1 /*
   2  * Copyright (c) 1998, 2019, 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/shared/collectedHeap.hpp"
  28 #include "jfr/jfrEvents.hpp"
  29 #include "jfr/support/jfrThreadId.hpp"
  30 #include "logging/log.hpp"
  31 #include "logging/logStream.hpp"
  32 #include "logging/logConfiguration.hpp"
  33 #include "memory/resourceArea.hpp"
  34 #include "memory/universe.hpp"
  35 #include "oops/method.hpp"
  36 #include "oops/oop.inline.hpp"
  37 #include "oops/verifyOopClosure.hpp"
  38 #include "runtime/atomic.hpp"
  39 #include "runtime/handles.inline.hpp"
  40 #include "runtime/interfaceSupport.inline.hpp"
  41 #include "runtime/mutexLocker.hpp"
  42 #include "runtime/os.hpp"
  43 #include "runtime/safepoint.hpp"
  44 #include "runtime/thread.inline.hpp"
  45 #include "runtime/vmThread.hpp"
  46 #include "runtime/vmOperations.hpp"
  47 #include "services/runtimeService.hpp"
  48 #include "utilities/dtrace.hpp"
  49 #include "utilities/events.hpp"
  50 #include "utilities/vmError.hpp"
  51 #include "utilities/xmlstream.hpp"
  52 
  53 VM_QueueHead VMOperationQueue::_queue_head[VMOperationQueue::nof_priorities];
  54 
  55 VMOperationQueue::VMOperationQueue() {
  56   // The queue is a circular doubled-linked list, which always contains
  57   // one element (i.e., one element means empty).
  58   for(int i = 0; i < nof_priorities; i++) {
  59     _queue_length[i] = 0;
  60     _queue_counter = 0;
  61     _queue[i] = &_queue_head[i];
  62     _queue[i]->set_next(_queue[i]);
  63     _queue[i]->set_prev(_queue[i]);
  64   }
  65   _drain_list = NULL;
  66 }
  67 
  68 
  69 bool VMOperationQueue::queue_empty(int prio) {
  70   // It is empty if there is exactly one element
  71   bool empty = (_queue[prio] == _queue[prio]->next());
  72   assert( (_queue_length[prio] == 0 && empty) ||
  73           (_queue_length[prio] > 0  && !empty), "sanity check");
  74   return _queue_length[prio] == 0;
  75 }
  76 
  77 // Inserts an element to the right of the q element
  78 void VMOperationQueue::insert(VM_Operation* q, VM_Operation* n) {
  79   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");
  80   n->set_prev(q);
  81   n->set_next(q->next());
  82   q->next()->set_prev(n);
  83   q->set_next(n);
  84 }
  85 
  86 void VMOperationQueue::queue_add(int prio, VM_Operation *op) {
  87   _queue_length[prio]++;
  88   insert(_queue[prio]->prev(), op);
  89 }
  90 
  91 
  92 void VMOperationQueue::unlink(VM_Operation* q) {
  93   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");
  94   q->prev()->set_next(q->next());
  95   q->next()->set_prev(q->prev());
  96 }
  97 
  98 VM_Operation* VMOperationQueue::queue_remove_front(int prio) {
  99   if (queue_empty(prio)) return NULL;
 100   assert(_queue_length[prio] >= 0, "sanity check");
 101   _queue_length[prio]--;
 102   VM_Operation* r = _queue[prio]->next();
 103   assert(r != _queue[prio], "cannot remove base element");
 104   unlink(r);
 105   return r;
 106 }
 107 
 108 VM_Operation* VMOperationQueue::queue_drain(int prio) {
 109   if (queue_empty(prio)) return NULL;
 110   DEBUG_ONLY(int length = _queue_length[prio];);
 111   assert(length >= 0, "sanity check");
 112   _queue_length[prio] = 0;
 113   VM_Operation* r = _queue[prio]->next();
 114   assert(r != _queue[prio], "cannot remove base element");
 115   // remove links to base element from head and tail
 116   r->set_prev(NULL);
