1 /*

     2  * Copyright 2001-2007 Sun Microsystems, Inc.  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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

    20  * CA 95054 USA or visit www.sun.com if you need additional information or

    21  * have any questions.

    22  *

    23  */

    24 

    25 # include "incls/_precompiled.incl"

    26 # include "incls/_referenceProcessor.cpp.incl"

    27 

    28 // List of discovered references.

    29 class DiscoveredList {

    30 public:

    31          DiscoveredList() : _head(NULL), _len(0) { }

    32   oop    head() const           { return _head; }

    33   oop*   head_ptr()             { return &_head; }

    34   void   set_head(oop o)        { _head = o; }

    35   bool   empty() const          { return _head == ReferenceProcessor::_sentinelRef; }

    36   size_t length()               { return _len; }

    37   void   set_length(size_t len) { _len = len; }

    38 private:

    39   size_t _len;

    40   oop   _head;

    41 };

    42 

    43 oop  ReferenceProcessor::_sentinelRef = NULL;

    44 

    45 const int subclasses_of_ref = REF_PHANTOM - REF_OTHER;

    46 

    47 void referenceProcessor_init() {

    48   ReferenceProcessor::init_statics();

    49 }

    50 

    51 void ReferenceProcessor::init_statics() {

    52   assert(_sentinelRef == NULL, "should be initialized precsiely once");

    53   EXCEPTION_MARK;

    54   _sentinelRef = instanceKlass::cast(

    55                    SystemDictionary::object_klass())->

    56                      allocate_permanent_instance(THREAD);

    57 

    58   // Initialize the master soft ref clock.

    59   java_lang_ref_SoftReference::set_clock(os::javaTimeMillis());

    60 

    61   if (HAS_PENDING_EXCEPTION) {

    62       Handle ex(THREAD, PENDING_EXCEPTION);

    63       vm_exit_during_initialization(ex);

    64   }

    65   assert(_sentinelRef != NULL && _sentinelRef->is_oop(),

    66          "Just constructed it!");

    67   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||

    68             RefDiscoveryPolicy == ReferentBasedDiscovery,

    69             "Unrecongnized RefDiscoveryPolicy");

    70 }

    71 

    72 

    73 ReferenceProcessor* ReferenceProcessor::create_ref_processor(

    74     MemRegion          span,

    75     bool               atomic_discovery,

    76     bool               mt_discovery,

    77     BoolObjectClosure* is_alive_non_header,

    78     int                parallel_gc_threads,

    79     bool               mt_processing)

    80 {

    81   int mt_degree = 1;

    82   if (parallel_gc_threads > 1) {

    83     mt_degree = parallel_gc_threads;

    84   }

    85   ReferenceProcessor* rp =

    86     new ReferenceProcessor(span, atomic_discovery,

    87                            mt_discovery, mt_degree,

    88                            mt_processing);

    89   if (rp == NULL) {

    90     vm_exit_during_initialization("Could not allocate ReferenceProcessor object");

    91   }

    92   rp->set_is_alive_non_header(is_alive_non_header);

    93   return rp;

    94 }

    95 

    96 

    97 ReferenceProcessor::ReferenceProcessor(MemRegion span,

    98   bool atomic_discovery, bool mt_discovery, int mt_degree,

    99   bool mt_processing) :

   100   _discovering_refs(false),

   101   _enqueuing_is_done(false),

   102   _is_alive_non_header(NULL),

   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               = mt_degree;

   110   _discoveredSoftRefs  = NEW_C_HEAP_ARRAY(DiscoveredList, _num_q * subclasses_of_ref);

   111   if (_discoveredSoftRefs == NULL) {

   112     vm_exit_during_initialization("Could not allocated RefProc Array");

   113   }

   114   _discoveredWeakRefs    = &_discoveredSoftRefs[_num_q];

   115   _discoveredFinalRefs   = &_discoveredWeakRefs[_num_q];

   116   _discoveredPhantomRefs = &_discoveredFinalRefs[_num_q];

   117   assert(_sentinelRef != NULL, "_sentinelRef is NULL");

   118   // Initialized all entries to _sentinelRef

   119   for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

   120         _discoveredSoftRefs[i].set_head(_sentinelRef);

   121     _discoveredSoftRefs[i].set_length(0);

   122   }

   123 }

   124 

   125 #ifndef PRODUCT

   126 void ReferenceProcessor::verify_no_references_recorded() {

   127   guarantee(!_discovering_refs, "Discovering refs?");

   128   for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

   129     guarantee(_discoveredSoftRefs[i].empty(),

   130               "Found non-empty discovered list");

   131   }

   132 }

   133 #endif

   134 

   135 void ReferenceProcessor::weak_oops_do(OopClosure* f) {

   136   for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

   137     f->do_oop(_discoveredSoftRefs[i].head_ptr());

   138   }

   139 }

   140 

   141 void ReferenceProcessor::oops_do(OopClosure* f) {

   142   f->do_oop(&_sentinelRef);

   143 }

   144 

   145 void ReferenceProcessor::update_soft_ref_master_clock()

   146 {

   147   // Update (advance) the soft ref master clock field. This must be done

   148   // after processing the soft ref list.

