1 /*
   2  * Copyright (c) 1998, 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 "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 "runtime/interfaceSupport.hpp"
  32 #include "runtime/mutexLocker.hpp"
  33 #include "runtime/os.hpp"
  34 #include "runtime/thread.inline.hpp"
  35 #include "runtime/vmThread.hpp"
  36 #include "runtime/vm_operations.hpp"
  37 #include "services/runtimeService.hpp"
  38 #include "trace/tracing.hpp"
  39 #include "utilities/dtrace.hpp"
  40 #include "utilities/events.hpp"
  41 #include "utilities/xmlstream.hpp"
  42 
  43 #ifndef USDT2
  44 HS_DTRACE_PROBE_DECL3(hotspot, vmops__request, char *, uintptr_t, int);
  45 HS_DTRACE_PROBE_DECL3(hotspot, vmops__begin, char *, uintptr_t, int);
  46 HS_DTRACE_PROBE_DECL3(hotspot, vmops__end, char *, uintptr_t, int);
  47 #endif /* !USDT2 */
  48 
  49 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
  50 
  51 // Dummy VM operation to act as first element in our circular double-linked list
  52 class VM_Dummy: public VM_Operation {
  53   VMOp_Type type() const { return VMOp_Dummy; }
  54   void  doit() {};
  55 };
  56 
  57 VMOperationQueue::VMOperationQueue() {
  58   // The queue is a circular doubled-linked list, which always contains
  59   // one element (i.e., one element means empty).
  60   for(int i = 0; i < nof_priorities; i++) {
  61     _queue_length[i] = 0;
  62     _queue_counter = 0;
  63     _queue[i] = new VM_Dummy();
  64     _queue[i]->set_next(_queue[i]);
  65     _queue[i]->set_prev(_queue[i]);
  66   }
  67   _drain_list = NULL;
  68 }
  69 
  70 
  71 bool VMOperationQueue::queue_empty(int prio) {
  72   // It is empty if there is exactly one element
  73   bool empty = (_queue[prio] == _queue[prio]->next());
  74   assert( (_queue_length[prio] == 0 && empty) ||
  75           (_queue_length[prio] > 0  && !empty), "sanity check");
  76   return _queue_length[prio] == 0;
  77 }
  78 
  79 // Inserts an element to the right of the q element
  80 void VMOperationQueue::insert(VM_Operation* q, VM_Operation* n) {
  81   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");
  82   n->set_prev(q);
  83   n->set_next(q->next());
  84   q->next()->set_prev(n);
  85   q->set_next(n);
  86 }
  87 
  88 void VMOperationQueue::queue_add_front(int prio, VM_Operation *op) {
  89   _queue_length[prio]++;
  90   insert(_queue[prio]->next(), op);
  91 }
  92 
  93 void VMOperationQueue::queue_add_back(int prio, VM_Operation *op) {
  94   _queue_length[prio]++;
  95   insert(_queue[prio]->prev(), op);
  96 }
  97 
  98 
  99 void VMOperationQueue::unlink(VM_Operation* q) {
 100   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");
 101   q->prev()->set_next(q->next());
 102   q->next()->set_prev(q->prev());
 103 }
 104 
 105 VM_Operation* VMOperationQueue::queue_remove_front(int prio) {
 106   if (queue_empty(prio)) return NULL;
 107   assert(_queue_length[prio] >= 0, "sanity check");
 108   _queue_length[prio]--;
 109   VM_Operation* r = _queue[prio]->next();
 110   assert(r != _queue[prio], "cannot remove base element");
 111   unlink(r);
 112   return r;
 113 }
 114 
 115 VM_Operation* VMOperationQueue::queue_drain(int prio) {
 116   if (queue_empty(prio)) return NULL;
 117   DEBUG_ONLY(int length = _queue_length[prio];);
 118   assert(length >= 0, "sanity check");
 119   _queue_length[prio] = 0;
 120   VM_Operation* r = _queue[prio]->next();
 121   assert(r != _queue[prio], "cannot remove base element");
 122   // remove links to base element from head and tail
 123   r->set_prev(NULL);
 124   _queue[prio]->prev()->set_next(NULL);
 125   // restore queue to empty state
 126   _queue[prio]->set_next(_queue[prio]);
 127   _queue[prio]->set_prev(_queue[prio]);
 128   assert(queue_empty(prio), "drain corrupted queue");
