1 /*
2 * Copyright (c) 1998, 2025, 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,
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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
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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/oopHandle.hpp"
32 #include "oops/weakHandle.hpp"
33 #include "runtime/javaThread.hpp"
34 #include "utilities/checkedCast.hpp"
35 #include "utilities/globalDefinitions.hpp"
36
37 class ObjectMonitor;
38 class ObjectMonitorContentionMark;
39 class ParkEvent;
40 class BasicLock;
41 class ContinuationWrapper;
42
43
44 class ObjectWaiter : public CHeapObj<mtThread> {
45 public:
46 enum TStates : uint8_t { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER };
47 ObjectWaiter* volatile _next;
48 ObjectWaiter* volatile _prev;
49 JavaThread* _thread;
50 OopHandle _vthread;
51 ObjectMonitor* _monitor;
52 uint64_t _notifier_tid;
53 int _recursions;
54 volatile TStates TState;
55 volatile bool _notified;
56 bool _is_wait;
57 bool _at_reenter;
58 bool _interrupted;
59 bool _active; // Contention monitoring is enabled
60 public:
61 ObjectWaiter(JavaThread* current);
62 ObjectWaiter(oop vthread, ObjectMonitor* mon);
63 ~ObjectWaiter();
64 JavaThread* thread() const { return _thread; }
65 bool is_vthread() const { return _thread == nullptr; }
66 uint8_t state() const { return TState; }
67 ObjectMonitor* monitor() const { return _monitor; }
68 bool is_wait() const { return _is_wait; }
69 bool notified() const { return _notified; }
70 bool at_reenter() const { return _at_reenter; }
71 bool at_monitorenter() const { return !_is_wait || _at_reenter || _notified; }
72 oop vthread() const;
73 void wait_reenter_begin(ObjectMonitor *mon);
74 void wait_reenter_end(ObjectMonitor *mon);
75
76 void set_bad_pointers() {
77 #ifdef ASSERT
78 this->_prev = (ObjectWaiter*) badAddressVal;
79 this->_next = (ObjectWaiter*) badAddressVal;
80 this->TState = ObjectWaiter::TS_RUN;
81 #endif
82 }
83 ObjectWaiter* next() {
84 assert (_next != (ObjectWaiter*) badAddressVal, "corrupted list!");
85 return _next;
86 }
87 ObjectWaiter* prev() {
88 assert (_prev != (ObjectWaiter*) badAddressVal, "corrupted list!");
89 return _prev;
90 }
91 };
92
93 // The ObjectMonitor class implements the heavyweight version of a
94 // JavaMonitor. The lightweight BasicLock/stack lock version has been
95 // inflated into an ObjectMonitor. This inflation is typically due to
96 // contention or use of Object.wait().
97 //
98 // WARNING: This is a very sensitive and fragile class. DO NOT make any
99 // changes unless you are fully aware of the underlying semantics.
100 //
101 // ObjectMonitor Layout Overview/Highlights/Restrictions:
102 //
103 // - For performance reasons we ensure the _metadata field is located at offset 0,
104 // which in turn means that ObjectMonitor can't inherit from any other class nor use
105 // any virtual member functions.
106 // - The _metadata and _owner fields should be separated by enough space
107 // to avoid false sharing due to parallel access by different threads.
108 // This is an advisory recommendation.
109 // - The general layout of the fields in ObjectMonitor is:
110 // _metadata
111 // <lightly_used_fields>
112 // <optional padding>
113 // _owner
114 // <optional padding>
115 // <remaining_fields>
116 // - The VM assumes write ordering and machine word alignment with
117 // respect to the _owner field and the <remaining_fields> that can
118 // be read in parallel by other threads.
119 // - Generally fields that are accessed closely together in time should
120 // be placed proximally in space to promote data cache locality. That
121 // is, temporal locality should condition spatial locality.
122 // - We have to balance avoiding false sharing with excessive invalidation
123 // from coherence traffic. As such, we try to cluster fields that tend
124 // to be _written_ at approximately the same time onto the same data
125 // cache line.
126 // - We also have to balance the natural tension between minimizing
127 // single threaded capacity misses with excessive multi-threaded
128 // coherency misses. There is no single optimal layout for both
129 // single-threaded and multi-threaded environments.
130 //
131 // - See TEST_VM(ObjectMonitor, sanity) gtest for how critical restrictions are
132 // enforced.
133 //
134 // - Separating _owner from the <remaining_fields> by enough space to
135 // avoid false sharing might be profitable. Given that the CAS in
136 // monitorenter will invalidate the line underlying _owner. We want
137 // to avoid an L1 data cache miss on that same line for monitorexit.
