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
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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 bool _is_wait;
56 bool _at_reenter;
57 bool _interrupted;
58 bool _do_timed_park;
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 at_reenter() const { return _at_reenter; }
70 bool at_monitorenter() const { return !_is_wait || TState != TS_WAIT; }
71 oop vthread() const;
72 void wait_reenter_begin(ObjectMonitor *mon);
73 void wait_reenter_end(ObjectMonitor *mon);
74
75 void set_bad_pointers() {
76 #ifdef ASSERT
77 this->_prev = (ObjectWaiter*) badAddressVal;
78 this->_next = (ObjectWaiter*) badAddressVal;
79 this->TState = ObjectWaiter::TS_RUN;
80 #endif
81 }
82 ObjectWaiter* next() {
83 assert (_next != (ObjectWaiter*) badAddressVal, "corrupted list!");
84 return _next;
85 }
86 ObjectWaiter* prev() {
87 assert (_prev != (ObjectWaiter*) badAddressVal, "corrupted list!");
88 return _prev;
89 }
90 };
91
92 // The ObjectMonitor class implements the heavyweight version of a
93 // JavaMonitor. The lightweight BasicLock/stack lock version has been
94 // inflated into an ObjectMonitor. This inflation is typically due to
95 // contention or use of Object.wait().
96 //
97 // WARNING: This is a very sensitive and fragile class. DO NOT make any
98 // changes unless you are fully aware of the underlying semantics.
99 //
100 // ObjectMonitor Layout Overview/Highlights/Restrictions:
101 //
102 // - For performance reasons we ensure the _metadata field is located at offset 0,
103 // which in turn means that ObjectMonitor can't inherit from any other class nor use
104 // any virtual member functions.
105 // - The _metadata and _owner fields should be separated by enough space
106 // to avoid false sharing due to parallel access by different threads.
107 // This is an advisory recommendation.
108 // - The general layout of the fields in ObjectMonitor is:
109 // _metadata
110 // <lightly_used_fields>
111 // <optional padding>
112 // _owner
113 // <optional padding>
114 // <remaining_fields>
115 // - The VM assumes write ordering and machine word alignment with
116 // respect to the _owner field and the <remaining_fields> that can
117 // be read in parallel by other threads.
118 // - Generally fields that are accessed closely together in time should
119 // be placed proximally in space to promote data cache locality. That
120 // is, temporal locality should condition spatial locality.
121 // - We have to balance avoiding false sharing with excessive invalidation
122 // from coherence traffic. As such, we try to cluster fields that tend
123 // to be _written_ at approximately the same time onto the same data
124 // cache line.
125 // - We also have to balance the natural tension between minimizing
126 // single threaded capacity misses with excessive multi-threaded
127 // coherency misses. There is no single optimal layout for both
128 // single-threaded and multi-threaded environments.
129 //
130 // - See TEST_VM(ObjectMonitor, sanity) gtest for how critical restrictions are
131 // enforced.
132 //
133 // - Separating _owner from the <remaining_fields> by enough space to
134 // avoid false sharing might be profitable. Given that the CAS in
135 // monitorenter will invalidate the line underlying _owner. We want
136 // to avoid an L1 data cache miss on that same line for monitorexit.
137 // Putting these <remaining_fields>:
138 // _recursions, _entry_list and _succ, all of which may be
139 // fetched in the inflated unlock path, on a different cache line
140 // would make them immune to CAS-based invalidation from the _owner
141 // field.
142 //
143 // - TODO: The _recursions field should be of type int, or int32_t but not
144 // intptr_t. There's no reason to use a 64-bit type for this field
145 // in a 64-bit JVM.
146
147 #define OM_CACHE_LINE_SIZE DEFAULT_CACHE_LINE_SIZE
148
149 class ObjectMonitor : public CHeapObj<mtObjectMonitor> {
150 friend class VMStructs;
151 JVMCI_ONLY(friend class JVMCIVMStructs;)
152
153 static OopStorage* _oop_storage;
154
155 // List of j.l.VirtualThread waiting to be unblocked by unblocker thread.
156 static OopHandle _vthread_list_head;
157 // ParkEvent of unblocker thread.
158 static ParkEvent* _vthread_unparker_ParkEvent;
159
160 // Because of frequent access, the metadata field is at offset zero (0).
