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src/hotspot/share/runtime/mutexLocker.hpp

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  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 #ifndef SHARE_RUNTIME_MUTEXLOCKER_HPP
 26 #define SHARE_RUNTIME_MUTEXLOCKER_HPP
 27 

 28 #include "memory/allocation.hpp"
 29 #include "runtime/flags/flagSetting.hpp"
 30 #include "runtime/mutex.hpp"

 31 
 32 class Thread;
 33 
 34 // Mutexes used in the VM.
 35 
 36 extern Mutex*   NMethodState_lock;               // a lock used to guard a compiled method state
 37 extern Monitor* SystemDictionary_lock;           // a lock on the system dictionary
 38 extern Mutex*   InvokeMethodTypeTable_lock;
 39 extern Monitor* InvokeMethodIntrinsicTable_lock;
 40 extern Mutex*   SharedDictionary_lock;           // a lock on the CDS shared dictionary
 41 extern Monitor* ClassInitError_lock;             // a lock on the class initialization error table
 42 extern Mutex*   Module_lock;                     // a lock on module and package related data structures
 43 extern Mutex*   CompiledIC_lock;                 // a lock used to guard compiled IC patching and access
 44 extern Mutex*   VMStatistic_lock;                // a lock used to guard statistics count increment
 45 extern Mutex*   JmethodIdCreation_lock;          // a lock on creating JNI method identifiers
 46 extern Mutex*   JfieldIdCreation_lock;           // a lock on creating JNI static field identifiers
 47 extern Monitor* JNICritical_lock;                // a lock used while synchronizing with threads entering/leaving JNI critical regions
 48 extern Mutex*   JvmtiThreadState_lock;           // a lock on modification of JVMTI thread data
 49 extern Monitor* EscapeBarrier_lock;              // a lock to sync reallocating and relocking objects because of JVMTI access
 50 extern Monitor* JvmtiVTMSTransition_lock;        // a lock for Virtual Thread Mount State transition (VTMS transition) management

 57 extern Mutex*   SignatureHandlerLibrary_lock;    // a lock on the SignatureHandlerLibrary
 58 extern Mutex*   VtableStubs_lock;                // a lock on the VtableStubs
 59 extern Mutex*   SymbolArena_lock;                // a lock on the symbol table arena
 60 extern Monitor* StringDedup_lock;                // a lock on the string deduplication facility
 61 extern Mutex*   StringDedupIntern_lock;          // a lock on StringTable notification of StringDedup
 62 extern Monitor* CodeCache_lock;                  // a lock on the CodeCache
 63 extern Mutex*   TouchedMethodLog_lock;           // a lock on allocation of LogExecutedMethods info
 64 extern Mutex*   RetData_lock;                    // a lock on installation of RetData inside method data
 65 extern Monitor* VMOperation_lock;                // a lock on queue of vm_operations waiting to execute
 66 extern Monitor* ThreadsLockThrottle_lock;        // used by Thread start/exit to reduce competition for Threads_lock,
 67                                                  // so a VM thread calling a safepoint is prioritized
 68 extern Monitor* Threads_lock;                    // a lock on the Threads table of active Java threads
 69                                                  // (also used by Safepoints too to block threads creation/destruction)
 70 extern Mutex*   NonJavaThreadsList_lock;         // a lock on the NonJavaThreads list
 71 extern Mutex*   NonJavaThreadsListSync_lock;     // a lock for NonJavaThreads list synchronization
 72 extern Monitor* STS_lock;                        // used for joining/leaving SuspendibleThreadSet.
 73 extern Mutex*   MonitoringSupport_lock;          // Protects updates to the serviceability memory pools and allocated memory high water mark.
 74 extern Monitor* ConcurrentGCBreakpoints_lock;    // Protects concurrent GC breakpoint management
 75 extern Mutex*   Compile_lock;                    // a lock held when Compilation is updating code (used to block CodeCache traversal, CHA updates, etc)
 76 extern Monitor* MethodCompileQueue_lock;         // a lock held when method compilations are enqueued, dequeued




