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                                        bool      discovered_list_needs_post_barrier)  :
 101   _discovering_refs(false),
 102   _enqueuing_is_done(false),
 103   _is_alive_non_header(is_alive_non_header),
 104   _discovered_list_needs_post_barrier(discovered_list_needs_post_barrier),
 105   _processing_is_mt(mt_processing),
 106   _next_id(0)
 107 {
 108   _span = span;
 109   _discovery_is_atomic = atomic_discovery;
 110   _discovery_is_mt     = mt_discovery;
 111   _num_q               = MAX2(1U, mt_processing_degree);
 112   _max_num_q           = MAX2(_num_q, mt_discovery_degree);
 113   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,
 114             _max_num_q * number_of_subclasses_of_ref(), mtGC);
 115 
 116   if (_discovered_refs == NULL) {
 117     vm_exit_during_initialization("Could not allocated RefProc Array");
 118   }
 119   _discoveredSoftRefs    = &_discovered_refs[0];
 120   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];
 121   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];
 122   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
 123 
 124   // Initialize all entries to NULL
 125   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 126     _discovered_refs[i].set_head(NULL);
 127     _discovered_refs[i].set_length(0);
 128   }
 129 
 130   setup_policy(false /* default soft ref policy */);
 131 }
 132 
 133 #ifndef PRODUCT
 134 void ReferenceProcessor::verify_no_references_recorded() {
 135   guarantee(!_discovering_refs, "Discovering refs?");
 136   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 137     guarantee(_discovered_refs[i].is_empty(),
 138               "Found non-empty discovered list");
 139   }
 140 }
 141 #endif
 142 
 143 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 144   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 145     if (UseCompressedOops) {
 146       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
 147     } else {
 148       f->do_oop((oop*)_discovered_refs[i].adr_head());
 149     }
 150   }
 151 }
 152 
 153 void ReferenceProcessor::update_soft_ref_master_clock() {
 154   // Update (advance) the soft ref master clock field. This must be done
 155   // after processing the soft ref list.
 156 
 157   // We need a monotonically non-decreasing time in ms but
 158   // os::javaTimeMillis() does not guarantee monotonicity.
 159   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 160   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
 161   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
 162 
 163   NOT_PRODUCT(
 164   if (now < _soft_ref_timestamp_clock) {
 165     warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
 166             _soft_ref_timestamp_clock, now);
 167   }
 168   )
 169   // The values of now and _soft_ref_timestamp_clock are set using
 170   // javaTimeNanos(), which is guaranteed to be monotonically
 171   // non-decreasing provided the underlying platform provides such
 172   // a time source (and it is bug free).
 173   // In product mode, however, protect ourselves from non-monotonicity.
 174   if (now > _soft_ref_timestamp_clock) {
 175     _soft_ref_timestamp_clock = now;
 176     java_lang_ref_SoftReference::set_clock(now);
 177   }
 178   // Else leave clock stalled at its old value until time progresses
 179   // past clock value.
 180 }
 181 
 182 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
 183   size_t total = 0;
 184   for (uint i = 0; i < _max_num_q; ++i) {
 185     total += lists[i].length();
 186   }
 187   return total;
 188 }
 189 
 190 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 191   BoolObjectClosure*           is_alive,
 192   OopClosure*                  keep_alive,
 193   VoidClosure*                 complete_gc,
 194   AbstractRefProcTaskExecutor* task_executor,
 195   GCTimer*                     gc_timer) {
 196   NOT_PRODUCT(verify_ok_to_handle_reflists());
 197 
 198   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 199   // Stop treating discovered references specially.
 200   disable_discovery();
 201 
 202   // If discovery was concurrent, someone could have modified
 203   // the value of the static field in the j.l.r.SoftReference
 204   // class that holds the soft reference timestamp clock using
 205   // reflection or Unsafe between when discovery was enabled and
 206   // now. Unconditionally update the static field in ReferenceProcessor
 207   // here so that we use the new value during processing of the
 208   // discovered soft refs.
 209 
 210   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 211 
 212   bool trace_time = PrintGCDetails && PrintReferenceGC;
 213 
 214   // Soft references
 215   size_t soft_count = 0;
 216   {
 217     GCTraceTime tt("SoftReference", trace_time, false, gc_timer);
 218     soft_count =
 219       process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 220                                  is_alive, keep_alive, complete_gc, task_executor);
 221   }
 222 
 223   update_soft_ref_master_clock();
 224 
 225   // Weak references
 226   size_t weak_count = 0;
 227   {
 228     GCTraceTime tt("WeakReference", trace_time, false, gc_timer);
 229     weak_count =
 230       process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 231                                  is_alive, keep_alive, complete_gc, task_executor);
 232   }
 233 
 234   // Final references
 235   size_t final_count = 0;
 236   {
 237     GCTraceTime tt("FinalReference", trace_time, false, gc_timer);
 238     final_count =
 239       process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 240                                  is_alive, keep_alive, complete_gc, task_executor);
 241   }
 242 
 243   // Phantom references
 244   size_t phantom_count = 0;
 245   {
 246     GCTraceTime tt("PhantomReference", trace_time, false, gc_timer);
 247     phantom_count =
 248       process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 249                                  is_alive, keep_alive, complete_gc, task_executor);
 250   }
 251 
 252   // Weak global JNI references. It would make more sense (semantically) to
 253   // traverse these simultaneously with the regular weak references above, but
 254   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 255   // thus use JNI weak references to circumvent the phantom references and
 256   // resurrect a "post-mortem" object.
 257   {
 258     GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
 259     if (task_executor != NULL) {
 260       task_executor->set_single_threaded_mode();
 261     }
 262     process_phaseJNI(is_alive, keep_alive, complete_gc);
 263   }
 264 
 265   return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
 266 }
 267 
 268 #ifndef PRODUCT
 269 // Calculate the number of jni handles.
