1 /*
   2  * Copyright (c) 1998, 2018, 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 "oops/method.hpp"
  35 #include "oops/oop.inline.hpp"
  36 #include "oops/verifyOopClosure.hpp"
  37 #include "runtime/interfaceSupport.inline.hpp"
  38 #include "runtime/mutexLocker.hpp"
  39 #include "runtime/os.hpp"
  40 #include "runtime/safepoint.hpp"
  41 #include "runtime/thread.inline.hpp"
  42 #include "runtime/vmThread.hpp"
  43 #include "runtime/vmOperations.hpp"
  44 #include "services/runtimeService.hpp"
  45 #include "utilities/dtrace.hpp"
  46 #include "utilities/events.hpp"
  47 #include "utilities/vmError.hpp"
  48 #include "utilities/xmlstream.hpp"
  49 
  50 // Dummy VM operation to act as first element in our circular double-linked list
  51 class VM_None: public VM_Operation {
  52   VMOp_Type type() const { return VMOp_None; }
  53   void  doit() {};
  54 };
  55 
  56 VMOperationQueue::VMOperationQueue() {
  57   // The queue is a circular doubled-linked list, which always contains
  58   // one element (i.e., one element means empty).
  59   for(int i = 0; i < nof_priorities; i++) {
  60     _queue_length[i] = 0;
  61     _queue_counter = 0;
  62     _queue[i] = new VM_None();
  63     _queue[i]->set_next(_queue[i]);
  64     _queue[i]->set_prev(_queue[i]);
  65   }
  66   _drain_list = NULL;
  67 }
  68 
  69 
  70 bool VMOperationQueue::queue_empty(int prio) {
  71   // It is empty if there is exactly one element
  72   bool empty = (_queue[prio] == _queue[prio]->next());
  73   assert( (_queue_length[prio] == 0 && empty) ||
  74           (_queue_length[prio] > 0  && !empty), "sanity check");
  75   return _queue_length[prio] == 0;
  76 }
  77 
  78 // Inserts an element to the right of the q element
  79 void VMOperationQueue::insert(VM_Operation* q, VM_Operation* n) {
  80   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");
  81   n->set_prev(q);
  82   n->set_next(q->next());
  83   q->next()->set_prev(n);
  84   q->set_next(n);
  85 }
  86 
  87 void VMOperationQueue::queue_add_front(int prio, VM_Operation *op) {
  88   _queue_length[prio]++;
  89   insert(_queue[prio]->next(), op);
  90 }
  91 
  92 void VMOperationQueue::queue_add_back(int prio, VM_Operation *op) {
  93   _queue_length[prio]++;
  94   insert(_queue[prio]->prev(), op);
  95 }
  96 
  97 
  98 void VMOperationQueue::unlink(VM_Operation* q) {
  99   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");
 100   q->prev()->set_next(q->next());
 101   q->next()->set_prev(q->prev());
 102 }
 103 
 104 VM_Operation* VMOperationQueue::queue_remove_front(int prio) {
 105   if (queue_empty(prio)) return NULL;
 106   assert(_queue_length[prio] >= 0, "sanity check");
 107   _queue_length[prio]--;
 108   VM_Operation* r = _queue[prio]->next();
 109   assert(r != _queue[prio], "cannot remove base element");
 110   unlink(r);
 111   return r;
 112 }
 113 
 114 VM_Operation* VMOperationQueue::queue_drain(int prio) {
 115   if (queue_empty(prio)) return NULL;
 116   DEBUG_ONLY(int length = _queue_length[prio];);
 117   assert(length >= 0, "sanity check");
 118   _queue_length[prio] = 0;
 119   VM_Operation* r = _queue[prio]->next();
 120   assert(r != _queue[prio], "cannot remove base element");
 121   // remove links to base element from head and tail
 122   r->set_prev(NULL);
 123   _queue[prio]->prev()->set_next(NULL);
 124   // restore queue to empty state
 125   _queue[prio]->set_next(_queue[prio]);
 126   _queue[prio]->set_prev(_queue[prio]);
 127   assert(queue_empty(prio), "drain corrupted queue");
 128 #ifdef ASSERT
 129   int len = 0;
 130   VM_Operation* cur;
 131   for(cur = r; cur != NULL; cur=cur->next()) len++;
 132   assert(len == length, "drain lost some ops");
 133 #endif
 134   return r;
 135 }
 136 
 137 void VMOperationQueue::queue_oops_do(int queue, OopClosure* f) {
 138   VM_Operation* cur = _queue[queue];
 139   cur = cur->next();
 140   while (cur != _queue[queue]) {
