Mercurial > jdk8-mips64-public > hotspot / file revision

 /*

  * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

  * or visit www.oracle.com if you need additional information or have any

  * questions.

  *

  */

 #include "precompiled.hpp"

 #include "compiler/compileBroker.hpp"

 #include "gc_interface/collectedHeap.hpp"

 #include "memory/resourceArea.hpp"

 #include "oops/method.hpp"

 #include "oops/oop.inline.hpp"

 #include "runtime/interfaceSupport.hpp"

 #include "runtime/mutexLocker.hpp"

 #include "runtime/os.hpp"

 #include "runtime/thread.inline.hpp"

 #include "runtime/vmThread.hpp"

 #include "runtime/vm_operations.hpp"

 #include "services/runtimeService.hpp"

 #include "trace/tracing.hpp"

 #include "utilities/dtrace.hpp"

 #include "utilities/events.hpp"

 #include "utilities/xmlstream.hpp"

 #ifndef USDT2

 HS_DTRACE_PROBE_DECL3(hotspot, vmops__request, char *, uintptr_t, int);

 HS_DTRACE_PROBE_DECL3(hotspot, vmops__begin, char *, uintptr_t, int);

 HS_DTRACE_PROBE_DECL3(hotspot, vmops__end, char *, uintptr_t, int);

 #endif /* !USDT2 */

 // Dummy VM operation to act as first element in our circular double-linked list

 class VM_Dummy: public VM_Operation {

   VMOp_Type type() const { return VMOp_Dummy; }

   void  doit() {};

 };

 VMOperationQueue::VMOperationQueue() {

   // The queue is a circular doubled-linked list, which always contains

   // one element (i.e., one element means empty).

   for(int i = 0; i < nof_priorities; i++) {

     _queue_length[i] = 0;

     _queue_counter = 0;

     _queue[i] = new VM_Dummy();

     _queue[i]->set_next(_queue[i]);

     _queue[i]->set_prev(_queue[i]);

   }

   _drain_list = NULL;

 }

 bool VMOperationQueue::queue_empty(int prio) {

   // It is empty if there is exactly one element

   bool empty = (_queue[prio] == _queue[prio]->next());

   assert( (_queue_length[prio] == 0 && empty) ||

           (_queue_length[prio] > 0  && !empty), "sanity check");

   return _queue_length[prio] == 0;

 }

 // Inserts an element to the right of the q element

 void VMOperationQueue::insert(VM_Operation* q, VM_Operation* n) {

   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");

   n->set_prev(q);

   n->set_next(q->next());

   q->next()->set_prev(n);

   q->set_next(n);

 }

 void VMOperationQueue::queue_add_front(int prio, VM_Operation *op) {

   _queue_length[prio]++;

   insert(_queue[prio]->next(), op);

 }

 void VMOperationQueue::queue_add_back(int prio, VM_Operation *op) {

   _queue_length[prio]++;

   insert(_queue[prio]->prev(), op);

 }

 void VMOperationQueue::unlink(VM_Operation* q) {

   assert(q->next()->prev() == q && q->prev()->next() == q, "sanity check");

   q->prev()->set_next(q->next());

   q->next()->set_prev(q->prev());

 }

 VM_Operation* VMOperationQueue::queue_remove_front(int prio) {

   if (queue_empty(prio)) return NULL;

   assert(_queue_length[prio] >= 0, "sanity check");

   _queue_length[prio]--;

   VM_Operation* r = _queue[prio]->next();

   assert(r != _queue[prio], "cannot remove base element");

   unlink(r);

   return r;

 }

 VM_Operation* VMOperationQueue::queue_drain(int prio) {

   if (queue_empty(prio)) return NULL;

   DEBUG_ONLY(int length = _queue_length[prio];);

   assert(length >= 0, "sanity check");

   _queue_length[prio] = 0;

   VM_Operation* r = _queue[prio]->next();

   assert(r != _queue[prio], "cannot remove base element");

   // remove links to base element from head and tail

   r->set_prev(NULL);

   _queue[prio]->prev()->set_next(NULL);

   // restore queue to empty state

   _queue[prio]->set_next(_queue[prio]);

   _queue[prio]->set_prev(_queue[prio]);

   assert(queue_empty(prio), "drain corrupted queue");