 270 uint ReferenceProcessor::count_jni_refs() {
 271   class AlwaysAliveClosure: public BoolObjectClosure {
 272   public:
 273     virtual bool do_object_b(oop obj) { return true; }
 274   };
 275 
 276   class CountHandleClosure: public OopClosure {
 277   private:
 278     int _count;
 279   public:
 280     CountHandleClosure(): _count(0) {}
 281     void do_oop(oop* unused)       { _count++; }
 282     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 283     int count() { return _count; }
 284   };
 285   CountHandleClosure global_handle_count;
 286   AlwaysAliveClosure always_alive;
 287   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
 288   return global_handle_count.count();
 289 }
 290 #endif
 291 
 292 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 293                                           OopClosure*        keep_alive,
 294                                           VoidClosure*       complete_gc) {
 295 #ifndef PRODUCT
 296   if (PrintGCDetails && PrintReferenceGC) {
 297     unsigned int count = count_jni_refs();
 298     gclog_or_tty->print(", %u refs", count);
 299   }
 300 #endif
 301   JNIHandles::weak_oops_do(is_alive, keep_alive);
 302   complete_gc->do_void();
 303 }
 304 
 305 
 306 template <class T>
 307 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 308                                    AbstractRefProcTaskExecutor* task_executor) {
 309 
 310   // Remember old value of pending references list
 311   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
 312   T old_pending_list_value = *pending_list_addr;
 313 
 314   // Enqueue references that are not made active again, and
 315   // clear the decks for the next collection (cycle).
 316   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
 317   // Do the post-barrier on pending_list_addr missed in
 318   // enqueue_discovered_reflist.
 319   oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
 320 
 321   // Stop treating discovered references specially.
 322   ref->disable_discovery();
 323 
 324   // Return true if new pending references were added
 325   return old_pending_list_value != *pending_list_addr;
 326 }
 327 
 328 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
 329   NOT_PRODUCT(verify_ok_to_handle_reflists());
 330   if (UseCompressedOops) {
 331     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 332   } else {
 333     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 334   }
 335 }
 336 
 337 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 338                                                     HeapWord* pending_list_addr) {
 339   // Given a list of refs linked through the "discovered" field
 340   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 341   // thus distinguishing them from active References, then
 342   // prepend them to the pending list.
 343   // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
 344   // the "next" field is used to chain the pending list, not the discovered
 345   // field.
 346 
 347   if (TraceReferenceGC && PrintGCDetails) {
 348     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
 349                            INTPTR_FORMAT, (address)refs_list.head());
 350   }
 351 
 352   oop obj = NULL;
 353   oop next_d = refs_list.head();
 354   if (pending_list_uses_discovered_field()) { // New behavior
 355     // Walk down the list, self-looping the next field
 356     // so that the References are not considered active.
 357     while (obj != next_d) {
 358       obj = next_d;
 359       assert(obj->is_instanceRef(), "should be reference object");
 360       next_d = java_lang_ref_Reference::discovered(obj);
 361       if (TraceReferenceGC && PrintGCDetails) {
 362         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 363                                (void *)obj, (void *)next_d);
 364       }
 365       assert(java_lang_ref_Reference::next(obj) == NULL,
 366              "Reference not active; should not be discovered");
 367       // Self-loop next, so as to make Ref not active.
 368       // Post-barrier not needed when looping to self.
 369       java_lang_ref_Reference::set_next_raw(obj, obj);
 370       if (next_d == obj) {  // obj is last








 371         // Swap refs_list into pending_list_addr and
 372         // set obj's discovered to what we read from pending_list_addr.
 373         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 374         // Need post-barrier on pending_list_addr above;
 375         // see special post-barrier code at the end of
 376         // enqueue_discovered_reflists() further below.
 377         java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
 378         oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
 379       }
 380     }
 381   } else { // Old behavior
 382     // Walk down the list, copying the discovered field into
 383     // the next field and clearing the discovered field.
 384     while (obj != next_d) {
 385       obj = next_d;
 386       assert(obj->is_instanceRef(), "should be reference object");
 387       next_d = java_lang_ref_Reference::discovered(obj);
 388       if (TraceReferenceGC && PrintGCDetails) {
 389         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 390                                (void *)obj, (void *)next_d);
 391       }
 392       assert(java_lang_ref_Reference::next(obj) == NULL,
 393              "The reference should not be enqueued");
 394       if (next_d == obj) {  // obj is last
 395         // Swap refs_list into pending_list_addr and
 396         // set obj's next to what we read from pending_list_addr.
 397         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 398         // Need oop_check on pending_list_addr above;
 399         // see special oop-check code at the end of
 400         // enqueue_discovered_reflists() further below.
 401         if (old == NULL) {
 402           // obj should be made to point to itself, since
 403           // pending list was empty.
 404           java_lang_ref_Reference::set_next(obj, obj);
 405         } else {
 406           java_lang_ref_Reference::set_next(obj, old);
 407         }
 408       } else {
 409         java_lang_ref_Reference::set_next(obj, next_d);
 410       }
 411       java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
 412     }
 413   }
 414 }
 415 
 416 // Parallel enqueue task
 417 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 418 public:
 419   RefProcEnqueueTask(ReferenceProcessor& ref_processor,
 420                      DiscoveredList      discovered_refs[],
 421                      HeapWord*           pending_list_addr,
 422                      int                 n_queues)
 423     : EnqueueTask(ref_processor, discovered_refs,
 424                   pending_list_addr, n_queues)
 425   { }
 426 
 427   virtual void work(unsigned int work_id) {
 428     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 429     // Simplest first cut: static partitioning.
 430     int index = work_id;
 431     // The increment on "index" must correspond to the maximum number of queues
 432     // (n_queues) with which that ReferenceProcessor was created.  That
 433     // is because of the "clever" way the discovered references lists were
 434     // allocated and are indexed into.