 129 #ifdef ASSERT
 130   int len = 0;
 131   VM_Operation* cur;
 132   for(cur = r; cur != NULL; cur=cur->next()) len++;
 133   assert(len == length, "drain lost some ops");
 134 #endif
 135   return r;
 136 }
 137 
 138 void VMOperationQueue::queue_oops_do(int queue, OopClosure* f) {
 139   VM_Operation* cur = _queue[queue];
 140   cur = cur->next();
 141   while (cur != _queue[queue]) {
 142     cur->oops_do(f);
 143     cur = cur->next();
 144   }
 145 }
 146 
 147 void VMOperationQueue::drain_list_oops_do(OopClosure* f) {
 148   VM_Operation* cur = _drain_list;
 149   while (cur != NULL) {
 150     cur->oops_do(f);
 151     cur = cur->next();
 152   }
 153 }
 154 
 155 //-----------------------------------------------------------------
 156 // High-level interface
 157 bool VMOperationQueue::add(VM_Operation *op) {
 158 
 159 #ifndef USDT2
 160   HS_DTRACE_PROBE3(hotspot, vmops__request, op->name(), strlen(op->name()),
 161                    op->evaluation_mode());
 162 #else /* USDT2 */
 163   HOTSPOT_VMOPS_REQUEST(
 164                    (char *) op->name(), strlen(op->name()),
 165                    op->evaluation_mode());
 166 #endif /* USDT2 */
 167 
 168   // Encapsulates VM queue policy. Currently, that
 169   // only involves putting them on the right list
 170   if (op->evaluate_at_safepoint()) {
 171     queue_add_back(SafepointPriority, op);
 172     return true;
 173   }
 174 
 175   queue_add_back(MediumPriority, op);
 176   return true;
 177 }
 178 
 179 VM_Operation* VMOperationQueue::remove_next() {
 180   // Assuming VMOperation queue is two-level priority queue. If there are
 181   // more than two priorities, we need a different scheduling algorithm.
 182   assert(SafepointPriority == 0 && MediumPriority == 1 && nof_priorities == 2,
 183          "current algorithm does not work");
 184 
 185   // simple counter based scheduling to prevent starvation of lower priority
 186   // queue. -- see 4390175
 187   int high_prio, low_prio;
 188   if (_queue_counter++ < 10) {
 189       high_prio = SafepointPriority;
 190       low_prio  = MediumPriority;
 191   } else {
 192       _queue_counter = 0;
 193       high_prio = MediumPriority;
 194       low_prio  = SafepointPriority;
 195   }
 196 
 197   return queue_remove_front(queue_empty(high_prio) ? low_prio : high_prio);
 198 }
 199 
 200 void VMOperationQueue::oops_do(OopClosure* f) {
 201   for(int i = 0; i < nof_priorities; i++) {
 202     queue_oops_do(i, f);
 203   }
 204   drain_list_oops_do(f);
 205 }
 206 
 207 
 208 //------------------------------------------------------------------------------------------------------------------
 209 // Implementation of VMThread stuff
 210 
 211 bool                VMThread::_should_terminate   = false;
 212 bool              VMThread::_terminated         = false;
 213 Monitor*          VMThread::_terminate_lock     = NULL;
 214 VMThread*         VMThread::_vm_thread          = NULL;
 215 VM_Operation*     VMThread::_cur_vm_operation   = NULL;
 216 VMOperationQueue* VMThread::_vm_queue           = NULL;
 217 PerfCounter*      VMThread::_perf_accumulated_vm_operation_time = NULL;
 218 
 219 
 220 void VMThread::create() {
 221   assert(vm_thread() == NULL, "we can only allocate one VMThread");
 222   _vm_thread = new VMThread();
 223 
 224   // Create VM operation queue
 225   _vm_queue = new VMOperationQueue();
 226   guarantee(_vm_queue != NULL, "just checking");
 227 
 228   _terminate_lock = new Monitor(Mutex::safepoint, "VMThread::_terminate_lock", true);
 229 
 230   if (UsePerfData) {
 231     // jvmstat performance counters
 232     Thread* THREAD = Thread::current();
 233     _perf_accumulated_vm_operation_time =