 #ifdef ASSERT

   int len = 0;

   VM_Operation* cur;

   for(cur = r; cur != NULL; cur=cur->next()) len++;

   assert(len == length, "drain lost some ops");

 #endif

   return r;

 }

 void VMOperationQueue::queue_oops_do(int queue, OopClosure* f) {

   VM_Operation* cur = _queue[queue];

   cur = cur->next();

   while (cur != _queue[queue]) {

     cur->oops_do(f);

     cur = cur->next();

   }

 }

 void VMOperationQueue::drain_list_oops_do(OopClosure* f) {

   VM_Operation* cur = _drain_list;

   while (cur != NULL) {

     cur->oops_do(f);

     cur = cur->next();

   }

 }

 //-----------------------------------------------------------------

 // High-level interface

 bool VMOperationQueue::add(VM_Operation *op) {

 #ifndef USDT2

   HS_DTRACE_PROBE3(hotspot, vmops__request, op->name(), strlen(op->name()),

                    op->evaluation_mode());

 #else /* USDT2 */

   HOTSPOT_VMOPS_REQUEST(

                    (char *) op->name(), strlen(op->name()),

                    op->evaluation_mode());

 #endif /* USDT2 */

   // Encapsulates VM queue policy. Currently, that

   // only involves putting them on the right list

   if (op->evaluate_at_safepoint()) {

     queue_add_back(SafepointPriority, op);

     return true;

   }

   queue_add_back(MediumPriority, op);

   return true;

 }

 VM_Operation* VMOperationQueue::remove_next() {

   // Assuming VMOperation queue is two-level priority queue. If there are

   // more than two priorities, we need a different scheduling algorithm.

   assert(SafepointPriority == 0 && MediumPriority == 1 && nof_priorities == 2,

          "current algorithm does not work");

   // simple counter based scheduling to prevent starvation of lower priority

   // queue. -- see 4390175

   int high_prio, low_prio;

   if (_queue_counter++ < 10) {

       high_prio = SafepointPriority;

       low_prio  = MediumPriority;

   } else {

       _queue_counter = 0;

       high_prio = MediumPriority;

       low_prio  = SafepointPriority;

   }

   return queue_remove_front(queue_empty(high_prio) ? low_prio : high_prio);

 }

 void VMOperationQueue::oops_do(OopClosure* f) {

   for(int i = 0; i < nof_priorities; i++) {

     queue_oops_do(i, f);

   }

   drain_list_oops_do(f);

 }

 //------------------------------------------------------------------------------------------------------------------

 // Implementation of VMThread stuff

 bool                VMThread::_should_terminate   = false;

 bool              VMThread::_terminated         = false;

 Monitor*          VMThread::_terminate_lock     = NULL;

 VMThread*         VMThread::_vm_thread          = NULL;

 VM_Operation*     VMThread::_cur_vm_operation   = NULL;

 VMOperationQueue* VMThread::_vm_queue           = NULL;

 PerfCounter*      VMThread::_perf_accumulated_vm_operation_time = NULL;

 void VMThread::create() {

   assert(vm_thread() == NULL, "we can only allocate one VMThread");

   _vm_thread = new VMThread();

   // Create VM operation queue

   _vm_queue = new VMOperationQueue();

   guarantee(_vm_queue != NULL, "just checking");

   _terminate_lock = new Monitor(Mutex::safepoint, "VMThread::_terminate_lock", true);

   if (UsePerfData) {

     // jvmstat performance counters

     Thread* THREAD = Thread::current();

     _perf_accumulated_vm_operation_time =

                  PerfDataManager::create_counter(SUN_THREADS, "vmOperationTime",

                                                  PerfData::U_Ticks, CHECK);

   }

 }

 VMThread::VMThread() : NamedThread() {

   set_name("VM Thread");

 }

 void VMThread::destroy() {

   if (_vm_thread != NULL) {

     delete _vm_thread;

     _vm_thread = NULL;      // VM thread is gone

   }

 }

 void VMThread::run() {

   assert(this == vm_thread(), "check");

   this->initialize_thread_local_storage();

   this->record_stack_base_and_size();