 435     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 436     for (int j = 0;
 437          j < ReferenceProcessor::number_of_subclasses_of_ref();
 438          j++, index += _n_queues) {
 439       _ref_processor.enqueue_discovered_reflist(
 440         _refs_lists[index], _pending_list_addr);
 441       _refs_lists[index].set_head(NULL);
 442       _refs_lists[index].set_length(0);
 443     }
 444   }
 445 };
 446 
 447 // Enqueue references that are not made active again
 448 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
 449   AbstractRefProcTaskExecutor* task_executor) {
 450   if (_processing_is_mt && task_executor != NULL) {
 451     // Parallel code
 452     RefProcEnqueueTask tsk(*this, _discovered_refs,
 453                            pending_list_addr, _max_num_q);
 454     task_executor->execute(tsk);
 455   } else {
 456     // Serial code: call the parent class's implementation
 457     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 458       enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
 459       _discovered_refs[i].set_head(NULL);
 460       _discovered_refs[i].set_length(0);
 461     }
 462   }
 463 }
 464 
 465 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 466   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 467   oop discovered = java_lang_ref_Reference::discovered(_ref);
 468   assert(_discovered_addr && discovered->is_oop_or_null(),
 469          "discovered field is bad");
 470   _next = discovered;
 471   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 472   _referent = java_lang_ref_Reference::referent(_ref);
 473   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 474          "Wrong oop found in java.lang.Reference object");
 475   assert(allow_null_referent ?
 476              _referent->is_oop_or_null()
 477            : _referent->is_oop(),
 478          "bad referent");
 479 }
 480 
 481 void DiscoveredListIterator::remove() {
 482   assert(_ref->is_oop(), "Dropping a bad reference");
 483   oop_store_raw(_discovered_addr, NULL);
 484 
 485   // First _prev_next ref actually points into DiscoveredList (gross).
 486   oop new_next;
 487   if (_next == _ref) {
 488     // At the end of the list, we should make _prev point to itself.
 489     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 490     // and _prev will be NULL.
 491     new_next = _prev;
 492   } else {
 493     new_next = _next;
 494   }
 495   // Remove Reference object from discovered list. Note that G1 does not need a
 496   // pre-barrier here because we know the Reference has already been found/marked,
 497   // that's how it ended up in the discovered list in the first place.
 498   oop_store_raw(_prev_next, new_next);
 499   if (_discovered_list_needs_post_barrier && _prev_next != _refs_list.adr_head()) {
 500     // Needs post-barrier and this is not the list head (which is not on the heap)
 501     oopDesc::bs()->write_ref_field(_prev_next, new_next);
 502   }
 503   NOT_PRODUCT(_removed++);
 504   _refs_list.dec_length(1);
 505 }
 506 
 507 // Make the Reference object active again.
 508 void DiscoveredListIterator::make_active() {
 509   // For G1 we don't want to use set_next - it
 510   // will dirty the card for the next field of
 511   // the reference object and will fail
 512   // CT verification.
 513   if (UseG1GC) {
 514     HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
 515     if (UseCompressedOops) {
 516       oopDesc::bs()->write_ref_field_pre((narrowOop*)next_addr, NULL);
 517     } else {
 518       oopDesc::bs()->write_ref_field_pre((oop*)next_addr, NULL);
 519     }
 520     java_lang_ref_Reference::set_next_raw(_ref, NULL);
 521   } else {
 522     java_lang_ref_Reference::set_next(_ref, NULL);
 523   }

 524 }
 525 
 526 void DiscoveredListIterator::clear_referent() {
 527   oop_store_raw(_referent_addr, NULL);
 528 }
 529 
 530 // NOTE: process_phase*() are largely similar, and at a high level
 531 // merely iterate over the extant list applying a predicate to
 532 // each of its elements and possibly removing that element from the
 533 // list and applying some further closures to that element.
 534 // We should consider the possibility of replacing these
 535 // process_phase*() methods by abstracting them into
 536 // a single general iterator invocation that receives appropriate
 537 // closures that accomplish this work.
 538 
 539 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 540 // referents are not alive, but that should be kept alive for policy reasons.
 541 // Keep alive the transitive closure of all such referents.
 542 void
 543 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 544                                    ReferencePolicy*   policy,
 545                                    BoolObjectClosure* is_alive,
 546                                    OopClosure*        keep_alive,
 547                                    VoidClosure*       complete_gc) {
 548   assert(policy != NULL, "Must have a non-NULL policy");
 549   DiscoveredListIterator iter(refs_list, keep_alive, is_alive, _discovered_list_needs_post_barrier);
 550   // Decide which softly reachable refs should be kept alive.
 551   while (iter.has_next()) {
 552     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 553     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 554     if (referent_is_dead &&
 555         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 556       if (TraceReferenceGC) {
 557         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 558                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 559       }
 560       // Remove Reference object from list
 561       iter.remove();
 562       // Make the Reference object active again
 563       iter.make_active();
 564       // keep the referent around
 565       iter.make_referent_alive();
 566       iter.move_to_next();
 567     } else {
 568       iter.next();
 569     }
 570   }
 571   // Close the reachable set
 572   complete_gc->do_void();
 573   NOT_PRODUCT(
 574     if (PrintGCDetails && TraceReferenceGC) {
 575       gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
 576         "discovered Refs by policy, from list " INTPTR_FORMAT,
 577         iter.removed(), iter.processed(), (address)refs_list.head());
 578     }
 579   )
 580 }
 581 
 582 // Traverse the list and remove any Refs that are not active, or
 583 // whose referents are either alive or NULL.