 141     cur->oops_do(f);
 142     cur = cur->next();
 143   }
 144 }
 145 
 146 void VMOperationQueue::drain_list_oops_do(OopClosure* f) {
 147   VM_Operation* cur = _drain_list;
 148   while (cur != NULL) {
 149     cur->oops_do(f);
 150     cur = cur->next();
 151   }
 152 }
 153 
 154 //-----------------------------------------------------------------
 155 // High-level interface
 156 bool VMOperationQueue::add(VM_Operation *op) {
 157 
 158   HOTSPOT_VMOPS_REQUEST(
 159                    (char *) op->name(), strlen(op->name()),
 160                    op->evaluation_mode());
 161 
 162   // Encapsulates VM queue policy. Currently, that
 163   // only involves putting them on the right list
 164   if (op->evaluate_at_safepoint()) {
 165     queue_add_back(SafepointPriority, op);
 166     return true;
 167   }
 168 
 169   queue_add_back(MediumPriority, op);
 170   return true;
 171 }
 172 
 173 VM_Operation* VMOperationQueue::remove_next() {
 174   // Assuming VMOperation queue is two-level priority queue. If there are
 175   // more than two priorities, we need a different scheduling algorithm.
 176   assert(SafepointPriority == 0 && MediumPriority == 1 && nof_priorities == 2,
 177          "current algorithm does not work");
 178 
 179   // simple counter based scheduling to prevent starvation of lower priority
 180   // queue. -- see 4390175
 181   int high_prio, low_prio;
 182   if (_queue_counter++ < 10) {
 183       high_prio = SafepointPriority;
 184       low_prio  = MediumPriority;
 185   } else {
 186       _queue_counter = 0;
 187       high_prio = MediumPriority;
 188       low_prio  = SafepointPriority;
 189   }
 190 
 191   return queue_remove_front(queue_empty(high_prio) ? low_prio : high_prio);
 192 }
 193 
 194 void VMOperationQueue::oops_do(OopClosure* f) {
 195   for(int i = 0; i < nof_priorities; i++) {
 196     queue_oops_do(i, f);
 197   }
 198   drain_list_oops_do(f);
 199 }
 200 
 201 //------------------------------------------------------------------------------------------------------------------
 202 // Timeout machinery
 203 
 204 void VMOperationTimeoutTask::task() {
 205   assert(AbortVMOnVMOperationTimeout, "only if enabled");
 206   if (is_armed()) {
 207     jlong delay = (os::javaTimeMillis() - _arm_time);
 208     if (delay > AbortVMOnVMOperationTimeoutDelay) {
 209       fatal("VM operation took too long: " SIZE_FORMAT " ms (timeout: " SIZE_FORMAT " ms)",
 210             delay, AbortVMOnVMOperationTimeoutDelay);
 211     }
 212   }
 213 }
 214 
 215 bool VMOperationTimeoutTask::is_armed() {
 216   return OrderAccess::load_acquire(&_armed) != 0;
 217 }
 218 
 219 void VMOperationTimeoutTask::arm() {
 220   _arm_time = os::javaTimeMillis();
 221   OrderAccess::release_store_fence(&_armed, 1);
 222 }
 223 
 224 void VMOperationTimeoutTask::disarm() {
 225   OrderAccess::release_store_fence(&_armed, 0);
 226 }
 227 
 228 //------------------------------------------------------------------------------------------------------------------
 229 // Implementation of VMThread stuff
 230 
 231 bool                VMThread::_should_terminate   = false;
 232 bool              VMThread::_terminated         = false;
 233 Monitor*          VMThread::_terminate_lock     = NULL;
 234 VMThread*         VMThread::_vm_thread          = NULL;
 235 VM_Operation*     VMThread::_cur_vm_operation   = NULL;
 236 VMOperationQueue* VMThread::_vm_queue           = NULL;
 237 PerfCounter*      VMThread::_perf_accumulated_vm_operation_time = NULL;
 238 const char*       VMThread::_no_op_reason       = NULL;
 239 VMOperationTimeoutTask* VMThread::_timeout_task = NULL;
 240 
 241 
 242 void VMThread::create() {
 243   assert(vm_thread() == NULL, "we can only allocate one VMThread");
 244   _vm_thread = new VMThread();
 245 
 246   if (AbortVMOnVMOperationTimeout) {
 247     // Make sure we call the timeout task frequently enough, but not too frequent.