 234                  PerfDataManager::create_counter(SUN_THREADS, "vmOperationTime",
 235                                                  PerfData::U_Ticks, CHECK);
 236   }
 237 }
 238 
 239 
 240 VMThread::VMThread() : NamedThread() {
 241   set_name("VM Thread");
 242 }
 243 
 244 void VMThread::destroy() {
 245   if (_vm_thread != NULL) {
 246     delete _vm_thread;
 247     _vm_thread = NULL;      // VM thread is gone
 248   }
 249 }
 250 
 251 void VMThread::run() {
 252   assert(this == vm_thread(), "check");
 253 
 254   this->initialize_thread_local_storage();
 255   this->set_native_thread_name(this->name());
 256   this->record_stack_base_and_size();
 257   // Notify_lock wait checks on active_handles() to rewait in
 258   // case of spurious wakeup, it should wait on the last
 259   // value set prior to the notify
 260   this->set_active_handles(JNIHandleBlock::allocate_block());
 261 
 262   {
 263     MutexLocker ml(Notify_lock);
 264     Notify_lock->notify();
 265   }
 266   // Notify_lock is destroyed by Threads::create_vm()
 267 
 268   int prio = (VMThreadPriority == -1)
 269     ? os::java_to_os_priority[NearMaxPriority]
 270     : VMThreadPriority;
 271   // Note that I cannot call os::set_priority because it expects Java
 272   // priorities and I am *explicitly* using OS priorities so that it's
 273   // possible to set the VM thread priority higher than any Java thread.
 274   os::set_native_priority( this, prio );
 275 
 276   // Wait for VM_Operations until termination
 277   this->loop();
 278 
 279   // Note the intention to exit before safepointing.
 280   // 6295565  This has the effect of waiting for any large tty
 281   // outputs to finish.
 282   if (xtty != NULL) {
 283     ttyLocker ttyl;
 284     xtty->begin_elem("destroy_vm");
 285     xtty->stamp();
 286     xtty->end_elem();
 287     assert(should_terminate(), "termination flag must be set");
 288   }
 289 
 290   // 4526887 let VM thread exit at Safepoint
 291   SafepointSynchronize::begin();
 292 
 293   if (VerifyBeforeExit) {
 294     HandleMark hm(VMThread::vm_thread());
 295     // Among other things, this ensures that Eden top is correct.
 296     Universe::heap()->prepare_for_verify();
 297     os::check_heap();
 298     // Silent verification so as not to pollute normal output,
 299     // unless we really asked for it.
 300     Universe::verify(!(PrintGCDetails || Verbose) || VerifySilently);
 301   }
 302 
 303   CompileBroker::set_should_block();
 304 
 305   // wait for threads (compiler threads or daemon threads) in the
 306   // _thread_in_native state to block.
 307   VM_Exit::wait_for_threads_in_native_to_block();
 308 
 309   // signal other threads that VM process is gone
 310   {
 311     // Note: we must have the _no_safepoint_check_flag. Mutex::lock() allows
 312     // VM thread to enter any lock at Safepoint as long as its _owner is NULL.
 313     // If that happens after _terminate_lock->wait() has unset _owner
 314     // but before it actually drops the lock and waits, the notification below
 315     // may get lost and we will have a hang. To avoid this, we need to use
 316     // Mutex::lock_without_safepoint_check().
 317     MutexLockerEx ml(_terminate_lock, Mutex::_no_safepoint_check_flag);
 318     _terminated = true;
 319     _terminate_lock->notify();
 320   }
 321 
 322   // Thread destructor usually does this.
 323   ThreadLocalStorage::set_thread(NULL);
 324 
 325   // Deletion must be done synchronously by the JNI DestroyJavaVM thread
 326   // so that the VMThread deletion completes before the main thread frees
 327   // up the CodeHeap.
 328 
 329 }
 330 
 331 
 332 // Notify the VMThread that the last non-daemon JavaThread has terminated,
 333 // and wait until operation is performed.
