1 /*
   2  * Copyright (c) 1998, 2015, 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 "compiler/compileBroker.hpp"
  27 #include "gc_interface/collectedHeap.hpp"
  28 #include "memory/resourceArea.hpp"
  29 #include "oops/method.hpp"
  30 #include "oops/oop.inline.hpp"
  31 #include "oops/verifyOop.hpp"
  32 #include "runtime/interfaceSupport.hpp"
  33 #include "runtime/mutexLocker.hpp"
  34 #include "runtime/os.hpp"
  35 #include "runtime/thread.inline.hpp"
  36 #include "runtime/vmThread.hpp"
  37 #include "runtime/vm_operations.hpp"
  38 #include "services/runtimeService.hpp"
  39 #include "trace/tracing.hpp"
  40 #include "utilities/dtrace.hpp"
  41 #include "utilities/events.hpp"
  42 #include "utilities/xmlstream.hpp"
  43 
  44 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
  45 
  46 // Dummy VM operation to act as first element in our circular double-linked list
  47 class VM_Dummy: public VM_Operation {
  48   VMOp_Type type() const { return VMOp_Dummy; }
  49   void  doit() {};
  50 };
  51 
  52 VMOperationQueue::VMOperationQueue() {
  53   // The queue is a circular doubled-linked list, which always contains
  54   // one element (i.e., one element means empty).
  55   for(int i = 0; i < nof_priorities; i++) {
  56     _queue_length[i] = 0;
  57     _queue_counter = 0;
  58     _queue[i] = new VM_Dummy();
  59     _queue[i]->set_next(_queue[i]);
  60     _queue[i]->set_prev(_queue[i]);
  61   }
  62   _drain_list = NULL;
  63 }
  64 
  65 
  66 bool VMOperationQueue::queue_empty(int prio) {
  67   // It is empty if there is exactly one element
  68   bool empty = (_queue[prio] == _queue[prio]->next());
  69   assert( (_queue_length[prio] == 0 && empty) ||
  70           (_queue_length[prio] > 0  && !empty), "sanity check");
  71   return _queue_length[prio] == 0;
  72 }
  73 
  74 // Inserts an element to the right of the q element
  75 void VMOperationQueue::insert(VM_Operation* q, VM_Operation* n) {
  76   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");
  77   n->set_prev(q);
  78   n->set_next(q->next());
  79   q->next()->set_prev(n);
  80   q->set_next(n);
  81 }
  82 
  83 void VMOperationQueue::queue_add_front(int prio, VM_Operation *op) {
  84   _queue_length[prio]++;
  85   insert(_queue[prio]->next(), op);
  86 }
  87 
  88 void VMOperationQueue::queue_add_back(int prio, VM_Operation *op) {
  89   _queue_length[prio]++;
  90   insert(_queue[prio]->prev(), op);
  91 }
  92 
  93 
  94 void VMOperationQueue::unlink(VM_Operation* q) {
  95   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");
  96   q->prev()->set_next(q->next());
  97   q->next()->set_prev(q->prev());
  98 }
  99 
 100 VM_Operation* VMOperationQueue::queue_remove_front(int prio) {
 101   if (queue_empty(prio)) return NULL;
 102   assert(_queue_length[prio] >= 0, "sanity check");
 103   _queue_length[prio]--;
 104   VM_Operation* r = _queue[prio]->next();
 105   assert(r != _queue[prio], "cannot remove base element");
 106   unlink(r);
 107   return r;
 108 }
 109 
 110 VM_Operation* VMOperationQueue::queue_drain(int prio) {
 111   if (queue_empty(prio)) return NULL;
 112   DEBUG_ONLY(int length = _queue_length[prio];);
 113   assert(length >= 0, "sanity check");
 114   _queue_length[prio] = 0;
 115   VM_Operation* r = _queue[prio]->next();
 116   assert(r != _queue[prio], "cannot remove base element");
 117   // remove links to base element from head and tail
 118   r->set_prev(NULL);
 119   _queue[prio]->prev()->set_next(NULL);
 120   // restore queue to empty state
 121   _queue[prio]->set_next(_queue[prio]);
 122   _queue[prio]->set_prev(_queue[prio]);