138 // Putting these <remaining_fields>:
139 // _recursions, _entry_list and _succ, all of which may be
140 // fetched in the inflated unlock path, on a different cache line
141 // would make them immune to CAS-based invalidation from the _owner
142 // field.
143 //
144 // - TODO: The _recursions field should be of type int, or int32_t but not
145 // intptr_t. There's no reason to use a 64-bit type for this field
146 // in a 64-bit JVM.
147
148 #define OM_CACHE_LINE_SIZE DEFAULT_CACHE_LINE_SIZE
149
150 class ObjectMonitor : public CHeapObj<mtObjectMonitor> {
151 friend class VMStructs;
152 JVMCI_ONLY(friend class JVMCIVMStructs;)
153
154 static OopStorage* _oop_storage;
155
156 // List of j.l.VirtualThread waiting to be unblocked by unblocker thread.
157 static OopHandle _vthread_list_head;
158 // ParkEvent of unblocker thread.
159 static ParkEvent* _vthread_unparker_ParkEvent;
160
161 // Because of frequent access, the metadata field is at offset zero (0).
162 // Enforced by the assert() in metadata_addr().
163 // * Lightweight locking with UseObjectMonitorTable:
164 // Contains the _object's hashCode.
165 // * * Lightweight locking without UseObjectMonitorTable:
166 // Contains the displaced object header word - mark
167 volatile uintptr_t _metadata; // metadata
168 WeakHandle _object; // backward object pointer
169 // Separate _metadata and _owner on different cache lines since both can
170 // have busy multi-threaded access. _metadata and _object are set at initial
171 // inflation. The _object does not change, so it is a good choice to share
172 // its cache line with _metadata.
173 DEFINE_PAD_MINUS_SIZE(0, OM_CACHE_LINE_SIZE, sizeof(_metadata) +
174 sizeof(WeakHandle));
175
176 static const int64_t NO_OWNER = 0;
177 static const int64_t ANONYMOUS_OWNER = 1;
178 static const int64_t DEFLATER_MARKER = 2;
179
180 int64_t volatile _owner; // Either owner_id of owner, NO_OWNER, ANONYMOUS_OWNER or DEFLATER_MARKER.
181 volatile uint64_t _previous_owner_tid; // thread id of the previous owner of the monitor
182 // Separate _owner and _next_om on different cache lines since
183 // both can have busy multi-threaded access. _previous_owner_tid is only
184 // changed by ObjectMonitor::exit() so it is a good choice to share the
185 // cache line with _owner.
186 DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(void* volatile) +
187 sizeof(volatile uint64_t));
188 ObjectMonitor* _next_om; // Next ObjectMonitor* linkage
189 volatile intx _recursions; // recursion count, 0 for first entry
190 ObjectWaiter* volatile _entry_list; // Threads blocked on entry or reentry.
191 // The list is actually composed of wait-nodes,
192 // acting as proxies for Threads.
193 ObjectWaiter* volatile _entry_list_tail; // _entry_list is the head, this is the tail.
194 int64_t volatile _succ; // Heir presumptive thread - used for futile wakeup throttling
195
196 volatile int _SpinDuration;
197
198 int _contentions; // Number of active contentions in enter(). It is used by is_busy()
199 // along with other fields to determine if an ObjectMonitor can be
200 // deflated. It is also used by the async deflation protocol. See
201 // ObjectMonitor::deflate_monitor().
202
203 ObjectWaiter* volatile _wait_set; // LL of threads waiting on the monitor - wait()
204 volatile int _waiters; // number of waiting threads
205 volatile int _wait_set_lock; // protects wait set queue - simple spinlock
206
207 public:
208
209 static void Initialize();
210 static void Initialize2();
211
212 static OopHandle& vthread_list_head() { return _vthread_list_head; }
213 static ParkEvent* vthread_unparker_ParkEvent() { return _vthread_unparker_ParkEvent; }
214
215 static int Knob_SpinLimit;
216
217 static ByteSize metadata_offset() { return byte_offset_of(ObjectMonitor, _metadata); }
218 static ByteSize owner_offset() { return byte_offset_of(ObjectMonitor, _owner); }
219 static ByteSize recursions_offset() { return byte_offset_of(ObjectMonitor, _recursions); }
220 static ByteSize succ_offset() { return byte_offset_of(ObjectMonitor, _succ); }
221 static ByteSize entry_list_offset() { return byte_offset_of(ObjectMonitor, _entry_list); }
222
223 // ObjectMonitor references can be ORed with markWord::monitor_value
224 // as part of the ObjectMonitor tagging mechanism. When we combine an
225 // ObjectMonitor reference with an offset, we need to remove the tag
226 // value in order to generate the proper address.
