1 /*
2 * Copyright (c) 1998, 2024, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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5 * This code is free software; you can redistribute it and/or modify it
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7 * published by the Free Software Foundation.
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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 *
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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/perfDataTypes.hpp"
33 #include "utilities/checkedCast.hpp"
34
35 class ObjectMonitor;
36 class ObjectMonitorContentionMark;
37 class ParkEvent;
38
39 // ObjectWaiter serves as a "proxy" or surrogate thread.
40 // TODO-FIXME: Eliminate ObjectWaiter and use the thread-specific
41 // ParkEvent instead. Beware, however, that the JVMTI code
42 // knows about ObjectWaiters, so we'll have to reconcile that code.
43 // See next_waiter(), first_waiter(), etc.
44
45 class ObjectWaiter : public StackObj {
46 public:
47 enum TStates { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER, TS_CXQ };
48 ObjectWaiter* volatile _next;
49 ObjectWaiter* volatile _prev;
50 JavaThread* _thread;
51 uint64_t _notifier_tid;
52 ParkEvent * _event;
53 volatile int _notified;
54 volatile TStates TState;
55 bool _active; // Contention monitoring is enabled
56 public:
57 ObjectWaiter(JavaThread* current);
58
59 void wait_reenter_begin(ObjectMonitor *mon);
60 void wait_reenter_end(ObjectMonitor *mon);
61 };
62
63 // The ObjectMonitor class implements the heavyweight version of a
64 // JavaMonitor. The lightweight BasicLock/stack lock version has been
65 // inflated into an ObjectMonitor. This inflation is typically due to
66 // contention or use of Object.wait().
67 //
68 // WARNING: This is a very sensitive and fragile class. DO NOT make any
69 // changes unless you are fully aware of the underlying semantics.
70 //
71 // ObjectMonitor Layout Overview/Highlights/Restrictions:
72 //
73 // - The _metadata field must be at offset 0 because the displaced header
74 // from markWord is stored there. We do not want markWord.hpp to include
75 // ObjectMonitor.hpp to avoid exposing ObjectMonitor everywhere. This
76 // means that ObjectMonitor cannot inherit from any other class nor can
77 // it use any virtual member functions. This restriction is critical to
78 // the proper functioning of the VM.
79 // - The _metadata and _owner fields should be separated by enough space
80 // to avoid false sharing due to parallel access by different threads.
81 // This is an advisory recommendation.
82 // - The general layout of the fields in ObjectMonitor is:
83 // _metadata
84 // <lightly_used_fields>
85 // <optional padding>
86 // _owner
87 // <optional padding>
88 // <remaining_fields>
89 // - The VM assumes write ordering and machine word alignment with
90 // respect to the _owner field and the <remaining_fields> that can
91 // be read in parallel by other threads.
92 // - Generally fields that are accessed closely together in time should
93 // be placed proximally in space to promote data cache locality. That
94 // is, temporal locality should condition spatial locality.
95 // - We have to balance avoiding false sharing with excessive invalidation
96 // from coherence traffic. As such, we try to cluster fields that tend
97 // to be _written_ at approximately the same time onto the same data
98 // cache line.
99 // - We also have to balance the natural tension between minimizing
100 // single threaded capacity misses with excessive multi-threaded
101 // coherency misses. There is no single optimal layout for both
102 // single-threaded and multi-threaded environments.
103 //
104 // - See TEST_VM(ObjectMonitor, sanity) gtest for how critical restrictions are
105 // enforced.
106 // - Adjacent ObjectMonitors should be separated by enough space to avoid
107 // false sharing. This is handled by the ObjectMonitor allocation code
108 // in synchronizer.cpp. Also see TEST_VM(SynchronizerTest, sanity) gtest.
109 //
110 // Futures notes:
111 // - Separating _owner from the <remaining_fields> by enough space to
112 // avoid false sharing might be profitable. Given that the CAS in
113 // monitorenter will invalidate the line underlying _owner. We want
114 // to avoid an L1 data cache miss on that same line for monitorexit.
115 // Putting these <remaining_fields>:
116 // _recursions, _EntryList, _cxq, and _succ, all of which may be
117 // fetched in the inflated unlock path, on a different cache line
118 // would make them immune to CAS-based invalidation from the _owner
119 // field.