 248     // Try to make the interval 10% of the timeout delay, so that we miss the timeout
 249     // by those 10% at max. Periodic task also expects it to fit min/max intervals.
 250     size_t interval = (size_t)AbortVMOnVMOperationTimeoutDelay / 10;
 251     interval = interval / PeriodicTask::interval_gran * PeriodicTask::interval_gran;
 252     interval = MAX2<size_t>(interval, PeriodicTask::min_interval);
 253     interval = MIN2<size_t>(interval, PeriodicTask::max_interval);
 254 
 255     _timeout_task = new VMOperationTimeoutTask(interval);
 256     _timeout_task->enroll();
 257   } else {
 258     assert(_timeout_task == NULL, "sanity");
 259   }
 260 
 261   // Create VM operation queue
 262   _vm_queue = new VMOperationQueue();
 263   guarantee(_vm_queue != NULL, "just checking");
 264 
 265   _terminate_lock = new Monitor(Mutex::safepoint, "VMThread::_terminate_lock", true,
 266                                 Monitor::_safepoint_check_never);
 267 
 268   if (UsePerfData) {
 269     // jvmstat performance counters
 270     Thread* THREAD = Thread::current();
 271     _perf_accumulated_vm_operation_time =
 272                  PerfDataManager::create_counter(SUN_THREADS, "vmOperationTime",
 273                                                  PerfData::U_Ticks, CHECK);
 274   }
 275 }
 276 
 277 VMThread::VMThread() : NamedThread() {
 278   set_name("VM Thread");
 279 }
 280 
 281 void VMThread::destroy() {
 282   _vm_thread = NULL;      // VM thread is gone
 283 }
 284 
 285 void VMThread::run() {
 286   assert(this == vm_thread(), "check");
 287 
 288   this->initialize_named_thread();
 289 
 290   // Notify_lock wait checks on active_handles() to rewait in
 291   // case of spurious wakeup, it should wait on the last
 292   // value set prior to the notify
 293   this->set_active_handles(JNIHandleBlock::allocate_block());
 294 
 295   {
 296     MutexLocker ml(Notify_lock);
 297     Notify_lock->notify();
 298   }
 299   // Notify_lock is destroyed by Threads::create_vm()
 300 
 301   int prio = (VMThreadPriority == -1)
 302     ? os::java_to_os_priority[NearMaxPriority]
 303     : VMThreadPriority;
 304   // Note that I cannot call os::set_priority because it expects Java
 305   // priorities and I am *explicitly* using OS priorities so that it's
 306   // possible to set the VM thread priority higher than any Java thread.
 307   os::set_native_priority( this, prio );
 308 
 309   // Wait for VM_Operations until termination
 310   this->loop();
 311 
 312   // Note the intention to exit before safepointing.
 313   // 6295565  This has the effect of waiting for any large tty
 314   // outputs to finish.
