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