 584 void
 585 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 586                              BoolObjectClosure* is_alive,
 587                              OopClosure*        keep_alive) {
 588   assert(discovery_is_atomic(), "Error");
 589   DiscoveredListIterator iter(refs_list, keep_alive, is_alive, _discovered_list_needs_post_barrier);
 590   while (iter.has_next()) {
 591     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 592     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 593     assert(next == NULL, "Should not discover inactive Reference");
 594     if (iter.is_referent_alive()) {
 595       if (TraceReferenceGC) {
 596         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 597                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
 598       }
 599       // The referent is reachable after all.
 600       // Remove Reference object from list.
 601       iter.remove();
 602       // Update the referent pointer as necessary: Note that this
 603       // should not entail any recursive marking because the
 604       // referent must already have been traversed.
 605       iter.make_referent_alive();
 606       iter.move_to_next();
 607     } else {
 608       iter.next();
 609     }
 610   }
 611   NOT_PRODUCT(
 612     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 613       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 614         "Refs in discovered list " INTPTR_FORMAT,
 615         iter.removed(), iter.processed(), (address)refs_list.head());
 616     }
 617   )
 618 }
 619 
 620 void
 621 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 622                                                   BoolObjectClosure* is_alive,
 623                                                   OopClosure*        keep_alive,
 624                                                   VoidClosure*       complete_gc) {
 625   assert(!discovery_is_atomic(), "Error");
 626   DiscoveredListIterator iter(refs_list, keep_alive, is_alive, _discovered_list_needs_post_barrier);
 627   while (iter.has_next()) {
 628     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 629     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 630     oop next = java_lang_ref_Reference::next(iter.obj());
 631     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 632          next != NULL)) {
 633       assert(next->is_oop_or_null(), "bad next field");
 634       // Remove Reference object from list
 635       iter.remove();
 636       // Trace the cohorts
 637       iter.make_referent_alive();
 638       if (UseCompressedOops) {
 639         keep_alive->do_oop((narrowOop*)next_addr);
 640       } else {
 641         keep_alive->do_oop((oop*)next_addr);
 642       }
 643       iter.move_to_next();
 644     } else {
 645       iter.next();
 646     }
 647   }
 648   // Now close the newly reachable set
 649   complete_gc->do_void();
 650   NOT_PRODUCT(
 651     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 652       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 653         "Refs in discovered list " INTPTR_FORMAT,
 654         iter.removed(), iter.processed(), (address)refs_list.head());
 655     }
 656   )
 657 }
 658 
 659 // Traverse the list and process the referents, by either
 660 // clearing them or keeping them (and their reachable
 661 // closure) alive.
 662 void
 663 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 664                                    bool               clear_referent,
 665                                    BoolObjectClosure* is_alive,
 666                                    OopClosure*        keep_alive,
 667                                    VoidClosure*       complete_gc) {
 668   ResourceMark rm;
 669   DiscoveredListIterator iter(refs_list, keep_alive, is_alive, _discovered_list_needs_post_barrier);
 670   while (iter.has_next()) {
 671     iter.update_discovered();
 672     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 673     if (clear_referent) {
 674       // NULL out referent pointer
 675       iter.clear_referent();
 676     } else {
 677       // keep the referent around
 678       iter.make_referent_alive();
 679     }
 680     if (TraceReferenceGC) {
 681       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 682                              clear_referent ? "cleared " : "",
 683                              (void *)iter.obj(), iter.obj()->klass()->internal_name());
 684     }
 685     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 686     iter.next();
 687   }
 688   // Remember to update the next pointer of the last ref.
 689   iter.update_discovered();
 690   // Close the reachable set
 691   complete_gc->do_void();
 692 }
 693 
 694 void
 695 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 696   oop obj = NULL;
 697   oop next = refs_list.head();
 698   while (next != obj) {
 699     obj = next;
 700     next = java_lang_ref_Reference::discovered(obj);
 701     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 702   }
 703   refs_list.set_head(NULL);
 704   refs_list.set_length(0);
 705 }
 706 
 707 void
 708 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
 709   clear_discovered_references(refs_list);
 710 }
 711 
 712 void ReferenceProcessor::abandon_partial_discovery() {
 713   // loop over the lists
 714   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 715     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 716       gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
 717     }
 718     abandon_partial_discovered_list(_discovered_refs[i]);
 719   }
 720 }
 721 
 722 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 723 public:
 724   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 725                     DiscoveredList      refs_lists[],
 726                     ReferencePolicy*    policy,
 727                     bool                marks_oops_alive)
 728     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 729       _policy(policy)
 730   { }
 731   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 732                     OopClosure& keep_alive,
 733                     VoidClosure& complete_gc)
 734   {
 735     Thread* thr = Thread::current();
 736     int refs_list_index = ((WorkerThread*)thr)->id();
 737     _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
 738                                   &is_alive, &keep_alive, &complete_gc);
 739   }
 740 private:
 741   ReferencePolicy* _policy;
 742 };
 743 
 744 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 745 public:
 746   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 747                     DiscoveredList      refs_lists[],
 748                     bool                marks_oops_alive)
 749     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
 750   { }
 751   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 752                     OopClosure& keep_alive,
 753                     VoidClosure& complete_gc)
 754   {
 755     _ref_processor.process_phase2(_refs_lists[i],
 756                                   &is_alive, &keep_alive, &complete_gc);
 757   }
 758 };
 759 
 760 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 761 public:
 762   RefProcPhase3Task(ReferenceProcessor& ref_processor,
 763                     DiscoveredList      refs_lists[],
 764                     bool                clear_referent,
 765                     bool                marks_oops_alive)
 766     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 767       _clear_referent(clear_referent)
 768   { }
 769   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 770                     OopClosure& keep_alive,
 771                     VoidClosure& complete_gc)
 772   {
 773     // Don't use "refs_list_index" calculated in this way because
 774     // balance_queues() has moved the Ref's into the first n queues.
