1 /*
   2  * Copyright (c) 1997, 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 "runtime/mutexLocker.hpp"
  27 #include "runtime/os.inline.hpp"
  28 #include "runtime/safepoint.hpp"
  29 #include "runtime/thread.inline.hpp"
  30 #include "runtime/vmThread.hpp"
  31 
  32 // Mutexes used in the VM (see comment in mutexLocker.hpp):
  33 //
  34 // Note that the following pointers are effectively final -- after having been
  35 // set at JVM startup-time, they should never be subsequently mutated.
  36 // Instead of using pointers to malloc()ed monitors and mutexes we should consider
  37 // eliminating the indirection and using instances instead.
  38 // Consider using GCC's __read_mostly.
  39 
  40 Mutex*   Patching_lock                = NULL;
  41 Monitor* SystemDictionary_lock        = NULL;
  42 Mutex*   SharedDictionary_lock        = NULL;
  43 Mutex*   Module_lock                  = NULL;
  44 Mutex*   CompiledIC_lock              = NULL;
  45 Mutex*   InlineCacheBuffer_lock       = NULL;
  46 Mutex*   VMStatistic_lock             = NULL;
  47 Mutex*   JNIGlobalAlloc_lock          = NULL;
  48 Mutex*   JNIGlobalActive_lock         = NULL;
  49 Mutex*   JNIWeakAlloc_lock            = NULL;
  50 Mutex*   JNIWeakActive_lock           = NULL;
  51 Mutex*   StringTableWeakAlloc_lock    = NULL;
  52 Mutex*   StringTableWeakActive_lock   = NULL;
  53 Mutex*   JNIHandleBlockFreeList_lock  = NULL;
  54 Mutex*   VMWeakAlloc_lock             = NULL;
  55 Mutex*   VMWeakActive_lock            = NULL;
  56 Mutex*   ResolvedMethodTable_lock     = NULL;
  57 Mutex*   JmethodIdCreation_lock       = NULL;
  58 Mutex*   JfieldIdCreation_lock        = NULL;
  59 Monitor* JNICritical_lock             = NULL;
  60 Mutex*   JvmtiThreadState_lock        = NULL;
  61 Monitor* Heap_lock                    = NULL;
  62 Mutex*   ExpandHeap_lock              = NULL;
  63 Mutex*   AdapterHandlerLibrary_lock   = NULL;
  64 Mutex*   SignatureHandlerLibrary_lock = NULL;
  65 Mutex*   VtableStubs_lock             = NULL;
  66 Mutex*   SymbolTable_lock             = NULL;
  67 Mutex*   StringTable_lock             = NULL;
  68 Monitor* StringDedupQueue_lock        = NULL;
  69 Mutex*   StringDedupTable_lock        = NULL;
  70 Monitor* CodeCache_lock               = NULL;
  71 Mutex*   MethodData_lock              = NULL;
  72 Mutex*   TouchedMethodLog_lock        = NULL;
  73 Mutex*   RetData_lock                 = NULL;
  74 Monitor* VMOperationQueue_lock        = NULL;
  75 Monitor* VMOperationRequest_lock      = NULL;
  76 Monitor* Safepoint_lock               = NULL;
  77 Monitor* SerializePage_lock           = NULL;
  78 Monitor* Threads_lock                 = NULL;
  79 Mutex*   NonJavaThreadsList_lock      = NULL;
  80 Monitor* CGC_lock                     = NULL;
  81 Monitor* STS_lock                     = NULL;
  82 Monitor* FullGCCount_lock             = NULL;
  83 Mutex*   SATB_Q_FL_lock               = NULL;
  84 Monitor* SATB_Q_CBL_mon               = NULL;
  85 Mutex*   Shared_SATB_Q_lock           = NULL;
  86 Mutex*   DirtyCardQ_FL_lock           = NULL;
  87 Monitor* DirtyCardQ_CBL_mon           = NULL;
  88 Mutex*   Shared_DirtyCardQ_lock       = NULL;
  89 Mutex*   MarkStackFreeList_lock       = NULL;
  90 Mutex*   MarkStackChunkList_lock      = NULL;
  91 Mutex*   ParGCRareEvent_lock          = NULL;
  92 Mutex*   DerivedPointerTableGC_lock   = NULL;
  93 Monitor* CGCPhaseManager_lock         = NULL;
  94 Mutex*   Compile_lock                 = NULL;
  95 Monitor* MethodCompileQueue_lock      = NULL;