 315   if (xtty != NULL) {
 316     ttyLocker ttyl;
 317     xtty->begin_elem("destroy_vm");
 318     xtty->stamp();
 319     xtty->end_elem();
 320     assert(should_terminate(), "termination flag must be set");
 321   }
 322 
 323   // 4526887 let VM thread exit at Safepoint
 324   _no_op_reason = "Halt";
 325   SafepointSynchronize::begin();
 326 
 327   if (VerifyBeforeExit) {
 328     HandleMark hm(VMThread::vm_thread());
 329     // Among other things, this ensures that Eden top is correct.
 330     Universe::heap()->prepare_for_verify();
 331     // Silent verification so as not to pollute normal output,
 332     // unless we really asked for it.
 333     Universe::verify();
 334   }
 335 
 336   CompileBroker::set_should_block();
 337 
 338   // wait for threads (compiler threads or daemon threads) in the
 339   // _thread_in_native state to block.
 340   VM_Exit::wait_for_threads_in_native_to_block();
 341 
 342   // signal other threads that VM process is gone
 343   {
 344     // Note: we must have the _no_safepoint_check_flag. Mutex::lock() allows
 345     // VM thread to enter any lock at Safepoint as long as its _owner is NULL.
 346     // If that happens after _terminate_lock->wait() has unset _owner
 347     // but before it actually drops the lock and waits, the notification below
 348     // may get lost and we will have a hang. To avoid this, we need to use
 349     // Mutex::lock_without_safepoint_check().
 350     MutexLockerEx ml(_terminate_lock, Mutex::_no_safepoint_check_flag);
 351     _terminated = true;
 352     _terminate_lock->notify();
 353   }
 354 
 355   // We are now racing with the VM termination being carried out in
 356   // another thread, so we don't "delete this". Numerous threads don't
 357   // get deleted when the VM terminates
 358 
 359 }
 360 
 361 
 362 // Notify the VMThread that the last non-daemon JavaThread has terminated,
 363 // and wait until operation is performed.
 364 void VMThread::wait_for_vm_thread_exit() {
 365   assert(Thread::current()->is_Java_thread(), "Should be a JavaThread");
 366   assert(((JavaThread*)Thread::current())->is_terminated(), "Should be terminated");
 367   { MutexLockerEx mu(VMOperationQueue_lock, Mutex::_no_safepoint_check_flag);
 368     _should_terminate = true;
 369     VMOperationQueue_lock->notify();
 370   }
 371 
 372   // Note: VM thread leaves at Safepoint. We are not stopped by Safepoint
 373   // because this thread has been removed from the threads list. But anything
 374   // that could get blocked by Safepoint should not be used after this point,
 375   // otherwise we will hang, since there is no one can end the safepoint.
 376 
 377   // Wait until VM thread is terminated
 378   // Note: it should be OK to use Terminator_lock here. But this is called
 379   // at a very delicate time (VM shutdown) and we are operating in non- VM
 380   // thread at Safepoint. It's safer to not share lock with other threads.
 381   { MutexLockerEx ml(_terminate_lock, Mutex::_no_safepoint_check_flag);
 382     while(!VMThread::is_terminated()) {
 383         _terminate_lock->wait(Mutex::_no_safepoint_check_flag);
 384     }
 385   }
 386 }
 387 
 388 static void post_vm_operation_event(EventExecuteVMOperation* event, VM_Operation* op) {
 389   assert(event != NULL, "invariant");
 390   assert(event->should_commit(), "invariant");
 391   assert(op != NULL, "invariant");
 392   const bool is_concurrent = op->evaluate_concurrently();
 393   const bool evaluate_at_safepoint = op->evaluate_at_safepoint();
 394   event->set_operation(op->type());
 395   event->set_safepoint(evaluate_at_safepoint);
 396   event->set_blocking(!is_concurrent);
 397   // Only write caller thread information for non-concurrent vm operations.
 398   // For concurrent vm operations, the thread id is set to 0 indicating thread is unknown.
 399   // This is because the caller thread could have exited already.