 117   _queue[prio]->prev()->set_next(NULL);
 118   // restore queue to empty state
 119   _queue[prio]->set_next(_queue[prio]);
 120   _queue[prio]->set_prev(_queue[prio]);
 121   assert(queue_empty(prio), "drain corrupted queue");
 122 #ifdef ASSERT
 123   int len = 0;
 124   VM_Operation* cur;
 125   for(cur = r; cur != NULL; cur=cur->next()) len++;
 126   assert(len == length, "drain lost some ops");
 127 #endif
 128   return r;
 129 }
 130 
 131 void VMOperationQueue::queue_oops_do(int queue, OopClosure* f) {
 132   VM_Operation* cur = _queue[queue];
 133   cur = cur->next();
 134   while (cur != _queue[queue]) {
 135     cur->oops_do(f);
 136     cur = cur->next();
 137   }
 138 }
 139 
 140 void VMOperationQueue::drain_list_oops_do(OopClosure* f) {
 141   VM_Operation* cur = _drain_list;
 142   while (cur != NULL) {
 143     cur->oops_do(f);
 144     cur = cur->next();
 145   }
 146 }
 147 
 148 //-----------------------------------------------------------------
 149 // High-level interface
 150 void VMOperationQueue::add(VM_Operation *op) {
 151 
 152   HOTSPOT_VMOPS_REQUEST(
 153                    (char *) op->name(), strlen(op->name()),
 154                    op->evaluate_at_safepoint() ? 0 : 1);
 155 
 156   // Encapsulates VM queue policy. Currently, that
 157   // only involves putting them on the right list
 158   queue_add(op->evaluate_at_safepoint() ? SafepointPriority : MediumPriority, op);
 159 }
 160 
 161 VM_Operation* VMOperationQueue::remove_next() {
 162   // Assuming VMOperation queue is two-level priority queue. If there are
 163   // more than two priorities, we need a different scheduling algorithm.
 164   assert(SafepointPriority == 0 && MediumPriority == 1 && nof_priorities == 2,
 165          "current algorithm does not work");
 166 
 167   // simple counter based scheduling to prevent starvation of lower priority
 168   // queue. -- see 4390175
 169   int high_prio, low_prio;
 170   if (_queue_counter++ < 10) {
 171       high_prio = SafepointPriority;
 172       low_prio  = MediumPriority;
 173   } else {
 174       _queue_counter = 0;
 175       high_prio = MediumPriority;
 176       low_prio  = SafepointPriority;
 177   }
 178 
 179   return queue_remove_front(queue_empty(high_prio) ? low_prio : high_prio);
 180 }
 181 
 182 void VMOperationQueue::oops_do(OopClosure* f) {
 183   for(int i = 0; i < nof_priorities; i++) {
 184     queue_oops_do(i, f);
 185   }
 186   drain_list_oops_do(f);
 187 }
 188 
 189 //------------------------------------------------------------------------------------------------------------------
 190 // Timeout machinery
 191 
 192 void VMOperationTimeoutTask::task() {
 193   assert(AbortVMOnVMOperationTimeout, "only if enabled");
 194   if (is_armed()) {
 195     jlong delay = (os::javaTimeMillis() - _arm_time);
 196     if (delay > AbortVMOnVMOperationTimeoutDelay) {
 197       fatal("VM operation took too long: " JLONG_FORMAT " ms (timeout: " INTX_FORMAT " ms)",
 198             delay, AbortVMOnVMOperationTimeoutDelay);
 199     }
 200   }
 201 }
 202 
 203 bool VMOperationTimeoutTask::is_armed() {
 204   return Atomic::load_acquire(&_armed) != 0;
 205 }
 206 
 207 void VMOperationTimeoutTask::arm() {
 208   _arm_time = os::javaTimeMillis();
 209   Atomic::release_store_fence(&_armed, 1);
 210 }
 211 
 212 void VMOperationTimeoutTask::disarm() {
 213   Atomic::release_store_fence(&_armed, 0);