 123   assert(queue_empty(prio), "drain corrupted queue");
 124 #ifdef ASSERT
 125   int len = 0;
 126   VM_Operation* cur;
 127   for(cur = r; cur != NULL; cur=cur->next()) len++;
 128   assert(len == length, "drain lost some ops");
 129 #endif
 130   return r;
 131 }
 132 
 133 void VMOperationQueue::queue_oops_do(int queue, OopClosure* f) {
 134   VM_Operation* cur = _queue[queue];
 135   cur = cur->next();
 136   while (cur != _queue[queue]) {
 137     cur->oops_do(f);
 138     cur = cur->next();
 139   }
 140 }
 141 
 142 void VMOperationQueue::drain_list_oops_do(OopClosure* f) {
 143   VM_Operation* cur = _drain_list;
 144   while (cur != NULL) {
 145     cur->oops_do(f);
 146     cur = cur->next();
 147   }
 148 }
 149 
 150 //-----------------------------------------------------------------
 151 // High-level interface
 152 bool VMOperationQueue::add(VM_Operation *op) {
 153 
 154   HOTSPOT_VMOPS_REQUEST(
 155                    (char *) op->name(), strlen(op->name()),
 156                    op->evaluation_mode());
 157 
 158   // Encapsulates VM queue policy. Currently, that
 159   // only involves putting them on the right list
 160   if (op->evaluate_at_safepoint()) {
 161     queue_add_back(SafepointPriority, op);
 162     return true;
 163   }
 164 
 165   queue_add_back(MediumPriority, op);
 166   return true;
 167 }
 168 
 169 VM_Operation* VMOperationQueue::remove_next() {
 170   // Assuming VMOperation queue is two-level priority queue. If there are
 171   // more than two priorities, we need a different scheduling algorithm.
 172   assert(SafepointPriority == 0 && MediumPriority == 1 && nof_priorities == 2,
 173          "current algorithm does not work");
 174 
 175   // simple counter based scheduling to prevent starvation of lower priority
 176   // queue. -- see 4390175
 177   int high_prio, low_prio;
 178   if (_queue_counter++ < 10) {
 179       high_prio = SafepointPriority;
 180       low_prio  = MediumPriority;
 181   } else {
 182       _queue_counter = 0;
 183       high_prio = MediumPriority;
 184       low_prio  = SafepointPriority;
 185   }
 186 
 187   return queue_remove_front(queue_empty(high_prio) ? low_prio : high_prio);
 188 }
 189 
 190 void VMOperationQueue::oops_do(OopClosure* f) {
 191   for(int i = 0; i < nof_priorities; i++) {
 192     queue_oops_do(i, f);
 193   }
 194   drain_list_oops_do(f);
 195 }
 196 
 197 
 198 //------------------------------------------------------------------------------------------------------------------
 199 // Implementation of VMThread stuff
 200 
 201 bool                VMThread::_should_terminate   = false;
 202 bool              VMThread::_terminated         = false;
 203 Monitor*          VMThread::_terminate_lock     = NULL;
 204 VMThread*         VMThread::_vm_thread          = NULL;
 205 VM_Operation*     VMThread::_cur_vm_operation   = NULL;
 206 VMOperationQueue* VMThread::_vm_queue           = NULL;
 207 PerfCounter*      VMThread::_perf_accumulated_vm_operation_time = NULL;
 208 
 209 
 210 void VMThread::create() {
 211   assert(vm_thread() == NULL, "we can only allocate one VMThread");
 212   _vm_thread = new VMThread();
 213 
 214   // Create VM operation queue
 215   _vm_queue = new VMOperationQueue();
 216   guarantee(_vm_queue != NULL, "just checking");
 217 
 218   _terminate_lock = new Monitor(Mutex::safepoint, "VMThread::_terminate_lock", true,
 219                                 Monitor::_safepoint_check_never);
 220 
 221   if (UsePerfData) {
 222     // jvmstat performance counters
 223     Thread* THREAD = Thread::current();
 224     _perf_accumulated_vm_operation_time =