   // Notify_lock wait checks on active_handles() to rewait in

   // case of spurious wakeup, it should wait on the last

   // value set prior to the notify

   this->set_active_handles(JNIHandleBlock::allocate_block());

   {

     MutexLocker ml(Notify_lock);

     Notify_lock->notify();

   }

   // Notify_lock is destroyed by Threads::create_vm()

   int prio = (VMThreadPriority == -1)

     ? os::java_to_os_priority[NearMaxPriority]

     : VMThreadPriority;

   // Note that I cannot call os::set_priority because it expects Java

   // priorities and I am *explicitly* using OS priorities so that it's

   // possible to set the VM thread priority higher than any Java thread.

   os::set_native_priority( this, prio );

   // Wait for VM_Operations until termination

   this->loop();

   // Note the intention to exit before safepointing.

   // 6295565  This has the effect of waiting for any large tty

   // outputs to finish.

   if (xtty != NULL) {

     ttyLocker ttyl;

     xtty->begin_elem("destroy_vm");

     xtty->stamp();

     xtty->end_elem();

     assert(should_terminate(), "termination flag must be set");

   }

   // 4526887 let VM thread exit at Safepoint

   SafepointSynchronize::begin();

   if (VerifyBeforeExit) {

     HandleMark hm(VMThread::vm_thread());

     // Among other things, this ensures that Eden top is correct.

     Universe::heap()->prepare_for_verify();

     os::check_heap();

     // Silent verification so as not to pollute normal output,

     // unless we really asked for it.

     Universe::verify(!(PrintGCDetails || Verbose) || VerifySilently);

   }

   CompileBroker::set_should_block();

   // wait for threads (compiler threads or daemon threads) in the

   // _thread_in_native state to block.

   VM_Exit::wait_for_threads_in_native_to_block();

   // signal other threads that VM process is gone

   {

     // Note: we must have the _no_safepoint_check_flag. Mutex::lock() allows

     // VM thread to enter any lock at Safepoint as long as its _owner is NULL.

     // If that happens after _terminate_lock->wait() has unset _owner

     // but before it actually drops the lock and waits, the notification below

     // may get lost and we will have a hang. To avoid this, we need to use

     // Mutex::lock_without_safepoint_check().

     MutexLockerEx ml(_terminate_lock, Mutex::_no_safepoint_check_flag);

     _terminated = true;

     _terminate_lock->notify();

   }

   // Thread destructor usually does this.

   ThreadLocalStorage::set_thread(NULL);

   // Deletion must be done synchronously by the JNI DestroyJavaVM thread

   // so that the VMThread deletion completes before the main thread frees

   // up the CodeHeap.

 }

 // Notify the VMThread that the last non-daemon JavaThread has terminated,

 // and wait until operation is performed.

 void VMThread::wait_for_vm_thread_exit() {

   { MutexLocker mu(VMOperationQueue_lock);

     _should_terminate = true;

     VMOperationQueue_lock->notify();

   }

   // Note: VM thread leaves at Safepoint. We are not stopped by Safepoint

   // because this thread has been removed from the threads list. But anything

   // that could get blocked by Safepoint should not be used after this point,

   // otherwise we will hang, since there is no one can end the safepoint.

   // Wait until VM thread is terminated

   // Note: it should be OK to use Terminator_lock here. But this is called

   // at a very delicate time (VM shutdown) and we are operating in non- VM

   // thread at Safepoint. It's safer to not share lock with other threads.

   { MutexLockerEx ml(_terminate_lock, Mutex::_no_safepoint_check_flag);

     while(!VMThread::is_terminated()) {

         _terminate_lock->wait(Mutex::_no_safepoint_check_flag);

     }

   }

 }

 void VMThread::print_on(outputStream* st) const {

   st->print("\"%s\" ", name());

   Thread::print_on(st);

   st->cr();

 }

 void VMThread::evaluate_operation(VM_Operation* op) {

   ResourceMark rm;

   {

     PerfTraceTime vm_op_timer(perf_accumulated_vm_operation_time());

 #ifndef USDT2

     HS_DTRACE_PROBE3(hotspot, vmops__begin, op->name(), strlen(op->name()),

                      op->evaluation_mode());

 #else /* USDT2 */

     HOTSPOT_VMOPS_BEGIN(

                      (char *) op->name(), strlen(op->name()),

                      op->evaluation_mode());

 #endif /* USDT2 */

     EventExecuteVMOperation event;

     op->evaluate();

     if (event.should_commit()) {

       bool is_concurrent = op->evaluate_concurrently();

       event.set_operation(op->type());

       event.set_safepoint(op->evaluate_at_safepoint());

       event.set_blocking(!is_concurrent);

       // Only write caller thread information for non-concurrent vm operations.