 77 extern Monitor* CompileThread_lock;              // a lock held by compile threads during compilation system initialization
 78 extern Monitor* Compilation_lock;                // a lock used to pause compilation
 79 extern Mutex*   TrainingData_lock;               // a lock used when accessing training records
 80 extern Monitor* TrainingReplayQueue_lock;        // a lock held when class are added/removed to the training replay queue
 81 extern Monitor* CompileTaskWait_lock;            // a lock held when CompileTasks are waited/notified
 82 extern Mutex*   CompileStatistics_lock;          // a lock held when updating compilation statistics
 83 extern Mutex*   DirectivesStack_lock;            // a lock held when mutating the dirstack and ref counting directives
 84 extern Monitor* Terminator_lock;                 // a lock used to guard termination of the vm
 85 extern Monitor* InitCompleted_lock;              // a lock used to signal threads waiting on init completed
 86 extern Monitor* BeforeExit_lock;                 // a lock used to guard cleanups and shutdown hooks
 87 extern Monitor* Notify_lock;                     // a lock used to synchronize the start-up of the vm
 88 extern Mutex*   ExceptionCache_lock;             // a lock used to synchronize exception cache updates
 89 
 90 #ifndef PRODUCT
 91 extern Mutex*   FullGCALot_lock;                 // a lock to make FullGCALot MT safe
 92 #endif // PRODUCT
 93 
 94 #if INCLUDE_G1GC
 95 extern Monitor* G1CGC_lock;                      // used for coordination between fore- & background G1 concurrent GC threads.
 96 extern Mutex*   G1FreeList_lock;                 // protects the G1 free region list during safepoints

116 extern Monitor* RedefineClasses_lock;            // locks classes from parallel redefinition
117 extern Mutex*   Verify_lock;                     // synchronize initialization of verify library
118 extern Monitor* ThreadsSMRDelete_lock;           // Used by ThreadsSMRSupport to take pressure off the Threads_lock
119 extern Mutex*   ThreadIdTableCreate_lock;        // Used by ThreadIdTable to lazily create the thread id table
120 extern Mutex*   SharedDecoder_lock;              // serializes access to the decoder during normal (not error reporting) use
121 extern Mutex*   DCmdFactory_lock;                // serialize access to DCmdFactory information
122 extern Mutex*   NMTQuery_lock;                   // serialize NMT Dcmd queries
123 extern Mutex*   NMTCompilationCostHistory_lock;  // guards NMT compilation cost history
124 extern Mutex*   NmtVirtualMemory_lock;           // guards NMT virtual memory updates
125 #if INCLUDE_CDS
126 #if INCLUDE_JVMTI
127 extern Mutex*   CDSClassFileStream_lock;         // FileMapInfo::open_stream_for_jvmti
128 #endif
129 extern Mutex*   DumpTimeTable_lock;              // SystemDictionaryShared::_dumptime_table
130 extern Mutex*   CDSLambda_lock;                  // LambdaProxyClassDictionary::find_lambda_proxy_class
131 extern Mutex*   DumpRegion_lock;                 // Symbol::operator new(size_t sz, int len)
132 extern Mutex*   ClassListFile_lock;              // ClassListWriter()
133 extern Mutex*   UnregisteredClassesTable_lock;   // UnregisteredClassesTableTable
134 extern Mutex*   LambdaFormInvokers_lock;         // Protecting LambdaFormInvokers::_lambdaform_lines
135 extern Mutex*   ScratchObjects_lock;             // Protecting _scratch_xxx_table in heapShared.cpp

136 extern Mutex*   FinalImageRecipes_lock;          // Protecting the tables used by FinalImageRecipes.
137 #endif // INCLUDE_CDS
138 #if INCLUDE_JFR
139 extern Mutex*   JfrStacktrace_lock;              // used to guard access to the JFR stacktrace table
140 extern Monitor* JfrMsg_lock;                     // protects JFR messaging
141 extern Mutex*   JfrBuffer_lock;                  // protects JFR buffer operations
142 #endif
143 
144 extern Mutex*   Metaspace_lock;                  // protects Metaspace virtualspace and chunk expansions
145 extern Monitor* MetaspaceCritical_lock;          // synchronizes failed metaspace allocations that risk throwing metaspace OOM
146 extern Mutex*   ClassLoaderDataGraph_lock;       // protects CLDG list, needed for concurrent unloading
147 
148 
149 extern Mutex*   CodeHeapStateAnalytics_lock;     // lock print functions against concurrent analyze functions.
150                                                  // Only used locally in PrintCodeCacheLayout processing.
151 
152 extern Mutex*   ExternalsRecorder_lock;          // used to guard access to the external addresses table
153 
154 extern Mutex*   AOTCodeCStrings_lock;            // used to guard access to the AOT code C strings table
155 

