1 /*
   2  * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "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 "utilities/dtrace.hpp"
  39 #include "utilities/events.hpp"
  40 #include "utilities/xmlstream.hpp"
  41 
  42 #ifndef USDT2
  43 HS_DTRACE_PROBE_DECL3(hotspot, vmops__request, char *, uintptr_t, int);
  44 HS_DTRACE_PROBE_DECL3(hotspot, vmops__begin, char *, uintptr_t, int);
  45 HS_DTRACE_PROBE_DECL3(hotspot, vmops__end, char *, uintptr_t, int);
  46 #endif /* !USDT2 */
  47 
  48 // Dummy VM operation to act as first element in our circular double-linked list
  49 class VM_Dummy: public VM_Operation {
  50   VMOp_Type type() const { return VMOp_Dummy; }
  51   void  doit() {};
  52 };
  53 
  54 VMOperationQueue::VMOperationQueue() {
  55   // The queue is a circular doubled-linked list, which always contains
  56   // one element (i.e., one element means empty).
  57   for(int i = 0; i < nof_priorities; i++) {
  58     _queue_length[i] = 0;
  59     _queue_counter = 0;
  60     _queue[i] = new VM_Dummy();
  61     _queue[i]->set_next(_queue[i]);
  62     _queue[i]->set_prev(_queue[i]);
  63   }
  64   _drain_list = NULL;
  65 }
  66 
  67 
  68 bool VMOperationQueue::queue_empty(int prio) {
  69   // It is empty if there is exactly one element
  70   bool empty = (_queue[prio] == _queue[prio]->next());
  71   assert( (_queue_length[prio] == 0 && empty) ||
  72           (_queue_length[prio] > 0  && !empty), "sanity check");
  73   return _queue_length[prio] == 0;
  74 }
  75 
  76 // Inserts an element to the right of the q element
  77 void VMOperationQueue::insert(VM_Operation* q, VM_Operation* n) {
  78   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");
  79   n->set_prev(q);
  80   n->set_next(q->next());
  81   q->next()->set_prev(n);
  82   q->set_next(n);
  83 }
  84 
  85 void VMOperationQueue::queue_add_front(int prio, VM_Operation *op) {
  86   _queue_length[prio]++;
  87   insert(_queue[prio]->next(), op);
  88 }
  89 
  90 void VMOperationQueue::queue_add_back(int prio, VM_Operation *op) {
  91   _queue_length[prio]++;
  92   insert(_queue[prio]->prev(), op);
  93 }
  94 
  95 
  96 void VMOperationQueue::unlink(VM_Operation* q) {
  97   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");
  98   q->prev()->set_next(q->next());
  99   q->next()->set_prev(q->prev());
 100 }
 101 
 102 VM_Operation* VMOperationQueue::queue_remove_front(int prio) {
 103   if (queue_empty(prio)) return NULL;
 104   assert(_queue_length[prio] >= 0, "sanity check");
 105   _queue_length[prio]--;
 106   VM_Operation* r = _queue[prio]->next();
 107   assert(r != _queue[prio], "cannot remove base element");
 108   unlink(r);
 109   return r;
 110 }
 111 
 112 VM_Operation* VMOperationQueue::queue_drain(int prio) {
 113   if (queue_empty(prio)) return NULL;
 114   DEBUG_ONLY(int length = _queue_length[prio];);
 115   assert(length >= 0, "sanity check");
 116   _queue_length[prio] = 0;
 117   VM_Operation* r = _queue[prio]->next();
 118   assert(r != _queue[prio], "cannot remove base element");
 119   // remove links to base element from head and tail
 120   r->set_prev(NULL);
 121   _queue[prio]->prev()->set_next(NULL);
 122   // restore queue to empty state
 123   _queue[prio]->set_next(_queue[prio]);
 124   _queue[prio]->set_prev(_queue[prio]);
 125   assert(queue_empty(prio), "drain corrupted queue");
 126 #ifdef DEBUG
 127   int len = 0;
 128   VM_Operation* cur;
 129   for(cur = r; cur != NULL; cur=cur->next()) len++;
 130   assert(len == length, "drain lost some ops");
 131 #endif
