop next, so as to make Ref not active.

 366       java_lang_ref_Reference::set_next_raw(obj, obj);
 367       if (next_d != obj) {
 368         // The Java threads will see the Reference objects linked together through
 369         // the discovered field. Instead of trying to do the write barrier updates
 370         // in all places in the reference processor where we manipulate the discovered
 371         // field we make sure to do the barrier here where we anyway iterate through
 372         // all linked Reference objects.
 373         oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
 374       } else {
 375         // This is the last object.
 376         // Swap refs_list into pending_list_addr and
 377         // set obj's discovered to what we read from pending_list_addr.
 378         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 379         // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.


 380         java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
 381         oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
 382       }
 383     }
 384   } else { // Old behavior
 385     // Walk down the list, copying the discovered field into
 386     // the next field and clearing the discovered field.
 387     while (obj != next_d) {
 388       obj = next_d;
 389       assert(obj->is_instanceRef(), "should be reference object");
 390       next_d = java_lang_ref_Reference::discovered(obj);
 391       if (TraceReferenceGC && PrintGCDetails) {
 392         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 393                                (void *)obj, (void *)next_d);
 394       }
 395       assert(java_lang_ref_Reference::next(obj) == NULL,
 396              "The reference should not be enqueued");
 397       if (next_d == obj) {  // obj is last
 398         // Swap refs_list into pending_list_addr and
 399         // set obj's next to what we read from pending_list_addr.
 400         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 401         // Need oop_check on pending_list_addr above;
 402         // see special oop-check code at the end of
 403         // enqueue_discovered_reflists() further below.
 404         if (old == NULL) {
 405           // obj should be made to point to itself, since
 406           // pending list was empty.
 407           java_lang_ref_Reference::set_next(obj, obj);
 408         } else {
 409           java_lang_ref_Reference::set_next(obj, old);
 410         }
 411       } else {
 412         java_lang_ref_Reference::set_next(obj, next_d);
 413       }
 414       java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
 415     }
 416   }
 417 }
 418 
 419 // Parallel enqueue task
 420 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 421 public:
 422   RefProcEnqueueTask(ReferenceProcessor& ref_processor,
 423                      DiscoveredList      discovered_refs[],
 424                      HeapWord*           pending_list_addr,
 425                      int                 n_queues)
 426     : EnqueueTask(ref_processor, discovered_refs,
 427                   pending_list_addr, n_queues)
 428   { }
 429 
 430   virtual void work(unsigned int work_id) {
 431     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 432     // Simplest first cut: static partitioning.
 433     int index = work_id;
 434     // The increment on "index" must correspond to the maximum number of queues
 435     // (n_queues) with which that ReferenceProcessor was created.  That
 436     // is because of the "clever" way the discovered references lists were
 437     // allocated and are indexed into.
 438     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 439     for (int j = 0;
 440          j < ReferenceProcessor::number_of_subclasses_of_ref();
 441          j++, index += _n_queues) {
 442       _ref_processor.enqueue_discovered_reflist(
 443         _refs_lists[index], _pending_list_addr);
 444       _refs_lists[index].set_head(NULL);
 445       _refs_lists[index].set_length(0);
 446     }
 447   }
 448 };
 449 
 450 // Enqueue references that are not made active again
 451 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
 452   AbstractRefProcTaskExecutor* task_executor) {
 453   if (_processing_is_mt && task_executor != NULL) {
 454     // Parallel code
 455     RefProcEnqueueTask tsk(*this, _discovered_refs,
 456                            pending_list_addr, _max_num_q);
 457     task_executor->execute(tsk);
 458   } else {
 459     // Serial code: call the parent class's implementation
 460     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 461       enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
 462       _discovered_refs[i].set_head(NULL);
 463       _discovered_refs[i].set_length(0);
 464     }
 465   }
 466 }
 467 
 468 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 469   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 470   oop discovered = java_lang_ref_Reference::discovered(_ref);
 471   assert(_discovered_addr && discovered->is_oop_or_null(),
 472          "discovered field is bad");
 473   _next = discovered;
 474   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 475   _referent = java_lang_ref_Reference::referent(_ref);
 476   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 477          "Wrong oop found in java.lang.Reference object");
 478   assert(allow_null_referent ?