  96 Monitor* CompileThread_lock           = NULL;
  97 Monitor* Compilation_lock             = NULL;
  98 Mutex*   CompileTaskAlloc_lock        = NULL;
  99 Mutex*   CompileStatistics_lock       = NULL;
 100 Mutex*   DirectivesStack_lock         = NULL;
 101 Mutex*   MultiArray_lock              = NULL;
 102 Monitor* Terminator_lock              = NULL;
 103 Monitor* BeforeExit_lock              = NULL;
 104 Monitor* Notify_lock                  = NULL;
 105 Mutex*   ProfilePrint_lock            = NULL;
 106 Mutex*   ExceptionCache_lock          = NULL;
 107 Mutex*   OsrList_lock                 = NULL;
 108 
 109 #ifndef PRODUCT
 110 Mutex*   FullGCALot_lock              = NULL;
 111 #endif
 112 
 113 Mutex*   Debug1_lock                  = NULL;
 114 Mutex*   Debug2_lock                  = NULL;
 115 Mutex*   Debug3_lock                  = NULL;
 116 
 117 Mutex*   tty_lock                     = NULL;
 118 
 119 Mutex*   RawMonitor_lock              = NULL;
 120 Mutex*   PerfDataMemAlloc_lock        = NULL;
 121 Mutex*   PerfDataManager_lock         = NULL;
 122 Mutex*   OopMapCacheAlloc_lock        = NULL;
 123 
 124 Mutex*   FreeList_lock                = NULL;
 125 Mutex*   OldSets_lock                 = NULL;
 126 Monitor* RootRegionScan_lock          = NULL;
 127 
 128 Monitor* GCTaskManager_lock           = NULL;
 129 
 130 Mutex*   Management_lock              = NULL;
 131 Monitor* Service_lock                 = NULL;
 132 Monitor* PeriodicTask_lock            = NULL;
 133 Monitor* RedefineClasses_lock         = NULL;
 134 
 135 Mutex*   ThreadHeapSampler_lock       = NULL;
 136 
 137 #if INCLUDE_JFR
 138 Mutex*   JfrStacktrace_lock           = NULL;
 139 Monitor* JfrMsg_lock                  = NULL;
 140 Mutex*   JfrBuffer_lock               = NULL;
 141 Mutex*   JfrStream_lock               = NULL;
 142 #endif
 143 
 144 #ifndef SUPPORTS_NATIVE_CX8
 145 Mutex*   UnsafeJlong_lock             = NULL;
 146 #endif
 147 Monitor* CodeHeapStateAnalytics_lock  = NULL;
 148 
 149 Mutex*   MetaspaceExpand_lock         = NULL;
 150 Mutex*   ThreadIdTableCreate_lock     = NULL;
 151 
 152 #define MAX_NUM_MUTEX 128
 153 static Monitor * _mutex_array[MAX_NUM_MUTEX];
 154 static int _num_mutex;
 155 
 156 #ifdef ASSERT
 157 void assert_locked_or_safepoint(const Monitor * lock) {
 158   // check if this thread owns the lock (common case)
 159   if (IgnoreLockingAssertions) return;
 160   assert(lock != NULL, "Need non-NULL lock");
 161   if (lock->owned_by_self()) return;
 162   if (SafepointSynchronize::is_at_safepoint()) return;
 163   if (!Universe::is_fully_initialized()) return;
 164   // see if invoker of VM operation owns it
 165   VM_Operation* op = VMThread::vm_operation();
 166   if (op != NULL && op->calling_thread() == lock->owner()) return;
 167   fatal("must own lock %s", lock->name());
 168 }
 169 
 170 // a stronger assertion than the above
 171 void assert_lock_strong(const Monitor * lock) {
 172   if (IgnoreLockingAssertions) return;
 173   assert(lock != NULL, "Need non-NULL lock");
 174   if (lock->owned_by_self()) return;
 175   fatal("must own lock %s", lock->name());
 176 }
 177 #endif
 178 
 179 #define def(var, type, pri, vm_block, safepoint_check_allowed ) {      \
 180   var = new type(Mutex::pri, #var, vm_block, safepoint_check_allowed); \
 181   assert(_num_mutex < MAX_NUM_MUTEX, "increase MAX_NUM_MUTEX");        \
 182   _mutex_array[_num_mutex++] = var;                                      \
 183 }
 184 
 185 // Using Padded subclasses to prevent false sharing of these global monitors and mutexes.