 132   return r;
 133 }
 134 
 135 void VMOperationQueue::queue_oops_do(int queue, OopClosure* f) {
 136   VM_Operation* cur = _queue[queue];
 137   cur = cur->next();
 138   while (cur != _queue[queue]) {
 139     cur->oops_do(f);
 140     cur = cur->next();
 141   }
 142 }
 143 
 144 void VMOperationQueue::drain_list_oops_do(OopClosure* f) {
 145   VM_Operation* cur = _drain_list;
 146   while (cur != NULL) {
 147     cur->oops_do(f);
 148     cur = cur->next();
 149   }
 150 }
 151 
 152 //-----------------------------------------------------------------
 153 // High-level interface
 154 bool VMOperationQueue::add(VM_Operation *op) {
 155 
 156 #ifndef USDT2
 157   HS_DTRACE_PROBE3(hotspot, vmops__request, op->name(), strlen(op->name()),
 158                    op->evaluation_mode());
 159 #else /* USDT2 */
 160   HOTSPOT_VMOPS_REQUEST(
 161                    (char *) op->name(), strlen(op->name()),
 162                    op->evaluation_mode());
 163 #endif /* USDT2 */
 164 
 165   // Encapsulates VM queue policy. Currently, that
 166   // only involves putting them on the right list
 167   if (op->evaluate_at_safepoint()) {
 168     queue_add_back(SafepointPriority, op);
 169     return true;
 170   }
 171 
 172   queue_add_back(MediumPriority, op);
 173   return true;
 174 }
 175 
 176 VM_Operation* VMOperationQueue::remove_next() {
 177   // Assuming VMOperation queue is two-level priority queue. If there are
 178   // more than two priorities, we need a different scheduling algorithm.
 179   assert(SafepointPriority == 0 && MediumPriority == 1 && nof_priorities == 2,
 180          "current algorithm does not work");
 181 
 182   // simple counter based scheduling to prevent starvation of lower priority
 183   // queue. -- see 4390175
 184   int high_prio, low_prio;
 185   if (_queue_counter++ < 10) {
 186       high_prio = SafepointPriority;
 187       low_prio  = MediumPriority;
 188   } else {
 189       _queue_counter = 0;
 190       high_prio = MediumPriority;
 191       low_prio  = SafepointPriority;
 192   }
 193 
 194   return queue_remove_front(queue_empty(high_prio) ? low_prio : high_prio);
 195 }
 196 
 197 void VMOperationQueue::oops_do(OopClosure* f) {
 198   for(int i = 0; i < nof_priorities; i++) {
 199     queue_oops_do(i, f);
 200   }
 201   drain_list_oops_do(f);
 202 }
 203 
 204 
 205 //------------------------------------------------------------------------------------------------------------------
 206 // Implementation of VMThread stuff
 207 
 208 bool                VMThread::_should_terminate   = false;
 209 bool              VMThread::_terminated         = false;
 210 Monitor*          VMThread::_terminate_lock     = NULL;
 211 VMThread*         VMThread::_vm_thread          = NULL;
 212 VM_Operation*     VMThread::_cur_vm_operation   = NULL;
 213 VMOperationQueue* VMThread::_vm_queue           = NULL;
 214 PerfCounter*      VMThread::_perf_accumulated_vm_operation_time = NULL;
 215 
 216 
 217 void VMThread::create() {
 218   assert(vm_thread() == NULL, "we can only allocate one VMThread");
 219   _vm_thread = new VMThread();
 220 
 221   // Create VM operation queue
 222   _vm_queue = new VMOperationQueue();
 223   guarantee(_vm_queue != NULL, "just checking");
 224 
 225   _terminate_lock = new Monitor(Mutex::safepoint, "VMThread::_terminate_lock", true);
 226 
 227   if (UsePerfData) {
 228     // jvmstat performance counters
 229     Thread* THREAD = Thread::current();
 230     _perf_accumulated_vm_operation_time =
 231                  PerfDataManager::create_counter(SUN_THREADS, "vmOperationTime",