161 // Enforced by the assert() in metadata_addr().
162 // * Lightweight locking with UseObjectMonitorTable:
163 // Contains the _object's hashCode.
164 // * * Lightweight locking without UseObjectMonitorTable:
165 // Contains the displaced object header word - mark
166 volatile uintptr_t _metadata; // metadata
167 WeakHandle _object; // backward object pointer
168 // Separate _metadata and _owner on different cache lines since both can
169 // have busy multi-threaded access. _metadata and _object are set at initial
170 // inflation. The _object does not change, so it is a good choice to share
171 // its cache line with _metadata.
172 DEFINE_PAD_MINUS_SIZE(0, OM_CACHE_LINE_SIZE, sizeof(_metadata) +
173 sizeof(WeakHandle));
174
175 static const int64_t NO_OWNER = 0;
176 static const int64_t ANONYMOUS_OWNER = 1;
177 static const int64_t DEFLATER_MARKER = 2;
178
179 int64_t volatile _owner; // Either owner_id of owner, NO_OWNER, ANONYMOUS_OWNER or DEFLATER_MARKER.
180 volatile uint64_t _previous_owner_tid; // thread id of the previous owner of the monitor
181 // Separate _owner and _next_om on different cache lines since
182 // both can have busy multi-threaded access. _previous_owner_tid is only
183 // changed by ObjectMonitor::exit() so it is a good choice to share the
184 // cache line with _owner.
185 DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(void* volatile) +
186 sizeof(volatile uint64_t));
187 ObjectMonitor* _next_om; // Next ObjectMonitor* linkage
188 volatile intx _recursions; // recursion count, 0 for first entry
189 ObjectWaiter* volatile _entry_list; // Threads blocked on entry or reentry.
190 // The list is actually composed of wait-nodes,
191 // acting as proxies for Threads.
192 ObjectWaiter* volatile _entry_list_tail; // _entry_list is the head, this is the tail.
193 int64_t volatile _succ; // Heir presumptive thread - used for futile wakeup throttling
194
195 volatile int _SpinDuration;
196
197 int _contentions; // Number of active contentions in enter(). It is used by is_busy()
198 // along with other fields to determine if an ObjectMonitor can be
199 // deflated. It is also used by the async deflation protocol. See
200 // ObjectMonitor::deflate_monitor().
201 int64_t _unmounted_vthreads; // Number of nodes in the _entry_list associated with unmounted vthreads.
202 // It might be temporarily more than the actual number but never less.
203
204 ObjectWaiter* volatile _wait_set; // LL of threads waiting on the monitor - wait()
205 volatile int _waiters; // number of waiting threads
206 volatile int _wait_set_lock; // protects wait set queue - simple spinlock
207
208 public:
209
210 static void Initialize();
211 static void Initialize2();
212
213 static OopHandle& vthread_list_head() { return _vthread_list_head; }
214 static ParkEvent* vthread_unparker_ParkEvent() { return _vthread_unparker_ParkEvent; }
215
216 static int Knob_SpinLimit;
217
218 static ByteSize metadata_offset() { return byte_offset_of(ObjectMonitor, _metadata); }
219 static ByteSize owner_offset() { return byte_offset_of(ObjectMonitor, _owner); }
220 static ByteSize recursions_offset() { return byte_offset_of(ObjectMonitor, _recursions); }
221 static ByteSize succ_offset() { return byte_offset_of(ObjectMonitor, _succ); }
222 static ByteSize entry_list_offset() { return byte_offset_of(ObjectMonitor, _entry_list); }
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 ((in_bytes(ObjectMonitor::f ## _offset())) - checked_cast<int>(markWord::monitor_value))
237
238 uintptr_t metadata() const;
239 void set_metadata(uintptr_t value);
240 volatile uintptr_t* metadata_addr();
241
242 markWord header() const;
243 void set_header(markWord hdr);
244
245 intptr_t hash() const;
246 void set_hash(intptr_t hash);
247
248 bool is_busy() const {
249 // TODO-FIXME: assert _owner == NO_OWNER implies _recursions = 0
250 intptr_t ret_code = intptr_t(_waiters) | intptr_t(_entry_list);
251 int cnts = contentions(); // read once
252 if (cnts > 0) {
253 ret_code |= intptr_t(cnts);
254 }
255 if (!owner_is_DEFLATER_MARKER()) {
256 ret_code |= intptr_t(owner_raw());
257 }
258 return ret_code != 0;
259 }
260 const char* is_busy_to_string(stringStream* ss);
261
262 bool is_entered(JavaThread* current) const;
263
264 // Returns true if this OM has an owner, false otherwise.