   149   jlong now = os::javaTimeMillis();

   150   jlong clock = java_lang_ref_SoftReference::clock();

   151   NOT_PRODUCT(

   152   if (now < clock) {

   153     warning("time warp: %d to %d", clock, now);

   154   }

   155   )

   156   // In product mode, protect ourselves from system time being adjusted

   157   // externally and going backward; see note in the implementation of

   158   // GenCollectedHeap::time_since_last_gc() for the right way to fix

   159   // this uniformly throughout the VM; see bug-id 4741166. XXX

   160   if (now > clock) {

   161     java_lang_ref_SoftReference::set_clock(now);

   162   }

   163   // Else leave clock stalled at its old value until time progresses

   164   // past clock value.

   165 }

   166 

   167 

   168 void

   169 ReferenceProcessor::process_discovered_references(

   170   ReferencePolicy*             policy,

   171   BoolObjectClosure*           is_alive,

   172   OopClosure*                  keep_alive,

   173   VoidClosure*                 complete_gc,

   174   AbstractRefProcTaskExecutor* task_executor) {

   175   NOT_PRODUCT(verify_ok_to_handle_reflists());

   176 

   177   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");

   178   // Stop treating discovered references specially.

   179   disable_discovery();

   180 

   181   bool trace_time = PrintGCDetails && PrintReferenceGC;

   182   // Soft references

   183   {

   184     TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);

   185     process_discovered_reflist(_discoveredSoftRefs, policy, true,

   186                                is_alive, keep_alive, complete_gc, task_executor);

   187   }

   188 

   189   update_soft_ref_master_clock();

   190 

   191   // Weak references

   192   {

   193     TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);

   194     process_discovered_reflist(_discoveredWeakRefs, NULL, true,

   195                                is_alive, keep_alive, complete_gc, task_executor);

   196   }

   197 

   198   // Final references

   199   {

   200     TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);

   201     process_discovered_reflist(_discoveredFinalRefs, NULL, false,

   202                                is_alive, keep_alive, complete_gc, task_executor);

   203   }

   204 

   205   // Phantom references

   206   {

   207     TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);

   208     process_discovered_reflist(_discoveredPhantomRefs, NULL, false,

   209                                is_alive, keep_alive, complete_gc, task_executor);

   210   }

   211 

   212   // Weak global JNI references. It would make more sense (semantically) to

   213   // traverse these simultaneously with the regular weak references above, but

   214   // that is not how the JDK1.2 specification is. See #4126360. Native code can

   215   // thus use JNI weak references to circumvent the phantom references and

   216   // resurrect a "post-mortem" object.

   217   {

   218     TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);

   219     if (task_executor != NULL) {

   220       task_executor->set_single_threaded_mode();

   221     }

   222     process_phaseJNI(is_alive, keep_alive, complete_gc);

   223   }

   224 }

   225 

   226 

   227 #ifndef PRODUCT

   228 // Calculate the number of jni handles.

   229 unsigned int ReferenceProcessor::count_jni_refs()

   230 {

   231   class AlwaysAliveClosure: public BoolObjectClosure {

   232   public:

   233     bool do_object_b(oop obj) { return true; }

   234     void do_object(oop obj) { assert(false, "Don't call"); }

   235   };

   236 

   237   class CountHandleClosure: public OopClosure {

   238   private:

   239     int _count;

   240   public:

   241     CountHandleClosure(): _count(0) {}

   242     void do_oop(oop* unused) {

   243       _count++;

   244     }

   245     int count() { return _count; }

   246   };

   247   CountHandleClosure global_handle_count;

   248   AlwaysAliveClosure always_alive;

   249   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);

   250   return global_handle_count.count();

   251 }

   252 #endif

   253 

   254 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,

   255                                           OopClosure*        keep_alive,

   256                                           VoidClosure*       complete_gc) {

   257 #ifndef PRODUCT

   258   if (PrintGCDetails && PrintReferenceGC) {

   259     unsigned int count = count_jni_refs();

   260     gclog_or_tty->print(", %u refs", count);

   261   }

   262 #endif

   263   JNIHandles::weak_oops_do(is_alive, keep_alive);

   264   // Finally remember to keep sentinel around

   265   keep_alive->do_oop(&_sentinelRef);

   266   complete_gc->do_void();

   267 }

   268 

   269 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {

   270   NOT_PRODUCT(verify_ok_to_handle_reflists());

   271   // Remember old value of pending references list

   272   oop* pending_list_addr = java_lang_ref_Reference::pending_list_addr();

   273   oop old_pending_list_value = *pending_list_addr;

   274 

   275   // Enqueue references that are not made active again, and

   276   // clear the decks for the next collection (cycle).

   277   enqueue_discovered_reflists(pending_list_addr, task_executor);

   278   // Do the oop-check on pending_list_addr missed in

   279   // enqueue_discovered_reflist. We should probably

   280   // do a raw oop_check so that future such idempotent

   281   // oop_stores relying on the oop-check side-effect

   282   // may be elided automatically and safely without

   283   // affecting correctness.

   284   oop_store(pending_list_addr, *(pending_list_addr));

   285 

   286   // Stop treating discovered references specially.

   287   disable_discovery();

   288 

   289   // Return true if new pending references were added

   290   return old_pending_list_value != *pending_list_addr;

   291 }

   292 

   293 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,

   294   oop* pending_list_addr) {

   295   // Given a list of refs linked through the "discovered" field

   296   // (java.lang.ref.Reference.discovered) chain them through the

   297   // "next" field (java.lang.ref.Reference.next) and prepend

   298   // to the pending list.