 232                                                  PerfData::U_Ticks, CHECK);
 233   }
 234 }
 235 
 236 
 237 VMThread::VMThread() : NamedThread() {
 238   set_name("VM Thread");
 239 }
 240 
 241 void VMThread::destroy() {
 242   if (_vm_thread != NULL) {
 243     delete _vm_thread;
 244     _vm_thread = NULL;      // VM thread is gone
 245   }
 246 }
 247 
 248 void VMThread::run() {
 249   assert(this == vm_thread(), "check");
 250 
 251   this->initialize_thread_local_storage();
 252   this->record_stack_base_and_size();
 253   // Notify_lock wait checks on active_handles() to rewait in
 254   // case of spurious wakeup, it should wait on the last
 255   // value set prior to the notify
 256   this->set_active_handles(JNIHandleBlock::allocate_block());
 257 
 258   {
 259     MutexLocker ml(Notify_lock);
 260     Notify_lock->notify();
 261   }
 262   // Notify_lock is destroyed by Threads::create_vm()
 263 
 264   int prio = (VMThreadPriority == -1)
 265     ? os::java_to_os_priority[NearMaxPriority]
 266     : VMThreadPriority;
 267   // Note that I cannot call os::set_priority because it expects Java
 268   // priorities and I am *explicitly* using OS priorities so that it's
 269   // possible to set the VM thread priority higher than any Java thread.
 270   os::set_native_priority( this, prio );
 271 
 272   // Wait for VM_Operations until termination
 273   this->loop();
 274 
 275   // Note the intention to exit before safepointing.
 276   // 6295565  This has the effect of waiting for any large tty
 277   // outputs to finish.
 278   if (xtty != NULL) {
 279     ttyLocker ttyl;
 280     xtty->begin_elem("destroy_vm");
 281     xtty->stamp();
 282     xtty->end_elem();
 283     assert(should_terminate(), "termination flag must be set");
 284   }
 285 
 286   // 4526887 let VM thread exit at Safepoint
 287   SafepointSynchronize::begin();
 288 
 289   if (VerifyBeforeExit) {
 290     HandleMark hm(VMThread::vm_thread());
 291     // Among other things, this ensures that Eden top is correct.
 292     Universe::heap()->prepare_for_verify();
 293     os::check_heap();
 294     // Silent verification so as not to pollute normal output,
 295     // unless we really asked for it.
 296     Universe::verify(!(PrintGCDetails || Verbose));
 297   }
 298 
 299   CompileBroker::set_should_block();
 300 
 301   // wait for threads (compiler threads or daemon threads) in the
 302   // _thread_in_native state to block.
 303   VM_Exit::wait_for_threads_in_native_to_block();
 304 
 305   // signal other threads that VM process is gone
 306   {
 307     // Note: we must have the _no_safepoint_check_flag. Mutex::lock() allows
 308     // VM thread to enter any lock at Safepoint as long as its _owner is NULL.
 309     // If that happens after _terminate_lock->wait() has unset _owner
 310     // but before it actually drops the lock and waits, the notification below
 311     // may get lost and we will have a hang. To avoid this, we need to use
 312     // Mutex::lock_without_safepoint_check().
 313     MutexLockerEx ml(_terminate_lock, Mutex::_no_safepoint_check_flag);
 314     _terminated = true;
 315     _terminate_lock->notify();
 316   }
 317 
 318   // Deletion must be done synchronously by the JNI DestroyJavaVM thread
 319   // so that the VMThread deletion completes before the main thread frees
 320   // up the CodeHeap.
 321 
 322 }
 323 
 324 
 325 // Notify the VMThread that the last non-daemon JavaThread has terminated,
 326 // and wait until operation is performed.