265 bool has_owner() const;
266 int64_t owner() const; // Returns NO_OWNER if DEFLATER_MARKER is observed.
267 int64_t owner_raw() const;
268
269 // These methods return the value we set in _owner when acquiring
270 // the monitor with the given thread/vthread, AKA owner_id.
271 static int64_t owner_id_from(JavaThread* thread);
272 static int64_t owner_id_from(oop vthread);
273
274 // Returns true if owner field == DEFLATER_MARKER and false otherwise.
275 bool owner_is_DEFLATER_MARKER() const;
276 // Returns true if 'this' is being async deflated and false otherwise.
277 bool is_being_async_deflated();
278 // Clear _owner field; current value must match thread's owner_id.
279 void release_clear_owner(JavaThread* thread);
280 // Simply set _owner field to new_value; current value must match old_value.
281 void set_owner_from_raw(int64_t old_value, int64_t new_value);
282 // Same as above but uses owner_id of current as new value.
283 void set_owner_from(int64_t old_value, JavaThread* current);
284 // Try to set _owner field to new_value if the current value matches
285 // old_value, using AtomicAccess::cmpxchg(). Otherwise, does not change the
286 // _owner field. Returns the prior value of the _owner field.
287 int64_t try_set_owner_from_raw(int64_t old_value, int64_t new_value);
288 // Same as above but uses owner_id of current as new_value.
289 int64_t try_set_owner_from(int64_t old_value, JavaThread* current);
290
291 // Methods to check and set _succ. The successor is the thread selected
292 // from _entry_list by the current owner when releasing the monitor,
293 // to run again and re-try acquiring the monitor. It is used to avoid
294 // unnecessary wake-ups if there is already a successor set.
295 bool has_successor() const;
296 bool has_successor(JavaThread* thread) const;
297 void set_successor(JavaThread* thread);
298 void set_successor(oop vthread);
299 void clear_successor();
300 int64_t successor() const;
301
302 // Returns true if _owner field == owner_id of thread, false otherwise.
303 bool has_owner(JavaThread* thread) const { return owner() == owner_id_from(thread); }
304 // Set _owner field to owner_id of thread; current value must be NO_OWNER.
305 void set_owner(JavaThread* thread) { set_owner_from(NO_OWNER, thread); }
306 // Try to set _owner field from NO_OWNER to owner_id of thread.
307 bool try_set_owner(JavaThread* thread) {
308 return try_set_owner_from(NO_OWNER, thread) == NO_OWNER;
309 }
310
311 bool has_anonymous_owner() const { return owner_raw() == ANONYMOUS_OWNER; }
312 void set_anonymous_owner() {
313 set_owner_from_raw(NO_OWNER, ANONYMOUS_OWNER);
314 }
315 void set_owner_from_anonymous(JavaThread* owner) {
316 set_owner_from(ANONYMOUS_OWNER, owner);
317 }
318
319 // Simply get _next_om field.
320 ObjectMonitor* next_om() const;
321 // Simply set _next_om field to new_value.
322 void set_next_om(ObjectMonitor* new_value);
323
324 int contentions() const;
325 void add_to_contentions(int value);
326 intx recursions() const { return _recursions; }
327 void set_recursions(size_t recursions);
328 void increment_recursions(JavaThread* current);
329 void inc_unmounted_vthreads();
330 void dec_unmounted_vthreads();
331 bool has_unmounted_vthreads() const;
332
333 // JVM/TI GetObjectMonitorUsage() needs this:
334 int waiters() const;
335 ObjectWaiter* first_waiter() { return _wait_set; }
336 ObjectWaiter* next_waiter(ObjectWaiter* o) { return o->_next; }
337 JavaThread* thread_of_waiter(ObjectWaiter* o) { return o->_thread; }
338
339 ObjectMonitor(oop object);
340 ~ObjectMonitor();
341
342 oop object() const;
343 oop object_peek() const;
344 bool object_is_dead() const;
345 bool object_refers_to(oop obj) const;
346
347 // Returns true if the specified thread owns the ObjectMonitor. Otherwise
348 // returns false and throws IllegalMonitorStateException (IMSE).