 334 void VMThread::wait_for_vm_thread_exit() {
 335   { MutexLocker mu(VMOperationQueue_lock);
 336     _should_terminate = true;
 337     VMOperationQueue_lock->notify();
 338   }
 339 
 340   // Note: VM thread leaves at Safepoint. We are not stopped by Safepoint
 341   // because this thread has been removed from the threads list. But anything
 342   // that could get blocked by Safepoint should not be used after this point,
 343   // otherwise we will hang, since there is no one can end the safepoint.
 344 
 345   // Wait until VM thread is terminated
 346   // Note: it should be OK to use Terminator_lock here. But this is called
 347   // at a very delicate time (VM shutdown) and we are operating in non- VM
 348   // thread at Safepoint. It's safer to not share lock with other threads.
 349   { MutexLockerEx ml(_terminate_lock, Mutex::_no_safepoint_check_flag);
 350     while(!VMThread::is_terminated()) {
 351         _terminate_lock->wait(Mutex::_no_safepoint_check_flag);
 352     }
 353   }
 354 }
 355 
 356 void VMThread::print_on(outputStream* st) const {
 357   st->print("\"%s\" ", name());
 358   Thread::print_on(st);
 359   st->cr();
 360 }
 361 
 362 void VMThread::evaluate_operation(VM_Operation* op) {
 363   ResourceMark rm;
 364 
 365   {
 366     PerfTraceTime vm_op_timer(perf_accumulated_vm_operation_time());
 367 #ifndef USDT2
 368     HS_DTRACE_PROBE3(hotspot, vmops__begin, op->name(), strlen(op->name()),
 369                      op->evaluation_mode());
 370 #else /* USDT2 */
 371     HOTSPOT_VMOPS_BEGIN(
 372                      (char *) op->name(), strlen(op->name()),
 373                      op->evaluation_mode());
 374 #endif /* USDT2 */
 375 
 376     EventExecuteVMOperation event;
 377 
 378     op->evaluate();
 379 
 380     if (event.should_commit()) {
 381       bool is_concurrent = op->evaluate_concurrently();
 382       event.set_operation(op->type());
 383       event.set_safepoint(op->evaluate_at_safepoint());
 384       event.set_blocking(!is_concurrent);
 385       // Only write caller thread information for non-concurrent vm operations.
 386       // For concurrent vm operations, the thread id is set to 0 indicating thread is unknown.
 387       // This is because the caller thread could have exited already.
 388       event.set_caller(is_concurrent ? 0 : op->calling_thread()->osthread()->thread_id());
 389       event.commit();
 390     }
 391 
 392 #ifndef USDT2
 393     HS_DTRACE_PROBE3(hotspot, vmops__end, op->name(), strlen(op->name()),
 394                      op->evaluation_mode());
 395 #else /* USDT2 */
 396     HOTSPOT_VMOPS_END(
 397                      (char *) op->name(), strlen(op->name()),
 398                      op->evaluation_mode());
 399 #endif /* USDT2 */
 400   }
 401 
 402   // Last access of info in _cur_vm_operation!