   299   if (TraceReferenceGC && PrintGCDetails) {

   300     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "

   301                            INTPTR_FORMAT, (address)refs_list.head());

   302   }

   303   oop obj = refs_list.head();

   304   // Walk down the list, copying the discovered field into

   305   // the next field and clearing it (except for the last

   306   // non-sentinel object which is treated specially to avoid

   307   // confusion with an active reference).

   308   while (obj != _sentinelRef) {

   309     assert(obj->is_instanceRef(), "should be reference object");

   310     oop next = java_lang_ref_Reference::discovered(obj);

   311     if (TraceReferenceGC && PrintGCDetails) {

   312       gclog_or_tty->print_cr("  obj " INTPTR_FORMAT "/next " INTPTR_FORMAT,

   313                              (oopDesc*) obj, (oopDesc*) next);

   314     }

   315     assert(*java_lang_ref_Reference::next_addr(obj) == NULL,

   316       "The reference should not be enqueued");

   317     if (next == _sentinelRef) {  // obj is last

   318       // Swap refs_list into pendling_list_addr and

   319       // set obj's next to what we read from pending_list_addr.

   320       oop old = (oop)Atomic::xchg_ptr(refs_list.head(), pending_list_addr);

   321       // Need oop_check on pending_list_addr above;

   322       // see special oop-check code at the end of

   323       // enqueue_discovered_reflists() further below.

   324       if (old == NULL) {

   325         // obj should be made to point to itself, since

   326         // pending list was empty.

   327         java_lang_ref_Reference::set_next(obj, obj);

   328       } else {

   329         java_lang_ref_Reference::set_next(obj, old);

   330       }

   331     } else {

   332       java_lang_ref_Reference::set_next(obj, next);

   333     }

   334     java_lang_ref_Reference::set_discovered(obj, (oop) NULL);

   335     obj = next;

   336   }

   337 }

   338 

   339 // Parallel enqueue task

   340 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {

   341 public:

   342   RefProcEnqueueTask(ReferenceProcessor& ref_processor,

   343                      DiscoveredList      discovered_refs[],

   344                      oop*                pending_list_addr,

   345                      oop                 sentinel_ref,

   346                      int                 n_queues)

   347     : EnqueueTask(ref_processor, discovered_refs,

   348                   pending_list_addr, sentinel_ref, n_queues)

   349   { }

   350 

   351   virtual void work(unsigned int work_id)

   352   {

   353     assert(work_id < (unsigned int)_ref_processor.num_q(), "Index out-of-bounds");

   354     // Simplest first cut: static partitioning.

   355     int index = work_id;

   356     for (int j = 0; j < subclasses_of_ref; j++, index += _n_queues) {

   357       _ref_processor.enqueue_discovered_reflist(

   358         _refs_lists[index], _pending_list_addr);

   359       _refs_lists[index].set_head(_sentinel_ref);

   360       _refs_lists[index].set_length(0);

   361     }

   362   }

   363 };

   364 

   365 // Enqueue references that are not made active again

   366 void ReferenceProcessor::enqueue_discovered_reflists(oop* pending_list_addr,

   367   AbstractRefProcTaskExecutor* task_executor) {

   368   if (_processing_is_mt && task_executor != NULL) {

   369     // Parallel code

   370     RefProcEnqueueTask tsk(*this, _discoveredSoftRefs,

   371                            pending_list_addr, _sentinelRef, _num_q);

   372     task_executor->execute(tsk);

   373   } else {

   374     // Serial code: call the parent class's implementation

   375     for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

   376       enqueue_discovered_reflist(_discoveredSoftRefs[i], pending_list_addr);

   377       _discoveredSoftRefs[i].set_head(_sentinelRef);

   378       _discoveredSoftRefs[i].set_length(0);

   379     }

   380   }

   381 }

   382 

   383 // Iterator for the list of discovered references.

   384 class DiscoveredListIterator {

   385 public:

   386   inline DiscoveredListIterator(DiscoveredList&    refs_list,

   387                                 OopClosure*        keep_alive,

   388                                 BoolObjectClosure* is_alive);

   389 

   390   // End Of List.

   391   inline bool has_next() const

   392   { return _next != ReferenceProcessor::_sentinelRef; }

   393 

   394   // Get oop to the Reference object.

   395   inline oop  obj() const { return _ref; }

   396 

   397   // Get oop to the referent object.

   398   inline oop  referent() const { return _referent; }

   399 

   400   // Returns true if referent is alive.

   401   inline bool is_referent_alive() const;

   402 

   403   // Loads data for the current reference.

   404   // The "allow_null_referent" argument tells us to allow for the possibility

   405   // of a NULL referent in the discovered Reference object. This typically

   406   // happens in the case of concurrent collectors that may have done the

   407   // discovery concurrently or interleaved with mutator execution.

   408   inline void load_ptrs(DEBUG_ONLY(bool allow_null_referent));

   409 

   410   // Move to the next discovered reference.

   411   inline void next();

   412 

   413   // Remove the current reference from the list and move to the next.

   414   inline void remove();

   415 

   416   // Make the Reference object active again.

   417   inline void make_active() { java_lang_ref_Reference::set_next(_ref, NULL); }

   418 

   419   // Make the referent alive.

   420   inline void make_referent_alive() { _keep_alive->do_oop(_referent_addr); }

   421 

   422   // Update the discovered field.

   423   inline void update_discovered() { _keep_alive->do_oop(_prev_next); }

   424 

   425   // NULL out referent pointer.