 186 void mutex_init() {
 187   def(tty_lock                     , PaddedMutex  , tty,         true,  Monitor::_safepoint_check_never);      // allow to lock in VM
 188 
 189   def(CGC_lock                     , PaddedMonitor, special,     true,  Monitor::_safepoint_check_never);      // coordinate between fore- and background GC
 190   def(STS_lock                     , PaddedMonitor, leaf,        true,  Monitor::_safepoint_check_never);
 191 
 192   def(VMWeakAlloc_lock             , PaddedMutex  , vmweak,      true,  Monitor::_safepoint_check_never);
 193   def(VMWeakActive_lock            , PaddedMutex  , vmweak-1,    true,  Monitor::_safepoint_check_never);
 194 
 195   def(StringTableWeakAlloc_lock    , PaddedMutex  , vmweak,      true,  Monitor::_safepoint_check_never);
 196   def(StringTableWeakActive_lock   , PaddedMutex  , vmweak-1,    true,  Monitor::_safepoint_check_never);
 197 
 198   if (UseConcMarkSweepGC || UseG1GC) {
 199     def(FullGCCount_lock           , PaddedMonitor, leaf,        true,  Monitor::_safepoint_check_never);      // in support of ExplicitGCInvokesConcurrent
 200   }
 201   if (UseG1GC) {
 202     def(SATB_Q_FL_lock             , PaddedMutex  , access,      true,  Monitor::_safepoint_check_never);
 203     def(SATB_Q_CBL_mon             , PaddedMonitor, access,      true,  Monitor::_safepoint_check_never);
 204     def(Shared_SATB_Q_lock         , PaddedMutex  , access + 1,  true,  Monitor::_safepoint_check_never);
 205 
 206     def(DirtyCardQ_FL_lock         , PaddedMutex  , access,      true,  Monitor::_safepoint_check_never);
 207     def(DirtyCardQ_CBL_mon         , PaddedMonitor, access,      true,  Monitor::_safepoint_check_never);
 208     def(Shared_DirtyCardQ_lock     , PaddedMutex  , access + 1,  true,  Monitor::_safepoint_check_never);
 209 
 210     def(FreeList_lock              , PaddedMutex  , leaf     ,   true,  Monitor::_safepoint_check_never);
 211     def(OldSets_lock               , PaddedMutex  , leaf     ,   true,  Monitor::_safepoint_check_never);
 212     def(RootRegionScan_lock        , PaddedMonitor, leaf     ,   true,  Monitor::_safepoint_check_never);
 213 
 214     def(StringDedupQueue_lock      , PaddedMonitor, leaf,        true,  Monitor::_safepoint_check_never);
 215     def(StringDedupTable_lock      , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_never);
 216 
 217     def(MarkStackFreeList_lock     , PaddedMutex  , leaf     ,   true,  Monitor::_safepoint_check_never);
 218     def(MarkStackChunkList_lock    , PaddedMutex  , leaf     ,   true,  Monitor::_safepoint_check_never);
 219   }
 220 #if INCLUDE_SHENANDOAHGC
 221   if (UseShenandoahGC) {
 222     def(SATB_Q_FL_lock             , PaddedMutex  , access,      true,  Monitor::_safepoint_check_never);
 223     def(SATB_Q_CBL_mon             , PaddedMonitor, access,      true,  Monitor::_safepoint_check_never);
 224     def(Shared_SATB_Q_lock         , PaddedMutex  , access + 1,  true,  Monitor::_safepoint_check_never);
 225 
 226     def(StringDedupQueue_lock      , PaddedMonitor, leaf,        true,  Monitor::_safepoint_check_never);
 227     def(StringDedupTable_lock      , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_never);
 228   }
 229 #endif
 230   def(ParGCRareEvent_lock          , PaddedMutex  , leaf     ,   true,  Monitor::_safepoint_check_sometimes);
 231   def(DerivedPointerTableGC_lock   , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_never);