 775     // Thread* thr = Thread::current();
 776     // int refs_list_index = ((WorkerThread*)thr)->id();
 777     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 778     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 779                                   &is_alive, &keep_alive, &complete_gc);
 780   }
 781 private:
 782   bool _clear_referent;
 783 };
 784 
 785 void ReferenceProcessor::set_discovered(oop ref, oop value) {
 786   java_lang_ref_Reference::set_discovered_raw(ref, value);
 787   if (_discovered_list_needs_post_barrier) {
 788     oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(ref), value);
 789   }
 790 }
 791 
 792 // Balances reference queues.
 793 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 794 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 795 // corresponding to the active workers will be processed.
 796 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 797 {
 798   // calculate total length
 799   size_t total_refs = 0;
 800   if (TraceReferenceGC && PrintGCDetails) {
 801     gclog_or_tty->print_cr("\nBalance ref_lists ");
 802   }
 803 
 804   for (uint i = 0; i < _max_num_q; ++i) {
 805     total_refs += ref_lists[i].length();
 806     if (TraceReferenceGC && PrintGCDetails) {
 807       gclog_or_tty->print("%d ", ref_lists[i].length());
 808     }
 809   }
 810   if (TraceReferenceGC && PrintGCDetails) {
 811     gclog_or_tty->print_cr(" = %d", total_refs);
 812   }
 813   size_t avg_refs = total_refs / _num_q + 1;
 814   uint to_idx = 0;
 815   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 816     bool move_all = false;
 817     if (from_idx >= _num_q) {
 818       move_all = ref_lists[from_idx].length() > 0;
 819     }
 820     while ((ref_lists[from_idx].length() > avg_refs) ||
 821            move_all) {
 822       assert(to_idx < _num_q, "Sanity Check!");
 823       if (ref_lists[to_idx].length() < avg_refs) {
 824         // move superfluous refs
 825         size_t refs_to_move;
 826         // Move all the Ref's if the from queue will not be processed.
 827         if (move_all) {
 828           refs_to_move = MIN2(ref_lists[from_idx].length(),
 829                               avg_refs - ref_lists[to_idx].length());
 830         } else {
 831           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 832                               avg_refs - ref_lists[to_idx].length());
 833         }
 834 
 835         assert(refs_to_move > 0, "otherwise the code below will fail");
 836 
 837         oop move_head = ref_lists[from_idx].head();
 838         oop move_tail = move_head;
 839         oop new_head  = move_head;
 840         // find an element to split the list on
 841         for (size_t j = 0; j < refs_to_move; ++j) {
 842           move_tail = new_head;
 843           new_head = java_lang_ref_Reference::discovered(new_head);
 844         }
 845 
 846         // Add the chain to the to list.
 847         if (ref_lists[to_idx].head() == NULL) {
 848           // to list is empty. Make a loop at the end.
 849           set_discovered(move_tail, move_tail);
 850         } else {
 851           set_discovered(move_tail, ref_lists[to_idx].head());
 852         }
 853         ref_lists[to_idx].set_head(move_head);
 854         ref_lists[to_idx].inc_length(refs_to_move);
 855 
 856         // Remove the chain from the from list.
 857         if (move_tail == new_head) {
 858           // We found the end of the from list.
 859           ref_lists[from_idx].set_head(NULL);
 860         } else {
 861           ref_lists[from_idx].set_head(new_head);
 862         }
 863         ref_lists[from_idx].dec_length(refs_to_move);
 864         if (ref_lists[from_idx].length() == 0) {
 865           break;
 866         }
 867       } else {
 868         to_idx = (to_idx + 1) % _num_q;
 869       }
 870     }
 871   }
 872 #ifdef ASSERT
 873   size_t balanced_total_refs = 0;
 874   for (uint i = 0; i < _max_num_q; ++i) {
 875     balanced_total_refs += ref_lists[i].length();
 876     if (TraceReferenceGC && PrintGCDetails) {
 877       gclog_or_tty->print("%d ", ref_lists[i].length());
 878     }
 879   }
 880   if (TraceReferenceGC && PrintGCDetails) {
 881     gclog_or_tty->print_cr(" = %d", balanced_total_refs);
 882     gclog_or_tty->flush();
 883   }
 884   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 885 #endif
 886 }
 887 
 888 void ReferenceProcessor::balance_all_queues() {
 889   balance_queues(_discoveredSoftRefs);
 890   balance_queues(_discoveredWeakRefs);
 891   balance_queues(_discoveredFinalRefs);
 892   balance_queues(_discoveredPhantomRefs);
 893 }
 894 
 895 size_t
 896 ReferenceProcessor::process_discovered_reflist(
 897   DiscoveredList               refs_lists[],
 898   ReferencePolicy*             policy,
 899   bool                         clear_referent,
 900   BoolObjectClosure*           is_alive,
 901   OopClosure*                  keep_alive,
 902   VoidClosure*                 complete_gc,
 903   AbstractRefProcTaskExecutor* task_executor)
 904 {
 905   bool mt_processing = task_executor != NULL && _processing_is_mt;
 906   // If discovery used MT and a dynamic number of GC threads, then
 907   // the queues must be balanced for correctness if fewer than the
 908   // maximum number of queues were used.  The number of queue used
 909   // during discovery may be different than the number to be used
 910   // for processing so don't depend of _num_q < _max_num_q as part
 911   // of the test.
 912   bool must_balance = _discovery_is_mt;
 913 
 914   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 915       must_balance) {
 916     balance_queues(refs_lists);
 917   }
 918 
 919   size_t total_list_count = total_count(refs_lists);
 920 
 921   if (PrintReferenceGC && PrintGCDetails) {
 922     gclog_or_tty->print(", %u refs", total_list_count);
 923   }
 924 
 925   // Phase 1 (soft refs only):
 926   // . Traverse the list and remove any SoftReferences whose
 927   //   referents are not alive, but that should be kept alive for
 928   //   policy reasons. Keep alive the transitive closure of all
 929   //   such referents.