   426   inline void clear_referent() { *_referent_addr = NULL; }

   427 

   428   // Statistics

   429   NOT_PRODUCT(

   430   inline size_t processed() const { return _processed; }

   431   inline size_t removed() const   { return _removed; }

   432   )

   433 

   434 private:

   435   inline void move_to_next();

   436 

   437 private:

   438   DiscoveredList&    _refs_list;

   439   oop*               _prev_next;

   440   oop                _ref;

   441   oop*               _discovered_addr;

   442   oop                _next;

   443   oop*               _referent_addr;

   444   oop                _referent;

   445   OopClosure*        _keep_alive;

   446   BoolObjectClosure* _is_alive;

   447   DEBUG_ONLY(

   448   oop                _first_seen; // cyclic linked list check

   449   )

   450   NOT_PRODUCT(

   451   size_t             _processed;

   452   size_t             _removed;

   453   )

   454 };

   455 

   456 inline DiscoveredListIterator::DiscoveredListIterator(DiscoveredList&    refs_list,

   457                                                       OopClosure*        keep_alive,

   458                                                       BoolObjectClosure* is_alive)

   459   : _refs_list(refs_list),

   460     _prev_next(refs_list.head_ptr()),

   461     _ref(refs_list.head()),

   462 #ifdef ASSERT

   463     _first_seen(refs_list.head()),

   464 #endif

   465 #ifndef PRODUCT

   466     _processed(0),

   467     _removed(0),

   468 #endif

   469     _next(refs_list.head()),

   470     _keep_alive(keep_alive),

   471     _is_alive(is_alive)

   472 { }

   473 

   474 inline bool DiscoveredListIterator::is_referent_alive() const

   475 {

   476   return _is_alive->do_object_b(_referent);

   477 }

   478 

   479 inline void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent))

   480 {

   481   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);

   482   assert(_discovered_addr && (*_discovered_addr)->is_oop_or_null(),

   483          "discovered field is bad");

   484   _next = *_discovered_addr;

   485   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);

   486   _referent = *_referent_addr;

   487   assert(Universe::heap()->is_in_reserved_or_null(_referent),

   488          "Wrong oop found in java.lang.Reference object");

   489   assert(allow_null_referent ?

   490              _referent->is_oop_or_null()

   491            : _referent->is_oop(),

   492          "bad referent");

   493 }

   494 

   495 inline void DiscoveredListIterator::next()

   496 {

   497   _prev_next = _discovered_addr;

   498   move_to_next();

   499 }

   500 

   501 inline void DiscoveredListIterator::remove()

   502 {

   503   assert(_ref->is_oop(), "Dropping a bad reference");

   504   // Clear the discovered_addr field so that the object does

   505   // not look like it has been discovered.

   506   *_discovered_addr = NULL;

   507   // Remove Reference object from list.

   508   *_prev_next = _next;

   509   NOT_PRODUCT(_removed++);

   510   move_to_next();

   511 }

   512 

   513 inline void DiscoveredListIterator::move_to_next()

   514 {

   515   _ref = _next;

   516   assert(_ref != _first_seen, "cyclic ref_list found");

   517   NOT_PRODUCT(_processed++);

   518 }

   519 

   520 

   521 // NOTE: process_phase*() are largely similar, and at a high level

   522 // merely iterate over the extant list applying a predicate to

   523 // each of its elements and possibly removing that element from the

   524 // list and applying some further closures to that element.

   525 // We should consider the possibility of replacing these

   526 // process_phase*() methods by abstracting them into

   527 // a single general iterator invocation that receives appropriate

   528 // closures that accomplish this work.

   529 

   530 // (SoftReferences only) Traverse the list and remove any SoftReferences whose

   531 // referents are not alive, but that should be kept alive for policy reasons.

   532 // Keep alive the transitive closure of all such referents.

   533 void

   534 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list_addr,

   535                                    ReferencePolicy*   policy,

   536                                    BoolObjectClosure* is_alive,

   537                                    OopClosure*        keep_alive,

   538                                    VoidClosure*       complete_gc) {

   539   assert(policy != NULL, "Must have a non-NULL policy");

   540   DiscoveredListIterator iter(refs_list_addr, keep_alive, is_alive);

   541   // Decide which softly reachable refs should be kept alive.

   542   while (iter.has_next()) {

   543     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));

   544     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();

   545     if (referent_is_dead && !policy->should_clear_reference(iter.obj())) {

   546       if (TraceReferenceGC) {

   547         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",

   548                                (address)iter.obj(), iter.obj()->blueprint()->internal_name());

   549       }

   550       // Make the Reference object active again

   551       iter.make_active();

   552       // keep the referent around

   553       iter.make_referent_alive();

   554       // Remove Reference object from list

   555       iter.remove();

   556     } else {

   557       iter.next();

   558     }

   559   }

   560   // Close the reachable set

   561   complete_gc->do_void();

   562   NOT_PRODUCT(

   563     if (PrintGCDetails && TraceReferenceGC) {

   564       gclog_or_tty->print(" Dropped %d dead Refs out of %d "

   565         "discovered Refs by policy ", iter.removed(), iter.processed());

   566     }

   567   )

   568 }

   569 

   570 // Traverse the list and remove any Refs that are not active, or

   571 // whose referents are either alive or NULL.