 214 }
 215 
 216 //------------------------------------------------------------------------------------------------------------------
 217 // Implementation of VMThread stuff
 218 
 219 bool              VMThread::_should_terminate   = false;
 220 bool              VMThread::_terminated         = false;
 221 Monitor*          VMThread::_terminate_lock     = NULL;
 222 VMThread*         VMThread::_vm_thread          = NULL;
 223 VM_Operation*     VMThread::_cur_vm_operation   = NULL;
 224 VMOperationQueue* VMThread::_vm_queue           = NULL;
 225 PerfCounter*      VMThread::_perf_accumulated_vm_operation_time = NULL;
 226 uint64_t          VMThread::_coalesced_count = 0;
 227 VMOperationTimeoutTask* VMThread::_timeout_task = NULL;
 228 
 229 
 230 void VMThread::create() {
 231   assert(vm_thread() == NULL, "we can only allocate one VMThread");
 232   _vm_thread = new VMThread();
 233 
 234   if (AbortVMOnVMOperationTimeout) {
 235     // Make sure we call the timeout task frequently enough, but not too frequent.
 236     // Try to make the interval 10% of the timeout delay, so that we miss the timeout
 237     // by those 10% at max. Periodic task also expects it to fit min/max intervals.
 238     size_t interval = (size_t)AbortVMOnVMOperationTimeoutDelay / 10;
 239     interval = interval / PeriodicTask::interval_gran * PeriodicTask::interval_gran;
 240     interval = MAX2<size_t>(interval, PeriodicTask::min_interval);
 241     interval = MIN2<size_t>(interval, PeriodicTask::max_interval);
 242 
 243     _timeout_task = new VMOperationTimeoutTask(interval);
 244     _timeout_task->enroll();
 245   } else {
 246     assert(_timeout_task == NULL, "sanity");
 247   }
 248 
 249   // Create VM operation queue
 250   _vm_queue = new VMOperationQueue();
 251   guarantee(_vm_queue != NULL, "just checking");
 252 
 253   _terminate_lock = new Monitor(Mutex::safepoint, "VMThread::_terminate_lock", true,
 254                                 Monitor::_safepoint_check_never);
 255 
 256   if (UsePerfData) {
 257     // jvmstat performance counters
 258     Thread* THREAD = Thread::current();
 259     _perf_accumulated_vm_operation_time =
 260                  PerfDataManager::create_counter(SUN_THREADS, "vmOperationTime",
 261                                                  PerfData::U_Ticks, CHECK);
 262   }
 263 }
 264 
 265 VMThread::VMThread() : NamedThread() {
 266   set_name("VM Thread");
 267 }
 268 
 269 void VMThread::destroy() {
 270   _vm_thread = NULL;      // VM thread is gone
 271 }
 272 
 273 static VM_None halt_op("Halt");
 274 
 275 void VMThread::run() {
 276   assert(this == vm_thread(), "check");
 277 
 278   // Notify_lock wait checks on active_handles() to rewait in
 279   // case of spurious wakeup, it should wait on the last
 280   // value set prior to the notify
 281   this->set_active_handles(JNIHandleBlock::allocate_block());
 282 
 283   {
 284     MutexLocker ml(Notify_lock);
 285     Notify_lock->notify();
 286   }
 287   // Notify_lock is destroyed by Threads::create_vm()
 288 
 289   int prio = (VMThreadPriority == -1)
 290     ? os::java_to_os_priority[NearMaxPriority]
 291     : VMThreadPriority;
 292   // Note that I cannot call os::set_priority because it expects Java
 293   // priorities and I am *explicitly* using OS priorities so that it's
 294   // possible to set the VM thread priority higher than any Java thread.