 479              _referent->is_oop_or_null()
 480            : _referent->is_oop(),
 481          "bad referent");
 482 }
 483 
 484 void DiscoveredListIterator::remove() {
 485   assert(_ref->is_oop(), "Dropping a bad reference");
 486   oop_store_raw(_discovered_addr, NULL);
 487 
 488   // First _prev_next ref actually points into DiscoveredList (gross).
 489   oop new_next;
 490   if (_next == _ref) {
 491     // At the end of the list, we should make _prev point to itself.
 492     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 493     // and _prev will be NULL.
 494     new_next = _prev;
 495   } else {
 496     new_next = _next;
 497   }
 498   // Remove Reference object from discovered list. Note that G1 does not need a
 499   // pre-barrier here because we know the Reference has already been found/marked,
 500   // that's how it ended up in the discovered list in the first place.
 501   oop_store_raw(_prev_next, new_next);




 502   NOT_PRODUCT(_removed++);
 503   _refs_list.dec_length(1);
 504 }
 505 
 506 // Make the Reference object active again.
 507 void DiscoveredListIterator::make_active() {
 508   // The pre barrier for G1 is probably just needed for the old
 509   // reference processing behavior. Should we guard this with
 510   // ReferenceProcessor::pending_list_uses_discovered_field() ?

 511   if (UseG1GC) {
 512     HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
 513     if (UseCompressedOops) {
 514       oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
 515     } else {
 516       oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
 517     }



 518   }
 519   java_lang_ref_Reference::set_next_raw(_ref, NULL);
 520 }
 521 
 522 void DiscoveredListIterator::clear_referent() {
 523   oop_store_raw(_referent_addr, NULL);
 524 }
 525 
 526 // NOTE: process_phase*() are largely similar, and at a high level
 527 // merely iterate over the extant list applying a predicate to
 528 // each of its elements and possibly removing that element from the
 529 // list and applying some further closures to that element.
 530 // We should consider the possibility of replacing these
 531 // process_phase*() methods by abstracting them into
 532 // a single general iterator invocation that receives appropriate
 533 // closures that accomplish this work.
 534 
 535 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 536 // referents are not alive, but that should be kept alive for policy reasons.
 537 // Keep alive the transitive closure of all such referents.
 538 void
 539 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 540                                    ReferencePolicy*   policy,
 541                                    BoolObjectClosure* is_alive,
 542                                    OopClosure*        keep_alive,
 543                                    VoidClosure*       complete_gc) {
 544   assert(policy != NULL, "Must have a non-NULL policy");
 545   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 546   // Decide which softly reachable refs should be kept alive.
 547   while (iter.has_next()) {
 548     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 549     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 550     if (referent_is_dead &&
 551         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 552       if (TraceReferenceGC) {
 553         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 554                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 555       }
 556       // Remove Reference object from list
 557       iter.remove();
 558       // Make the Reference object active again
 559       iter.make_active();
 560       // keep the referent around
 561       iter.make_referent_alive();
 562       iter.move_to_next();
 563     } else {
 564       iter.next();
 565     }
 566   }
 567   // Close the reachable set
 568   complete_gc->do_void();
 569   NOT_PRODUCT(
 570     if (PrintGCDetails && TraceReferenceGC) {
 571       gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
 572         "discovered Refs by policy, from list " INTPTR_FORMAT,
 573         iter.removed(), iter.processed(), (address)refs_list.head());
 574     }
 575   )
 576 }
 577 
 578 // Traverse the list and remove any Refs that are not active, or
 579 // whose referents are either alive or NULL.
 580 void
 581 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 582                              BoolObjectClosure* is_alive,
 583                              OopClosure*        keep_alive) {
 584   assert(discovery_is_atomic(), "Error");
 585   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 586   while (iter.has_next()) {
 587     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 588     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 589     assert(next == NULL, "Should not discover inactive Reference");
 590     if (iter.is_referent_alive()) {
 591       if (TraceReferenceGC) {
 592         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 593                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 594       }
 595       // The referent is reachable after all.
 596       // Remove Reference object from list.
 597       iter.remove();
 598       // Update the referent pointer as necessary: Note that this
 599       // should not entail any recursive marking because the
 600       // referent must already have been traversed.