   572 void

   573 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list_addr,

   574                              BoolObjectClosure* is_alive,

   575                              OopClosure*        keep_alive)

   576 {

   577   assert(discovery_is_atomic(), "Error");

   578   DiscoveredListIterator iter(refs_list_addr, keep_alive, is_alive);

   579   while (iter.has_next()) {

   580     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));

   581     DEBUG_ONLY(oop* next_addr = java_lang_ref_Reference::next_addr(iter.obj());)

   582     assert(*next_addr == NULL, "Should not discover inactive Reference");

   583     if (iter.is_referent_alive()) {

   584       if (TraceReferenceGC) {

   585         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",

   586                                (address)iter.obj(), iter.obj()->blueprint()->internal_name());

   587       }

   588       // The referent is reachable after all.

   589       // Update the referent pointer as necessary: Note that this

   590       // should not entail any recursive marking because the

   591       // referent must already have been traversed.

   592       iter.make_referent_alive();

   593       // Remove Reference object from list

   594       iter.remove();

   595     } else {

   596       iter.next();

   597     }

   598   }

   599   NOT_PRODUCT(

   600     if (PrintGCDetails && TraceReferenceGC) {

   601       gclog_or_tty->print(" Dropped %d active Refs out of %d "

   602         "Refs in discovered list ", iter.removed(), iter.processed());

   603     }

   604   )

   605 }

   606 

   607 void

   608 ReferenceProcessor::pp2_work_concurrent_discovery(

   609   DiscoveredList&    refs_list_addr,

   610   BoolObjectClosure* is_alive,

   611   OopClosure*        keep_alive,

   612   VoidClosure*       complete_gc)

   613 {

   614   assert(!discovery_is_atomic(), "Error");

   615   DiscoveredListIterator iter(refs_list_addr, keep_alive, is_alive);

   616   while (iter.has_next()) {

   617     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));

   618     oop* next_addr = java_lang_ref_Reference::next_addr(iter.obj());

   619     if ((iter.referent() == NULL || iter.is_referent_alive() ||

   620          *next_addr != NULL)) {

   621       assert((*next_addr)->is_oop_or_null(), "bad next field");

   622       // Remove Reference object from list

   623       iter.remove();

   624       // Trace the cohorts

   625       iter.make_referent_alive();

   626       keep_alive->do_oop(next_addr);

   627     } else {

   628       iter.next();

   629     }

   630   }

   631   // Now close the newly reachable set

   632   complete_gc->do_void();

   633   NOT_PRODUCT(

   634     if (PrintGCDetails && TraceReferenceGC) {

   635       gclog_or_tty->print(" Dropped %d active Refs out of %d "

   636         "Refs in discovered list ", iter.removed(), iter.processed());

   637     }

   638   )

   639 }

   640 

   641 // Traverse the list and process the referents, by either

   642 // either clearing them or keeping them (and their reachable

   643 // closure) alive.

   644 void

   645 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list_addr,

   646                                    bool               clear_referent,

   647                                    BoolObjectClosure* is_alive,

   648                                    OopClosure*        keep_alive,

   649                                    VoidClosure*       complete_gc) {

   650   DiscoveredListIterator iter(refs_list_addr, keep_alive, is_alive);

   651   while (iter.has_next()) {

   652     iter.update_discovered();

   653     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));

   654     if (clear_referent) {

   655       // NULL out referent pointer

   656       iter.clear_referent();

   657     } else {

   658       // keep the referent around

   659       iter.make_referent_alive();

   660     }

   661     if (TraceReferenceGC) {

   662       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",

   663                              clear_referent ? "cleared " : "",

   664                              (address)iter.obj(), iter.obj()->blueprint()->internal_name());

   665     }

   666     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");

   667     // If discovery is concurrent, we may have objects with null referents,

   668     // being those that were concurrently cleared after they were discovered

   669     // (and not subsequently precleaned).

   670     assert(   (discovery_is_atomic() && iter.referent()->is_oop())

   671            || (!discovery_is_atomic() && iter.referent()->is_oop_or_null(UseConcMarkSweepGC)),

   672            "Adding a bad referent");

   673     iter.next();

   674   }

   675   // Remember to keep sentinel pointer around

   676   iter.update_discovered();

   677   // Close the reachable set

   678   complete_gc->do_void();

   679 }

   680 

   681 void

   682 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& ref_list) {

   683   oop obj = ref_list.head();

   684   while (obj != _sentinelRef) {

   685     oop* discovered_addr = java_lang_ref_Reference::discovered_addr(obj);

   686     obj = *discovered_addr;

   687     *discovered_addr = NULL;

   688   }

   689   ref_list.set_head(_sentinelRef);

   690   ref_list.set_length(0);

   691 }

   692 

   693 void

   694 ReferenceProcessor::abandon_partial_discovered_list_arr(DiscoveredList refs_lists[]) {

   695   for (int i = 0; i < _num_q; i++) {

   696     abandon_partial_discovered_list(refs_lists[i]);

   697   }

   698 }

   699 

   700 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {

   701 public:

   702   RefProcPhase1Task(ReferenceProcessor& ref_processor,

   703                     DiscoveredList      refs_lists[],

   704                     ReferencePolicy*    policy,

   705                     bool                marks_oops_alive)

   706     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),

   707       _policy(policy)

   708   { }

   709   virtual void work(unsigned int i, BoolObjectClosure& is_alive,

   710                     OopClosure& keep_alive,

   711                     VoidClosure& complete_gc)

   712   {

   713     _ref_processor.process_phase1(_refs_lists[i], _policy,

   714                                   &is_alive, &keep_alive, &complete_gc);

   715   }

   716 private:

   717   ReferencePolicy* _policy;

   718 };

   719 

   720 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {

   721 public:

   722   RefProcPhase2Task(ReferenceProcessor& ref_processor,

   723                     DiscoveredList      refs_lists[],

   724                     bool                marks_oops_alive)

   725     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)

   726   { }

   727   virtual void work(unsigned int i, BoolObjectClosure& is_alive,

   728                     OopClosure& keep_alive,

   729                     VoidClosure& complete_gc)

   730   {

   731     _ref_processor.process_phase2(_refs_lists[i],

   732                                   &is_alive, &keep_alive, &complete_gc);

   733   }

   734 };

   735 

   736 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {

   737 public:

   738   RefProcPhase3Task(ReferenceProcessor& ref_processor,

   739                     DiscoveredList      refs_lists[],

   740                     bool                clear_referent,

   741                     bool                marks_oops_alive)

   742     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),

   743       _clear_referent(clear_referent)

   744   { }

   745   virtual void work(unsigned int i, BoolObjectClosure& is_alive,

   746                     OopClosure& keep_alive,

   747                     VoidClosure& complete_gc)

   748   {

   749     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,

   750                                   &is_alive, &keep_alive, &complete_gc);

   751   }

   752 private:

   753   bool _clear_referent;

   754 };

   755 

   756 // Balances reference queues.

   757 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])

   758 {

   759   // calculate total length

   760   size_t total_refs = 0;

   761   for (int i = 0; i < _num_q; ++i) {

   762     total_refs += ref_lists[i].length();

   763   }

   764   size_t avg_refs = total_refs / _num_q + 1;

   765   int to_idx = 0;

   766   for (int from_idx = 0; from_idx < _num_q; from_idx++) {

   767     while (ref_lists[from_idx].length() > avg_refs) {

   768       assert(to_idx < _num_q, "Sanity Check!");

   769       if (ref_lists[to_idx].length() < avg_refs) {

   770         // move superfluous refs

   771         size_t refs_to_move =

   772           MIN2(ref_lists[from_idx].length() - avg_refs,

   773                avg_refs - ref_lists[to_idx].length());

   774         oop move_head = ref_lists[from_idx].head();

   775         oop move_tail = move_head;

   776         oop new_head  = move_head;

   777         // find an element to split the list on

   778         for (size_t j = 0; j < refs_to_move; ++j) {

   779           move_tail = new_head;

   780           new_head = *java_lang_ref_Reference::discovered_addr(new_head);

   781         }

   782         java_lang_ref_Reference::set_discovered(move_tail, ref_lists[to_idx].head());

   783         ref_lists[to_idx].set_head(move_head);

   784         ref_lists[to_idx].set_length(ref_lists[to_idx].length() + refs_to_move);

   785         ref_lists[from_idx].set_head(new_head);

   786         ref_lists[from_idx].set_length(ref_lists[from_idx].length() - refs_to_move);

   787       } else {

   788         ++to_idx;

   789       }

   790     }

   791   }

   792 }

   793 

   794 void

   795 ReferenceProcessor::process_discovered_reflist(

   796   DiscoveredList               refs_lists[],

   797   ReferencePolicy*             policy,

   798   bool                         clear_referent,

   799   BoolObjectClosure*           is_alive,

   800   OopClosure*                  keep_alive,

   801   VoidClosure*                 complete_gc,

   802   AbstractRefProcTaskExecutor* task_executor)

   803 {

   804   bool mt = task_executor != NULL && _processing_is_mt;

   805   if (mt && ParallelRefProcBalancingEnabled) {

   806     balance_queues(refs_lists);

   807   }

   808   if (PrintReferenceGC && PrintGCDetails) {

   809     size_t total = 0;

   810     for (int i = 0; i < _num_q; ++i) {

   811       total += refs_lists[i].length();

   812     }

   813     gclog_or_tty->print(", %u refs", total);

   814   }

   815 

   816   // Phase 1 (soft refs only):

   817   // . Traverse the list and remove any SoftReferences whose

   818   //   referents are not alive, but that should be kept alive for

   819   //   policy reasons. Keep alive the transitive closure of all

   820   //   such referents.

   821   if (policy != NULL) {

   822     if (mt) {

   823       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);

   824       task_executor->execute(phase1);

   825     } else {

   826       for (int i = 0; i < _num_q; i++) {

   827         process_phase1(refs_lists[i], policy,

   828                        is_alive, keep_alive, complete_gc);

   829       }

   830     }

   831   } else { // policy == NULL

   832     assert(refs_lists != _discoveredSoftRefs,

   833            "Policy must be specified for soft references.");

   834   }

   835 

   836   // Phase 2:

   837   // . Traverse the list and remove any refs whose referents are alive.

   838   if (mt) {

   839     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);

   840     task_executor->execute(phase2);

   841   } else {

   842     for (int i = 0; i < _num_q; i++) {

   843       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);

   844     }

   845   }

   846 

   847   // Phase 3:

   848   // . Traverse the list and process referents as appropriate.