 400   event->set_caller(is_concurrent ? 0 : JFR_THREAD_ID(op->calling_thread()));
 401   event->set_safepointId(evaluate_at_safepoint ? SafepointSynchronize::safepoint_counter() : 0);
 402   event->commit();
 403 }
 404 
 405 void VMThread::evaluate_operation(VM_Operation* op) {
 406   ResourceMark rm;
 407 
 408   {
 409     PerfTraceTime vm_op_timer(perf_accumulated_vm_operation_time());
 410     HOTSPOT_VMOPS_BEGIN(
 411                      (char *) op->name(), strlen(op->name()),
 412                      op->evaluation_mode());
 413 
 414     EventExecuteVMOperation event;
 415     op->evaluate();
 416     if (event.should_commit()) {
 417       post_vm_operation_event(&event, op);
 418     }
 419 
 420     HOTSPOT_VMOPS_END(
 421                      (char *) op->name(), strlen(op->name()),
 422                      op->evaluation_mode());
 423   }
 424 
 425   // Last access of info in _cur_vm_operation!
 426   bool c_heap_allocated = op->is_cheap_allocated();
 427 
 428   // Mark as completed
 429   if (!op->evaluate_concurrently()) {
 430     op->calling_thread()->increment_vm_operation_completed_count();
 431   }
 432   // It is unsafe to access the _cur_vm_operation after the 'increment_vm_operation_completed_count' call,
 433   // since if it is stack allocated the calling thread might have deallocated
 434   if (c_heap_allocated) {
 435     delete _cur_vm_operation;
 436   }
 437 }
 438 
 439 bool VMThread::no_op_safepoint_needed(bool check_time) {
 440   if (SafepointALot) {
 441     _no_op_reason = "SafepointALot";
 442     return true;
 443   }
 444   if (!SafepointSynchronize::is_cleanup_needed()) {
 445     return false;
 446   }
 447   if (check_time) {
 448     long interval = SafepointSynchronize::last_non_safepoint_interval();
 449     bool max_time_exceeded = GuaranteedSafepointInterval != 0 &&
 450                              (interval > GuaranteedSafepointInterval);
 451     if (!max_time_exceeded) {
 452       return false;
 453     }
 454   }
 455   _no_op_reason = "Cleanup";
 456   return true;
 457 }
 458 
 459 void VMThread::loop() {
 460   assert(_cur_vm_operation == NULL, "no current one should be executing");
 461 
 462   while(true) {
 463     VM_Operation* safepoint_ops = NULL;
 464     //
 465     // Wait for VM operation
 466     //
 467     // use no_safepoint_check to get lock without attempting to "sneak"
 468     { MutexLockerEx mu_queue(VMOperationQueue_lock,
 469                              Mutex::_no_safepoint_check_flag);
 470 
 471       // Look for new operation
 472       assert(_cur_vm_operation == NULL, "no current one should be executing");
 473       _cur_vm_operation = _vm_queue->remove_next();
 474 
 475       // Stall time tracking code
 476       if (PrintVMQWaitTime && _cur_vm_operation != NULL &&
 477           !_cur_vm_operation->evaluate_concurrently()) {
 478         long stall = os::javaTimeMillis() - _cur_vm_operation->timestamp();
 479         if (stall > 0)
 480           tty->print_cr("%s stall: %ld",  _cur_vm_operation->name(), stall);
 481       }
 482 
 483       while (!should_terminate() && _cur_vm_operation == NULL) {
 484         // wait with a timeout to guarantee safepoints at regular intervals
 485         bool timedout =
 486           VMOperationQueue_lock->wait(Mutex::_no_safepoint_check_flag,
 487                                       GuaranteedSafepointInterval);
 488 
 489         // Support for self destruction
 490         if ((SelfDestructTimer != 0) && !VMError::is_error_reported() &&
 491             (os::elapsedTime() > (double)SelfDestructTimer * 60.0)) {
 492           tty->print_cr("VM self-destructed");
 493           exit(-1);
 494         }
 495 