 930   if (policy != NULL) {
 931     if (mt_processing) {
 932       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
 933       task_executor->execute(phase1);
 934     } else {
 935       for (uint i = 0; i < _max_num_q; i++) {
 936         process_phase1(refs_lists[i], policy,
 937                        is_alive, keep_alive, complete_gc);
 938       }
 939     }
 940   } else { // policy == NULL
 941     assert(refs_lists != _discoveredSoftRefs,
 942            "Policy must be specified for soft references.");
 943   }
 944 
 945   // Phase 2:
 946   // . Traverse the list and remove any refs whose referents are alive.
 947   if (mt_processing) {
 948     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
 949     task_executor->execute(phase2);
 950   } else {
 951     for (uint i = 0; i < _max_num_q; i++) {
 952       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 953     }
 954   }
 955 
 956   // Phase 3:
 957   // . Traverse the list and process referents as appropriate.
 958   if (mt_processing) {
 959     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
 960     task_executor->execute(phase3);
 961   } else {
 962     for (uint i = 0; i < _max_num_q; i++) {
 963       process_phase3(refs_lists[i], clear_referent,
 964                      is_alive, keep_alive, complete_gc);
 965     }
 966   }
 967 
 968   return total_list_count;
 969 }
 970 
 971 void ReferenceProcessor::clean_up_discovered_references() {
 972   // loop over the lists
 973   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 974     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 975       gclog_or_tty->print_cr(
 976         "\nScrubbing %s discovered list of Null referents",
 977         list_name(i));
 978     }
 979     clean_up_discovered_reflist(_discovered_refs[i]);
 980   }
 981 }
 982 
 983 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
 984   assert(!discovery_is_atomic(), "Else why call this method?");
 985   DiscoveredListIterator iter(refs_list, NULL, NULL, _discovered_list_needs_post_barrier);
 986   while (iter.has_next()) {
 987     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 988     oop next = java_lang_ref_Reference::next(iter.obj());
 989     assert(next->is_oop_or_null(), "bad next field");
 990     // If referent has been cleared or Reference is not active,
 991     // drop it.
 992     if (iter.referent() == NULL || next != NULL) {
 993       debug_only(
 994         if (PrintGCDetails && TraceReferenceGC) {
 995           gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
 996             INTPTR_FORMAT " with next field: " INTPTR_FORMAT
 997             " and referent: " INTPTR_FORMAT,
 998             (void *)iter.obj(), (void *)next, (void *)iter.referent());
 999         }
1000       )
1001       // Remove Reference object from list
1002       iter.remove();
1003       iter.move_to_next();
1004     } else {
1005       iter.next();
1006     }
1007   }
1008   NOT_PRODUCT(
1009     if (PrintGCDetails && TraceReferenceGC) {
1010       gclog_or_tty->print(
1011         " Removed %d Refs with NULL referents out of %d discovered Refs",
1012         iter.removed(), iter.processed());
1013     }
1014   )
1015 }
1016 
1017 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1018   uint id = 0;
1019   // Determine the queue index to use for this object.
1020   if (_discovery_is_mt) {
1021     // During a multi-threaded discovery phase,
1022     // each thread saves to its "own" list.
1023     Thread* thr = Thread::current();
1024     id = thr->as_Worker_thread()->id();
1025   } else {
1026     // single-threaded discovery, we save in round-robin
1027     // fashion to each of the lists.
1028     if (_processing_is_mt) {
1029       id = next_id();
1030     }
1031   }
1032   assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1033 
1034   // Get the discovered queue to which we will add
1035   DiscoveredList* list = NULL;
1036   switch (rt) {
1037     case REF_OTHER:
1038       // Unknown reference type, no special treatment
1039       break;
1040     case REF_SOFT:
1041       list = &_discoveredSoftRefs[id];
1042       break;
1043     case REF_WEAK:
1044       list = &_discoveredWeakRefs[id];
1045       break;
1046     case REF_FINAL:
1047       list = &_discoveredFinalRefs[id];
1048       break;
1049     case REF_PHANTOM:
1050       list = &_discoveredPhantomRefs[id];
1051       break;
1052     case REF_NONE:
1053       // we should not reach here if we are an InstanceRefKlass
1054     default:
1055       ShouldNotReachHere();
1056   }
1057   if (TraceReferenceGC && PrintGCDetails) {
1058     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1059   }
1060   return list;
1061 }
1062 
1063 inline void
1064 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1065                                               oop             obj,
1066                                               HeapWord*       discovered_addr) {
1067   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1068   // First we must make sure this object is only enqueued once. CAS in a non null
1069   // discovered_addr.
1070   oop current_head = refs_list.head();
1071   // The last ref must have its discovered field pointing to itself.
1072   oop next_discovered = (current_head != NULL) ? current_head : obj;
1073 
1074   // Note: In the case of G1, this specific pre-barrier is strictly
1075   // not necessary because the only case we are interested in
1076   // here is when *discovered_addr is NULL (see the CAS further below),
1077   // so this will expand to nothing. As a result, we have manually
1078   // elided this out for G1, but left in the test for some future
1079   // collector that might have need for a pre-barrier here, e.g.:-
1080   // oopDesc::bs()->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1081   assert(!_discovered_list_needs_post_barrier || UseG1GC,
1082          "Need to check non-G1 collector: "
1083          "may need a pre-write-barrier for CAS from NULL below");
1084   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1085                                                     NULL);
1086   if (retest == NULL) {
1087     // This thread just won the right to enqueue the object.
1088     // We have separate lists for enqueueing, so no synchronization
1089     // is necessary.