 403   bool c_heap_allocated = op->is_cheap_allocated();
 404 
 405   // Mark as completed
 406   if (!op->evaluate_concurrently()) {
 407     op->calling_thread()->increment_vm_operation_completed_count();
 408   }
 409   // It is unsafe to access the _cur_vm_operation after the 'increment_vm_operation_completed_count' call,
 410   // since if it is stack allocated the calling thread might have deallocated
 411   if (c_heap_allocated) {
 412     delete _cur_vm_operation;
 413   }
 414 }
 415 
 416 
 417 void VMThread::loop() {
 418   assert(_cur_vm_operation == NULL, "no current one should be executing");
 419 
 420   while(true) {
 421     VM_Operation* safepoint_ops = NULL;
 422     //
 423     // Wait for VM operation
 424     //
 425     // use no_safepoint_check to get lock without attempting to "sneak"
 426     { MutexLockerEx mu_queue(VMOperationQueue_lock,
 427                              Mutex::_no_safepoint_check_flag);
 428 
 429       // Look for new operation
 430       assert(_cur_vm_operation == NULL, "no current one should be executing");
 431       _cur_vm_operation = _vm_queue->remove_next();
 432 
 433       // Stall time tracking code
 434       if (PrintVMQWaitTime && _cur_vm_operation != NULL &&
 435           !_cur_vm_operation->evaluate_concurrently()) {
 436         long stall = os::javaTimeMillis() - _cur_vm_operation->timestamp();
 437         if (stall > 0)
 438           tty->print_cr("%s stall: %Ld",  _cur_vm_operation->name(), stall);
 439       }
 440 
 441       while (!should_terminate() && _cur_vm_operation == NULL) {
 442         // wait with a timeout to guarantee safepoints at regular intervals
 443         bool timedout =
 444           VMOperationQueue_lock->wait(Mutex::_no_safepoint_check_flag,
 445                                       GuaranteedSafepointInterval);
 446 
 447         // Support for self destruction
 448         if ((SelfDestructTimer != 0) && !is_error_reported() &&
 449             (os::elapsedTime() > SelfDestructTimer * 60)) {
 450           tty->print_cr("VM self-destructed");
 451           exit(-1);
 452         }
 453 
 454         if (timedout && (SafepointALot ||
 455                          SafepointSynchronize::is_cleanup_needed())) {
 456           MutexUnlockerEx mul(VMOperationQueue_lock,
 457                               Mutex::_no_safepoint_check_flag);
 458           // Force a safepoint since we have not had one for at least
 459           // 'GuaranteedSafepointInterval' milliseconds.  This will run all
 460           // the clean-up processing that needs to be done regularly at a
 461           // safepoint
 462           SafepointSynchronize::begin();
 463           #ifdef ASSERT
 464             if (GCALotAtAllSafepoints) InterfaceSupport::check_gc_alot();
 465           #endif
 466           SafepointSynchronize::end();
 467         }
 468         _cur_vm_operation = _vm_queue->remove_next();
 469 
 470         // If we are at a safepoint we will evaluate all the operations that
 471         // follow that also require a safepoint
 472         if (_cur_vm_operation != NULL &&
 473             _cur_vm_operation->evaluate_at_safepoint()) {
 474           safepoint_ops = _vm_queue->drain_at_safepoint_priority();
 475         }
 476       }
 477 
 478       if (should_terminate()) break;
 479     } // Release mu_queue_lock
 480 
 481     //
 482     // Execute VM operation
 483     //
 484     { HandleMark hm(VMThread::vm_thread());
 485 
 486       EventMark em("Executing VM operation: %s", vm_operation()->name());
 487       assert(_cur_vm_operation != NULL, "we should have found an operation to execute");
 488 
 489       // Give the VM thread an extra quantum.  Jobs tend to be bursty and this
 490       // helps the VM thread to finish up the job.
 491       // FIXME: When this is enabled and there are many threads, this can degrade
 492       // performance significantly.
 493       if( VMThreadHintNoPreempt )
 494         os::hint_no_preempt();
 495 
 496       // If we are at a safepoint we will evaluate all the operations that
 497       // follow that also require a safepoint
 498       if (_cur_vm_operation->evaluate_at_safepoint()) {
 499 
 500         _vm_queue->set_drain_list(safepoint_ops); // ensure ops can be scanned
 501 
 502         SafepointSynchronize::begin();
 503         evaluate_operation(_cur_vm_operation);
 504         // now process all queued safepoint ops, iteratively draining
 505         // the queue until there are none left
 506         do {
 507           _cur_vm_operation = safepoint_ops;
 508           if (_cur_vm_operation != NULL) {
 509             do {
 510               // evaluate_operation deletes the op object so we have
 511               // to grab the next op now
 512               VM_Operation* next = _cur_vm_operation->next();
 513               _vm_queue->set_drain_list(next);
 514               evaluate_operation(_cur_vm_operation);
 515               _cur_vm_operation = next;
 516               if (PrintSafepointStatistics) {
 517                 SafepointSynchronize::inc_vmop_coalesced_count();
 518               }
 519             } while (_cur_vm_operation != NULL);
 520           }
 521           // There is a chance that a thread enqueued a safepoint op
 522           // since we released the op-queue lock and initiated the safepoint.
