1 /*
  2  * Copyright (c) 1998, 2022, 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 #ifndef SHARE_RUNTIME_OBJECTMONITOR_HPP
 26 #define SHARE_RUNTIME_OBJECTMONITOR_HPP
 27 
 28 #include "memory/allocation.hpp"
 29 #include "memory/padded.hpp"
 30 #include "oops/markWord.hpp"
 31 #include "oops/weakHandle.hpp"
 32 #include "runtime/os.hpp"
 33 #include "runtime/park.hpp"
 34 #include "runtime/perfDataTypes.hpp"
 35 
 36 class ObjectMonitor;
 37 
 38 // ObjectWaiter serves as a "proxy" or surrogate thread.
 39 // TODO-FIXME: Eliminate ObjectWaiter and use the thread-specific
 40 // ParkEvent instead.  Beware, however, that the JVMTI code
 41 // knows about ObjectWaiters, so we'll have to reconcile that code.
 42 // See next_waiter(), first_waiter(), etc.
 43 
 44 class ObjectWaiter : public StackObj {
 45  public:
 46   enum TStates { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER, TS_CXQ };
 47   ObjectWaiter* volatile _next;
 48   ObjectWaiter* volatile _prev;
 49   JavaThread*   _thread;
 50   jlong         _notifier_tid;
 51   ParkEvent *   _event;
 52   volatile int  _notified;
 53   volatile TStates TState;
 54   bool          _active;           // Contention monitoring is enabled
 55  public:
 56   ObjectWaiter(JavaThread* current);
 57 
 58   void wait_reenter_begin(ObjectMonitor *mon);
 59   void wait_reenter_end(ObjectMonitor *mon);
 60 };
 61 
 62 // The ObjectMonitor class implements the heavyweight version of a
 63 // JavaMonitor. The lightweight BasicLock/stack lock version has been
 64 // inflated into an ObjectMonitor. This inflation is typically due to
 65 // contention or use of Object.wait().
 66 //
 67 // WARNING: This is a very sensitive and fragile class. DO NOT make any
 68 // changes unless you are fully aware of the underlying semantics.
 69 //
 70 // ObjectMonitor Layout Overview/Highlights/Restrictions:
 71 //
 72 // - The _header field must be at offset 0 because the displaced header
 73 //   from markWord is stored there. We do not want markWord.hpp to include
 74 //   ObjectMonitor.hpp to avoid exposing ObjectMonitor everywhere. This
 75 //   means that ObjectMonitor cannot inherit from any other class nor can
 76 //   it use any virtual member functions. This restriction is critical to
 77 //   the proper functioning of the VM.
 78 // - The _header and _owner fields should be separated by enough space
 79 //   to avoid false sharing due to parallel access by different threads.
 80 //   This is an advisory recommendation.
 81 // - The general layout of the fields in ObjectMonitor is:
 82 //     _header
 83 //     <lightly_used_fields>
 84 //     <optional padding>
 85 //     _owner
 86 //     <remaining_fields>
 87 // - The VM assumes write ordering and machine word alignment with
 88 //   respect to the _owner field and the <remaining_fields> that can
 89 //   be read in parallel by other threads.
 90 // - Generally fields that are accessed closely together in time should
 91 //   be placed proximally in space to promote data cache locality. That
 92 //   is, temporal locality should condition spatial locality.
 93 // - We have to balance avoiding false sharing with excessive invalidation
 94 //   from coherence traffic. As such, we try to cluster fields that tend
 95 //   to be _written_ at approximately the same time onto the same data
 96 //   cache line.
 97 // - We also have to balance the natural tension between minimizing
 98 //   single threaded capacity misses with excessive multi-threaded
 99 //   coherency misses. There is no single optimal layout for both
100 //   single-threaded and multi-threaded environments.
101 //
102 // - See TEST_VM(ObjectMonitor, sanity) gtest for how critical restrictions are
103 //   enforced.
104 // - Adjacent ObjectMonitors should be separated by enough space to avoid
105 //   false sharing. This is handled by the ObjectMonitor allocation code
106 //   in synchronizer.cpp. Also see TEST_VM(SynchronizerTest, sanity) gtest.