 225                  PerfDataManager::create_counter(SUN_THREADS, "vmOperationTime",
 226                                                  PerfData::U_Ticks, CHECK);
 227   }
 228 }
 229 
 230 
 231 VMThread::VMThread() : NamedThread() {
 232   set_name("VM Thread");
 233 }
 234 
 235 void VMThread::destroy() {
 236   if (_vm_thread != NULL) {
 237     delete _vm_thread;
 238     _vm_thread = NULL;      // VM thread is gone
 239   }
 240 }
 241 
 242 void VMThread::run() {
 243   assert(this == vm_thread(), "check");
 244 
 245   this->initialize_thread_local_storage();
 246   this->set_native_thread_name(this->name());
 247   this->record_stack_base_and_size();
 248   // Notify_lock wait checks on active_handles() to rewait in
 249   // case of spurious wakeup, it should wait on the last
 250   // value set prior to the notify
 251   this->set_active_handles(JNIHandleBlock::allocate_block());
 252 
 253   {
 254     MutexLocker ml(Notify_lock);
 255     Notify_lock->notify();
 256   }
 257   // Notify_lock is destroyed by Threads::create_vm()
 258 
 259   int prio = (VMThreadPriority == -1)
 260     ? os::java_to_os_priority[NearMaxPriority]
 261     : VMThreadPriority;
 262   // Note that I cannot call os::set_priority because it expects Java
 263   // priorities and I am *explicitly* using OS priorities so that it's
 264   // possible to set the VM thread priority higher than any Java thread.
 265   os::set_native_priority( this, prio );
 266 
 267   // Wait for VM_Operations until termination
 268   this->loop();
 269 
 270   // Note the intention to exit before safepointing.
 271   // 6295565  This has the effect of waiting for any large tty
 272   // outputs to finish.
 273   if (xtty != NULL) {
 274     ttyLocker ttyl;
 275     xtty->begin_elem("destroy_vm");
 276     xtty->stamp();
 277     xtty->end_elem();
 278     assert(should_terminate(), "termination flag must be set");
 279   }
 280 
 281   // 4526887 let VM thread exit at Safepoint
 282   SafepointSynchronize::begin();
 283 
 284   if (VerifyBeforeExit) {
 285     HandleMark hm(VMThread::vm_thread());
 286     // Among other things, this ensures that Eden top is correct.
 287     Universe::heap()->prepare_for_verify();
 288     os::check_heap();
 289     // Silent verification so as not to pollute normal output,
 290     // unless we really asked for it.
 291     Universe::verify(!(PrintGCDetails || Verbose) || VerifySilently);
 292   }
 293 
 294   CompileBroker::set_should_block();
 295 
 296   // wait for threads (compiler threads or daemon threads) in the
 297   // _thread_in_native state to block.
 298   VM_Exit::wait_for_threads_in_native_to_block();
 299 
 300   // signal other threads that VM process is gone
 301   {
 302     // Note: we must have the _no_safepoint_check_flag. Mutex::lock() allows
 303     // VM thread to enter any lock at Safepoint as long as its _owner is NULL.
 304     // If that happens after _terminate_lock->wait() has unset _owner
 305     // but before it actually drops the lock and waits, the notification below
 306     // may get lost and we will have a hang. To avoid this, we need to use
 307     // Mutex::lock_without_safepoint_check().
 308     MutexLockerEx ml(_terminate_lock, Mutex::_no_safepoint_check_flag);
 309     _terminated = true;
 310     _terminate_lock->notify();
 311   }
 312 
 313   // Thread destructor usually does this.
 314   ThreadLocalStorage::set_thread(NULL);
 315 
 316   // Deletion must be done synchronously by the JNI DestroyJavaVM thread
 317   // so that the VMThread deletion completes before the main thread frees
 318   // up the CodeHeap.