 295   os::set_native_priority( this, prio );
 296 
 297   // Wait for VM_Operations until termination
 298   this->loop();
 299 
 300   // Note the intention to exit before safepointing.
 301   // 6295565  This has the effect of waiting for any large tty
 302   // outputs to finish.
 303   if (xtty != NULL) {
 304     ttyLocker ttyl;
 305     xtty->begin_elem("destroy_vm");
 306     xtty->stamp();
 307     xtty->end_elem();
 308     assert(should_terminate(), "termination flag must be set");
 309   }
 310 
 311   // 4526887 let VM thread exit at Safepoint
 312   _cur_vm_operation = &halt_op;
 313   SafepointSynchronize::begin();
 314 
 315   if (VerifyBeforeExit) {
 316     HandleMark hm(VMThread::vm_thread());
 317     // Among other things, this ensures that Eden top is correct.
 318     Universe::heap()->prepare_for_verify();
 319     // Silent verification so as not to pollute normal output,
 320     // unless we really asked for it.
 321     Universe::verify();
 322   }
 323 
 324   CompileBroker::set_should_block();
 325 
 326   // wait for threads (compiler threads or daemon threads) in the
 327   // _thread_in_native state to block.
 328   VM_Exit::wait_for_threads_in_native_to_block();
 329 
 330   // signal other threads that VM process is gone
 331   {
 332     // Note: we must have the _no_safepoint_check_flag. Mutex::lock() allows
 333     // VM thread to enter any lock at Safepoint as long as its _owner is NULL.
 334     // If that happens after _terminate_lock->wait() has unset _owner
 335     // but before it actually drops the lock and waits, the notification below
 336     // may get lost and we will have a hang. To avoid this, we need to use
 337     // Mutex::lock_without_safepoint_check().
 338     MonitorLocker ml(_terminate_lock, Mutex::_no_safepoint_check_flag);
 339     _terminated = true;
 340     ml.notify();
 341   }
 342 
 343   // We are now racing with the VM termination being carried out in
 344   // another thread, so we don't "delete this". Numerous threads don't
 345   // get deleted when the VM terminates
 346 
 347 }
 348 
 349 
 350 // Notify the VMThread that the last non-daemon JavaThread has terminated,
 351 // and wait until operation is performed.
 352 void VMThread::wait_for_vm_thread_exit() {
 353   assert(Thread::current()->is_Java_thread(), "Should be a JavaThread");
 354   assert(((JavaThread*)Thread::current())->is_terminated(), "Should be terminated");
 355   { MonitorLocker mu(VMOperationQueue_lock, Mutex::_no_safepoint_check_flag);
 356     _should_terminate = true;
 357     mu.notify();
 358   }
 359 
 360   // Note: VM thread leaves at Safepoint. We are not stopped by Safepoint
 361   // because this thread has been removed from the threads list. But anything
 362   // that could get blocked by Safepoint should not be used after this point,
 363   // otherwise we will hang, since there is no one can end the safepoint.
 364 
 365   // Wait until VM thread is terminated
 366   // Note: it should be OK to use Terminator_lock here. But this is called
 367   // at a very delicate time (VM shutdown) and we are operating in non- VM
 368   // thread at Safepoint. It's safer to not share lock with other threads.