120 //
121 // - The _recursions field should be of type int, or int32_t but not
122 // intptr_t. There's no reason to use a 64-bit type for this field
123 // in a 64-bit JVM.
124
125 #define OM_CACHE_LINE_SIZE DEFAULT_CACHE_LINE_SIZE
126
127 class ObjectMonitor : public CHeapObj<mtObjectMonitor> {
128 friend class ObjectSynchronizer;
129 friend class ObjectWaiter;
130 friend class VMStructs;
131 JVMCI_ONLY(friend class JVMCIVMStructs;)
132
133 static OopStorage* _oop_storage;
134
135 // The sync code expects the metadata field to be at offset zero (0).
136 // Enforced by the assert() in metadata_addr().
137 // * LM_LIGHTWEIGHT with UseObjectMonitorTable:
138 // Contains the _object's hashCode.
139 // * LM_LEGACY, LM_MONITOR, LM_LIGHTWEIGHT without UseObjectMonitorTable:
140 // Contains the displaced object header word - mark
141 volatile uintptr_t _metadata; // metadata
142 WeakHandle _object; // backward object pointer
143 // Separate _metadata and _owner on different cache lines since both can
144 // have busy multi-threaded access. _metadata and _object are set at initial
145 // inflation. The _object does not change, so it is a good choice to share
146 // its cache line with _metadata.
147 DEFINE_PAD_MINUS_SIZE(0, OM_CACHE_LINE_SIZE, sizeof(_metadata) +
148 sizeof(WeakHandle));
149 // Used by async deflation as a marker in the _owner field.
150 // Note that the choice of the two markers is peculiar:
151 // - They need to represent values that cannot be pointers. In particular,
152 // we achieve this by using the lowest two bits.
153 // - ANONYMOUS_OWNER should be a small value, it is used in generated code
154 // and small values encode much better.
155 // - We test for anonymous owner by testing for the lowest bit, therefore
156 // DEFLATER_MARKER must *not* have that bit set.
157 static const uintptr_t DEFLATER_MARKER_VALUE = 2;
158 #define DEFLATER_MARKER reinterpret_cast<void*>(DEFLATER_MARKER_VALUE)
159 public:
160 // NOTE: Typed as uintptr_t so that we can pick it up in SA, via vmStructs.
161 static const uintptr_t ANONYMOUS_OWNER = 1;
162
163 private:
164 static void* anon_owner_ptr() { return reinterpret_cast<void*>(ANONYMOUS_OWNER); }
165
166 void* volatile _owner; // pointer to owning thread OR BasicLock
167 volatile uint64_t _previous_owner_tid; // thread id of the previous owner of the monitor
168 // Separate _owner and _next_om on different cache lines since
169 // both can have busy multi-threaded access. _previous_owner_tid is only
170 // changed by ObjectMonitor::exit() so it is a good choice to share the
171 // cache line with _owner.
172 DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(void* volatile) +
173 sizeof(volatile uint64_t));
174 ObjectMonitor* _next_om; // Next ObjectMonitor* linkage
175 volatile intx _recursions; // recursion count, 0 for first entry
176 ObjectWaiter* volatile _EntryList; // Threads blocked on entry or reentry.
177 // The list is actually composed of WaitNodes,
178 // acting as proxies for Threads.
179
180 ObjectWaiter* volatile _cxq; // LL of recently-arrived threads blocked on entry.
181 JavaThread* volatile _succ; // Heir presumptive thread - used for futile wakeup throttling
182 JavaThread* volatile _Responsible;
183
184 volatile int _SpinDuration;
185
186 int _contentions; // Number of active contentions in enter(). It is used by is_busy()
187 // along with other fields to determine if an ObjectMonitor can be
188 // deflated. It is also used by the async deflation protocol. See
189 // ObjectMonitor::deflate_monitor().
190
191 ObjectWaiter* volatile _WaitSet; // LL of threads wait()ing on the monitor
192 volatile int _waiters; // number of waiting threads
193 private:
194 volatile int _WaitSetLock; // protects Wait Queue - simple spinlock
195
196 public:
197
198 static void Initialize();
199
200 // Only perform a PerfData operation if the PerfData object has been
201 // allocated and if the PerfDataManager has not freed the PerfData
202 // objects which can happen at normal VM shutdown.