 601       iter.make_referent_alive();
 602       iter.move_to_next();
 603     } else {
 604       iter.next();
 605     }
 606   }
 607   NOT_PRODUCT(
 608     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 609       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 610         "Refs in discovered list " INTPTR_FORMAT,
 611         iter.removed(), iter.processed(), (address)refs_list.head());
 612     }
 613   )
 614 }
 615 
 616 void
 617 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 618                                                   BoolObjectClosure* is_alive,
 619                                                   OopClosure*        keep_alive,
 620                                                   VoidClosure*       complete_gc) {
 621   assert(!discovery_is_atomic(), "Error");
 622   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 623   while (iter.has_next()) {
 624     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 625     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 626     oop next = java_lang_ref_Reference::next(iter.obj());
 627     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 628          next != NULL)) {
 629       assert(next->is_oop_or_null(), "bad next field");
 630       // Remove Reference object from list
 631       iter.remove();
 632       // Trace the cohorts
 633       iter.make_referent_alive();
 634       if (UseCompressedOops) {
 635         keep_alive->do_oop((narrowOop*)next_addr);
 636       } else {
 637         keep_alive->do_oop((oop*)next_addr);
 638       }
 639       iter.move_to_next();
 640     } else {
 641       iter.next();
 642     }
 643   }
 644   // Now close the newly reachable set
 645   complete_gc->do_void();
 646   NOT_PRODUCT(
 647     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 648       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 649         "Refs in discovered list " INTPTR_FORMAT,
 650         iter.removed(), iter.processed(), (address)refs_list.head());
 651     }
 652   )
 653 }
 654 
 655 // Traverse the list and process the referents, by either
 656 // clearing them or keeping them (and their reachable
 657 // closure) alive.
 658 void
 659 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 660                                    bool               clear_referent,
 661                                    BoolObjectClosure* is_alive,
 662                                    OopClosure*        keep_alive,
 663                                    VoidClosure*       complete_gc) {
 664   ResourceMark rm;
 665   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 666   while (iter.has_next()) {
 667     iter.update_discovered();
 668     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 669     if (clear_referent) {
 670       // NULL out referent pointer
 671       iter.clear_referent();
 672     } else {
 673       // keep the referent around
 674       iter.make_referent_alive();
 675     }
 676     if (TraceReferenceGC) {
 677       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 678                              clear_referent ? "cleared " : "",
 679                              (void *)iter.obj(), iter.obj()->klass()->internal_name());
 680     }
 681     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 682     iter.next();
 683   }
 684   // Remember to update the next pointer of the last ref.
 685   iter.update_discovered();
 686   // Close the reachable set
 687   complete_gc->do_void();
 688 }
 689 
 690 void
 691 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 692   oop obj = NULL;
 693   oop next = refs_list.head();
 694   while (next != obj) {
 695     obj = next;
 696     next = java_lang_ref_Reference::discovered(obj);
 697     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 698   }
 699   refs_list.set_head(NULL);
 700   refs_list.set_length(0);
 701 }
 702 
 703 void
 704 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
 705   clear_discovered_references(refs_list);
 706 }
 707 
 708 void ReferenceProcessor::abandon_partial_discovery() {
 709   // loop over the lists
 710   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 711     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 712       gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
 713     }
 714     abandon_partial_discovered_list(_discovered_refs[i]);
 715   }
 716 }
 717 
 718 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 719 public:
 720   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 721                     DiscoveredList      refs_lists[],
 722                     ReferencePolicy*    policy,
 723                     bool                marks_oops_alive)
 724     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 725       _policy(policy)
 726   { }
 727   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 728                     OopClosure& keep_alive,
 729                     VoidClosure& complete_gc)
 730   {
 731     Thread* thr = Thread::current();
 732     int refs_list_index = ((WorkerThread*)thr)->id();
 733     _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
 734                                   &is_alive, &keep_alive, &complete_gc);
 735   }
 736 private:
 737   ReferencePolicy* _policy;
 738 };
 739 
 740 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 741 public:
 742   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 743                     DiscoveredList      refs_lists[],
 744                     bool                marks_oops_alive)
 745     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
 746   { }
 747   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 748                     OopClosure& keep_alive,
 749                     VoidClosure& complete_gc)
 750   {
 751     _ref_processor.process_phase2(_refs_lists[i],
 752                                   &is_alive, &keep_alive, &complete_gc);
 753   }
 754 };
 755 
 756 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 757 public:
 758   RefProcPhase3Task(ReferenceProcessor& ref_processor,
 759                     DiscoveredList      refs_lists[],
 760                     bool                clear_referent,
 761                     bool                marks_oops_alive)
 762     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 763       _clear_referent(clear_referent)
 764   { }
 765   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 766                     OopClosure& keep_alive,
 767                     VoidClosure& complete_gc)
 768   {
 769     // Don't use "refs_list_index" calculated in this way because
 770     // balance_queues() has moved the Ref's into the first n queues.