 369   { MonitorLocker ml(_terminate_lock, Mutex::_no_safepoint_check_flag);
 370     while(!VMThread::is_terminated()) {
 371       ml.wait();
 372     }
 373   }
 374 }
 375 
 376 static void post_vm_operation_event(EventExecuteVMOperation* event, VM_Operation* op) {
 377   assert(event != NULL, "invariant");
 378   assert(event->should_commit(), "invariant");
 379   assert(op != NULL, "invariant");
 380   const bool evaluate_at_safepoint = op->evaluate_at_safepoint();
 381   event->set_operation(op->type());
 382   event->set_safepoint(evaluate_at_safepoint);
 383   event->set_blocking(true);
 384   event->set_caller(JFR_THREAD_ID(op->calling_thread()));
 385   event->set_safepointId(evaluate_at_safepoint ? SafepointSynchronize::safepoint_id() : 0);
 386   event->commit();
 387 }
 388 
 389 void VMThread::evaluate_operation(VM_Operation* op) {
 390   ResourceMark rm;
 391 
 392   {
 393     PerfTraceTime vm_op_timer(perf_accumulated_vm_operation_time());
 394     HOTSPOT_VMOPS_BEGIN(
 395                      (char *) op->name(), strlen(op->name()),
 396                      op->evaluate_at_safepoint() ? 0 : 1);
 397 
 398     EventExecuteVMOperation event;
 399     op->evaluate();
 400     if (event.should_commit()) {
 401       post_vm_operation_event(&event, op);
 402     }
 403 
 404     HOTSPOT_VMOPS_END(
 405                      (char *) op->name(), strlen(op->name()),
 406                      op->evaluate_at_safepoint() ? 0 : 1);
 407   }
 408 
 409   // Mark as completed
 410   op->calling_thread()->increment_vm_operation_completed_count();
 411 }
 412 
 413 static VM_None    safepointALot_op("SafepointALot");
 414 static VM_Cleanup cleanup_op;
 415 
 416 class HandshakeALotClosure : public HandshakeClosure {
 417  public:
 418   HandshakeALotClosure() : HandshakeClosure("HandshakeALot") {}
 419   void do_thread(Thread* thread) {
 420 #ifdef ASSERT
 421     assert(thread->is_Java_thread(), "must be");
 422     JavaThread* jt = (JavaThread*)thread;
 423     jt->verify_states_for_handshake();
 424 #endif
 425   }
 426 };
 427 
 428 void VMThread::check_for_forced_cleanup() {
 429   MonitorLocker mq(VMOperationQueue_lock,  Mutex::_no_safepoint_check_flag);
 430   mq.notify();
 431 }
 432 
 433 VM_Operation* VMThread::no_op_safepoint() {
 434   // Check for handshakes first since we may need to return a VMop.
 435   if (HandshakeALot) {
 436     HandshakeALotClosure hal_cl;
 437     Handshake::execute(&hal_cl);
 438   }
 439   // Check for a cleanup before SafepointALot to keep stats correct.
 440   long interval_ms = SafepointTracing::time_since_last_safepoint_ms();
 441   bool max_time_exceeded = GuaranteedSafepointInterval != 0 &&
 442                            (interval_ms >= GuaranteedSafepointInterval);
 443   if ((max_time_exceeded && SafepointSynchronize::is_cleanup_needed()) ||
 444       SafepointSynchronize::is_forced_cleanup_needed()) {
 445     return &cleanup_op;
 446   }
 447   if (SafepointALot) {
 448     return &safepointALot_op;
 449   }
 450   // Nothing to be done.
 451   return NULL;
 452 }
 453 
 454 void VMThread::loop() {
 455   assert(_cur_vm_operation == NULL, "no current one should be executing");
 456 
 457   SafepointSynchronize::init(_vm_thread);
 458 
 459   while(true) {
 460     VM_Operation* safepoint_ops = NULL;
 461     //
 462     // Wait for VM operation
 463     //
 464     // use no_safepoint_check to get lock without attempting to "sneak"
 465     { MonitorLocker mu_queue(VMOperationQueue_lock,
 466                              Mutex::_no_safepoint_check_flag);
 467 
 468       // Look for new operation
 469       assert(_cur_vm_operation == NULL, "no current one should be executing");
 470       _cur_vm_operation = _vm_queue->remove_next();
 471 
 472       // Stall time tracking code
 473       if (PrintVMQWaitTime && _cur_vm_operation != NULL) {
 474         long stall = os::javaTimeMillis() - _cur_vm_operation->timestamp();
 475         if (stall > 0)
 476           tty->print_cr("%s stall: %ld",  _cur_vm_operation->name(), stall);
 477       }
 478 
 479       while (!should_terminate() && _cur_vm_operation == NULL) {
 480         // wait with a timeout to guarantee safepoints at regular intervals
 481         // (if there is cleanup work to do)
 482         (void)mu_queue.wait(GuaranteedSafepointInterval);
 483 
 484         // Support for self destruction
 485         if ((SelfDestructTimer != 0) && !VMError::is_error_reported() &&
 486             (os::elapsedTime() > (double)SelfDestructTimer * 60.0)) {
 487           tty->print_cr("VM self-destructed");
 488           exit(-1);
 489         }
 490 
 491         // If the queue contains a safepoint VM op,
 492         // clean up will be done so we can skip this part.