227 //
228 // We can either adjust the ObjectMonitor reference and then add the
229 // offset or we can adjust the offset that is added to the ObjectMonitor
230 // reference. The latter avoids an AGI (Address Generation Interlock)
231 // stall so the helper macro adjusts the offset value that is returned
232 // to the ObjectMonitor reference manipulation code:
233 //
234 #define OM_OFFSET_NO_MONITOR_VALUE_TAG(f) \
235 ((in_bytes(ObjectMonitor::f ## _offset())) - checked_cast<int>(markWord::monitor_value))
236
237 uintptr_t metadata() const;
238 void set_metadata(uintptr_t value);
239 volatile uintptr_t* metadata_addr();
240
241 markWord header() const;
242 void set_header(markWord hdr);
243
244 intptr_t hash() const;
245 void set_hash(intptr_t hash);
246
247 bool is_busy() const {
248 // TODO-FIXME: assert _owner == NO_OWNER implies _recursions = 0
249 intptr_t ret_code = intptr_t(_waiters) | intptr_t(_entry_list);
250 int cnts = contentions(); // read once
251 if (cnts > 0) {
252 ret_code |= intptr_t(cnts);
253 }
254 if (!owner_is_DEFLATER_MARKER()) {
255 ret_code |= intptr_t(owner_raw());
256 }
257 return ret_code != 0;
258 }
259 const char* is_busy_to_string(stringStream* ss);
260
261 bool is_entered(JavaThread* current) const;
262
263 // Returns true if this OM has an owner, false otherwise.
264 bool has_owner() const;
265 int64_t owner() const; // Returns NO_OWNER if DEFLATER_MARKER is observed.
266 int64_t owner_raw() const;
267
268 // These methods return the value we set in _owner when acquiring
269 // the monitor with the given thread/vthread, AKA owner_id.
270 static int64_t owner_id_from(JavaThread* thread);
271 static int64_t owner_id_from(oop vthread);
272
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 thread's owner_id.
278 void release_clear_owner(JavaThread* thread);
279 // Simply set _owner field to new_value; current value must match old_value.
280 void set_owner_from_raw(int64_t old_value, int64_t new_value);
281 // Same as above but uses owner_id of current as new value.
282 void set_owner_from(int64_t old_value, JavaThread* current);
283 // Try to set _owner field to new_value if the current value matches
284 // old_value, using AtomicAccess::cmpxchg(). Otherwise, does not change the
285 // _owner field. Returns the prior value of the _owner field.
286 int64_t try_set_owner_from_raw(int64_t old_value, int64_t new_value);
287 // Same as above but uses owner_id of current as new_value.
288 int64_t try_set_owner_from(int64_t old_value, JavaThread* current);
289
290 // Methods to check and set _succ. The successor is the thread selected
291 // from _entry_list by the current owner when releasing the monitor,
292 // to run again and re-try acquiring the monitor. It is used to avoid
293 // unnecessary wake-ups if there is already a successor set.
294 bool has_successor() const;
295 bool has_successor(JavaThread* thread) const;
296 void set_successor(JavaThread* thread);
297 void set_successor(oop vthread);
298 void clear_successor();
299 int64_t successor() const;
300
301 // Returns true if _owner field == owner_id of thread, false otherwise.
302 bool has_owner(JavaThread* thread) const { return owner() == owner_id_from(thread); }
303 // Set _owner field to owner_id of thread; current value must be NO_OWNER.
304 void set_owner(JavaThread* thread) { set_owner_from(NO_OWNER, thread); }
305 // Try to set _owner field from NO_OWNER to owner_id of thread.
306 bool try_set_owner(JavaThread* thread) {
307 return try_set_owner_from(NO_OWNER, thread) == NO_OWNER;
308 }
309
310 bool has_anonymous_owner() const { return owner_raw() == ANONYMOUS_OWNER; }
311 void set_anonymous_owner() {
312 set_owner_from_raw(NO_OWNER, ANONYMOUS_OWNER);
313 }
314 void set_owner_from_anonymous(JavaThread* owner) {
315 set_owner_from(ANONYMOUS_OWNER, owner);
316 }
317
318 // Simply get _next_om field.
319 ObjectMonitor* next_om() const;
320 // Simply set _next_om field to new_value.