       // For concurrent vm operations, the thread id is set to 0 indicating thread is unknown.

       // This is because the caller thread could have exited already.

       event.set_caller(is_concurrent ? 0 : op->calling_thread()->osthread()->thread_id());

       event.commit();

     }

 #ifndef USDT2

     HS_DTRACE_PROBE3(hotspot, vmops__end, op->name(), strlen(op->name()),

                      op->evaluation_mode());

 #else /* USDT2 */

     HOTSPOT_VMOPS_END(

                      (char *) op->name(), strlen(op->name()),

                      op->evaluation_mode());

 #endif /* USDT2 */

   }

   // Last access of info in _cur_vm_operation!

   bool c_heap_allocated = op->is_cheap_allocated();

   // Mark as completed

   if (!op->evaluate_concurrently()) {

     op->calling_thread()->increment_vm_operation_completed_count();

   }

   // It is unsafe to access the _cur_vm_operation after the 'increment_vm_operation_completed_count' call,

   // since if it is stack allocated the calling thread might have deallocated

   if (c_heap_allocated) {

     delete _cur_vm_operation;

   }

 }

 void VMThread::loop() {

   assert(_cur_vm_operation == NULL, "no current one should be executing");

   while(true) {

     VM_Operation* safepoint_ops = NULL;

     //

     // Wait for VM operation

     //

     // use no_safepoint_check to get lock without attempting to "sneak"

     { MutexLockerEx mu_queue(VMOperationQueue_lock,

                              Mutex::_no_safepoint_check_flag);

       // Look for new operation

       assert(_cur_vm_operation == NULL, "no current one should be executing");

       _cur_vm_operation = _vm_queue->remove_next();

       // Stall time tracking code

       if (PrintVMQWaitTime && _cur_vm_operation != NULL &&

           !_cur_vm_operation->evaluate_concurrently()) {

         long stall = os::javaTimeMillis() - _cur_vm_operation->timestamp();

         if (stall > 0)

           tty->print_cr("%s stall: %Ld",  _cur_vm_operation->name(), stall);

       }

       while (!should_terminate() && _cur_vm_operation == NULL) {

         // wait with a timeout to guarantee safepoints at regular intervals

         bool timedout =

           VMOperationQueue_lock->wait(Mutex::_no_safepoint_check_flag,

                                       GuaranteedSafepointInterval);

         // Support for self destruction

         if ((SelfDestructTimer != 0) && !is_error_reported() &&

             (os::elapsedTime() > SelfDestructTimer * 60)) {

           tty->print_cr("VM self-destructed");

           exit(-1);

         }

         if (timedout && (SafepointALot ||

                          SafepointSynchronize::is_cleanup_needed())) {

           MutexUnlockerEx mul(VMOperationQueue_lock,

                               Mutex::_no_safepoint_check_flag);

           // Force a safepoint since we have not had one for at least

           // 'GuaranteedSafepointInterval' milliseconds.  This will run all

           // the clean-up processing that needs to be done regularly at a

           // safepoint

           SafepointSynchronize::begin();

           #ifdef ASSERT

             if (GCALotAtAllSafepoints) InterfaceSupport::check_gc_alot();

           #endif

           SafepointSynchronize::end();

         }

         _cur_vm_operation = _vm_queue->remove_next();

         // If we are at a safepoint we will evaluate all the operations that

         // follow that also require a safepoint

         if (_cur_vm_operation != NULL &&

             _cur_vm_operation->evaluate_at_safepoint()) {

           safepoint_ops = _vm_queue->drain_at_safepoint_priority();

         }

       }

       if (should_terminate()) break;

     } // Release mu_queue_lock

     //

     // Execute VM operation

     //

     { HandleMark hm(VMThread::vm_thread());

       EventMark em("Executing VM operation: %s", vm_operation()->name());

       assert(_cur_vm_operation != NULL, "we should have found an operation to execute");

       // Give the VM thread an extra quantum.  Jobs tend to be bursty and this

       // helps the VM thread to finish up the job.