107 //
108 // Futures notes:
109 //   - Separating _owner from the <remaining_fields> by enough space to
110 //     avoid false sharing might be profitable. Given
111 //     http://blogs.oracle.com/dave/entry/cas_and_cache_trivia_invalidate
112 //     we know that the CAS in monitorenter will invalidate the line
113 //     underlying _owner. We want to avoid an L1 data cache miss on that
114 //     same line for monitorexit. Putting these <remaining_fields>:
115 //     _recursions, _EntryList, _cxq, and _succ, all of which may be
116 //     fetched in the inflated unlock path, on a different cache line
117 //     would make them immune to CAS-based invalidation from the _owner
118 //     field.
119 //
120 //   - The _recursions field should be of type int, or int32_t but not
121 //     intptr_t. There's no reason to use a 64-bit type for this field
122 //     in a 64-bit JVM.
123 
124 #ifndef OM_CACHE_LINE_SIZE
125 // Use DEFAULT_CACHE_LINE_SIZE if not already specified for
126 // the current build platform.
127 #define OM_CACHE_LINE_SIZE DEFAULT_CACHE_LINE_SIZE
128 #endif
129 
130 class ObjectMonitor : public CHeapObj<mtObjectMonitor> {
131   friend class ObjectSynchronizer;
132   friend class ObjectWaiter;
133   friend class VMStructs;
134   JVMCI_ONLY(friend class JVMCIVMStructs;)
135 
136   static OopStorage* _oop_storage;
137 
138   // The sync code expects the header field to be at offset zero (0).
139   // Enforced by the assert() in header_addr().
140   volatile markWord _header;        // displaced object header word - mark
141   WeakHandle _object;               // backward object pointer
142   // Separate _header and _owner on different cache lines since both can
143   // have busy multi-threaded access. _header and _object are set at initial
144   // inflation. The _object does not change, so it is a good choice to share
145   // its cache line with _header.
146   DEFINE_PAD_MINUS_SIZE(0, OM_CACHE_LINE_SIZE, sizeof(volatile markWord) +
147                         sizeof(WeakHandle));
148   // Used by async deflation as a marker in the _owner field.
149   // Note that the choice of the two markers is peculiar:
150   // - They need to represent values that cannot be pointers. In particular,
151   //   we achieve this by using the lowest two bits
152   // - ANONYMOUS_OWNER should be a small value, it is used in generated code
153   //   and small values encode much better
154   // - We test for anonymous owner by testing for the lowest bit, therefore
155   //   DEFLATER_MARKER must *not* have that bit set.
156   #define DEFLATER_MARKER reinterpret_cast<void*>(2)
157   #define ANONYMOUS_OWNER reinterpret_cast<void*>(1)
158   void* volatile _owner;            // pointer to owning thread OR BasicLock
159   volatile jlong _previous_owner_tid;  // thread id of the previous owner of the monitor
160   // Separate _owner and _next_om on different cache lines since
161   // both can have busy multi-threaded access. _previous_owner_tid is only
162   // changed by ObjectMonitor::exit() so it is a good choice to share the
163   // cache line with _owner.
164   DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(void* volatile) +
165                         sizeof(volatile jlong));
166   ObjectMonitor* _next_om;          // Next ObjectMonitor* linkage
167   volatile intx _recursions;        // recursion count, 0 for first entry
168   ObjectWaiter* volatile _EntryList;  // Threads blocked on entry or reentry.
169                                       // The list is actually composed of WaitNodes,
170                                       // acting as proxies for Threads.
171 
172   ObjectWaiter* volatile _cxq;      // LL of recently-arrived threads blocked on entry.
173   JavaThread* volatile _succ;       // Heir presumptive thread - used for futile wakeup throttling
174   JavaThread* volatile _Responsible;
175 
176   volatile int _Spinner;            // for exit->spinner handoff optimization
177   volatile int _SpinDuration;
178 
179   jint  _contentions;               // Number of active contentions in enter(). It is used by is_busy()
180                                     // along with other fields to determine if an ObjectMonitor can be
181                                     // deflated. It is also used by the async deflation protocol. See
182                                     // ObjectMonitor::deflate_monitor().