1090     refs_list.set_head(obj);
1091     refs_list.inc_length(1);
1092     if (_discovered_list_needs_post_barrier) {
1093       oopDesc::bs()->write_ref_field((void*)discovered_addr, next_discovered);
1094     }
1095 
1096     if (TraceReferenceGC) {
1097       gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1098                              (void *)obj, obj->klass()->internal_name());
1099     }
1100   } else {
1101     // If retest was non NULL, another thread beat us to it:
1102     // The reference has already been discovered...
1103     if (TraceReferenceGC) {
1104       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1105                              (void *)obj, obj->klass()->internal_name());
1106     }
1107   }
1108 }
1109 
1110 #ifndef PRODUCT
1111 // Non-atomic (i.e. concurrent) discovery might allow us
1112 // to observe j.l.References with NULL referents, being those
1113 // cleared concurrently by mutators during (or after) discovery.
1114 void ReferenceProcessor::verify_referent(oop obj) {
1115   bool da = discovery_is_atomic();
1116   oop referent = java_lang_ref_Reference::referent(obj);
1117   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1118          err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1119                  INTPTR_FORMAT " during %satomic discovery ",
1120                  (void *)referent, (void *)obj, da ? "" : "non-"));
1121 }
1122 #endif
1123 
1124 // We mention two of several possible choices here:
1125 // #0: if the reference object is not in the "originating generation"
1126 //     (or part of the heap being collected, indicated by our "span"
1127 //     we don't treat it specially (i.e. we scan it as we would
1128 //     a normal oop, treating its references as strong references).
1129 //     This means that references can't be discovered unless their
1130 //     referent is also in the same span. This is the simplest,
1131 //     most "local" and most conservative approach, albeit one
1132 //     that may cause weak references to be enqueued least promptly.
1133 //     We call this choice the "ReferenceBasedDiscovery" policy.
1134 // #1: the reference object may be in any generation (span), but if
1135 //     the referent is in the generation (span) being currently collected
1136 //     then we can discover the reference object, provided
1137 //     the object has not already been discovered by
1138 //     a different concurrently running collector (as may be the
1139 //     case, for instance, if the reference object is in CMS and
1140 //     the referent in DefNewGeneration), and provided the processing
1141 //     of this reference object by the current collector will
1142 //     appear atomic to every other collector in the system.
1143 //     (Thus, for instance, a concurrent collector may not
1144 //     discover references in other generations even if the
1145 //     referent is in its own generation). This policy may,
1146 //     in certain cases, enqueue references somewhat sooner than
1147 //     might Policy #0 above, but at marginally increased cost
1148 //     and complexity in processing these references.
1149 //     We call this choice the "RefeferentBasedDiscovery" policy.
1150 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1151   // Make sure we are discovering refs (rather than processing discovered refs).
1152   if (!_discovering_refs || !RegisterReferences) {
1153     return false;
1154   }
1155   // We only discover active references.
1156   oop next = java_lang_ref_Reference::next(obj);
1157   if (next != NULL) {   // Ref is no longer active
1158     return false;
1159   }
1160 
1161   HeapWord* obj_addr = (HeapWord*)obj;
1162   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1163       !_span.contains(obj_addr)) {
1164     // Reference is not in the originating generation;
1165     // don't treat it specially (i.e. we want to scan it as a normal
1166     // object with strong references).
1167     return false;
1168   }
1169 
1170   // We only discover references whose referents are not (yet)
1171   // known to be strongly reachable.
1172   if (is_alive_non_header() != NULL) {
1173     verify_referent(obj);
1174     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1175       return false;  // referent is reachable
1176     }
1177   }
1178   if (rt == REF_SOFT) {
1179     // For soft refs we can decide now if these are not
1180     // current candidates for clearing, in which case we
1181     // can mark through them now, rather than delaying that
1182     // to the reference-processing phase. Since all current
1183     // time-stamp policies advance the soft-ref clock only
1184     // at a major collection cycle, this is always currently
1185     // accurate.
1186     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1187       return false;
1188     }
1189   }
1190 
1191   ResourceMark rm;      // Needed for tracing.
1192 
1193   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1194   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1195   assert(discovered->is_oop_or_null(), "bad discovered field");
1196   if (discovered != NULL) {
1197     // The reference has already been discovered...
1198     if (TraceReferenceGC) {
1199       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1200                              (void *)obj, obj->klass()->internal_name());
1201     }
1202     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1203       // assumes that an object is not processed twice;
1204       // if it's been already discovered it must be on another
1205       // generation's discovered list; so we won't discover it.
1206       return false;
1207     } else {
1208       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1209              "Unrecognized policy");
1210       // Check assumption that an object is not potentially
1211       // discovered twice except by concurrent collectors that potentially
1212       // trace the same Reference object twice.
1213       assert(UseConcMarkSweepGC || UseG1GC,
1214              "Only possible with a concurrent marking collector");
1215       return true;
1216     }
1217   }
1218 
1219   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1220     verify_referent(obj);
1221     // Discover if and only if EITHER:
1222     // .. reference is in our span, OR
1223     // .. we are an atomic collector and referent is in our span
1224     if (_span.contains(obj_addr) ||
1225         (discovery_is_atomic() &&
1226          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1227       // should_enqueue = true;
1228     } else {
1229       return false;
1230     }
1231   } else {
1232     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1233            _span.contains(obj_addr), "code inconsistency");
1234   }
1235 
1236   // Get the right type of discovered queue head.
1237   DiscoveredList* list = get_discovered_list(rt);
1238   if (list == NULL) {
1239     return false;   // nothing special needs to be done
1240   }
1241 
1242   if (_discovery_is_mt) {
1243     add_to_discovered_list_mt(*list, obj, discovered_addr);
1244   } else {
1245     // If "_discovered_list_needs_post_barrier", we do write barriers when
1246     // updating the discovered reference list.  Otherwise, we do a raw store
1247     // here: the field will be visited later when processing the discovered
1248     // references.