 771     // Thread* thr = Thread::current();
 772     // int refs_list_index = ((WorkerThread*)thr)->id();
 773     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 774     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 775                                   &is_alive, &keep_alive, &complete_gc);
 776   }
 777 private:
 778   bool _clear_referent;
 779 };
 780 







 781 // Balances reference queues.
 782 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 783 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 784 // corresponding to the active workers will be processed.
 785 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 786 {
 787   // calculate total length
 788   size_t total_refs = 0;
 789   if (TraceReferenceGC && PrintGCDetails) {
 790     gclog_or_tty->print_cr("\nBalance ref_lists ");
 791   }
 792 
 793   for (uint i = 0; i < _max_num_q; ++i) {
 794     total_refs += ref_lists[i].length();
 795     if (TraceReferenceGC && PrintGCDetails) {
 796       gclog_or_tty->print("%d ", ref_lists[i].length());
 797     }
 798   }
 799   if (TraceReferenceGC && PrintGCDetails) {
 800     gclog_or_tty->print_cr(" = %d", total_refs);
 801   }
 802   size_t avg_refs = total_refs / _num_q + 1;
 803   uint to_idx = 0;
 804   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 805     bool move_all = false;
 806     if (from_idx >= _num_q) {
 807       move_all = ref_lists[from_idx].length() > 0;
 808     }
 809     while ((ref_lists[from_idx].length() > avg_refs) ||
 810            move_all) {
 811       assert(to_idx < _num_q, "Sanity Check!");
 812       if (ref_lists[to_idx].length() < avg_refs) {
 813         // move superfluous refs
 814         size_t refs_to_move;
 815         // Move all the Ref's if the from queue will not be processed.
 816         if (move_all) {
 817           refs_to_move = MIN2(ref_lists[from_idx].length(),
 818                               avg_refs - ref_lists[to_idx].length());
 819         } else {
 820           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 821                               avg_refs - ref_lists[to_idx].length());
 822         }
 823 
 824         assert(refs_to_move > 0, "otherwise the code below will fail");
 825 
 826         oop move_head = ref_lists[from_idx].head();
 827         oop move_tail = move_head;
 828         oop new_head  = move_head;
 829         // find an element to split the list on
 830         for (size_t j = 0; j < refs_to_move; ++j) {
 831           move_tail = new_head;
 832           new_head = java_lang_ref_Reference::discovered(new_head);
 833         }
 834 
 835         // Add the chain to the to list.
 836         if (ref_lists[to_idx].head() == NULL) {
 837           // to list is empty. Make a loop at the end.
 838           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
 839         } else {
 840           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
 841         }
 842         ref_lists[to_idx].set_head(move_head);
 843         ref_lists[to_idx].inc_length(refs_to_move);
 844 
 845         // Remove the chain from the from list.
 846         if (move_tail == new_head) {
 847           // We found the end of the from list.
 848           ref_lists[from_idx].set_head(NULL);
 849         } else {
 850           ref_lists[from_idx].set_head(new_head);
 851         }
 852         ref_lists[from_idx].dec_length(refs_to_move);
 853         if (ref_lists[from_idx].length() == 0) {
 854           break;
 855         }
 856       } else {
 857         to_idx = (to_idx + 1) % _num_q;
 858       }
 859     }
 860   }
 861 #ifdef ASSERT
 862   size_t balanced_total_refs = 0;
 863   for (uint i = 0; i < _max_num_q; ++i) {
 864     balanced_total_refs += ref_lists[i].length();
 865     if (TraceReferenceGC && PrintGCDetails) {
 866       gclog_or_tty->print("%d ", ref_lists[i].length());
 867     }
 868   }
 869   if (TraceReferenceGC && PrintGCDetails) {
 870     gclog_or_tty->print_cr(" = %d", balanced_total_refs);
 871     gclog_or_tty->flush();
 872   }
 873   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 874 #endif
 875 }
 876 
 877 void ReferenceProcessor::balance_all_queues() {
 878   balance_queues(_discoveredSoftRefs);
 879   balance_queues(_discoveredWeakRefs);
 880   balance_queues(_discoveredFinalRefs);
 881   balance_queues(_discoveredPhantomRefs);
 882 }
 883 
 884 size_t
 885 ReferenceProcessor::process_discovered_reflist(
 886   DiscoveredList               refs_lists[],
 887   ReferencePolicy*             policy,
 888   bool                         clear_referent,
 889   BoolObjectClosure*           is_alive,
 890   OopClosure*                  keep_alive,
 891   VoidClosure*                 complete_gc,
 892   AbstractRefProcTaskExecutor* task_executor)
 893 {
 894   bool mt_processing = task_executor != NULL && _processing_is_mt;
 895   // If discovery used MT and a dynamic number of GC threads, then
 896   // the queues must be balanced for correctness if fewer than the
 897   // maximum number of queues were used.  The number of queue used
 898   // during discovery may be different than the number to be used
 899   // for processing so don't depend of _num_q < _max_num_q as part
 900   // of the test.