 327 void VMThread::wait_for_vm_thread_exit() {
 328   { MutexLocker mu(VMOperationQueue_lock);
 329     _should_terminate = true;
 330     VMOperationQueue_lock->notify();
 331   }
 332 
 333   // Note: VM thread leaves at Safepoint. We are not stopped by Safepoint
 334   // because this thread has been removed from the threads list. But anything
 335   // that could get blocked by Safepoint should not be used after this point,
 336   // otherwise we will hang, since there is no one can end the safepoint.
 337 
 338   // Wait until VM thread is terminated
 339   // Note: it should be OK to use Terminator_lock here. But this is called
 340   // at a very delicate time (VM shutdown) and we are operating in non- VM
 341   // thread at Safepoint. It's safer to not share lock with other threads.
 342   { MutexLockerEx ml(_terminate_lock, Mutex::_no_safepoint_check_flag);
 343     while(!VMThread::is_terminated()) {
 344         _terminate_lock->wait(Mutex::_no_safepoint_check_flag);
 345     }
 346   }
 347 }
 348 
 349 void VMThread::print_on(outputStream* st) const {
 350   st->print("\"%s\" ", name());
 351   Thread::print_on(st);
 352   st->cr();
 353 }
 354 
 355 void VMThread::evaluate_operation(VM_Operation* op) {
 356   ResourceMark rm;
 357 
 358   {
 359     PerfTraceTime vm_op_timer(perf_accumulated_vm_operation_time());
 360 #ifndef USDT2
 361     HS_DTRACE_PROBE3(hotspot, vmops__begin, op->name(), strlen(op->name()),
 362                      op->evaluation_mode());
 363 #else /* USDT2 */
 364     HOTSPOT_VMOPS_BEGIN(
 365                      (char *) op->name(), strlen(op->name()),
 366                      op->evaluation_mode());
 367 #endif /* USDT2 */
 368     op->evaluate();
 369 #ifndef USDT2
 370     HS_DTRACE_PROBE3(hotspot, vmops__end, op->name(), strlen(op->name()),
 371                      op->evaluation_mode());
 372 #else /* USDT2 */
 373     HOTSPOT_VMOPS_END(
 374                      (char *) op->name(), strlen(op->name()),
 375                      op->evaluation_mode());
 376 #endif /* USDT2 */
 377   }
 378 
 379   // Last access of info in _cur_vm_operation!
 380   bool c_heap_allocated = op->is_cheap_allocated();
 381 
 382   // Mark as completed
 383   if (!op->evaluate_concurrently()) {
 384     op->calling_thread()->increment_vm_operation_completed_count();
 385   }
 386   // It is unsafe to access the _cur_vm_operation after the 'increment_vm_operation_completed_count' call,
 387   // since if it is stack allocated the calling thread might have deallocated
 388   if (c_heap_allocated) {
 389     delete _cur_vm_operation;
 390   }
 391 }
 392 
 393 
 394 void VMThread::loop() {
 395   assert(_cur_vm_operation == NULL, "no current one should be executing");
 396 
 397   while(true) {
 398     VM_Operation* safepoint_ops = NULL;
 399     //
 400     // Wait for VM operation
 401     //
 402     // use no_safepoint_check to get lock without attempting to "sneak"
 403     { MutexLockerEx mu_queue(VMOperationQueue_lock,
 404                              Mutex::_no_safepoint_check_flag);
 405 
 406       // Look for new operation
 407       assert(_cur_vm_operation == NULL, "no current one should be executing");
 408       _cur_vm_operation = _vm_queue->remove_next();
 409 
 410       // Stall time tracking code
 411       if (PrintVMQWaitTime && _cur_vm_operation != NULL &&
 412           !_cur_vm_operation->evaluate_concurrently()) {
 413         long stall = os::javaTimeMillis() - _cur_vm_operation->timestamp();
 414         if (stall > 0)
 415           tty->print_cr("%s stall: %Ld",  _cur_vm_operation->name(), stall);
 416       }
 417 
 418       while (!should_terminate() && _cur_vm_operation == NULL) {
 419         // wait with a timeout to guarantee safepoints at regular intervals
 420         bool timedout =