 319 
 320 }
 321 
 322 
 323 // Notify the VMThread that the last non-daemon JavaThread has terminated,
 324 // and wait until operation is performed.
 325 void VMThread::wait_for_vm_thread_exit() {
 326   { MutexLocker mu(VMOperationQueue_lock);
 327     _should_terminate = true;
 328     VMOperationQueue_lock->notify();
 329   }
 330 
 331   // Note: VM thread leaves at Safepoint. We are not stopped by Safepoint
 332   // because this thread has been removed from the threads list. But anything
 333   // that could get blocked by Safepoint should not be used after this point,
 334   // otherwise we will hang, since there is no one can end the safepoint.
 335 
 336   // Wait until VM thread is terminated
 337   // Note: it should be OK to use Terminator_lock here. But this is called
 338   // at a very delicate time (VM shutdown) and we are operating in non- VM
 339   // thread at Safepoint. It's safer to not share lock with other threads.
 340   { MutexLockerEx ml(_terminate_lock, Mutex::_no_safepoint_check_flag);
 341     while(!VMThread::is_terminated()) {
 342         _terminate_lock->wait(Mutex::_no_safepoint_check_flag);
 343     }
 344   }
 345 }
 346 
 347 void VMThread::evaluate_operation(VM_Operation* op) {
 348   ResourceMark rm;
 349 
 350   {
 351     PerfTraceTime vm_op_timer(perf_accumulated_vm_operation_time());
 352     HOTSPOT_VMOPS_BEGIN(
 353                      (char *) op->name(), strlen(op->name()),
 354                      op->evaluation_mode());
 355 
 356     EventExecuteVMOperation event;
 357 
 358     op->evaluate();
 359 
 360     if (event.should_commit()) {
 361       bool is_concurrent = op->evaluate_concurrently();
 362       event.set_operation(op->type());
 363       event.set_safepoint(op->evaluate_at_safepoint());
 364       event.set_blocking(!is_concurrent);
 365       // Only write caller thread information for non-concurrent vm operations.
 366       // For concurrent vm operations, the thread id is set to 0 indicating thread is unknown.
 367       // This is because the caller thread could have exited already.
 368       event.set_caller(is_concurrent ? 0 : op->calling_thread()->osthread()->thread_id());
 369       event.commit();
 370     }
 371 
 372     HOTSPOT_VMOPS_END(
 373                      (char *) op->name(), strlen(op->name()),
 374                      op->evaluation_mode());
 375   }
 376 
 377   // Last access of info in _cur_vm_operation!
 378   bool c_heap_allocated = op->is_cheap_allocated();
 379 
 380   // Mark as completed
 381   if (!op->evaluate_concurrently()) {
 382     op->calling_thread()->increment_vm_operation_completed_count();
 383   }
 384   // It is unsafe to access the _cur_vm_operation after the 'increment_vm_operation_completed_count' call,
 385   // since if it is stack allocated the calling thread might have deallocated
 386   if (c_heap_allocated) {
 387     delete _cur_vm_operation;
 388   }
 389 }
 390 
 391 
 392 void VMThread::loop() {
 393   assert(_cur_vm_operation == NULL, "no current one should be executing");
 394 
 395   while(true) {
 396     VM_Operation* safepoint_ops = NULL;
 397     //
 398     // Wait for VM operation
 399     //
 400     // use no_safepoint_check to get lock without attempting to "sneak"
 401     { MutexLockerEx mu_queue(VMOperationQueue_lock,
 402                              Mutex::_no_safepoint_check_flag);
 403 
 404       // Look for new operation
 405       assert(_cur_vm_operation == NULL, "no current one should be executing");
 406       _cur_vm_operation = _vm_queue->remove_next();
 407 
 408       // Stall time tracking code
 409       if (PrintVMQWaitTime && _cur_vm_operation != NULL &&
 410           !_cur_vm_operation->evaluate_concurrently()) {
 411         long stall = os::javaTimeMillis() - _cur_vm_operation->timestamp();
 412         if (stall > 0)
 413           tty->print_cr("%s stall: %Ld",  _cur_vm_operation->name(), stall);
 414       }
 415 