175 // See orderAccess.hpp.  We assume throughout the VM that MutexLocker's
176 // and friends constructors do a fence, a lock and an acquire *in that
177 // order*.  And that their destructors do a release and unlock, in *that*
178 // order.  If their implementations change such that these assumptions
179 // are violated, a whole lot of code will break.
180 
181 // for debugging: check that we're already owning this lock (or are at a safepoint / handshake)
182 #ifdef ASSERT
183 void assert_locked_or_safepoint(const Mutex* lock);
184 void assert_lock_strong(const Mutex* lock);
185 #else
186 #define assert_locked_or_safepoint(lock)
187 #define assert_lock_strong(lock)
188 #endif
189 
190 // Internal implementation. Skips on null Mutex.
191 // Subclasses enforce stronger invariants.
192 class MutexLockerImpl: public StackObj {
193  protected:
194   Mutex* _mutex;



195 
196   MutexLockerImpl(Mutex* mutex, Mutex::SafepointCheckFlag flag = Mutex::_safepoint_check_flag) :
197     _mutex(mutex) {
198     bool no_safepoint_check = flag == Mutex::_no_safepoint_check_flag;
199     if (_mutex != nullptr) {
200       if (no_safepoint_check) {
201         _mutex->lock_without_safepoint_check();
202       } else {
203         _mutex->lock();
204       }
205     }
206   }
207 
208   MutexLockerImpl(Thread* thread, Mutex* mutex, Mutex::SafepointCheckFlag flag = Mutex::_safepoint_check_flag) :
209     _mutex(mutex) {
210     bool no_safepoint_check = flag == Mutex::_no_safepoint_check_flag;
211     if (_mutex != nullptr) {
212       if (no_safepoint_check) {
213         _mutex->lock_without_safepoint_check(thread);
214       } else {
215         _mutex->lock(thread);
216       }
217     }
218   }
219 
220   ~MutexLockerImpl() {
221     if (_mutex != nullptr) {
222       assert_lock_strong(_mutex);
223       _mutex->unlock();












224     }
225   }
226 



227  public:


228   static void post_initialize();


229 };
230 
231 // Simplest mutex locker.
232 // Does not allow null mutexes.
233 class MutexLocker: public MutexLockerImpl {
234  public:
235    MutexLocker(Mutex* mutex, Mutex::SafepointCheckFlag flag = Mutex::_safepoint_check_flag) :
236      MutexLockerImpl(mutex, flag) {
237      assert(mutex != nullptr, "null mutex not allowed");
238    }
239 
240    MutexLocker(Thread* thread, Mutex* mutex, Mutex::SafepointCheckFlag flag = Mutex::_safepoint_check_flag) :
241      MutexLockerImpl(thread, mutex, flag) {
242      assert(mutex != nullptr, "null mutex not allowed");
243    }
244 };
245 
246 // Conditional mutex locker.
247 // Like MutexLocker above, but only locks when condition is true.
248 class ConditionalMutexLocker: public MutexLockerImpl {