 901   bool must_balance = _discovery_is_mt;
 902 
 903   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 904       must_balance) {
 905     balance_queues(refs_lists);
 906   }
 907 
 908   size_t total_list_count = total_count(refs_lists);
 909 
 910   if (PrintReferenceGC && PrintGCDetails) {
 911     gclog_or_tty->print(", %u refs", total_list_count);
 912   }
 913 
 914   // Phase 1 (soft refs only):
 915   // . Traverse the list and remove any SoftReferences whose
 916   //   referents are not alive, but that should be kept alive for
 917   //   policy reasons. Keep alive the transitive closure of all
 918   //   such referents.
 919   if (policy != NULL) {
 920     if (mt_processing) {
 921       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
 922       task_executor->execute(phase1);
 923     } else {
 924       for (uint i = 0; i < _max_num_q; i++) {
 925         process_phase1(refs_lists[i], policy,
 926                        is_alive, keep_alive, complete_gc);
 927       }
 928     }
 929   } else { // policy == NULL
 930     assert(refs_lists != _discoveredSoftRefs,
 931            "Policy must be specified for soft references.");
 932   }
 933 
 934   // Phase 2:
 935   // . Traverse the list and remove any refs whose referents are alive.
 936   if (mt_processing) {
 937     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
 938     task_executor->execute(phase2);
 939   } else {
 940     for (uint i = 0; i < _max_num_q; i++) {
 941       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 942     }
 943   }
 944 
 945   // Phase 3:
 946   // . Traverse the list and process referents as appropriate.
 947   if (mt_processing) {
 948     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
 949     task_executor->execute(phase3);
 950   } else {
 951     for (uint i = 0; i < _max_num_q; i++) {
 952       process_phase3(refs_lists[i], clear_referent,
 953                      is_alive, keep_alive, complete_gc);
 954     }
 955   }
 956 
 957   return total_list_count;
 958 }
 959 
 960 void ReferenceProcessor::clean_up_discovered_references() {
 961   // loop over the lists
 962   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 963     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 964       gclog_or_tty->print_cr(
 965         "\nScrubbing %s discovered list of Null referents",
 966         list_name(i));
 967     }
 968     clean_up_discovered_reflist(_discovered_refs[i]);
 969   }
 970 }
 971 
 972 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
 973   assert(!discovery_is_atomic(), "Else why call this method?");
 974   DiscoveredListIterator iter(refs_list, NULL, NULL);
 975   while (iter.has_next()) {
 976     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 977     oop next = java_lang_ref_Reference::next(iter.obj());
 978     assert(next->is_oop_or_null(), "bad next field");
 979     // If referent has been cleared or Reference is not active,
 980     // drop it.
 981     if (iter.referent() == NULL || next != NULL) {
 982       debug_only(
 983         if (PrintGCDetails && TraceReferenceGC) {
 984           gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
 985             INTPTR_FORMAT " with next field: " INTPTR_FORMAT
 986             " and referent: " INTPTR_FORMAT,
 987             (void *)iter.obj(), (void *)next, (void *)iter.referent());
 988         }
 989       )
 990       // Remove Reference object from list
 991       iter.remove();
 992       iter.move_to_next();
 993     } else {
 994       iter.next();
 995     }
 996   }
 997   NOT_PRODUCT(
 998     if (PrintGCDetails && TraceReferenceGC) {
 999       gclog_or_tty->print(
1000         " Removed %d Refs with NULL referents out of %d discovered Refs",
1001         iter.removed(), iter.processed());
1002     }
1003   )
1004 }
1005 
1006 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1007   uint id = 0;
1008   // Determine the queue index to use for this object.