 493         if (!_vm_queue->peek_at_safepoint_priority()) {
 494 
 495           // Have to unlock VMOperationQueue_lock just in case no_op_safepoint()
 496           // has to do a handshake when HandshakeALot is enabled.
 497           MutexUnlocker mul(VMOperationQueue_lock, Mutex::_no_safepoint_check_flag);
 498           if ((_cur_vm_operation = VMThread::no_op_safepoint()) != NULL) {
 499             // Force a safepoint since we have not had one for at least
 500             // 'GuaranteedSafepointInterval' milliseconds and we need to clean
 501             // something. This will run all the clean-up processing that needs
 502             // to be done at a safepoint.
 503             SafepointSynchronize::begin();
 504             #ifdef ASSERT
 505             if (GCALotAtAllSafepoints) InterfaceSupport::check_gc_alot();
 506             #endif
 507             SafepointSynchronize::end();
 508             _cur_vm_operation = NULL;
 509           }
 510         }
 511         _cur_vm_operation = _vm_queue->remove_next();
 512 
 513         // If we are at a safepoint we will evaluate all the operations that
 514         // follow that also require a safepoint
 515         if (_cur_vm_operation != NULL &&
 516             _cur_vm_operation->evaluate_at_safepoint()) {
 517           safepoint_ops = _vm_queue->drain_at_safepoint_priority();
 518         }
 519       }
 520 
 521       if (should_terminate()) break;
 522     } // Release mu_queue_lock
 523 
 524     //
 525     // Execute VM operation
 526     //
 527     { HandleMark hm(VMThread::vm_thread());
 528 
 529       EventMark em("Executing VM operation: %s", vm_operation()->name());
 530       assert(_cur_vm_operation != NULL, "we should have found an operation to execute");
 531 
 532       // If we are at a safepoint we will evaluate all the operations that
 533       // follow that also require a safepoint
 534       if (_cur_vm_operation->evaluate_at_safepoint()) {
 535         log_debug(vmthread)("Evaluating safepoint VM operation: %s", _cur_vm_operation->name());
 536 
 537         _vm_queue->set_drain_list(safepoint_ops); // ensure ops can be scanned
 538 
 539         SafepointSynchronize::begin();
 540 
 541         if (_timeout_task != NULL) {
 542           _timeout_task->arm();
 543         }
 544 
 545         evaluate_operation(_cur_vm_operation);
 546         // now process all queued safepoint ops, iteratively draining
 547         // the queue until there are none left
 548         do {
 549           _cur_vm_operation = safepoint_ops;
 550           if (_cur_vm_operation != NULL) {
 551             do {
 552               EventMark em("Executing coalesced safepoint VM operation: %s", _cur_vm_operation->name());
 553               log_debug(vmthread)("Evaluating coalesced safepoint VM operation: %s", _cur_vm_operation->name());
 554               // evaluate_operation deletes the op object so we have
 555               // to grab the next op now
 556               VM_Operation* next = _cur_vm_operation->next();
 557               _vm_queue->set_drain_list(next);
 558               evaluate_operation(_cur_vm_operation);
 559               _cur_vm_operation = next;
 560               _coalesced_count++;
 561             } while (_cur_vm_operation != NULL);
 562           }
 563           // There is a chance that a thread enqueued a safepoint op
 564           // since we released the op-queue lock and initiated the safepoint.