203 //
204 #define OM_PERFDATA_OP(f, op_str) \
205 do { \
206 if (ObjectMonitor::_sync_ ## f != nullptr && \
207 PerfDataManager::has_PerfData()) { \
208 ObjectMonitor::_sync_ ## f->op_str; \
209 } \
210 } while (0)
211
212 static PerfCounter * _sync_ContendedLockAttempts;
213 static PerfCounter * _sync_FutileWakeups;
214 static PerfCounter * _sync_Parks;
215 static PerfCounter * _sync_Notifications;
216 static PerfCounter * _sync_Inflations;
217 static PerfCounter * _sync_Deflations;
218 static PerfLongVariable * _sync_MonExtant;
219
220 static int Knob_SpinLimit;
221
222 static ByteSize metadata_offset() { return byte_offset_of(ObjectMonitor, _metadata); }
223 static ByteSize owner_offset() { return byte_offset_of(ObjectMonitor, _owner); }
224 static ByteSize recursions_offset() { return byte_offset_of(ObjectMonitor, _recursions); }
225 static ByteSize cxq_offset() { return byte_offset_of(ObjectMonitor, _cxq); }
226 static ByteSize succ_offset() { return byte_offset_of(ObjectMonitor, _succ); }
227 static ByteSize EntryList_offset() { return byte_offset_of(ObjectMonitor, _EntryList); }
228
229 // ObjectMonitor references can be ORed with markWord::monitor_value
230 // as part of the ObjectMonitor tagging mechanism. When we combine an
231 // ObjectMonitor reference with an offset, we need to remove the tag
232 // value in order to generate the proper address.
233 //
234 // We can either adjust the ObjectMonitor reference and then add the
235 // offset or we can adjust the offset that is added to the ObjectMonitor
236 // reference. The latter avoids an AGI (Address Generation Interlock)
237 // stall so the helper macro adjusts the offset value that is returned
238 // to the ObjectMonitor reference manipulation code:
239 //
240 #define OM_OFFSET_NO_MONITOR_VALUE_TAG(f) \
241 ((in_bytes(ObjectMonitor::f ## _offset())) - checked_cast<int>(markWord::monitor_value))
242
243 uintptr_t metadata() const;
244 void set_metadata(uintptr_t value);
245 volatile uintptr_t* metadata_addr();
246
247 markWord header() const;
248 void set_header(markWord hdr);
249
250 intptr_t hash() const;
251 void set_hash(intptr_t hash);
252
253 bool is_busy() const {
254 // TODO-FIXME: assert _owner == null implies _recursions = 0
255 intptr_t ret_code = intptr_t(_waiters) | intptr_t(_cxq) | intptr_t(_EntryList);
256 int cnts = contentions(); // read once
257 if (cnts > 0) {
258 ret_code |= intptr_t(cnts);
259 }
260 if (!owner_is_DEFLATER_MARKER()) {
261 ret_code |= intptr_t(owner_raw());
262 }
263 return ret_code != 0;
264 }
265 const char* is_busy_to_string(stringStream* ss);
266
267 bool is_entered(JavaThread* current) const;
268
269 // Returns true if this OM has an owner, false otherwise.
270 bool has_owner() const;
271 void* owner() const; // Returns null if DEFLATER_MARKER is observed.
272 void* owner_raw() const;
273 // Returns true if owner field == DEFLATER_MARKER and false otherwise.
274 bool owner_is_DEFLATER_MARKER() const;
275 // Returns true if 'this' is being async deflated and false otherwise.
276 bool is_being_async_deflated();
277 // Clear _owner field; current value must match old_value.
278 void release_clear_owner(void* old_value);
279 // Simply set _owner field to new_value; current value must match old_value.
280 void set_owner_from(void* old_value, void* new_value);
281 // Simply set _owner field to current; current value must match basic_lock_p.
282 void set_owner_from_BasicLock(void* basic_lock_p, JavaThread* current);
283 // Try to set _owner field to new_value if the current value matches
284 // old_value, using Atomic::cmpxchg(). Otherwise, does not change the
285 // _owner field. Returns the prior value of the _owner field.
286 void* try_set_owner_from(void* old_value, void* new_value);
287
288 void set_owner_anonymous() {
289 set_owner_from(nullptr, anon_owner_ptr());
290 }
291
292 bool is_owner_anonymous() const {
293 return owner_raw() == anon_owner_ptr();
294 }
295
296 void set_owner_from_anonymous(Thread* owner) {
297 set_owner_from(anon_owner_ptr(), owner);
298 }
299
300 // Simply get _next_om field.