 421           VMOperationQueue_lock->wait(Mutex::_no_safepoint_check_flag,
 422                                       GuaranteedSafepointInterval);
 423 
 424         // Support for self destruction
 425         if ((SelfDestructTimer != 0) && !is_error_reported() &&
 426             (os::elapsedTime() > SelfDestructTimer * 60)) {
 427           tty->print_cr("VM self-destructed");
 428           exit(-1);
 429         }
 430 
 431         if (timedout && (SafepointALot ||
 432                          SafepointSynchronize::is_cleanup_needed())) {
 433           MutexUnlockerEx mul(VMOperationQueue_lock,
 434                               Mutex::_no_safepoint_check_flag);
 435           // Force a safepoint since we have not had one for at least
 436           // 'GuaranteedSafepointInterval' milliseconds.  This will run all
 437           // the clean-up processing that needs to be done regularly at a
 438           // safepoint
 439           SafepointSynchronize::begin();
 440           #ifdef ASSERT
 441             if (GCALotAtAllSafepoints) InterfaceSupport::check_gc_alot();
 442           #endif
 443           SafepointSynchronize::end();
 444         }
 445         _cur_vm_operation = _vm_queue->remove_next();
 446 
 447         // If we are at a safepoint we will evaluate all the operations that
 448         // follow that also require a safepoint
 449         if (_cur_vm_operation != NULL &&
 450             _cur_vm_operation->evaluate_at_safepoint()) {
 451           safepoint_ops = _vm_queue->drain_at_safepoint_priority();
 452         }
 453       }
 454 
 455       if (should_terminate()) break;
 456     } // Release mu_queue_lock
 457 
 458     //
 459     // Execute VM operation
 460     //
 461     { HandleMark hm(VMThread::vm_thread());
 462 
 463       EventMark em("Executing VM operation: %s", vm_operation()->name());
 464       assert(_cur_vm_operation != NULL, "we should have found an operation to execute");
 465 
 466       // Give the VM thread an extra quantum.  Jobs tend to be bursty and this
 467       // helps the VM thread to finish up the job.
 468       // FIXME: When this is enabled and there are many threads, this can degrade
 469       // performance significantly.
 470       if( VMThreadHintNoPreempt )
 471         os::hint_no_preempt();
 472 
 473       // If we are at a safepoint we will evaluate all the operations that
 474       // follow that also require a safepoint
 475       if (_cur_vm_operation->evaluate_at_safepoint()) {
 476 
 477         _vm_queue->set_drain_list(safepoint_ops); // ensure ops can be scanned
 478 
 479         SafepointSynchronize::begin();
 480         evaluate_operation(_cur_vm_operation);
 481         // now process all queued safepoint ops, iteratively draining
 482         // the queue until there are none left
 483         do {
 484           _cur_vm_operation = safepoint_ops;
 485           if (_cur_vm_operation != NULL) {
 486             do {
 487               // evaluate_operation deletes the op object so we have
 488               // to grab the next op now
 489               VM_Operation* next = _cur_vm_operation->next();
 490               _vm_queue->set_drain_list(next);
 491               evaluate_operation(_cur_vm_operation);
 492               _cur_vm_operation = next;
 493               if (PrintSafepointStatistics) {
 494                 SafepointSynchronize::inc_vmop_coalesced_count();
 495               }
 496             } while (_cur_vm_operation != NULL);
 497           }
 498           // There is a chance that a thread enqueued a safepoint op
 499           // since we released the op-queue lock and initiated the safepoint.
 500           // So we drain the queue again if there is anything there, as an
 501           // optimization to try and reduce the number of safepoints.