 232   def(CGCPhaseManager_lock         , PaddedMonitor, leaf,        false, Monitor::_safepoint_check_sometimes);
 233   def(CodeCache_lock               , PaddedMutex  , special,     true,  Monitor::_safepoint_check_never);
 234   def(RawMonitor_lock              , PaddedMutex  , special,     true,  Monitor::_safepoint_check_never);
 235   def(OopMapCacheAlloc_lock        , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_always);     // used for oop_map_cache allocation.
 236 
 237   def(MetaspaceExpand_lock         , PaddedMutex  , leaf-1,      true,  Monitor::_safepoint_check_never);
 238 
 239   def(Patching_lock                , PaddedMutex  , special,     true,  Monitor::_safepoint_check_never);      // used for safepointing and code patching.
 240   def(Service_lock                 , PaddedMonitor, special,     true,  Monitor::_safepoint_check_never);      // used for service thread operations
 241   def(JmethodIdCreation_lock       , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_always);     // used for creating jmethodIDs.
 242 
 243   def(SystemDictionary_lock        , PaddedMonitor, leaf,        true,  Monitor::_safepoint_check_always);     // lookups done by VM thread
 244   def(SharedDictionary_lock        , PaddedMutex,   leaf,        true,  Monitor::_safepoint_check_always);     // lookups done by VM thread
 245   def(Module_lock                  , PaddedMutex  , leaf+2,      true,  Monitor::_safepoint_check_always);
 246   def(InlineCacheBuffer_lock       , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_always);
 247   def(VMStatistic_lock             , PaddedMutex  , leaf,        false, Monitor::_safepoint_check_always);
 248   def(ExpandHeap_lock              , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_always);     // Used during compilation by VM thread
 249   def(JNIHandleBlockFreeList_lock  , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_never);      // handles are used by VM thread
 250   def(SignatureHandlerLibrary_lock , PaddedMutex  , leaf,        false, Monitor::_safepoint_check_always);
 251   def(SymbolTable_lock             , PaddedMutex  , leaf+2,      true,  Monitor::_safepoint_check_always);
 252   def(StringTable_lock             , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_always);
 253   def(ProfilePrint_lock            , PaddedMutex  , leaf,        false, Monitor::_safepoint_check_always);     // serial profile printing
 254   def(ExceptionCache_lock          , PaddedMutex  , leaf,        false, Monitor::_safepoint_check_always);     // serial profile printing
 255   def(OsrList_lock                 , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_never);
 256   def(Debug1_lock                  , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_never);
 257 #ifndef PRODUCT
 258   def(FullGCALot_lock              , PaddedMutex  , leaf,        false, Monitor::_safepoint_check_always);     // a lock to make FullGCALot MT safe
 259 #endif
 260   def(BeforeExit_lock              , PaddedMonitor, leaf,        true,  Monitor::_safepoint_check_always);
 261   def(PerfDataMemAlloc_lock        , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_always);     // used for allocating PerfData memory for performance data
 262   def(PerfDataManager_lock         , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_always);     // used for synchronized access to PerfDataManager resources
 263 
 264   // CMS_modUnionTable_lock                   leaf