 416       while (!should_terminate() && _cur_vm_operation == NULL) {
 417         // wait with a timeout to guarantee safepoints at regular intervals
 418         bool timedout =
 419           VMOperationQueue_lock->wait(Mutex::_no_safepoint_check_flag,
 420                                       GuaranteedSafepointInterval);
 421 
 422         // Support for self destruction
 423         if ((SelfDestructTimer != 0) && !is_error_reported() &&
 424             (os::elapsedTime() > SelfDestructTimer * 60)) {
 425           tty->print_cr("VM self-destructed");
 426           exit(-1);
 427         }
 428 
 429         if (timedout && (SafepointALot ||
 430                          SafepointSynchronize::is_cleanup_needed())) {
 431           MutexUnlockerEx mul(VMOperationQueue_lock,
 432                               Mutex::_no_safepoint_check_flag);
 433           // Force a safepoint since we have not had one for at least
 434           // 'GuaranteedSafepointInterval' milliseconds.  This will run all
 435           // the clean-up processing that needs to be done regularly at a
 436           // safepoint
 437           SafepointSynchronize::begin();
 438           #ifdef ASSERT
 439             if (GCALotAtAllSafepoints) InterfaceSupport::check_gc_alot();
 440           #endif
 441           SafepointSynchronize::end();
 442         }
 443         _cur_vm_operation = _vm_queue->remove_next();
 444 
 445         // If we are at a safepoint we will evaluate all the operations that
 446         // follow that also require a safepoint
 447         if (_cur_vm_operation != NULL &&
 448             _cur_vm_operation->evaluate_at_safepoint()) {
 449           safepoint_ops = _vm_queue->drain_at_safepoint_priority();
 450         }
 451       }
 452 
 453       if (should_terminate()) break;
 454     } // Release mu_queue_lock
 455 
 456     //
 457     // Execute VM operation
 458     //
 459     { HandleMark hm(VMThread::vm_thread());
 460 
 461       EventMark em("Executing VM operation: %s", vm_operation()->name());
 462       assert(_cur_vm_operation != NULL, "we should have found an operation to execute");
 463 
 464       // Give the VM thread an extra quantum.  Jobs tend to be bursty and this
 465       // helps the VM thread to finish up the job.
 466       // FIXME: When this is enabled and there are many threads, this can degrade
 467       // performance significantly.
 468       if( VMThreadHintNoPreempt )
 469         os::hint_no_preempt();
 470 
 471       // If we are at a safepoint we will evaluate all the operations that
 472       // follow that also require a safepoint
 473       if (_cur_vm_operation->evaluate_at_safepoint()) {
 474 
 475         _vm_queue->set_drain_list(safepoint_ops); // ensure ops can be scanned
 476 
 477         SafepointSynchronize::begin();
 478         evaluate_operation(_cur_vm_operation);
 479         // now process all queued safepoint ops, iteratively draining
 480         // the queue until there are none left
 481         do {
 482           _cur_vm_operation = safepoint_ops;
 483           if (_cur_vm_operation != NULL) {
 484             do {
 485               // evaluate_operation deletes the op object so we have
 486               // to grab the next op now
 487               VM_Operation* next = _cur_vm_operation->next();
 488               _vm_queue->set_drain_list(next);
 489               evaluate_operation(_cur_vm_operation);
 490               _cur_vm_operation = next;
 491               if (PrintSafepointStatistics) {
 492                 SafepointSynchronize::inc_vmop_coalesced_count();
 493               }
 494             } while (_cur_vm_operation != NULL);
 495           }
 496           // There is a chance that a thread enqueued a safepoint op
 497           // since we released the op-queue lock and initiated the safepoint.
 498           // So we drain the queue again if there is anything there, as an
 499           // optimization to try and reduce the number of safepoints.