 502           // As the safepoint synchronizes us with JavaThreads we will see
 503           // any enqueue made by a JavaThread, but the peek will not
 504           // necessarily detect a concurrent enqueue by a GC thread, but
 505           // that simply means the op will wait for the next major cycle of the
 506           // VMThread - just as it would if the GC thread lost the race for
 507           // the lock.
 508           if (_vm_queue->peek_at_safepoint_priority()) {
 509             // must hold lock while draining queue
 510             MutexLockerEx mu_queue(VMOperationQueue_lock,
 511                                      Mutex::_no_safepoint_check_flag);
 512             safepoint_ops = _vm_queue->drain_at_safepoint_priority();
 513           } else {
 514             safepoint_ops = NULL;
 515           }
 516         } while(safepoint_ops != NULL);
 517 
 518         _vm_queue->set_drain_list(NULL);
 519 
 520         // Complete safepoint synchronization
 521         SafepointSynchronize::end();
 522 
 523       } else {  // not a safepoint operation
 524         if (TraceLongCompiles) {
 525           elapsedTimer t;
 526           t.start();
 527           evaluate_operation(_cur_vm_operation);
 528           t.stop();
 529           double secs = t.seconds();
 530           if (secs * 1e3 > LongCompileThreshold) {
 531             // XXX - _cur_vm_operation should not be accessed after
 532             // the completed count has been incremented; the waiting
 533             // thread may have already freed this memory.
 534             tty->print_cr("vm %s: %3.7f secs]", _cur_vm_operation->name(), secs);
 535           }
 536         } else {
 537           evaluate_operation(_cur_vm_operation);
 538         }
 539 
 540         _cur_vm_operation = NULL;
 541       }
 542     }
 543 
 544     //
 545     //  Notify (potential) waiting Java thread(s) - lock without safepoint
 546     //  check so that sneaking is not possible
 547     { MutexLockerEx mu(VMOperationRequest_lock,
 548                        Mutex::_no_safepoint_check_flag);
 549       VMOperationRequest_lock->notify_all();
 550     }
 551 
 552     //
 553     // We want to make sure that we get to a safepoint regularly.
 554     //
 555     if (SafepointALot || SafepointSynchronize::is_cleanup_needed()) {
 556       long interval          = SafepointSynchronize::last_non_safepoint_interval();
 557       bool max_time_exceeded = GuaranteedSafepointInterval != 0 && (interval > GuaranteedSafepointInterval);
 558       if (SafepointALot || max_time_exceeded) {
 559         HandleMark hm(VMThread::vm_thread());
 560         SafepointSynchronize::begin();
 561         SafepointSynchronize::end();
 562       }
 563     }
 564   }
 565 }
 566 
 567 void VMThread::execute(VM_Operation* op) {
 568   Thread* t = Thread::current();
 569 
 570   if (!t->is_VM_thread()) {
 571     SkipGCALot sgcalot(t);    // avoid re-entrant attempts to gc-a-lot
 572     // JavaThread or WatcherThread
 573     bool concurrent = op->evaluate_concurrently();
 574     // only blocking VM operations need to verify the caller's safepoint state:
 575     if (!concurrent) {
 576       t->check_for_valid_safepoint_state(true);
 577     }
 578 
 579     // New request from Java thread, evaluate prologue
 580     if (!op->doit_prologue()) {
 581       return;   // op was cancelled
 582     }
 583 
 584     // Setup VM_operations for execution
 585     op->set_calling_thread(t, Thread::get_priority(t));
 586 
 587     // It does not make sense to execute the epilogue, if the VM operation object is getting
 588     // deallocated by the VM thread.
 589     bool execute_epilog = !op->is_cheap_allocated();
 590     assert(!concurrent || op->is_cheap_allocated(), "concurrent => cheap_allocated");
 591 
 592     // Get ticket number for non-concurrent VM operations
 593     int ticket = 0;
 594     if (!concurrent) {
 595       ticket = t->vm_operation_ticket();
 596     }
 597 
 598     // Add VM operation to list of waiting threads. We are guaranteed not to block while holding the
 599     // VMOperationQueue_lock, so we can block without a safepoint check. This allows vm operation requests
 600     // to be queued up during a safepoint synchronization.