  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 #ifndef SHARE_RUNTIME_MUTEXLOCKER_HPP
 26 #define SHARE_RUNTIME_MUTEXLOCKER_HPP
 27 
 28 #include "logging/log.hpp"
 29 #include "memory/allocation.hpp"
 30 #include "runtime/flags/flagSetting.hpp"
 31 #include "runtime/mutex.hpp"
 32 #include "runtime/perfData.hpp"
 33 
 34 class Thread;
 35 
 36 // Mutexes used in the VM.
 37 
 38 extern Mutex*   NMethodState_lock;               // a lock used to guard a compiled method state
 39 extern Monitor* SystemDictionary_lock;           // a lock on the system dictionary
 40 extern Mutex*   InvokeMethodTypeTable_lock;
 41 extern Monitor* InvokeMethodIntrinsicTable_lock;
 42 extern Mutex*   SharedDictionary_lock;           // a lock on the CDS shared dictionary
 43 extern Monitor* ClassInitError_lock;             // a lock on the class initialization error table
 44 extern Mutex*   Module_lock;                     // a lock on module and package related data structures
 45 extern Mutex*   CompiledIC_lock;                 // a lock used to guard compiled IC patching and access
 46 extern Mutex*   VMStatistic_lock;                // a lock used to guard statistics count increment
 47 extern Mutex*   JmethodIdCreation_lock;          // a lock on creating JNI method identifiers
 48 extern Mutex*   JfieldIdCreation_lock;           // a lock on creating JNI static field identifiers
 49 extern Monitor* JNICritical_lock;                // a lock used while synchronizing with threads entering/leaving JNI critical regions
 50 extern Mutex*   JvmtiThreadState_lock;           // a lock on modification of JVMTI thread data
 51 extern Monitor* EscapeBarrier_lock;              // a lock to sync reallocating and relocking objects because of JVMTI access
 52 extern Monitor* JvmtiVTMSTransition_lock;        // a lock for Virtual Thread Mount State transition (VTMS transition) management

 59 extern Mutex*   SignatureHandlerLibrary_lock;    // a lock on the SignatureHandlerLibrary
 60 extern Mutex*   VtableStubs_lock;                // a lock on the VtableStubs
 61 extern Mutex*   SymbolArena_lock;                // a lock on the symbol table arena
 62 extern Monitor* StringDedup_lock;                // a lock on the string deduplication facility
 63 extern Mutex*   StringDedupIntern_lock;          // a lock on StringTable notification of StringDedup
 64 extern Monitor* CodeCache_lock;                  // a lock on the CodeCache
 65 extern Mutex*   TouchedMethodLog_lock;           // a lock on allocation of LogExecutedMethods info
 66 extern Mutex*   RetData_lock;                    // a lock on installation of RetData inside method data
 67 extern Monitor* VMOperation_lock;                // a lock on queue of vm_operations waiting to execute
 68 extern Monitor* ThreadsLockThrottle_lock;        // used by Thread start/exit to reduce competition for Threads_lock,
 69                                                  // so a VM thread calling a safepoint is prioritized
 70 extern Monitor* Threads_lock;                    // a lock on the Threads table of active Java threads
 71                                                  // (also used by Safepoints too to block threads creation/destruction)
 72 extern Mutex*   NonJavaThreadsList_lock;         // a lock on the NonJavaThreads list
 73 extern Mutex*   NonJavaThreadsListSync_lock;     // a lock for NonJavaThreads list synchronization
 74 extern Monitor* STS_lock;                        // used for joining/leaving SuspendibleThreadSet.
 75 extern Mutex*   MonitoringSupport_lock;          // Protects updates to the serviceability memory pools and allocated memory high water mark.
 76 extern Monitor* ConcurrentGCBreakpoints_lock;    // Protects concurrent GC breakpoint management
 77 extern Mutex*   Compile_lock;                    // a lock held when Compilation is updating code (used to block CodeCache traversal, CHA updates, etc)
 78 extern Monitor* MethodCompileQueue_lock;         // a lock held when method compilations are enqueued, dequeued
 79 extern Monitor* MethodCompileQueueC1_lock;       // a lock held when method compilations are enqueued, dequeued
 80 extern Monitor* MethodCompileQueueC2_lock;       // a lock held when method compilations are enqueued, dequeued
 81 extern Monitor* MethodCompileQueueSC1_lock;      // a lock held when method compilations are enqueued, dequeued
 82 extern Monitor* MethodCompileQueueSC2_lock;      // a lock held when method compilations are enqueued, dequeued
 83 extern Monitor* CompileThread_lock;              // a lock held by compile threads during compilation system initialization
 84 extern Monitor* Compilation_lock;                // a lock used to pause compilation
 85 extern Mutex*   TrainingData_lock;               // a lock used when accessing training records
 86 extern Monitor* TrainingReplayQueue_lock;        // a lock held when class are added/removed to the training replay queue
 87 extern Monitor* CompileTaskWait_lock;            // a lock held when CompileTasks are waited/notified
 88 extern Mutex*   CompileStatistics_lock;          // a lock held when updating compilation statistics
 89 extern Mutex*   DirectivesStack_lock;            // a lock held when mutating the dirstack and ref counting directives
 90 extern Monitor* Terminator_lock;                 // a lock used to guard termination of the vm
 91 extern Monitor* InitCompleted_lock;              // a lock used to signal threads waiting on init completed
 92 extern Monitor* BeforeExit_lock;                 // a lock used to guard cleanups and shutdown hooks
 93 extern Monitor* Notify_lock;                     // a lock used to synchronize the start-up of the vm
 94 extern Mutex*   ExceptionCache_lock;             // a lock used to synchronize exception cache updates
 95 
 96 #ifndef PRODUCT
 97 extern Mutex*   FullGCALot_lock;                 // a lock to make FullGCALot MT safe
 98 #endif // PRODUCT
 99 
100 #if INCLUDE_G1GC
101 extern Monitor* G1CGC_lock;                      // used for coordination between fore- & background G1 concurrent GC threads.
102 extern Mutex*   G1FreeList_lock;                 // protects the G1 free region list during safepoints