 565           // So we drain the queue again if there is anything there, as an
 566           // optimization to try and reduce the number of safepoints.
 567           // As the safepoint synchronizes us with JavaThreads we will see
 568           // any enqueue made by a JavaThread, but the peek will not
 569           // necessarily detect a concurrent enqueue by a GC thread, but
 570           // that simply means the op will wait for the next major cycle of the
 571           // VMThread - just as it would if the GC thread lost the race for
 572           // the lock.
 573           if (_vm_queue->peek_at_safepoint_priority()) {
 574             // must hold lock while draining queue
 575             MutexLocker mu_queue(VMOperationQueue_lock,
 576                                  Mutex::_no_safepoint_check_flag);
 577             safepoint_ops = _vm_queue->drain_at_safepoint_priority();
 578           } else {
 579             safepoint_ops = NULL;
 580           }
 581         } while(safepoint_ops != NULL);
 582 
 583         _vm_queue->set_drain_list(NULL);
 584 
 585         if (_timeout_task != NULL) {
 586           _timeout_task->disarm();
 587         }
 588 
 589         // Complete safepoint synchronization
 590         SafepointSynchronize::end();
 591 
 592       } else {  // not a safepoint operation
 593         log_debug(vmthread)("Evaluating non-safepoint VM operation: %s", _cur_vm_operation->name());
 594         if (TraceLongCompiles) {
 595           elapsedTimer t;
 596           t.start();
 597           evaluate_operation(_cur_vm_operation);
 598           t.stop();
 599           double secs = t.seconds();
 600           if (secs * 1e3 > LongCompileThreshold) {
 601             // XXX - _cur_vm_operation should not be accessed after
 602             // the completed count has been incremented; the waiting
 603             // thread may have already freed this memory.
 604             tty->print_cr("vm %s: %3.7f secs]", _cur_vm_operation->name(), secs);
 605           }
 606         } else {
 607           evaluate_operation(_cur_vm_operation);
 608         }
 609 
 610         _cur_vm_operation = NULL;
 611       }
 612     }
 613 
 614     //
 615     //  Notify (potential) waiting Java thread(s)
 616     { MonitorLocker mu(VMOperationRequest_lock, Mutex::_no_safepoint_check_flag);
 617       mu.notify_all();
 618     }
 619   }
 620 }
 621 
 622 // A SkipGCALot object is used to elide the usual effect of gc-a-lot
 623 // over a section of execution by a thread. Currently, it's used only to
 624 // prevent re-entrant calls to GC.
 625 class SkipGCALot : public StackObj {
 626   private:
 627    bool _saved;
 628    Thread* _t;
 629 
 630   public:
 631 #ifdef ASSERT
 632     SkipGCALot(Thread* t) : _t(t) {
 633       _saved = _t->skip_gcalot();
 634       _t->set_skip_gcalot(true);
 635     }
 636 
 637     ~SkipGCALot() {
 638       assert(_t->skip_gcalot(), "Save-restore protocol invariant");
 639       _t->set_skip_gcalot(_saved);
 640     }
 641 #else
 642     SkipGCALot(Thread* t) { }
 643     ~SkipGCALot() { }
 644 #endif
 645 };
 646 
 647 void VMThread::execute(VM_Operation* op) {
 648   Thread* t = Thread::current();
 649 
 650   if (!t->is_VM_thread()) {
 651     SkipGCALot sgcalot(t);    // avoid re-entrant attempts to gc-a-lot
 652     // JavaThread or WatcherThread
 653     t->check_for_valid_safepoint_state();
 654 
 655     // New request from Java thread, evaluate prologue
 656     if (!op->doit_prologue()) {
 657       return;   // op was cancelled
 658     }
 659 
 660     // Setup VM_operations for execution
 661     op->set_calling_thread(t);
 662 
 663     // Get ticket number for the VM operation
 664     int ticket = t->vm_operation_ticket();
 665 
 666     // Add VM operation to list of waiting threads. We are guaranteed not to block while holding the
 667     // VMOperationQueue_lock, so we can block without a safepoint check. This allows vm operation requests
 668     // to be queued up during a safepoint synchronization.
