rev 5823 : 8031703: Missing post-barrier in ReferenceProcessor

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