1249     oop current_head = list->head();
1250     // The last ref must have its discovered field pointing to itself.
1251     oop next_discovered = (current_head != NULL) ? current_head : obj;
1252 
1253     // As in the case further above, since we are over-writing a NULL
1254     // pre-value, we can safely elide the pre-barrier here for the case of G1.
1255     // e.g.:- oopDesc::bs()->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1256     assert(discovered == NULL, "control point invariant");
1257     assert(!_discovered_list_needs_post_barrier || UseG1GC,
1258            "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
1259     oop_store_raw(discovered_addr, next_discovered);
1260     if (_discovered_list_needs_post_barrier) {
1261       oopDesc::bs()->write_ref_field((void*)discovered_addr, next_discovered);
1262     }
1263     list->set_head(obj);
1264     list->inc_length(1);
1265 
1266     if (TraceReferenceGC) {
1267       gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1268                                 (void *)obj, obj->klass()->internal_name());
1269     }
1270   }
1271   assert(obj->is_oop(), "Discovered a bad reference");
1272   verify_referent(obj);
1273   return true;
1274 }
1275 
1276 // Preclean the discovered references by removing those
1277 // whose referents are alive, and by marking from those that
1278 // are not active. These lists can be handled here
1279 // in any order and, indeed, concurrently.
1280 void ReferenceProcessor::preclean_discovered_references(
1281   BoolObjectClosure* is_alive,
1282   OopClosure* keep_alive,
1283   VoidClosure* complete_gc,
1284   YieldClosure* yield,
1285   GCTimer* gc_timer) {
1286 
1287   NOT_PRODUCT(verify_ok_to_handle_reflists());
1288 
1289   // Soft references
1290   {
1291     GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1292               false, gc_timer);
1293     for (uint i = 0; i < _max_num_q; i++) {
1294       if (yield->should_return()) {
1295         return;
1296       }
1297       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1298                                   keep_alive, complete_gc, yield);
1299     }
1300   }
1301 
1302   // Weak references
1303   {
1304     GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1305               false, gc_timer);
1306     for (uint i = 0; i < _max_num_q; i++) {
1307       if (yield->should_return()) {
1308         return;
1309       }
1310       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1311                                   keep_alive, complete_gc, yield);
1312     }
1313   }
1314 
1315   // Final references
1316   {
1317     GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1318               false, gc_timer);
1319     for (uint i = 0; i < _max_num_q; i++) {
1320       if (yield->should_return()) {
1321         return;
1322       }
1323       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1324                                   keep_alive, complete_gc, yield);
1325     }
1326   }
1327 
1328   // Phantom references
1329   {
1330     GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1331               false, gc_timer);
1332     for (uint i = 0; i < _max_num_q; i++) {
1333       if (yield->should_return()) {
1334         return;
1335       }
1336       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1337                                   keep_alive, complete_gc, yield);
1338     }
1339   }
1340 }
1341 
1342 // Walk the given discovered ref list, and remove all reference objects
1343 // whose referents are still alive, whose referents are NULL or which
1344 // are not active (have a non-NULL next field). NOTE: When we are
1345 // thus precleaning the ref lists (which happens single-threaded today),
1346 // we do not disable refs discovery to honor the correct semantics of
1347 // java.lang.Reference. As a result, we need to be careful below
1348 // that ref removal steps interleave safely with ref discovery steps
1349 // (in this thread).
1350 void
1351 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1352                                                 BoolObjectClosure* is_alive,
1353                                                 OopClosure*        keep_alive,
1354                                                 VoidClosure*       complete_gc,
1355                                                 YieldClosure*      yield) {
1356   DiscoveredListIterator iter(refs_list, keep_alive, is_alive, _discovered_list_needs_post_barrier);
1357   while (iter.has_next()) {
1358     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1359     oop obj = iter.obj();
1360     oop next = java_lang_ref_Reference::next(obj);
1361     if (iter.referent() == NULL || iter.is_referent_alive() ||
1362         next != NULL) {
1363       // The referent has been cleared, or is alive, or the Reference is not
1364       // active; we need to trace and mark its cohort.
1365       if (TraceReferenceGC) {
1366         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1367                                (void *)iter.obj(), iter.obj()->klass()->internal_name());
1368       }
1369       // Remove Reference object from list
1370       iter.remove();
1371       // Keep alive its cohort.
1372       iter.make_referent_alive();
1373       if (UseCompressedOops) {
1374         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1375         keep_alive->do_oop(next_addr);
1376       } else {
1377         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1378         keep_alive->do_oop(next_addr);
1379       }
1380       iter.move_to_next();
1381     } else {
1382       iter.next();
1383     }
1384   }
1385   // Close the reachable set
1386   complete_gc->do_void();
1387 
1388   NOT_PRODUCT(
1389     if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1390       gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1391         "Refs in discovered list " INTPTR_FORMAT,
1392         iter.removed(), iter.processed(), (address)refs_list.head());
1393     }
1394   )
1395 }
1396 
1397 const char* ReferenceProcessor::list_name(uint i) {
1398    assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1399           "Out of bounds index");
1400 
1401    int j = i / _max_num_q;
1402    switch (j) {
1403      case 0: return "SoftRef";
1404      case 1: return "WeakRef";
1405      case 2: return "FinalRef";
1406      case 3: return "PhantomRef";
1407    }
1408    ShouldNotReachHere();
1409    return NULL;
1410 }
1411 
1412 #ifndef PRODUCT
1413 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1414   // empty for now
1415 }
1416 #endif
1417 
1418 #ifndef PRODUCT
1419 void ReferenceProcessor::clear_discovered_references() {
1420   guarantee(!_discovering_refs, "Discovering refs?");
1421   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1422     clear_discovered_references(_discovered_refs[i]);
1423   }
1424 }
1425 
1426 #endif // PRODUCT
--- EOF ---