1009   if (_discovery_is_mt) {
1010     // During a multi-threaded discovery phase,
1011     // each thread saves to its "own" list.
1012     Thread* thr = Thread::current();
1013     id = thr->as_Worker_thread()->id();
1014   } else {
1015     // single-threaded discovery, we save in round-robin
1016     // fashion to each of the lists.
1017     if (_processing_is_mt) {
1018       id = next_id();
1019     }
1020   }
1021   assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1022 
1023   // Get the discovered queue to which we will add
1024   DiscoveredList* list = NULL;
1025   switch (rt) {
1026     case REF_OTHER:
1027       // Unknown reference type, no special treatment
1028       break;
1029     case REF_SOFT:
1030       list = &_discoveredSoftRefs[id];
1031       break;
1032     case REF_WEAK:
1033       list = &_discoveredWeakRefs[id];
1034       break;
1035     case REF_FINAL:
1036       list = &_discoveredFinalRefs[id];
1037       break;
1038     case REF_PHANTOM:
1039       list = &_discoveredPhantomRefs[id];
1040       break;
1041     case REF_NONE:
1042       // we should not reach here if we are an InstanceRefKlass
1043     default:
1044       ShouldNotReachHere();
1045   }
1046   if (TraceReferenceGC && PrintGCDetails) {
1047     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1048   }
1049   return list;
1050 }
1051 
1052 inline void
1053 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1054                                               oop             obj,
1055                                               HeapWord*       discovered_addr) {
1056   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1057   // First we must make sure this object is only enqueued once. CAS in a non null
1058   // discovered_addr.
1059   oop current_head = refs_list.head();
1060   // The last ref must have its discovered field pointing to itself.
1061   oop next_discovered = (current_head != NULL) ? current_head : obj;
1062 










1063   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1064                                                     NULL);
1065   if (retest == NULL) {
1066     // This thread just won the right to enqueue the object.
1067     // We have separate lists for enqueueing, so no synchronization
1068     // is necessary.
1069     refs_list.set_head(obj);
1070     refs_list.inc_length(1);



1071 
1072     if (TraceReferenceGC) {
1073       gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1074                              (void *)obj, obj->klass()->internal_name());
1075     }
1076   } else {
1077     // If retest was non NULL, another thread beat us to it:
1078     // The reference has already been discovered...
1079     if (TraceReferenceGC) {
1080       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1081                              (void *)obj, obj->klass()->internal_name());
1082     }
1083   }
1084 }
1085 
1086 #ifndef PRODUCT
1087 // Non-atomic (i.e. concurrent) discovery might allow us
1088 // to observe j.l.References with NULL referents, being those
1089 // cleared concurrently by mutators during (or after) discovery.
1090 void ReferenceProcessor::verify_referent(oop obj) {
1091   bool da = discovery_is_atomic();
1092   oop referent = java_lang_ref_Reference::referent(obj);
1093   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1094          err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1095                  INTPTR_FORMAT " during %satomic discovery ",
1096                  (void *)referent, (void *)obj, da ? "" : "non-"));
1097 }
1098 #endif
1099 
1100 // We mention two of several possible choices here:
1101 // #0: if the reference object is not in the "originating generation"
1102 //     (or part of the heap being collected, indicated by our "span"
1103 //     we don't treat it specially (i.e. we scan it as we would
1104 //     a normal oop, treating its references as strong references).
1105 //     This means that references can't be discovered unless their
1106 //     referent is also in the same span. This is the simplest,
1107 //     most "local" and most conservative approach, albeit one
1108 //     that may cause weak references to be enqueued least promptly.
1109 //     We call this choice the "ReferenceBasedDiscovery" policy.
1110 // #1: the reference object may be in any generation (span), but if
1111 //     the referent is in the generation (span) being currently collected
1112 //     then we can discover the reference object, provided
1113 //     the object has not already been discovered by
1114 //     a different concurrently running collector (as may be the
1115 //     case, for instance, if the reference object is in CMS and
1116 //     the referent in DefNewGeneration), and provided the processing
1117 //     of this reference object by the current collector will
1118 //     appear atomic to every other collector in the system.