   849   if (mt) {

   850     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);

   851     task_executor->execute(phase3);

   852   } else {

   853     for (int i = 0; i < _num_q; i++) {

   854       process_phase3(refs_lists[i], clear_referent,

   855                      is_alive, keep_alive, complete_gc);

   856     }

   857   }

   858 }

   859 

   860 void ReferenceProcessor::clean_up_discovered_references() {

   861   // loop over the lists

   862   for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

   863     if (TraceReferenceGC && PrintGCDetails && ((i % _num_q) == 0)) {

   864       gclog_or_tty->print_cr(

   865         "\nScrubbing %s discovered list of Null referents",

   866         list_name(i));

   867     }

   868     clean_up_discovered_reflist(_discoveredSoftRefs[i]);

   869   }

   870 }

   871 

   872 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {

   873   assert(!discovery_is_atomic(), "Else why call this method?");

   874   DiscoveredListIterator iter(refs_list, NULL, NULL);

   875   size_t length = refs_list.length();

   876   while (iter.has_next()) {

   877     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));

   878     oop* next_addr = java_lang_ref_Reference::next_addr(iter.obj());

   879     assert((*next_addr)->is_oop_or_null(), "bad next field");

   880     // If referent has been cleared or Reference is not active,

   881     // drop it.

   882     if (iter.referent() == NULL || *next_addr != NULL) {

   883       debug_only(

   884         if (PrintGCDetails && TraceReferenceGC) {

   885           gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "

   886             INTPTR_FORMAT " with next field: " INTPTR_FORMAT

   887             " and referent: " INTPTR_FORMAT,

   888             (address)iter.obj(), (address)*next_addr, (address)iter.referent());

   889         }

   890       )

   891       // Remove Reference object from list

   892       iter.remove();

   893       --length;

   894     } else {

   895       iter.next();

   896     }

   897   }

   898   refs_list.set_length(length);

   899   NOT_PRODUCT(

   900     if (PrintGCDetails && TraceReferenceGC) {

   901       gclog_or_tty->print(

   902         " Removed %d Refs with NULL referents out of %d discovered Refs",

   903         iter.removed(), iter.processed());

   904     }

   905   )

   906 }

   907 

   908 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {

   909   int id = 0;

   910   // Determine the queue index to use for this object.

   911   if (_discovery_is_mt) {

   912     // During a multi-threaded discovery phase,

   913     // each thread saves to its "own" list.

   914     Thread* thr = Thread::current();

   915     assert(thr->is_GC_task_thread(),

   916            "Dubious cast from Thread* to WorkerThread*?");

   917     id = ((WorkerThread*)thr)->id();

   918   } else {

   919     // single-threaded discovery, we save in round-robin

   920     // fashion to each of the lists.

   921     if (_processing_is_mt) {

   922       id = next_id();

   923     }

   924   }

   925   assert(0 <= id && id < _num_q, "Id is out-of-bounds (call Freud?)");

   926 

   927   // Get the discovered queue to which we will add

   928   DiscoveredList* list = NULL;

   929   switch (rt) {

   930     case REF_OTHER:

   931       // Unknown reference type, no special treatment

   932       break;

   933     case REF_SOFT:

   934       list = &_discoveredSoftRefs[id];

   935       break;

   936     case REF_WEAK:

   937       list = &_discoveredWeakRefs[id];

   938       break;

   939     case REF_FINAL:

   940       list = &_discoveredFinalRefs[id];

   941       break;

   942     case REF_PHANTOM:

   943       list = &_discoveredPhantomRefs[id];

   944       break;

   945     case REF_NONE:

   946       // we should not reach here if we are an instanceRefKlass

   947     default:

   948       ShouldNotReachHere();

   949   }

   950   return list;

   951 }

   952 

   953 inline void ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& list,

   954   oop obj, oop* discovered_addr) {

   955   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");

   956   // First we must make sure this object is only enqueued once. CAS in a non null

   957   // discovered_addr.

   958   oop retest = (oop)Atomic::cmpxchg_ptr(list.head(), discovered_addr, NULL);

   959   if (retest == NULL) {

   960     // This thread just won the right to enqueue the object.

   961     // We have separate lists for enqueueing so no synchronization

   962     // is necessary.

   963     list.set_head(obj);

   964     list.set_length(list.length() + 1);

   965   } else {

   966     // If retest was non NULL, another thread beat us to it:

   967     // The reference has already been discovered...

   968     if (TraceReferenceGC) {

   969       gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",

   970                              obj, obj->blueprint()->internal_name());

   971     }

   972   }

   973 }

   974 

   975 

   976 // We mention two of several possible choices here:

   977 // #0: if the reference object is not in the "originating generation"

   978 //     (or part of the heap being collected, indicated by our "span"

   979 //     we don't treat it specially (i.e. we scan it as we would

   980 //     a normal oop, treating its references as strong references).

   981 //     This means that references can't be enqueued unless their

   982 //     referent is also in the same span. This is the simplest,

   983 //     most "local" and most conservative approach, albeit one

   984 //     that may cause weak references to be enqueued least promptly.

   985 //     We call this choice the "ReferenceBasedDiscovery" policy.

   986 // #1: the reference object may be in any generation (span), but if

   987 //     the referent is in the generation (span) being currently collected

   988 //     then we can discover the reference object, provided

   989 //     the object has not already been discovered by

   990 //     a different concurrently running collector (as may be the

   991 //     case, for instance, if the reference object is in CMS and

   992 //     the referent in DefNewGeneration), and provided the processing

   993 //     of this reference object by the current collector will

   994 //     appear atomic to every other collector in the system.

   995 //     (Thus, for instance, a concurrent collector may not

   996 //     discover references in other generations even if the

   997 //     referent is in its own generation). This policy may,

   998 //     in certain cases, enqueue references somewhat sooner than

   999 //     might Policy #0 above, but at marginally increased cost

  1000 //     and complexity in processing these references.

  1001 //     We call this choice the "RefeferentBasedDiscovery" policy.

  1002 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {

  1003   // We enqueue references only if we are discovering refs

  1004   // (rather than processing discovered refs).