 496         if (timedout && VMThread::no_op_safepoint_needed(false)) {
 497           MutexUnlockerEx mul(VMOperationQueue_lock,
 498                               Mutex::_no_safepoint_check_flag);
 499           // Force a safepoint since we have not had one for at least
 500           // 'GuaranteedSafepointInterval' milliseconds.  This will run all
 501           // the clean-up processing that needs to be done regularly at a
 502           // safepoint
 503           SafepointSynchronize::begin();
 504           #ifdef ASSERT
 505             if (GCALotAtAllSafepoints) InterfaceSupport::check_gc_alot();
 506           #endif
 507           SafepointSynchronize::end();
 508         }
 509         _cur_vm_operation = _vm_queue->remove_next();
 510 
 511         // If we are at a safepoint we will evaluate all the operations that
 512         // follow that also require a safepoint
 513         if (_cur_vm_operation != NULL &&
 514             _cur_vm_operation->evaluate_at_safepoint()) {
 515           safepoint_ops = _vm_queue->drain_at_safepoint_priority();
 516         }
 517       }
 518 
 519       if (should_terminate()) break;
 520     } // Release mu_queue_lock
 521 
 522     //
 523     // Execute VM operation
 524     //
 525     { HandleMark hm(VMThread::vm_thread());
 526 
 527       EventMark em("Executing VM operation: %s", vm_operation()->name());
 528       assert(_cur_vm_operation != NULL, "we should have found an operation to execute");
 529 
 530       // If we are at a safepoint we will evaluate all the operations that
 531       // follow that also require a safepoint
 532       if (_cur_vm_operation->evaluate_at_safepoint()) {
 533         log_debug(vmthread)("Evaluating safepoint VM operation: %s", _cur_vm_operation->name());
 534 
 535         _vm_queue->set_drain_list(safepoint_ops); // ensure ops can be scanned
 536 
 537         SafepointSynchronize::begin();
 538 
 539         if (_timeout_task != NULL) {
 540           _timeout_task->arm();
 541         }
 542 
 543         evaluate_operation(_cur_vm_operation);
 544         // now process all queued safepoint ops, iteratively draining
 545         // the queue until there are none left
 546         do {
 547           _cur_vm_operation = safepoint_ops;
 548           if (_cur_vm_operation != NULL) {
 549             do {
 550               log_debug(vmthread)("Evaluating coalesced safepoint VM operation: %s", _cur_vm_operation->name());
 551               // evaluate_operation deletes the op object so we have
 552               // to grab the next op now
 553               VM_Operation* next = _cur_vm_operation->next();
 554               _vm_queue->set_drain_list(next);
 555               evaluate_operation(_cur_vm_operation);
 556               _cur_vm_operation = next;
 557               if (log_is_enabled(Debug, safepoint, stats)) {
 558                 SafepointSynchronize::inc_vmop_coalesced_count();
 559               }
 560             } while (_cur_vm_operation != NULL);
 561           }
 562           // There is a chance that a thread enqueued a safepoint op
 563           // since we released the op-queue lock and initiated the safepoint.
 564           // So we drain the queue again if there is anything there, as an
 565           // optimization to try and reduce the number of safepoints.
 566           // As the safepoint synchronizes us with JavaThreads we will see
 567           // any enqueue made by a JavaThread, but the peek will not
 568           // necessarily detect a concurrent enqueue by a GC thread, but
 569           // that simply means the op will wait for the next major cycle of the
 570           // VMThread - just as it would if the GC thread lost the race for
 571           // the lock.