 265   // CMS_bitMap_lock                          leaf 1
 266   // CMS_freeList_lock                        leaf 2
 267 
 268   def(Safepoint_lock               , PaddedMonitor, safepoint,   true,  Monitor::_safepoint_check_sometimes);  // locks SnippetCache_lock/Threads_lock
 269 
 270   def(Threads_lock                 , PaddedMonitor, barrier,     true,  Monitor::_safepoint_check_sometimes);
 271   def(NonJavaThreadsList_lock      , PaddedMutex,   leaf,        true,  Monitor::_safepoint_check_never);
 272 
 273   def(VMOperationQueue_lock        , PaddedMonitor, nonleaf,     true,  Monitor::_safepoint_check_sometimes);  // VM_thread allowed to block on these
 274   def(VMOperationRequest_lock      , PaddedMonitor, nonleaf,     true,  Monitor::_safepoint_check_sometimes);
 275   def(RetData_lock                 , PaddedMutex  , nonleaf,     false, Monitor::_safepoint_check_always);
 276   def(Terminator_lock              , PaddedMonitor, nonleaf,     true,  Monitor::_safepoint_check_sometimes);
 277   def(VtableStubs_lock             , PaddedMutex  , nonleaf,     true,  Monitor::_safepoint_check_always);
 278   def(Notify_lock                  , PaddedMonitor, nonleaf,     true,  Monitor::_safepoint_check_always);
 279   // OopStorage-based JNI may lock the alloc_locks while releasing a handle,
 280   // while previous JNI didn't need a lock for handle release.  This runs afoul
 281   // of some places which hold other locks while releasing a handle, including
 282   // the Patching_lock, which is of "special" rank.  As a temporary workaround,
 283   // lower the JNI oopstorage lock ranks to make them super-special.
 284   def(JNIGlobalAlloc_lock          , PaddedMutex  , nonleaf,     true,  Monitor::_safepoint_check_never);
 285   def(JNIGlobalActive_lock         , PaddedMutex  , nonleaf-1,   true,  Monitor::_safepoint_check_never);
 286   def(JNIWeakAlloc_lock            , PaddedMutex  , nonleaf,     true,  Monitor::_safepoint_check_never);
 287   def(JNIWeakActive_lock           , PaddedMutex  , nonleaf-1,   true,  Monitor::_safepoint_check_never);
 288   def(JNICritical_lock             , PaddedMonitor, nonleaf,     true,  Monitor::_safepoint_check_always);     // used for JNI critical regions
 289   def(AdapterHandlerLibrary_lock   , PaddedMutex  , nonleaf,     true,  Monitor::_safepoint_check_always);
 290 
 291   def(Heap_lock                    , PaddedMonitor, nonleaf+1,   false, Monitor::_safepoint_check_sometimes);
 292   def(JfieldIdCreation_lock        , PaddedMutex  , nonleaf+1,   true,  Monitor::_safepoint_check_always);     // jfieldID, Used in VM_Operation
 293   def(ResolvedMethodTable_lock     , PaddedMutex  , nonleaf+1,   false, Monitor::_safepoint_check_always);     // Used to protect ResolvedMethodTable
 294 
 295   def(CompiledIC_lock              , PaddedMutex  , nonleaf+2,   false, Monitor::_safepoint_check_always);     // locks VtableStubs_lock, InlineCacheBuffer_lock
 296   def(CompileTaskAlloc_lock        , PaddedMutex  , nonleaf+2,   true,  Monitor::_safepoint_check_always);
 297   def(CompileStatistics_lock       , PaddedMutex  , nonleaf+2,   false, Monitor::_safepoint_check_always);
 298   def(DirectivesStack_lock         , PaddedMutex  , special,     true,  Monitor::_safepoint_check_never);
 299   def(MultiArray_lock              , PaddedMutex  , nonleaf+2,   false, Monitor::_safepoint_check_always);     // locks SymbolTable_lock
 300 