1119 //     (Thus, for instance, a concurrent collector may not
1120 //     discover references in other generations even if the
1121 //     referent is in its own generation). This policy may,
1122 //     in certain cases, enqueue references somewhat sooner than
1123 //     might Policy #0 above, but at marginally increased cost
1124 //     and complexity in processing these references.
1125 //     We call this choice the "RefeferentBasedDiscovery" policy.
1126 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1127   // Make sure we are discovering refs (rather than processing discovered refs).
1128   if (!_discovering_refs || !RegisterReferences) {
1129     return false;
1130   }
1131   // We only discover active references.
1132   oop next = java_lang_ref_Reference::next(obj);
1133   if (next != NULL) {   // Ref is no longer active
1134     return false;
1135   }
1136 
1137   HeapWord* obj_addr = (HeapWord*)obj;
1138   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1139       !_span.contains(obj_addr)) {
1140     // Reference is not in the originating generation;
1141     // don't treat it specially (i.e. we want to scan it as a normal
1142     // object with strong references).
1143     return false;
1144   }
1145 
1146   // We only discover references whose referents are not (yet)
1147   // known to be strongly reachable.
1148   if (is_alive_non_header() != NULL) {
1149     verify_referent(obj);
1150     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1151       return false;  // referent is reachable
1152     }
1153   }
1154   if (rt == REF_SOFT) {
1155     // For soft refs we can decide now if these are not
1156     // current candidates for clearing, in which case we
1157     // can mark through them now, rather than delaying that
1158     // to the reference-processing phase. Since all current
1159     // time-stamp policies advance the soft-ref clock only
1160     // at a major collection cycle, this is always currently
1161     // accurate.
1162     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1163       return false;
1164     }
1165   }
1166 
1167   ResourceMark rm;      // Needed for tracing.
1168 
1169   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1170   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1171   assert(discovered->is_oop_or_null(), "bad discovered field");
1172   if (discovered != NULL) {
1173     // The reference has already been discovered...
1174     if (TraceReferenceGC) {
1175       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1176                              (void *)obj, obj->klass()->internal_name());
1177     }
1178     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1179       // assumes that an object is not processed twice;
1180       // if it's been already discovered it must be on another
1181       // generation's discovered list; so we won't discover it.
1182       return false;
1183     } else {
1184       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1185              "Unrecognized policy");
1186       // Check assumption that an object is not potentially
1187       // discovered twice except by concurrent collectors that potentially
1188       // trace the same Reference object twice.
1189       assert(UseConcMarkSweepGC || UseG1GC,
1190              "Only possible with a concurrent marking collector");
1191       return true;
1192     }
1193   }
1194 
1195   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1196     verify_referent(obj);
1197     // Discover if and only if EITHER:
1198     // .. reference is in our span, OR
1199     // .. we are an atomic collector and referent is in our span
1200     if (_span.contains(obj_addr) ||
1201         (discovery_is_atomic() &&
1202          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1203       // should_enqueue = true;
1204     } else {
1205       return false;
1206     }
1207   } else {
1208     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1209            _span.contains(obj_addr), "code inconsistency");
1210   }
1211 
1212   // Get the right type of discovered queue head.
1213   DiscoveredList* list = get_discovered_list(rt);
1214   if (list == NULL) {
1215     return false;   // nothing special needs to be done
1216   }
1217 
1218   if (_discovery_is_mt) {
1219     add_to_discovered_list_mt(*list, obj, discovered_addr);
1220   } else {
1221     // We do a raw store here: the field will be visited later when processing
1222     // the discovered references.


1223     oop current_head = list->head();
1224     // The last ref must have its discovered field pointing to itself.
1225     oop next_discovered = (current_head != NULL) ? current_head : obj;
1226 



1227     assert(discovered == NULL, "control point invariant");


1228     oop_store_raw(discovered_addr, next_discovered);



1229     list->set_head(obj);
1230     list->inc_length(1);
1231 
1232     if (TraceReferenceGC) {
1233       gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1234                                 (void *)obj, obj->klass()->internal_name());
1235     }
1236   }
1237   assert(obj->is_oop(), "Discovered a bad reference");
1238   verify_referent(obj);
1239   return true;
1240 }
1241 
1242 // Preclean the discovered references by removing those
1243 // whose referents are alive, and by marking from those that
1244 // are not active. These lists can be handled here
1245 // in any order and, indeed, concurrently.