       // FIXME: When this is enabled and there are many threads, this can degrade

       // performance significantly.

       if( VMThreadHintNoPreempt )

         os::hint_no_preempt();

       // If we are at a safepoint we will evaluate all the operations that

       // follow that also require a safepoint

       if (_cur_vm_operation->evaluate_at_safepoint()) {

         _vm_queue->set_drain_list(safepoint_ops); // ensure ops can be scanned

         SafepointSynchronize::begin();

         evaluate_operation(_cur_vm_operation);

         // now process all queued safepoint ops, iteratively draining

         // the queue until there are none left

         do {

           _cur_vm_operation = safepoint_ops;

           if (_cur_vm_operation != NULL) {

             do {

               // evaluate_operation deletes the op object so we have

               // to grab the next op now

               VM_Operation* next = _cur_vm_operation->next();

               _vm_queue->set_drain_list(next);

               evaluate_operation(_cur_vm_operation);

               _cur_vm_operation = next;

               if (PrintSafepointStatistics) {

                 SafepointSynchronize::inc_vmop_coalesced_count();

               }

             } while (_cur_vm_operation != NULL);

           }

           // There is a chance that a thread enqueued a safepoint op

           // since we released the op-queue lock and initiated the safepoint.

           // So we drain the queue again if there is anything there, as an

           // optimization to try and reduce the number of safepoints.

           // As the safepoint synchronizes us with JavaThreads we will see

           // any enqueue made by a JavaThread, but the peek will not

           // necessarily detect a concurrent enqueue by a GC thread, but

           // that simply means the op will wait for the next major cycle of the

           // VMThread - just as it would if the GC thread lost the race for

           // the lock.

           if (_vm_queue->peek_at_safepoint_priority()) {

             // must hold lock while draining queue

             MutexLockerEx mu_queue(VMOperationQueue_lock,

                                      Mutex::_no_safepoint_check_flag);

             safepoint_ops = _vm_queue->drain_at_safepoint_priority();

           } else {

             safepoint_ops = NULL;

           }

         } while(safepoint_ops != NULL);

         _vm_queue->set_drain_list(NULL);

         // Complete safepoint synchronization

         SafepointSynchronize::end();

       } else {  // not a safepoint operation

         if (TraceLongCompiles) {

           elapsedTimer t;

           t.start();

           evaluate_operation(_cur_vm_operation);

           t.stop();

           double secs = t.seconds();

           if (secs * 1e3 > LongCompileThreshold) {

             // XXX - _cur_vm_operation should not be accessed after

             // the completed count has been incremented; the waiting

             // thread may have already freed this memory.

             tty->print_cr("vm %s: %3.7f secs]", _cur_vm_operation->name(), secs);

           }

         } else {

           evaluate_operation(_cur_vm_operation);

         }

         _cur_vm_operation = NULL;

       }

     }

     //

     //  Notify (potential) waiting Java thread(s) - lock without safepoint

     //  check so that sneaking is not possible

     { MutexLockerEx mu(VMOperationRequest_lock,

                        Mutex::_no_safepoint_check_flag);

       VMOperationRequest_lock->notify_all();

     }

     //

     // We want to make sure that we get to a safepoint regularly.

     //

     if (SafepointALot || SafepointSynchronize::is_cleanup_needed()) {

       long interval          = SafepointSynchronize::last_non_safepoint_interval();

       bool max_time_exceeded = GuaranteedSafepointInterval != 0 && (interval > GuaranteedSafepointInterval);

       if (SafepointALot || max_time_exceeded) {

         HandleMark hm(VMThread::vm_thread());

         SafepointSynchronize::begin();

         SafepointSynchronize::end();

       }

     }

   }

 }

 void VMThread::execute(VM_Operation* op) {

   Thread* t = Thread::current();

   if (!t->is_VM_thread()) {

     SkipGCALot sgcalot(t);    // avoid re-entrant attempts to gc-a-lot

     // JavaThread or WatcherThread

     bool concurrent = op->evaluate_concurrently();