 523           // So we drain the queue again if there is anything there, as an
 524           // optimization to try and reduce the number of safepoints.
 525           // As the safepoint synchronizes us with JavaThreads we will see
 526           // any enqueue made by a JavaThread, but the peek will not
 527           // necessarily detect a concurrent enqueue by a GC thread, but
 528           // that simply means the op will wait for the next major cycle of the
 529           // VMThread - just as it would if the GC thread lost the race for
 530           // the lock.
 531           if (_vm_queue->peek_at_safepoint_priority()) {
 532             // must hold lock while draining queue
 533             MutexLockerEx mu_queue(VMOperationQueue_lock,
 534                                      Mutex::_no_safepoint_check_flag);
 535             safepoint_ops = _vm_queue->drain_at_safepoint_priority();
 536           } else {
 537             safepoint_ops = NULL;
 538           }
 539         } while(safepoint_ops != NULL);
 540 
 541         _vm_queue->set_drain_list(NULL);
 542 
 543         // Complete safepoint synchronization
 544         SafepointSynchronize::end();
 545 
 546       } else {  // not a safepoint operation
 547         if (TraceLongCompiles) {
 548           elapsedTimer t;
 549           t.start();
 550           evaluate_operation(_cur_vm_operation);
 551           t.stop();
 552           double secs = t.seconds();
 553           if (secs * 1e3 > LongCompileThreshold) {
 554             // XXX - _cur_vm_operation should not be accessed after
 555             // the completed count has been incremented; the waiting
 556             // thread may have already freed this memory.
 557             tty->print_cr("vm %s: %3.7f secs]", _cur_vm_operation->name(), secs);
 558           }
 559         } else {
 560           evaluate_operation(_cur_vm_operation);
 561         }
 562 
 563         _cur_vm_operation = NULL;
 564       }
 565     }
 566 
 567     //
 568     //  Notify (potential) waiting Java thread(s) - lock without safepoint
 569     //  check so that sneaking is not possible
 570     { MutexLockerEx mu(VMOperationRequest_lock,
 571                        Mutex::_no_safepoint_check_flag);
 572       VMOperationRequest_lock->notify_all();
 573     }
 574 
 575     //
 576     // We want to make sure that we get to a safepoint regularly.
 577     //
 578     if (SafepointALot || SafepointSynchronize::is_cleanup_needed()) {
 579       long interval          = SafepointSynchronize::last_non_safepoint_interval();
 580       bool max_time_exceeded = GuaranteedSafepointInterval != 0 && (interval > GuaranteedSafepointInterval);
 581       if (SafepointALot || max_time_exceeded) {
 582         HandleMark hm(VMThread::vm_thread());
 583         SafepointSynchronize::begin();
 584         SafepointSynchronize::end();
 585       }
 586     }
 587   }
 588 }
 589 
 590 void VMThread::execute(VM_Operation* op) {
 591   Thread* t = Thread::current();
 592 
 593   if (!t->is_VM_thread()) {
 594     SkipGCALot sgcalot(t);    // avoid re-entrant attempts to gc-a-lot
 595     // JavaThread or WatcherThread
 596     bool concurrent = op->evaluate_concurrently();
 597     // only blocking VM operations need to verify the caller's safepoint state:
 598     if (!concurrent) {
 599       t->check_for_valid_safepoint_state(true);
 600     }
 601 
 602     // New request from Java thread, evaluate prologue
 603     if (!op->doit_prologue()) {
 604       return;   // op was cancelled
 605     }
 606 
 607     // Setup VM_operations for execution
 608     op->set_calling_thread(t, Thread::get_priority(t));
 609 
 610     // It does not make sense to execute the epilogue, if the VM operation object is getting
 611     // deallocated by the VM thread.