122 extern Monitor* RedefineClasses_lock;            // locks classes from parallel redefinition
123 extern Mutex*   Verify_lock;                     // synchronize initialization of verify library
124 extern Monitor* ThreadsSMRDelete_lock;           // Used by ThreadsSMRSupport to take pressure off the Threads_lock
125 extern Mutex*   ThreadIdTableCreate_lock;        // Used by ThreadIdTable to lazily create the thread id table
126 extern Mutex*   SharedDecoder_lock;              // serializes access to the decoder during normal (not error reporting) use
127 extern Mutex*   DCmdFactory_lock;                // serialize access to DCmdFactory information
128 extern Mutex*   NMTQuery_lock;                   // serialize NMT Dcmd queries
129 extern Mutex*   NMTCompilationCostHistory_lock;  // guards NMT compilation cost history
130 extern Mutex*   NmtVirtualMemory_lock;           // guards NMT virtual memory updates
131 #if INCLUDE_CDS
132 #if INCLUDE_JVMTI
133 extern Mutex*   CDSClassFileStream_lock;         // FileMapInfo::open_stream_for_jvmti
134 #endif
135 extern Mutex*   DumpTimeTable_lock;              // SystemDictionaryShared::_dumptime_table
136 extern Mutex*   CDSLambda_lock;                  // LambdaProxyClassDictionary::find_lambda_proxy_class
137 extern Mutex*   DumpRegion_lock;                 // Symbol::operator new(size_t sz, int len)
138 extern Mutex*   ClassListFile_lock;              // ClassListWriter()
139 extern Mutex*   UnregisteredClassesTable_lock;   // UnregisteredClassesTableTable
140 extern Mutex*   LambdaFormInvokers_lock;         // Protecting LambdaFormInvokers::_lambdaform_lines
141 extern Mutex*   ScratchObjects_lock;             // Protecting _scratch_xxx_table in heapShared.cpp
142 extern Mutex*   ArchivedObjectTables_lock;       // Protecting the table used by HeapShared::get_archived_object_permanent_index()
143 extern Mutex*   FinalImageRecipes_lock;          // Protecting the tables used by FinalImageRecipes.
144 #endif // INCLUDE_CDS
145 #if INCLUDE_JFR
146 extern Mutex*   JfrStacktrace_lock;              // used to guard access to the JFR stacktrace table
147 extern Monitor* JfrMsg_lock;                     // protects JFR messaging
148 extern Mutex*   JfrBuffer_lock;                  // protects JFR buffer operations
149 #endif
150 
151 extern Mutex*   Metaspace_lock;                  // protects Metaspace virtualspace and chunk expansions
152 extern Monitor* MetaspaceCritical_lock;          // synchronizes failed metaspace allocations that risk throwing metaspace OOM
153 extern Mutex*   ClassLoaderDataGraph_lock;       // protects CLDG list, needed for concurrent unloading
154 
155 
156 extern Mutex*   CodeHeapStateAnalytics_lock;     // lock print functions against concurrent analyze functions.
157                                                  // Only used locally in PrintCodeCacheLayout processing.
158 
159 extern Mutex*   ExternalsRecorder_lock;          // used to guard access to the external addresses table
160 
161 extern Mutex*   AOTCodeCStrings_lock;            // used to guard access to the AOT code C strings table
162 