183  protected:
184   ObjectWaiter* volatile _WaitSet;  // LL of threads wait()ing on the monitor
185   volatile jint  _waiters;          // number of waiting threads
186  private:
187   volatile int _WaitSetLock;        // protects Wait Queue - simple spinlock
188 
189  public:
190   static void Initialize();
191 
192   // Only perform a PerfData operation if the PerfData object has been
193   // allocated and if the PerfDataManager has not freed the PerfData
194   // objects which can happen at normal VM shutdown.
195   //
196   #define OM_PERFDATA_OP(f, op_str)              \
197     do {                                         \
198       if (ObjectMonitor::_sync_ ## f != NULL &&  \
199           PerfDataManager::has_PerfData()) {     \
200         ObjectMonitor::_sync_ ## f->op_str;      \
201       }                                          \
202     } while (0)
203 
204   static PerfCounter * _sync_ContendedLockAttempts;
205   static PerfCounter * _sync_FutileWakeups;
206   static PerfCounter * _sync_Parks;
207   static PerfCounter * _sync_Notifications;
208   static PerfCounter * _sync_Inflations;
209   static PerfCounter * _sync_Deflations;
210   static PerfLongVariable * _sync_MonExtant;
211 
212   static int Knob_SpinLimit;
213 
214   // TODO-FIXME: the "offset" routines should return a type of off_t instead of int ...
215   // ByteSize would also be an appropriate type.
216   static int header_offset_in_bytes()      { return offset_of(ObjectMonitor, _header); }
217   static int object_offset_in_bytes()      { return offset_of(ObjectMonitor, _object); }
218   static int owner_offset_in_bytes()       { return offset_of(ObjectMonitor, _owner); }
219   static int recursions_offset_in_bytes()  { return offset_of(ObjectMonitor, _recursions); }
220   static int cxq_offset_in_bytes()         { return offset_of(ObjectMonitor, _cxq); }
221   static int succ_offset_in_bytes()        { return offset_of(ObjectMonitor, _succ); }
222   static int EntryList_offset_in_bytes()   { return offset_of(ObjectMonitor, _EntryList); }
223 
224   // ObjectMonitor references can be ORed with markWord::monitor_value
225   // as part of the ObjectMonitor tagging mechanism. When we combine an
226   // ObjectMonitor reference with an offset, we need to remove the tag
227   // value in order to generate the proper address.
228   //
229   // We can either adjust the ObjectMonitor reference and then add the
230   // offset or we can adjust the offset that is added to the ObjectMonitor
231   // reference. The latter avoids an AGI (Address Generation Interlock)
232   // stall so the helper macro adjusts the offset value that is returned
233   // to the ObjectMonitor reference manipulation code:
234   //
235   #define OM_OFFSET_NO_MONITOR_VALUE_TAG(f) \
236     ((ObjectMonitor::f ## _offset_in_bytes()) - markWord::monitor_value)
237 
238   markWord           header() const;
239   volatile markWord* header_addr();
240   void               set_header(markWord hdr);
241 
242   bool is_busy() const {
243     // TODO-FIXME: assert _owner == null implies _recursions = 0
244     intptr_t ret_code = _waiters | intptr_t(_cxq) | intptr_t(_EntryList);
245     if (contentions() > 0) {
246       ret_code |= contentions();
247     }
248     if (!owner_is_DEFLATER_MARKER()) {
249       ret_code |= intptr_t(owner_raw());
250     }
251     return ret_code != 0;
252   }
253   const char* is_busy_to_string(stringStream* ss);
254 
255   intptr_t  is_entered(JavaThread* current) const;
256 
257   bool      has_owner() const;
258   void*     owner() const;  // Returns NULL if DEFLATER_MARKER is observed.
259   void*     owner_raw() const;
260   // Returns true if owner field == DEFLATER_MARKER and false otherwise.
261   bool      owner_is_DEFLATER_MARKER() const;
262   // Returns true if 'this' is being async deflated and false otherwise.
263   bool      is_being_async_deflated();
264   // Clear _owner field; current value must match old_value.
265   void      release_clear_owner(void* old_value);
266   // Simply set _owner field to new_value; current value must match old_value.
267   void      set_owner_from(void* old_value, void* new_value);
268   // Simply set _owner field to current; current value must match basic_lock_p.
269   void      set_owner_from_BasicLock(void* basic_lock_p, JavaThread* current);
270   // Try to set _owner field to new_value if the current value matches
271   // old_value, using Atomic::cmpxchg(). Otherwise, does not change the
272   // _owner field. Returns the prior value of the _owner field.