     // only blocking VM operations need to verify the caller's safepoint state:

     if (!concurrent) {

       t->check_for_valid_safepoint_state(true);

     }

     // New request from Java thread, evaluate prologue

     if (!op->doit_prologue()) {

       return;   // op was cancelled

     }

     // Setup VM_operations for execution

     op->set_calling_thread(t, Thread::get_priority(t));

     // It does not make sense to execute the epilogue, if the VM operation object is getting

     // deallocated by the VM thread.

     bool execute_epilog = !op->is_cheap_allocated();

     assert(!concurrent || op->is_cheap_allocated(), "concurrent => cheap_allocated");

     // Get ticket number for non-concurrent VM operations

     int ticket = 0;

     if (!concurrent) {

       ticket = t->vm_operation_ticket();

     }

     // Add VM operation to list of waiting threads. We are guaranteed not to block while holding the

     // VMOperationQueue_lock, so we can block without a safepoint check. This allows vm operation requests

     // to be queued up during a safepoint synchronization.

     {

       VMOperationQueue_lock->lock_without_safepoint_check();

       bool ok = _vm_queue->add(op);

     op->set_timestamp(os::javaTimeMillis());

       VMOperationQueue_lock->notify();

       VMOperationQueue_lock->unlock();

       // VM_Operation got skipped

       if (!ok) {

         assert(concurrent, "can only skip concurrent tasks");

         if (op->is_cheap_allocated()) delete op;

         return;

       }

     }

     if (!concurrent) {

       // Wait for completion of request (non-concurrent)

       // Note: only a JavaThread triggers the safepoint check when locking

       MutexLocker mu(VMOperationRequest_lock);

       while(t->vm_operation_completed_count() < ticket) {

         VMOperationRequest_lock->wait(!t->is_Java_thread());

       }

     }

     if (execute_epilog) {

       op->doit_epilogue();

     }

   } else {

     // invoked by VM thread; usually nested VM operation

     assert(t->is_VM_thread(), "must be a VM thread");

     VM_Operation* prev_vm_operation = vm_operation();

     if (prev_vm_operation != NULL) {

       // Check the VM operation allows nested VM operation. This normally not the case, e.g., the compiler

       // does not allow nested scavenges or compiles.

       if (!prev_vm_operation->allow_nested_vm_operations()) {

         fatal(err_msg("Nested VM operation %s requested by operation %s",

                       op->name(), vm_operation()->name()));

       }

       op->set_calling_thread(prev_vm_operation->calling_thread(), prev_vm_operation->priority());

     }

     EventMark em("Executing %s VM operation: %s", prev_vm_operation ? "nested" : "", op->name());

     // Release all internal handles after operation is evaluated

     HandleMark hm(t);

     _cur_vm_operation = op;

     if (op->evaluate_at_safepoint() && !SafepointSynchronize::is_at_safepoint()) {

       SafepointSynchronize::begin();

       op->evaluate();

       SafepointSynchronize::end();

     } else {

       op->evaluate();

     }

     // Free memory if needed

     if (op->is_cheap_allocated()) delete op;

     _cur_vm_operation = prev_vm_operation;

   }

 }

 void VMThread::oops_do(OopClosure* f, CLDToOopClosure* cld_f, CodeBlobClosure* cf) {

   Thread::oops_do(f, cld_f, cf);

   _vm_queue->oops_do(f);

 }

 //------------------------------------------------------------------------------------------------------------------

 #ifndef PRODUCT

 void VMOperationQueue::verify_queue(int prio) {

   // Check that list is correctly linked

   int length = _queue_length[prio];

   VM_Operation *cur = _queue[prio];

   int i;

   // Check forward links

   for(i = 0; i < length; i++) {

     cur = cur->next();

     assert(cur != _queue[prio], "list to short (forward)");

   }

   assert(cur->next() == _queue[prio], "list to long (forward)");

   // Check backwards links

   cur = _queue[prio];

   for(i = 0; i < length; i++) {

     cur = cur->prev();

     assert(cur != _queue[prio], "list to short (backwards)");

   }

   assert(cur->prev() == _queue[prio], "list to long (backwards)");

 }

 #endif

 void VMThread::verify() {

   oops_do(&VerifyOopClosure::verify_oop, NULL, NULL);

 }