182 // See orderAccess.hpp.  We assume throughout the VM that MutexLocker's
183 // and friends constructors do a fence, a lock and an acquire *in that
184 // order*.  And that their destructors do a release and unlock, in *that*
185 // order.  If their implementations change such that these assumptions
186 // are violated, a whole lot of code will break.
187 
188 // for debugging: check that we're already owning this lock (or are at a safepoint / handshake)
189 #ifdef ASSERT
190 void assert_locked_or_safepoint(const Mutex* lock);
191 void assert_lock_strong(const Mutex* lock);
192 #else
193 #define assert_locked_or_safepoint(lock)
194 #define assert_lock_strong(lock)
195 #endif
196 
197 // Internal implementation. Skips on null Mutex.
198 // Subclasses enforce stronger invariants.
199 class MutexLockerImpl: public StackObj {
200  protected:
201   Mutex* _mutex;
202   bool _prof;
203   elapsedTimer _before;
204   elapsedTimer _after;
205 
206 private:
207   static PerfCounter** _perf_lock_count;
208   static PerfCounter** _perf_lock_wait_time;
209   static PerfCounter** _perf_lock_hold_time;







210 
211 public:
212 
213   MutexLockerImpl(Mutex* mutex, Mutex::SafepointCheckFlag flag = Mutex::_safepoint_check_flag);
214 
215   MutexLockerImpl(Thread* thread, Mutex* mutex, Mutex::SafepointCheckFlag flag = Mutex::_safepoint_check_flag);






216 
217   ~MutexLockerImpl() {
218     if (_mutex != nullptr) {
219       assert_lock_strong(_mutex);
220       _mutex->unlock();
221 
222       if (_mutex->id() == -1) {
223         log_trace(init)("Unnamed unclassified lock: %s", _mutex->name());
224       }
225 
226       if (_prof) {
227         assert(UsePerfData, "required");
228         _after.stop();
229         _perf_lock_count    [_mutex->id() + 1]->inc();
230         _perf_lock_wait_time[_mutex->id() + 1]->inc(_before.ticks());
231         _perf_lock_hold_time[_mutex->id() + 1]->inc(_after.ticks());
232       }
233     }
234   }
235 
236  private:
237   static void print_counter_on(outputStream* st, const char* name, bool is_unique, int idx);
238 
239  public:
240   static int name2id(const char* name);
241 
242   static void post_initialize();
243   static void init_counters();
244   static void print_counters_on(outputStream* st);
245 };
246 
247 // Simplest mutex locker.
248 // Does not allow null mutexes.
249 class MutexLocker: public MutexLockerImpl {
250  public:
251    MutexLocker(Mutex* mutex, Mutex::SafepointCheckFlag flag = Mutex::_safepoint_check_flag) :
252      MutexLockerImpl(mutex, flag) {
253      assert(mutex != nullptr, "null mutex not allowed");
254    }
255 
256    MutexLocker(Thread* thread, Mutex* mutex, Mutex::SafepointCheckFlag flag = Mutex::_safepoint_check_flag) :
257      MutexLockerImpl(thread, mutex, flag) {
258      assert(mutex != nullptr, "null mutex not allowed");
259    }
260 };
261 
262 // Conditional mutex locker.
263 // Like MutexLocker above, but only locks when condition is true.
264 class ConditionalMutexLocker: public MutexLockerImpl {
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