1246 void ReferenceProcessor::preclean_discovered_references(
1247   BoolObjectClosure* is_alive,
1248   OopClosure* keep_alive,
1249   VoidClosure* complete_gc,
1250   YieldClosure* yield,
1251   GCTimer* gc_timer) {
1252 
1253   NOT_PRODUCT(verify_ok_to_handle_reflists());
1254 
1255   // Soft references
1256   {
1257     GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1258               false, gc_timer);
1259     for (uint i = 0; i < _max_num_q; i++) {
1260       if (yield->should_return()) {
1261         return;
1262       }
1263       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1264                                   keep_alive, complete_gc, yield);
1265     }
1266   }
1267 
1268   // Weak references
1269   {
1270     GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1271               false, gc_timer);
1272     for (uint i = 0; i < _max_num_q; i++) {
1273       if (yield->should_return()) {
1274         return;
1275       }
1276       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1277                                   keep_alive, complete_gc, yield);
1278     }
1279   }
1280 
1281   // Final references
1282   {
1283     GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1284               false, gc_timer);
1285     for (uint i = 0; i < _max_num_q; i++) {
1286       if (yield->should_return()) {
1287         return;
1288       }
1289       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1290                                   keep_alive, complete_gc, yield);
1291     }
1292   }
1293 
1294   // Phantom references
1295   {
1296     GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1297               false, gc_timer);
1298     for (uint i = 0; i < _max_num_q; i++) {
1299       if (yield->should_return()) {
1300         return;
1301       }
1302       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1303                                   keep_alive, complete_gc, yield);
1304     }
1305   }
1306 }
1307 
1308 // Walk the given discovered ref list, and remove all reference objects
1309 // whose referents are still alive, whose referents are NULL or which
1310 // are not active (have a non-NULL next field). NOTE: When we are
1311 // thus precleaning the ref lists (which happens single-threaded today),
1312 // we do not disable refs discovery to honor the correct semantics of
1313 // java.lang.Reference. As a result, we need to be careful below
1314 // that ref removal steps interleave safely with ref discovery steps
1315 // (in this thread).
1316 void
1317 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1318                                                 BoolObjectClosure* is_alive,
1319                                                 OopClosure*        keep_alive,
1320                                                 VoidClosure*       complete_gc,
1321                                                 YieldClosure*      yield) {
1322   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1323   while (iter.has_next()) {
1324     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1325     oop obj = iter.obj();
1326     oop next = java_lang_ref_Reference::next(obj);
1327     if (iter.referent() == NULL || iter.is_referent_alive() ||
1328         next != NULL) {
1329       // The referent has been cleared, or is alive, or the Reference is not
1330       // active; we need to trace and mark its cohort.
1331       if (TraceReferenceGC) {
1332         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1333                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
1334       }
1335       // Remove Reference object from list
1336       iter.remove();
1337       // Keep alive its cohort.
1338       iter.make_referent_alive();
1339       if (UseCompressedOops) {
1340         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1341         keep_alive->do_oop(next_addr);
1342       } else {
1343         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1344         keep_alive->do_oop(next_addr);
1345       }
1346       iter.move_to_next();
1347     } else {
1348       iter.next();
1349     }
1350   }
1351   // Close the reachable set
1352   complete_gc->do_void();
1353 
1354   NOT_PRODUCT(
1355     if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1356       gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1357         "Refs in discovered list " INTPTR_FORMAT,
1358         iter.removed(), iter.processed(), (address)refs_list.head());
1359     }
1360   )
1361 }
1362 
1363 const char* ReferenceProcessor::list_name(uint i) {
1364    assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1365           "Out of bounds index");
1366 
1367    int j = i / _max_num_q;
1368    switch (j) {
1369      case 0: return "SoftRef";
1370      case 1: return "WeakRef";
1371      case 2: return "FinalRef";
1372      case 3: return "PhantomRef";
1373    }
1374    ShouldNotReachHere();
1375    return NULL;
1376 }
1377 
1378 #ifndef PRODUCT
1379 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1380   // empty for now
1381 }
1382 #endif
1383 
1384 #ifndef PRODUCT
1385 void ReferenceProcessor::clear_discovered_references() {
1386   guarantee(!_discovering_refs, "Discovering refs?");
1387   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1388     clear_discovered_references(_discovered_refs[i]);
1389   }
1390 }
1391 
1392 #endif // PRODUCT
--- EOF ---