349 bool check_owner(TRAPS);
350
351 private:
352 class ExitOnSuspend {
353 protected:
354 ObjectMonitor* _om;
355 bool _om_exited;
356 public:
357 ExitOnSuspend(ObjectMonitor* om) : _om(om), _om_exited(false) {}
358 void operator()(JavaThread* current);
359 bool exited() { return _om_exited; }
360 };
361 class ClearSuccOnSuspend {
362 protected:
363 ObjectMonitor* _om;
364 public:
365 ClearSuccOnSuspend(ObjectMonitor* om) : _om(om) {}
366 void operator()(JavaThread* current);
367 };
368
369 bool enter_is_async_deflating();
370 void notify_contended_enter(JavaThread *current);
371 public:
372 void enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark);
373 bool enter_for(JavaThread* locking_thread);
374 bool enter(JavaThread* current);
375 bool try_enter(JavaThread* current, bool check_for_recursion = true);
376 bool spin_enter(JavaThread* current);
377 void enter_with_contention_mark(JavaThread* current, ObjectMonitorContentionMark& contention_mark);
378 void exit(JavaThread* current, bool not_suspended = true);
379 bool resume_operation(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont);
380 void wait(jlong millis, bool interruptible, TRAPS);
381 void notify(TRAPS);
382 void notifyAll(TRAPS);
383 void quick_notify(JavaThread* current);
384 void quick_notifyAll(JavaThread* current);
385
386 void print() const;
387 #ifdef ASSERT
388 void print_debug_style_on(outputStream* st) const;
389 #endif
390 void print_on(outputStream* st) const;
391
392 // Use the following at your own risk
393 intx complete_exit(JavaThread* current);
394
395 private:
396 void add_to_entry_list(JavaThread* current, ObjectWaiter* node);
397 void add_waiter(ObjectWaiter* waiter);
398 bool notify_internal(JavaThread* current);
399 ObjectWaiter* dequeue_waiter();
400 void dequeue_specific_waiter(ObjectWaiter* waiter);
401 void enter_internal(JavaThread* current);
402 void reenter_internal(JavaThread* current, ObjectWaiter* current_node);
403 void entry_list_build_dll(JavaThread* current);
404 void unlink_after_acquire(JavaThread* current, ObjectWaiter* current_node);
405 ObjectWaiter* entry_list_tail(JavaThread* current);
406
407 bool vthread_monitor_enter(JavaThread* current, ObjectWaiter* node = nullptr);
408 void vthread_wait(JavaThread* current, jlong millis);
409 bool vthread_wait_reenter(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont);
410 void vthread_epilog(JavaThread* current, ObjectWaiter* node);
411
412 enum class TryLockResult { Interference = -1, HasOwner = 0, Success = 1 };
413
414 bool try_lock_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark);
415 bool try_lock_or_add_to_entry_list(JavaThread* current, ObjectWaiter* node);
416 TryLockResult try_lock(JavaThread* current);
417
418 bool try_spin(JavaThread* current);
419 bool short_fixed_spin(JavaThread* current, int spin_count, bool adapt);
420 void exit_epilog(JavaThread* current, ObjectWaiter* Wakee);
421
422 public:
423 // Deflation support
424 bool deflate_monitor(Thread* current);
425 void install_displaced_markword_in_object(const oop obj);
426
427 // JFR support
428 static bool is_jfr_excluded(const Klass* monitor_klass);
429 };
430
431 // RAII object to ensure that ObjectMonitor::is_being_async_deflated() is
432 // stable within the context of this mark.
433 class ObjectMonitorContentionMark : StackObj {
434 DEBUG_ONLY(friend class ObjectMonitor;)
435
436 ObjectMonitor* _monitor;
437 bool _extended;
438
439 NONCOPYABLE(ObjectMonitorContentionMark);
440
441 public:
442 explicit ObjectMonitorContentionMark(ObjectMonitor* monitor);
443 ~ObjectMonitorContentionMark();
444
445 // Extends the contention scope beyond this objects lifetime.
446 // Requires manual decrement of the contentions counter.
447 void extend();
448 };
449
450 #endif // SHARE_RUNTIME_OBJECTMONITOR_HPP