 500           // As the safepoint synchronizes us with JavaThreads we will see
 501           // any enqueue made by a JavaThread, but the peek will not
 502           // necessarily detect a concurrent enqueue by a GC thread, but
 503           // that simply means the op will wait for the next major cycle of the
 504           // VMThread - just as it would if the GC thread lost the race for
 505           // the lock.
 506           if (_vm_queue->peek_at_safepoint_priority()) {
 507             // must hold lock while draining queue
 508             MutexLockerEx mu_queue(VMOperationQueue_lock,
 509                                      Mutex::_no_safepoint_check_flag);
 510             safepoint_ops = _vm_queue->drain_at_safepoint_priority();
 511           } else {
 512             safepoint_ops = NULL;
 513           }
 514         } while(safepoint_ops != NULL);
 515 
 516         _vm_queue->set_drain_list(NULL);
 517 
 518         // Complete safepoint synchronization
 519         SafepointSynchronize::end();
 520 
 521       } else {  // not a safepoint operation
 522         if (TraceLongCompiles) {
 523           elapsedTimer t;
 524           t.start();
 525           evaluate_operation(_cur_vm_operation);
 526           t.stop();
 527           double secs = t.seconds();
 528           if (secs * 1e3 > LongCompileThreshold) {
 529             // XXX - _cur_vm_operation should not be accessed after
 530             // the completed count has been incremented; the waiting
 531             // thread may have already freed this memory.
 532             tty->print_cr("vm %s: %3.7f secs]", _cur_vm_operation->name(), secs);
 533           }
 534         } else {
 535           evaluate_operation(_cur_vm_operation);
 536         }
 537 
 538         _cur_vm_operation = NULL;
 539       }
 540     }
 541 
 542     //
 543     //  Notify (potential) waiting Java thread(s) - lock without safepoint
 544     //  check so that sneaking is not possible
 545     { MutexLockerEx mu(VMOperationRequest_lock,
 546                        Mutex::_no_safepoint_check_flag);
 547       VMOperationRequest_lock->notify_all();
 548     }
 549 
 550     //
 551     // We want to make sure that we get to a safepoint regularly.
 552     //
 553     if (SafepointALot || SafepointSynchronize::is_cleanup_needed()) {
 554       long interval          = SafepointSynchronize::last_non_safepoint_interval();
 555       bool max_time_exceeded = GuaranteedSafepointInterval != 0 && (interval > GuaranteedSafepointInterval);
 556       if (SafepointALot || max_time_exceeded) {
 557         HandleMark hm(VMThread::vm_thread());
 558         SafepointSynchronize::begin();
 559         SafepointSynchronize::end();
 560       }
 561     }
 562   }
 563 }
 564 
 565 void VMThread::execute(VM_Operation* op) {
 566   Thread* t = Thread::current();
 567 
 568   if (!t->is_VM_thread()) {
 569     SkipGCALot sgcalot(t);    // avoid re-entrant attempts to gc-a-lot
 570     // JavaThread or WatcherThread
 571     bool concurrent = op->evaluate_concurrently();
 572     // only blocking VM operations need to verify the caller's safepoint state:
 573     if (!concurrent) {
 574       t->check_for_valid_safepoint_state(true);
 575     }
 576 
 577     // New request from Java thread, evaluate prologue
 578     if (!op->doit_prologue()) {
 579       return;   // op was cancelled
 580     }
 581 
 582     // Setup VM_operations for execution
 583     op->set_calling_thread(t, Thread::get_priority(t));
 584 
 585     // It does not make sense to execute the epilogue, if the VM operation object is getting
 586     // deallocated by the VM thread.
 587     bool execute_epilog = !op->is_cheap_allocated();
 588     assert(!concurrent || op->is_cheap_allocated(), "concurrent => cheap_allocated");
 589 
 590     // Get ticket number for non-concurrent VM operations
 591     int ticket = 0;
 592     if (!concurrent) {
 593       ticket = t->vm_operation_ticket();
 594     }
 595 
 596     // Add VM operation to list of waiting threads. We are guaranteed not to block while holding the
 597     // VMOperationQueue_lock, so we can block without a safepoint check. This allows vm operation requests
 598     // to be queued up during a safepoint synchronization.