 572           if (_vm_queue->peek_at_safepoint_priority()) {
 573             // must hold lock while draining queue
 574             MutexLockerEx mu_queue(VMOperationQueue_lock,
 575                                      Mutex::_no_safepoint_check_flag);
 576             safepoint_ops = _vm_queue->drain_at_safepoint_priority();
 577           } else {
 578             safepoint_ops = NULL;
 579           }
 580         } while(safepoint_ops != NULL);
 581 
 582         _vm_queue->set_drain_list(NULL);
 583 
 584         if (_timeout_task != NULL) {
 585           _timeout_task->disarm();
 586         }
 587 
 588         // Complete safepoint synchronization
 589         SafepointSynchronize::end();
 590 
 591       } else {  // not a safepoint operation
 592         log_debug(vmthread)("Evaluating non-safepoint VM operation: %s", _cur_vm_operation->name());
 593         if (TraceLongCompiles) {
 594           elapsedTimer t;
 595           t.start();
 596           evaluate_operation(_cur_vm_operation);
 597           t.stop();
 598           double secs = t.seconds();
 599           if (secs * 1e3 > LongCompileThreshold) {
 600             // XXX - _cur_vm_operation should not be accessed after
 601             // the completed count has been incremented; the waiting
 602             // thread may have already freed this memory.
 603             tty->print_cr("vm %s: %3.7f secs]", _cur_vm_operation->name(), secs);
 604           }
 605         } else {
 606           evaluate_operation(_cur_vm_operation);
 607         }
 608 
 609         _cur_vm_operation = NULL;
 610       }
 611     }
 612 
 613     //
 614     //  Notify (potential) waiting Java thread(s) - lock without safepoint
 615     //  check so that sneaking is not possible
 616     { MutexLockerEx mu(VMOperationRequest_lock,
 617                        Mutex::_no_safepoint_check_flag);
 618       VMOperationRequest_lock->notify_all();
 619     }
 620 
 621     //
 622     // We want to make sure that we get to a safepoint regularly.
 623     //
 624     if (VMThread::no_op_safepoint_needed(true)) {
 625       HandleMark hm(VMThread::vm_thread());
 626       SafepointSynchronize::begin();
 627       SafepointSynchronize::end();
 628     }
 629   }
 630 }
 631 
 632 // A SkipGCALot object is used to elide the usual effect of gc-a-lot
 633 // over a section of execution by a thread. Currently, it's used only to
 634 // prevent re-entrant calls to GC.
 635 class SkipGCALot : public StackObj {
 636   private:
 637    bool _saved;
 638    Thread* _t;
 639 
 640   public:
 641 #ifdef ASSERT
 642     SkipGCALot(Thread* t) : _t(t) {
 643       _saved = _t->skip_gcalot();
 644       _t->set_skip_gcalot(true);
 645     }
 646 
 647     ~SkipGCALot() {
 648       assert(_t->skip_gcalot(), "Save-restore protocol invariant");
 649       _t->set_skip_gcalot(_saved);
 650     }
 651 #else
 652     SkipGCALot(Thread* t) { }
 653     ~SkipGCALot() { }
 654 #endif
 655 };
 656 
 657 void VMThread::execute(VM_Operation* op) {
 658   Thread* t = Thread::current();
 659 
 660   if (!t->is_VM_thread()) {
 661     SkipGCALot sgcalot(t);    // avoid re-entrant attempts to gc-a-lot
 662     // JavaThread or WatcherThread
 663     bool concurrent = op->evaluate_concurrently();
 664     // only blocking VM operations need to verify the caller's safepoint state:
 665     if (!concurrent) {
 666       t->check_for_valid_safepoint_state(true);
 667     }
 668 
 669     // New request from Java thread, evaluate prologue
 670     if (!op->doit_prologue()) {
 671       return;   // op was cancelled
 672     }
 673 
 674     // Setup VM_operations for execution
 675     op->set_calling_thread(t, Thread::get_priority(t));
 676 
 677     // It does not make sense to execute the epilogue, if the VM operation object is getting
 678     // deallocated by the VM thread.
 679     bool execute_epilog = !op->is_cheap_allocated();
 680     assert(!concurrent || op->is_cheap_allocated(), "concurrent => cheap_allocated");
 681 
 682     // Get ticket number for non-concurrent VM operations
 683     int ticket = 0;
 684     if (!concurrent) {
 685       ticket = t->vm_operation_ticket();
 686     }
 687 
 688     // Add VM operation to list of waiting threads. We are guaranteed not to block while holding the
 689     // VMOperationQueue_lock, so we can block without a safepoint check. This allows vm operation requests
 690     // to be queued up during a safepoint synchronization.