273   void*     try_set_owner_from(void* old_value, void* new_value);
274 
275   void set_owner_anonymous() {
276     set_owner_from(NULL, ANONYMOUS_OWNER);
277   }
278 
279   bool is_owner_anonymous() const {
280     return owner_raw() == ANONYMOUS_OWNER;
281   }
282 
283   void set_owner_from_anonymous(Thread* owner) {
284     set_owner_from(ANONYMOUS_OWNER, owner);
285   }
286 
287   // Simply get _next_om field.
288   ObjectMonitor* next_om() const;
289   // Get _next_om field with acquire semantics.
290   ObjectMonitor* next_om_acquire() const;
291   // Simply set _next_om field to new_value.
292   void set_next_om(ObjectMonitor* new_value);
293   // Set _next_om field to new_value with release semantics.
294   void release_set_next_om(ObjectMonitor* new_value);
295   // Try to set _next_om field to new_value if the current value matches
296   // old_value, using Atomic::cmpxchg(). Otherwise, does not change the
297   // _next_om field. Returns the prior value of the _next_om field.
298   ObjectMonitor* try_set_next_om(ObjectMonitor* old_value, ObjectMonitor* new_value);
299 
300   jint      waiters() const;
301 
302   jint      contentions() const;
303   void      add_to_contentions(jint value);
304   intx      recursions() const                                         { return _recursions; }
305 
306   // JVM/TI GetObjectMonitorUsage() needs this:
307   ObjectWaiter* first_waiter()                                         { return _WaitSet; }
308   ObjectWaiter* next_waiter(ObjectWaiter* o)                           { return o->_next; }
309   JavaThread* thread_of_waiter(ObjectWaiter* o)                        { return o->_thread; }
310 
311   ObjectMonitor(oop object);
312   ~ObjectMonitor();
313 
314   oop       object() const;
315   oop       object_peek() const;
316 
317   // Returns true if the specified thread owns the ObjectMonitor. Otherwise
318   // returns false and throws IllegalMonitorStateException (IMSE).
319   bool      check_owner(TRAPS);
320 
321  private:
322   class ExitOnSuspend {
323    protected:
324     ObjectMonitor* _om;
325     bool _om_exited;
326    public:
327     ExitOnSuspend(ObjectMonitor* om) : _om(om), _om_exited(false) {}
328     void operator()(JavaThread* current);
329     bool exited() { return _om_exited; }
330   };
331   class ClearSuccOnSuspend {
332    protected:
333     ObjectMonitor* _om;
334    public:
335     ClearSuccOnSuspend(ObjectMonitor* om) : _om(om)  {}
336     void operator()(JavaThread* current);
337   };
338  public:
339   bool      enter(JavaThread* current);
340   void      exit(JavaThread* current, bool not_suspended = true);
341   void      wait(jlong millis, bool interruptible, TRAPS);
342   void      notify(TRAPS);
343   void      notifyAll(TRAPS);
344 
345   void      print() const;
346 #ifdef ASSERT
347   void      print_debug_style_on(outputStream* st) const;
348 #endif
349   void      print_on(outputStream* st) const;
350 
351   // Use the following at your own risk
352   intx      complete_exit(JavaThread* current);
353   bool      reenter(intx recursions, JavaThread* current);
354 
355   static void maybe_deflate_dead(oop* p);
356 
357  private:
358   void      AddWaiter(ObjectWaiter* waiter);
359   void      INotify(JavaThread* current);
360   ObjectWaiter* DequeueWaiter();
361   void      DequeueSpecificWaiter(ObjectWaiter* waiter);
362   void      EnterI(JavaThread* current);
363   void      ReenterI(JavaThread* current, ObjectWaiter* current_node);
364   void      UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* current_node);
365   int       TryLock(JavaThread* current);
366   int       NotRunnable(JavaThread* current, JavaThread* Owner);
367   int       TrySpin(JavaThread* current);
368   void      ExitEpilog(JavaThread* current, ObjectWaiter* Wakee);
369 
370   // Deflation support
371   bool      deflate_monitor();
372   void      install_displaced_markword_in_object(const oop obj);
373 };
374 
375 #endif // SHARE_RUNTIME_OBJECTMONITOR_HPP