 301   def(JvmtiThreadState_lock        , PaddedMutex  , nonleaf+2,   false, Monitor::_safepoint_check_always);     // Used by JvmtiThreadState/JvmtiEventController
 302   def(Management_lock              , PaddedMutex  , nonleaf+2,   false, Monitor::_safepoint_check_always);     // used for JVM management
 303 
 304   def(Compile_lock                 , PaddedMutex  , nonleaf+3,   true,  Monitor::_safepoint_check_sometimes);
 305   def(MethodData_lock              , PaddedMutex  , nonleaf+3,   false, Monitor::_safepoint_check_always);
 306   def(TouchedMethodLog_lock        , PaddedMutex  , nonleaf+3,   false, Monitor::_safepoint_check_always);
 307 
 308   def(MethodCompileQueue_lock      , PaddedMonitor, nonleaf+4,   true,  Monitor::_safepoint_check_always);
 309   def(Debug2_lock                  , PaddedMutex  , nonleaf+4,   true,  Monitor::_safepoint_check_never);
 310   def(Debug3_lock                  , PaddedMutex  , nonleaf+4,   true,  Monitor::_safepoint_check_never);
 311   def(CompileThread_lock           , PaddedMonitor, nonleaf+5,   false, Monitor::_safepoint_check_always);
 312   def(PeriodicTask_lock            , PaddedMonitor, nonleaf+5,   true,  Monitor::_safepoint_check_sometimes);
 313   def(RedefineClasses_lock         , PaddedMonitor, nonleaf+5,   true,  Monitor::_safepoint_check_always);
 314 
 315   def(ThreadHeapSampler_lock       , PaddedMutex,   nonleaf,     false, Monitor::_safepoint_check_never);
 316 
 317   if (WhiteBoxAPI) {
 318     def(Compilation_lock           , PaddedMonitor, leaf,        false, Monitor::_safepoint_check_never);
 319   }
 320 
 321 #if INCLUDE_JFR
 322   def(JfrMsg_lock                  , PaddedMonitor, leaf,        true,  Monitor::_safepoint_check_always);
 323   def(JfrBuffer_lock               , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_never);
 324   def(JfrStream_lock               , PaddedMutex  , leaf+1,      true,  Monitor::_safepoint_check_never);      // ensure to rank lower than 'safepoint'
 325   def(JfrStacktrace_lock           , PaddedMutex  , special,     true,  Monitor::_safepoint_check_sometimes);
 326 #endif
 327 
 328 #ifndef SUPPORTS_NATIVE_CX8
 329   def(UnsafeJlong_lock             , PaddedMutex  , special,     false, Monitor::_safepoint_check_never);
 330 #endif
 331 
 332   def(CodeHeapStateAnalytics_lock  , PaddedMutex  , leaf,        true,  Monitor::_safepoint_check_never);
 333   def(ThreadIdTableCreate_lock     , PaddedMutex  , leaf,        false, Monitor::_safepoint_check_always);
 334 }
 335 
 336 GCMutexLocker::GCMutexLocker(Monitor * mutex) {
 337   if (SafepointSynchronize::is_at_safepoint()) {
 338     _locked = false;
 339   } else {
 340     _mutex = mutex;
 341     _locked = true;
 342     _mutex->lock();
 343   }
 344 }
 345 
 346 // Print all mutexes/monitors that are currently owned by a thread; called
 347 // by fatal error handler.
 348 void print_owned_locks_on_error(outputStream* st) {
 349   st->print("VM Mutex/Monitor currently owned by a thread: ");
 350   bool none = true;
 351   for (int i = 0; i < _num_mutex; i++) {
 352      // see if it has an owner
 353      if (_mutex_array[i]->owner() != NULL) {
 354        if (none) {
 355           // print format used by Mutex::print_on_error()
 356           st->print_cr(" ([mutex/lock_event])");
 357           none = false;
 358        }
 359        _mutex_array[i]->print_on_error(st);
 360        st->cr();
 361      }
 362   }
 363   if (none) st->print_cr("None");
 364 }