 691     {
 692       VMOperationQueue_lock->lock_without_safepoint_check();
 693       log_debug(vmthread)("Adding VM operation: %s", op->name());
 694       bool ok = _vm_queue->add(op);
 695       op->set_timestamp(os::javaTimeMillis());
 696       VMOperationQueue_lock->notify();
 697       VMOperationQueue_lock->unlock();
 698       // VM_Operation got skipped
 699       if (!ok) {
 700         assert(concurrent, "can only skip concurrent tasks");
 701         if (op->is_cheap_allocated()) delete op;
 702         return;
 703       }
 704     }
 705 
 706     if (!concurrent) {
 707       // Wait for completion of request (non-concurrent)
 708       // Note: only a JavaThread triggers the safepoint check when locking
 709       MutexLocker mu(VMOperationRequest_lock);
 710       while(t->vm_operation_completed_count() < ticket) {
 711         VMOperationRequest_lock->wait(!t->is_Java_thread());
 712       }
 713     }
 714 
 715     if (execute_epilog) {
 716       op->doit_epilogue();
 717     }
 718   } else {
 719     // invoked by VM thread; usually nested VM operation
 720     assert(t->is_VM_thread(), "must be a VM thread");
 721     VM_Operation* prev_vm_operation = vm_operation();
 722     if (prev_vm_operation != NULL) {
 723       // Check the VM operation allows nested VM operation. This normally not the case, e.g., the compiler
 724       // does not allow nested scavenges or compiles.
 725       if (!prev_vm_operation->allow_nested_vm_operations()) {
 726         fatal("Nested VM operation %s requested by operation %s",
 727               op->name(), vm_operation()->name());
 728       }
 729       op->set_calling_thread(prev_vm_operation->calling_thread(), prev_vm_operation->priority());
 730     }
 731 
 732     EventMark em("Executing %s VM operation: %s", prev_vm_operation ? "nested" : "", op->name());
 733 
 734     // Release all internal handles after operation is evaluated
 735     HandleMark hm(t);
 736     _cur_vm_operation = op;
 737 
 738     if (op->evaluate_at_safepoint() && !SafepointSynchronize::is_at_safepoint()) {
 739       SafepointSynchronize::begin();
 740       op->evaluate();
 741       SafepointSynchronize::end();
 742     } else {
 743       op->evaluate();
 744     }
 745 
 746     // Free memory if needed
 747     if (op->is_cheap_allocated()) delete op;
 748 
 749     _cur_vm_operation = prev_vm_operation;
 750   }
 751 }
 752 
 753 
 754 void VMThread::oops_do(OopClosure* f, CodeBlobClosure* cf) {
 755   Thread::oops_do(f, cf);
 756   _vm_queue->oops_do(f);
 757 }
 758 
 759 //------------------------------------------------------------------------------------------------------------------
 760 #ifndef PRODUCT
 761 
 762 void VMOperationQueue::verify_queue(int prio) {
 763   // Check that list is correctly linked
 764   int length = _queue_length[prio];
 765   VM_Operation *cur = _queue[prio];
 766   int i;
 767 
 768   // Check forward links
 769   for(i = 0; i < length; i++) {
 770     cur = cur->next();
 771     assert(cur != _queue[prio], "list to short (forward)");
 772   }
 773   assert(cur->next() == _queue[prio], "list to long (forward)");
 774 
 775   // Check backwards links
 776   cur = _queue[prio];
 777   for(i = 0; i < length; i++) {
 778     cur = cur->prev();
 779     assert(cur != _queue[prio], "list to short (backwards)");
 780   }
 781   assert(cur->prev() == _queue[prio], "list to long (backwards)");
 782 }
 783 
 784 #endif
 785 
 786 void VMThread::verify() {
 787   oops_do(&VerifyOopClosure::verify_oop, NULL);
 788 }