 612     bool execute_epilog = !op->is_cheap_allocated();
 613     assert(!concurrent || op->is_cheap_allocated(), "concurrent => cheap_allocated");
 614 
 615     // Get ticket number for non-concurrent VM operations
 616     int ticket = 0;
 617     if (!concurrent) {
 618       ticket = t->vm_operation_ticket();
 619     }
 620 
 621     // Add VM operation to list of waiting threads. We are guaranteed not to block while holding the
 622     // VMOperationQueue_lock, so we can block without a safepoint check. This allows vm operation requests
 623     // to be queued up during a safepoint synchronization.
 624     {
 625       VMOperationQueue_lock->lock_without_safepoint_check();
 626       bool ok = _vm_queue->add(op);
 627     op->set_timestamp(os::javaTimeMillis());
 628       VMOperationQueue_lock->notify();
 629       VMOperationQueue_lock->unlock();
 630       // VM_Operation got skipped
 631       if (!ok) {
 632         assert(concurrent, "can only skip concurrent tasks");
 633         if (op->is_cheap_allocated()) delete op;
 634         return;
 635       }
 636     }
 637 
 638     if (!concurrent) {
 639       // Wait for completion of request (non-concurrent)
 640       // Note: only a JavaThread triggers the safepoint check when locking
 641       MutexLocker mu(VMOperationRequest_lock);
 642       while(t->vm_operation_completed_count() < ticket) {
 643         VMOperationRequest_lock->wait(!t->is_Java_thread());
 644       }
 645     }
 646 
 647     if (execute_epilog) {
 648       op->doit_epilogue();
 649     }
 650   } else {
 651     // invoked by VM thread; usually nested VM operation
 652     assert(t->is_VM_thread(), "must be a VM thread");
 653     VM_Operation* prev_vm_operation = vm_operation();
 654     if (prev_vm_operation != NULL) {
 655       // Check the VM operation allows nested VM operation. This normally not the case, e.g., the compiler
 656       // does not allow nested scavenges or compiles.
 657       if (!prev_vm_operation->allow_nested_vm_operations()) {
 658         fatal(err_msg("Nested VM operation %s requested by operation %s",
 659                       op->name(), vm_operation()->name()));
 660       }
 661       op->set_calling_thread(prev_vm_operation->calling_thread(), prev_vm_operation->priority());
 662     }
 663 
 664     EventMark em("Executing %s VM operation: %s", prev_vm_operation ? "nested" : "", op->name());
 665 
 666     // Release all internal handles after operation is evaluated
 667     HandleMark hm(t);
 668     _cur_vm_operation = op;
 669 
 670     if (op->evaluate_at_safepoint() && !SafepointSynchronize::is_at_safepoint()) {
 671       SafepointSynchronize::begin();
 672       op->evaluate();
 673       SafepointSynchronize::end();
 674     } else {
 675       op->evaluate();
 676     }
 677 
 678     // Free memory if needed
 679     if (op->is_cheap_allocated()) delete op;
 680 
 681     _cur_vm_operation = prev_vm_operation;
 682   }
 683 }
 684 
 685 
 686 void VMThread::oops_do(OopClosure* f, CLDClosure* cld_f, CodeBlobClosure* cf) {
 687   Thread::oops_do(f, cld_f, cf);
 688   _vm_queue->oops_do(f);
 689 }
 690 
 691 //------------------------------------------------------------------------------------------------------------------
 692 #ifndef PRODUCT
 693 
 694 void VMOperationQueue::verify_queue(int prio) {
 695   // Check that list is correctly linked
 696   int length = _queue_length[prio];
 697   VM_Operation *cur = _queue[prio];
 698   int i;
 699 
 700   // Check forward links
 701   for(i = 0; i < length; i++) {
 702     cur = cur->next();
 703     assert(cur != _queue[prio], "list to short (forward)");
 704   }
 705   assert(cur->next() == _queue[prio], "list to long (forward)");
 706 
 707   // Check backwards links
 708   cur = _queue[prio];
 709   for(i = 0; i < length; i++) {
 710     cur = cur->prev();
 711     assert(cur != _queue[prio], "list to short (backwards)");
 712   }
 713   assert(cur->prev() == _queue[prio], "list to long (backwards)");
 714 }
 715 
 716 #endif
 717 
 718 void VMThread::verify() {
 719   oops_do(&VerifyOopClosure::verify_oop, NULL, NULL);
 720 }