 601     {
 602       VMOperationQueue_lock->lock_without_safepoint_check();
 603       bool ok = _vm_queue->add(op);
 604       op->set_timestamp(os::javaTimeMillis());
 605       VMOperationQueue_lock->notify();
 606       VMOperationQueue_lock->unlock();
 607       // VM_Operation got skipped
 608       if (!ok) {
 609         assert(concurrent, "can only skip concurrent tasks");
 610         if (op->is_cheap_allocated()) delete op;
 611         return;
 612       }
 613     }
 614 
 615     if (!concurrent) {
 616       // Wait for completion of request (non-concurrent)
 617       // Note: only a JavaThread triggers the safepoint check when locking
 618       MutexLocker mu(VMOperationRequest_lock);
 619       while(t->vm_operation_completed_count() < ticket) {
 620         VMOperationRequest_lock->wait(!t->is_Java_thread());
 621       }
 622     }
 623 
 624     if (execute_epilog) {
 625       op->doit_epilogue();
 626     }
 627   } else {
 628     // invoked by VM thread; usually nested VM operation
 629     assert(t->is_VM_thread(), "must be a VM thread");
 630     VM_Operation* prev_vm_operation = vm_operation();
 631     if (prev_vm_operation != NULL) {
 632       // Check the VM operation allows nested VM operation. This normally not the case, e.g., the compiler
 633       // does not allow nested scavenges or compiles.
 634       if (!prev_vm_operation->allow_nested_vm_operations()) {
 635         fatal(err_msg("Nested VM operation %s requested by operation %s",
 636                       op->name(), vm_operation()->name()));
 637       }
 638       op->set_calling_thread(prev_vm_operation->calling_thread(), prev_vm_operation->priority());
 639     }
 640 
 641     EventMark em("Executing %s VM operation: %s", prev_vm_operation ? "nested" : "", op->name());
 642 
 643     // Release all internal handles after operation is evaluated
 644     HandleMark hm(t);
 645     _cur_vm_operation = op;
 646 
 647     if (op->evaluate_at_safepoint() && !SafepointSynchronize::is_at_safepoint()) {
 648       SafepointSynchronize::begin();
 649       op->evaluate();
 650       SafepointSynchronize::end();
 651     } else {
 652       op->evaluate();
 653     }
 654 
 655     // Free memory if needed
 656     if (op->is_cheap_allocated()) delete op;
 657 
 658     _cur_vm_operation = prev_vm_operation;
 659   }
 660 }
 661 
 662 
 663 void VMThread::oops_do(OopClosure* f, CLDToOopClosure* cld_f, CodeBlobClosure* cf) {
 664   Thread::oops_do(f, cld_f, cf);
 665   _vm_queue->oops_do(f);
 666 }
 667 
 668 //------------------------------------------------------------------------------------------------------------------
 669 #ifndef PRODUCT
 670 
 671 void VMOperationQueue::verify_queue(int prio) {
 672   // Check that list is correctly linked
 673   int length = _queue_length[prio];
 674   VM_Operation *cur = _queue[prio];
 675   int i;
 676 
 677   // Check forward links
 678   for(i = 0; i < length; i++) {
 679     cur = cur->next();
 680     assert(cur != _queue[prio], "list to short (forward)");
 681   }
 682   assert(cur->next() == _queue[prio], "list to long (forward)");
 683 
 684   // Check backwards links
 685   cur = _queue[prio];
 686   for(i = 0; i < length; i++) {
 687     cur = cur->prev();
 688     assert(cur != _queue[prio], "list to short (backwards)");
 689   }
 690   assert(cur->prev() == _queue[prio], "list to long (backwards)");
 691 }
 692 
 693 #endif
 694 
 695 void VMThread::verify() {
 696   oops_do(&VerifyOopClosure::verify_oop, NULL, NULL);
 697 }
--- EOF ---