301 ObjectMonitor* next_om() const;
302 // Simply set _next_om field to new_value.
303 void set_next_om(ObjectMonitor* new_value);
304
305 int waiters() const;
306
307 int contentions() const;
308 void add_to_contentions(int value);
309 intx recursions() const { return _recursions; }
310 void set_recursions(size_t recursions);
311
312 // JVM/TI GetObjectMonitorUsage() needs this:
313 ObjectWaiter* first_waiter() { return _WaitSet; }
314 ObjectWaiter* next_waiter(ObjectWaiter* o) { return o->_next; }
315 JavaThread* thread_of_waiter(ObjectWaiter* o) { return o->_thread; }
316
317 ObjectMonitor(oop object);
318 ~ObjectMonitor();
319
320 oop object() const;
321 oop object_peek() const;
322 bool object_is_cleared() const;
323 bool object_is_dead() const;
324 bool object_refers_to(oop obj) const;
325
326 // Returns true if the specified thread owns the ObjectMonitor. Otherwise
327 // returns false and throws IllegalMonitorStateException (IMSE).
328 bool check_owner(TRAPS);
329
330 private:
331 class ExitOnSuspend {
332 protected:
333 ObjectMonitor* _om;
334 bool _om_exited;
335 public:
336 ExitOnSuspend(ObjectMonitor* om) : _om(om), _om_exited(false) {}
337 void operator()(JavaThread* current);
338 bool exited() { return _om_exited; }
339 };
340 class ClearSuccOnSuspend {
341 protected:
342 ObjectMonitor* _om;
343 public:
344 ClearSuccOnSuspend(ObjectMonitor* om) : _om(om) {}
345 void operator()(JavaThread* current);
346 };
347
348 bool enter_is_async_deflating();
349 public:
350 void enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark);
351 bool enter_for(JavaThread* locking_thread);
352 bool enter(JavaThread* current);
353 bool try_enter(JavaThread* current);
354 bool spin_enter(JavaThread* current);
355 void enter_with_contention_mark(JavaThread* current, ObjectMonitorContentionMark& contention_mark);
356 void exit(JavaThread* current, bool not_suspended = true);
357 void wait(jlong millis, bool interruptible, TRAPS);
358 void notify(TRAPS);
359 void notifyAll(TRAPS);
360
361 void print() const;
362 #ifdef ASSERT
363 void print_debug_style_on(outputStream* st) const;
364 #endif
365 void print_on(outputStream* st) const;
366
367 // Use the following at your own risk
368 intx complete_exit(JavaThread* current);
369
370 private:
371 void AddWaiter(ObjectWaiter* waiter);
372 void INotify(JavaThread* current);
373 ObjectWaiter* DequeueWaiter();
374 void DequeueSpecificWaiter(ObjectWaiter* waiter);
375 void EnterI(JavaThread* current);
376 void ReenterI(JavaThread* current, ObjectWaiter* current_node);
377 void UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* current_node);
378
379
380 enum class TryLockResult { Interference = -1, HasOwner = 0, Success = 1 };
381
382 TryLockResult TryLock(JavaThread* current);
383
384 bool TrySpin(JavaThread* current);
385 bool short_fixed_spin(JavaThread* current, int spin_count, bool adapt);
386 void ExitEpilog(JavaThread* current, ObjectWaiter* Wakee);
387
388 // Deflation support
389 bool deflate_monitor(Thread* current);
390 private:
391 void install_displaced_markword_in_object(const oop obj);
392 };
393
394 // RAII object to ensure that ObjectMonitor::is_being_async_deflated() is
395 // stable within the context of this mark.
396 class ObjectMonitorContentionMark : StackObj {
397 DEBUG_ONLY(friend class ObjectMonitor;)
398
399 ObjectMonitor* _monitor;
400
401 NONCOPYABLE(ObjectMonitorContentionMark);
402
403 public:
404 explicit ObjectMonitorContentionMark(ObjectMonitor* monitor);
405 ~ObjectMonitorContentionMark();
406 };
407
408 #endif // SHARE_RUNTIME_OBJECTMONITOR_HPP