  1005   if (!_discovering_refs || !RegisterReferences) {

  1006     return false;

  1007   }

  1008   // We only enqueue active references.

  1009   oop* next_addr = java_lang_ref_Reference::next_addr(obj);

  1010   if (*next_addr != NULL) {

  1011     return false;

  1012   }

  1013 

  1014   HeapWord* obj_addr = (HeapWord*)obj;

  1015   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&

  1016       !_span.contains(obj_addr)) {

  1017     // Reference is not in the originating generation;

  1018     // don't treat it specially (i.e. we want to scan it as a normal

  1019     // object with strong references).

  1020     return false;

  1021   }

  1022 

  1023   // We only enqueue references whose referents are not (yet) strongly

  1024   // reachable.

  1025   if (is_alive_non_header() != NULL) {

  1026     oop referent = java_lang_ref_Reference::referent(obj);

  1027     // We'd like to assert the following:

  1028     // assert(referent != NULL, "Refs with null referents already filtered");

  1029     // However, since this code may be executed concurrently with

  1030     // mutators, which can clear() the referent, it is not

  1031     // guaranteed that the referent is non-NULL.

  1032     if (is_alive_non_header()->do_object_b(referent)) {

  1033       return false;  // referent is reachable

  1034     }

  1035   }

  1036 

  1037   oop* discovered_addr = java_lang_ref_Reference::discovered_addr(obj);

  1038   assert(discovered_addr != NULL && (*discovered_addr)->is_oop_or_null(),

  1039          "bad discovered field");

  1040   if (*discovered_addr != NULL) {

  1041     // The reference has already been discovered...

  1042     if (TraceReferenceGC) {

  1043       gclog_or_tty->print_cr("Already enqueued reference (" INTPTR_FORMAT ": %s)",

  1044                              (oopDesc*)obj, obj->blueprint()->internal_name());

  1045     }

  1046     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {

  1047       // assumes that an object is not processed twice;

  1048       // if it's been already discovered it must be on another

  1049       // generation's discovered list; so we won't discover it.

  1050       return false;

  1051     } else {

  1052       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,

  1053              "Unrecognized policy");

  1054       // Check assumption that an object is not potentially

  1055       // discovered twice except by concurrent collectors that potentially

  1056       // trace the same Reference object twice.

  1057       assert(UseConcMarkSweepGC,

  1058              "Only possible with a concurrent collector");

  1059       return true;

  1060     }

  1061   }

  1062 

  1063   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {

  1064     oop referent = java_lang_ref_Reference::referent(obj);

  1065     assert(referent->is_oop(), "bad referent");

  1066     // enqueue if and only if either:

  1067     // reference is in our span or

  1068     // we are an atomic collector and referent is in our span

  1069     if (_span.contains(obj_addr) ||

  1070         (discovery_is_atomic() && _span.contains(referent))) {

  1071       // should_enqueue = true;

  1072     } else {

  1073       return false;

  1074     }

  1075   } else {

  1076     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&

  1077            _span.contains(obj_addr), "code inconsistency");

  1078   }

  1079 

  1080   // Get the right type of discovered queue head.

  1081   DiscoveredList* list = get_discovered_list(rt);

  1082   if (list == NULL) {

  1083     return false;   // nothing special needs to be done

  1084   }

  1085 

  1086   // We do a raw store here, the field will be visited later when

  1087   // processing the discovered references.

  1088   if (_discovery_is_mt) {

  1089     add_to_discovered_list_mt(*list, obj, discovered_addr);

  1090   } else {

  1091     *discovered_addr = list->head();

  1092     list->set_head(obj);

  1093     list->set_length(list->length() + 1);

  1094   }

  1095 

  1096   // In the MT discovery case, it is currently possible to see

  1097   // the following message multiple times if several threads

  1098   // discover a reference about the same time. Only one will

  1099   // however have actually added it to the disocvered queue.

  1100   // One could let add_to_discovered_list_mt() return an

  1101   // indication for success in queueing (by 1 thread) or

  1102   // failure (by all other threads), but I decided the extra

  1103   // code was not worth the effort for something that is

  1104   // only used for debugging support.

  1105   if (TraceReferenceGC) {

  1106     oop referent = java_lang_ref_Reference::referent(obj);

  1107     if (PrintGCDetails) {

  1108       gclog_or_tty->print_cr("Enqueued reference (" INTPTR_FORMAT ": %s)",

  1109                              (oopDesc*) obj, obj->blueprint()->internal_name());

  1110     }

  1111     assert(referent->is_oop(), "Enqueued a bad referent");

  1112   }

  1113   assert(obj->is_oop(), "Enqueued a bad reference");

  1114   return true;

  1115 }

  1116 

  1117 // Preclean the discovered references by removing those

  1118 // whose referents are alive, and by marking from those that

  1119 // are not active. These lists can be handled here

  1120 // in any order and, indeed, concurrently.

  1121 void ReferenceProcessor::preclean_discovered_references(

  1122   BoolObjectClosure* is_alive,

  1123   OopClosure* keep_alive,

  1124   VoidClosure* complete_gc,

  1125   YieldClosure* yield) {

  1126 

  1127   NOT_PRODUCT(verify_ok_to_handle_reflists());

  1128 

  1129   // Soft references

  1130   {

  1131     TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,

  1132               false, gclog_or_tty);

  1133     for (int i = 0; i < _num_q; i++) {

  1134       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,

  1135                                   keep_alive, complete_gc, yield);