 599     {
 600       VMOperationQueue_lock->lock_without_safepoint_check();
 601       bool ok = _vm_queue->add(op);
 602     op->set_timestamp(os::javaTimeMillis());
 603       VMOperationQueue_lock->notify();
 604       VMOperationQueue_lock->unlock();
 605       // VM_Operation got skipped
 606       if (!ok) {
 607         assert(concurrent, "can only skip concurrent tasks");
 608         if (op->is_cheap_allocated()) delete op;
 609         return;
 610       }
 611     }
 612 
 613     if (!concurrent) {
 614       // Wait for completion of request (non-concurrent)
 615       // Note: only a JavaThread triggers the safepoint check when locking
 616       MutexLocker mu(VMOperationRequest_lock);
 617       while(t->vm_operation_completed_count() < ticket) {
 618         VMOperationRequest_lock->wait(!t->is_Java_thread());
 619       }
 620     }
 621 
 622     if (execute_epilog) {
 623       op->doit_epilogue();
 624     }
 625   } else {
 626     // invoked by VM thread; usually nested VM operation
 627     assert(t->is_VM_thread(), "must be a VM thread");
 628     VM_Operation* prev_vm_operation = vm_operation();
 629     if (prev_vm_operation != NULL) {
 630       // Check the VM operation allows nested VM operation. This normally not the case, e.g., the compiler
 631       // does not allow nested scavenges or compiles.
 632       if (!prev_vm_operation->allow_nested_vm_operations()) {
 633         fatal(err_msg("Nested VM operation %s requested by operation %s",
 634                       op->name(), vm_operation()->name()));
 635       }
 636       op->set_calling_thread(prev_vm_operation->calling_thread(), prev_vm_operation->priority());
 637     }
 638 
 639     EventMark em("Executing %s VM operation: %s", prev_vm_operation ? "nested" : "", op->name());
 640 
 641     // Release all internal handles after operation is evaluated
 642     HandleMark hm(t);
 643     _cur_vm_operation = op;
 644 
 645     if (op->evaluate_at_safepoint() && !SafepointSynchronize::is_at_safepoint()) {
 646       SafepointSynchronize::begin();
 647       op->evaluate();
 648       SafepointSynchronize::end();
 649     } else {
 650       op->evaluate();
 651     }
 652 
 653     // Free memory if needed
 654     if (op->is_cheap_allocated()) delete op;
 655 
 656     _cur_vm_operation = prev_vm_operation;
 657   }
 658 }
 659 
 660 
 661 void VMThread::oops_do(OopClosure* f, CLDClosure* cld_f, CodeBlobClosure* cf) {
 662   Thread::oops_do(f, cld_f, cf);
 663   _vm_queue->oops_do(f);
 664 }
 665 
 666 //------------------------------------------------------------------------------------------------------------------
 667 #ifndef PRODUCT
 668 
 669 void VMOperationQueue::verify_queue(int prio) {
 670   // Check that list is correctly linked
 671   int length = _queue_length[prio];
 672   VM_Operation *cur = _queue[prio];
 673   int i;
 674 
 675   // Check forward links
 676   for(i = 0; i < length; i++) {
 677     cur = cur->next();
 678     assert(cur != _queue[prio], "list to short (forward)");
 679   }
 680   assert(cur->next() == _queue[prio], "list to long (forward)");
 681 
 682   // Check backwards links
 683   cur = _queue[prio];
 684   for(i = 0; i < length; i++) {
 685     cur = cur->prev();
 686     assert(cur != _queue[prio], "list to short (backwards)");
 687   }
 688   assert(cur->prev() == _queue[prio], "list to long (backwards)");
 689 }
 690 
 691 #endif
 692 
 693 void VMThread::verify() {
 694   oops_do(&VerifyOopClosure::verify_oop, NULL, NULL);
 695 }