321 void set_next_om(ObjectMonitor* new_value);
322
323 int contentions() const;
324 void add_to_contentions(int value);
325 intx recursions() const { return _recursions; }
326 void set_recursions(size_t recursions);
327 void increment_recursions(JavaThread* current);
328
329 // JVM/TI GetObjectMonitorUsage() needs this:
330 int waiters() const;
331 ObjectWaiter* first_waiter() { return _wait_set; }
332 ObjectWaiter* next_waiter(ObjectWaiter* o) { return o->_next; }
333 JavaThread* thread_of_waiter(ObjectWaiter* o) { return o->_thread; }
334
335 ObjectMonitor(oop object);
336 ~ObjectMonitor();
337
338 oop object() const;
339 oop object_peek() const;
340 bool object_is_dead() const;
341 bool object_refers_to(oop obj) const;
342
343 // Returns true if the specified thread owns the ObjectMonitor. Otherwise
344 // returns false and throws IllegalMonitorStateException (IMSE).
345 bool check_owner(TRAPS);
346
347 private:
348 class ExitOnSuspend {
349 protected:
350 ObjectMonitor* _om;
351 bool _om_exited;
352 public:
353 ExitOnSuspend(ObjectMonitor* om) : _om(om), _om_exited(false) {}
354 void operator()(JavaThread* current);
355 bool exited() { return _om_exited; }
356 };
357 class ClearSuccOnSuspend {
358 protected:
359 ObjectMonitor* _om;
360 public:
361 ClearSuccOnSuspend(ObjectMonitor* om) : _om(om) {}
362 void operator()(JavaThread* current);
363 };
364
365 bool enter_is_async_deflating();
366 void notify_contended_enter(JavaThread *current);
367 public:
368 void enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark);
369 bool enter_for(JavaThread* locking_thread);
370 bool enter(JavaThread* current);
371 bool try_enter(JavaThread* current, bool check_for_recursion = true);
372 bool spin_enter(JavaThread* current);
373 void enter_with_contention_mark(JavaThread* current, ObjectMonitorContentionMark& contention_mark);
374 void exit(JavaThread* current, bool not_suspended = true);
375 bool resume_operation(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont);
376 void wait(jlong millis, bool interruptible, TRAPS);
377 void notify(TRAPS);
378 void notifyAll(TRAPS);
379 void quick_notify(JavaThread* current);
380 void quick_notifyAll(JavaThread* current);
381
382 void print() const;
383 #ifdef ASSERT
384 void print_debug_style_on(outputStream* st) const;
385 #endif
386 void print_on(outputStream* st) const;
387
388 // Use the following at your own risk
389 intx complete_exit(JavaThread* current);
390
391 private:
392 void add_to_entry_list(JavaThread* current, ObjectWaiter* node);
393 void add_waiter(ObjectWaiter* waiter);
394 bool notify_internal(JavaThread* current);
395 ObjectWaiter* dequeue_waiter();
396 void dequeue_specific_waiter(ObjectWaiter* waiter);
397 void enter_internal(JavaThread* current);
398 void reenter_internal(JavaThread* current, ObjectWaiter* current_node);
399 void entry_list_build_dll(JavaThread* current);
400 void unlink_after_acquire(JavaThread* current, ObjectWaiter* current_node);
401 ObjectWaiter* entry_list_tail(JavaThread* current);
402
403 bool vthread_monitor_enter(JavaThread* current, ObjectWaiter* node = nullptr);
404 void vthread_wait(JavaThread* current, jlong millis);
405 bool vthread_wait_reenter(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont);
406 void vthread_epilog(JavaThread* current, ObjectWaiter* node);
407
408 enum class TryLockResult { Interference = -1, HasOwner = 0, Success = 1 };
409
410 bool try_lock_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark);
411 bool try_lock_or_add_to_entry_list(JavaThread* current, ObjectWaiter* node);
412 TryLockResult try_lock(JavaThread* current);
413
414 bool try_spin(JavaThread* current);
415 bool short_fixed_spin(JavaThread* current, int spin_count, bool adapt);
416 void exit_epilog(JavaThread* current, ObjectWaiter* Wakee);
417
418 public:
419 // Deflation support
420 bool deflate_monitor(Thread* current);
421 void install_displaced_markword_in_object(const oop obj);
422
423 // JFR support
424 static bool is_jfr_excluded(const Klass* monitor_klass);
425 };
426
427 // RAII object to ensure that ObjectMonitor::is_being_async_deflated() is
428 // stable within the context of this mark.
429 class ObjectMonitorContentionMark : StackObj {
430 DEBUG_ONLY(friend class ObjectMonitor;)
431
432 ObjectMonitor* _monitor;
433 bool _extended;
434
435 NONCOPYABLE(ObjectMonitorContentionMark);
436
437 public:
438 explicit ObjectMonitorContentionMark(ObjectMonitor* monitor);
439 ~ObjectMonitorContentionMark();
440
441 // Extends the contention scope beyond this objects lifetime.
442 // Requires manual decrement of the contentions counter.
443 void extend();
444 };
445
446 #endif // SHARE_RUNTIME_OBJECTMONITOR_HPP