 669     {
 670       MonitorLocker ml(VMOperationQueue_lock, Mutex::_no_safepoint_check_flag);
 671       log_debug(vmthread)("Adding VM operation: %s", op->name());
 672       _vm_queue->add(op);
 673       op->set_timestamp(os::javaTimeMillis());
 674       ml.notify();
 675     }
 676     {
 677       // Wait for completion of request
 678       // Note: only a JavaThread triggers the safepoint check when locking
 679       MonitorLocker ml(VMOperationRequest_lock,
 680                        t->is_Java_thread() ? Mutex::_safepoint_check_flag : Mutex::_no_safepoint_check_flag);
 681       while(t->vm_operation_completed_count() < ticket) {
 682         ml.wait();
 683       }
 684     }
 685     op->doit_epilogue();
 686   } else {
 687     // invoked by VM thread; usually nested VM operation
 688     assert(t->is_VM_thread(), "must be a VM thread");
 689     VM_Operation* prev_vm_operation = vm_operation();
 690     if (prev_vm_operation != NULL) {
 691       // Check the VM operation allows nested VM operation. This normally not the case, e.g., the compiler
 692       // does not allow nested scavenges or compiles.
 693       if (!prev_vm_operation->allow_nested_vm_operations()) {
 694         fatal("Nested VM operation %s requested by operation %s",
 695               op->name(), vm_operation()->name());
 696       }
 697       op->set_calling_thread(prev_vm_operation->calling_thread());
 698     }
 699 
 700     EventMark em("Executing %s VM operation: %s", prev_vm_operation ? "nested" : "", op->name());
 701 
 702     // Release all internal handles after operation is evaluated
 703     HandleMark hm(t);
 704     _cur_vm_operation = op;
 705 
 706     if (op->evaluate_at_safepoint() && !SafepointSynchronize::is_at_safepoint()) {
 707       SafepointSynchronize::begin();
 708       op->evaluate();
 709       SafepointSynchronize::end();
 710     } else {
 711       op->evaluate();
 712     }
 713 
 714     _cur_vm_operation = prev_vm_operation;
 715   }
 716 }
 717 
 718 
 719 void VMThread::oops_do(OopClosure* f, CodeBlobClosure* cf) {
 720   Thread::oops_do(f, cf);
 721   _vm_queue->oops_do(f);
 722 }
 723 
 724 //------------------------------------------------------------------------------------------------------------------
 725 #ifndef PRODUCT
 726 
 727 void VMOperationQueue::verify_queue(int prio) {
 728   // Check that list is correctly linked
 729   int length = _queue_length[prio];
 730   VM_Operation *cur = _queue[prio];
 731   int i;
 732 
 733   // Check forward links
 734   for(i = 0; i < length; i++) {
 735     cur = cur->next();
 736     assert(cur != _queue[prio], "list to short (forward)");
 737   }
 738   assert(cur->next() == _queue[prio], "list to long (forward)");
 739 
 740   // Check backwards links
 741   cur = _queue[prio];
 742   for(i = 0; i < length; i++) {
 743     cur = cur->prev();
 744     assert(cur != _queue[prio], "list to short (backwards)");
 745   }
 746   assert(cur->prev() == _queue[prio], "list to long (backwards)");
 747 }
 748 
 749 #endif
 750 
 751 void VMThread::verify() {
 752   oops_do(&VerifyOopClosure::verify_oop, NULL);
 753 }