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
36 class ObjectMonitor;
37 class ObjectMonitorContentionMark;
38 class ParkEvent;
39 class BasicLock;
40 class ContinuationWrapper;
41
42
43 class ObjectWaiter : public CHeapObj<mtThread> {
44 public:
45 enum TStates : uint8_t { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER };
46 ObjectWaiter* volatile _next;
47 ObjectWaiter* volatile _prev;
48 JavaThread* _thread;
49 OopHandle _vthread;
50 ObjectMonitor* _monitor;
51 uint64_t _notifier_tid;
52 int _recursions;
53 volatile TStates TState;
54 volatile bool _notified;
55 bool _is_wait;
56 bool _at_reenter;
57 bool _interrupted;
58 bool _interruptible;
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 ObjectWaiter* const badObjectWaiterPtr = (ObjectWaiter*) 0xBAD;
77 void set_bad_pointers() {
78 #ifdef ASSERT
79 this->_prev = badObjectWaiterPtr;
80 this->_next = badObjectWaiterPtr;
81 this->TState = ObjectWaiter::TS_RUN;
82 #endif
83 }
84 ObjectWaiter* next() {
85 assert (_next != badObjectWaiterPtr, "corrupted list!");
86 return _next;
87 }
88 ObjectWaiter* prev() {
89 assert (_prev != badObjectWaiterPtr, "corrupted list!");
90 return _prev;
91 }
92 };
93
94 // The ObjectMonitor class implements the heavyweight version of a
95 // JavaMonitor. The lightweight BasicLock/stack lock version has been
96 // inflated into an ObjectMonitor. This inflation is typically due to
97 // contention or use of Object.wait().
98 //
99 // WARNING: This is a very sensitive and fragile class. DO NOT make any
100 // changes unless you are fully aware of the underlying semantics.
101 //
102 // ObjectMonitor Layout Overview/Highlights/Restrictions:
103 //
104 // - For performance reasons we ensure the _metadata field is located at offset 0,
105 // which in turn means that ObjectMonitor can't inherit from any other class nor use
106 // any virtual member functions.
107 // - The _metadata and _owner fields should be separated by enough space
108 // to avoid false sharing due to parallel access by different threads.
109 // This is an advisory recommendation.
110 // - The general layout of the fields in ObjectMonitor is:
111 // _metadata
112 // <lightly_used_fields>
113 // <optional padding>
114 // _owner
115 // <optional padding>
116 // <remaining_fields>
117 // - The VM assumes write ordering and machine word alignment with
118 // respect to the _owner field and the <remaining_fields> that can
119 // be read in parallel by other threads.
120 // - Generally fields that are accessed closely together in time should
121 // be placed proximally in space to promote data cache locality. That
122 // is, temporal locality should condition spatial locality.
123 // - We have to balance avoiding false sharing with excessive invalidation
124 // from coherence traffic. As such, we try to cluster fields that tend
125 // to be _written_ at approximately the same time onto the same data
126 // cache line.
127 // - We also have to balance the natural tension between minimizing
128 // single threaded capacity misses with excessive multi-threaded
129 // coherency misses. There is no single optimal layout for both
130 // single-threaded and multi-threaded environments.
131 //
132 // - See TEST_VM(ObjectMonitor, sanity) gtest for how critical restrictions are
133 // enforced.
134 //
135 // - Separating _owner from the <remaining_fields> by enough space to
136 // avoid false sharing might be profitable. Given that the CAS in
137 // monitorenter will invalidate the line underlying _owner. We want
138 // to avoid an L1 data cache miss on that same line for monitorexit.
139 // Putting these <remaining_fields>:
140 // _recursions, _entry_list and _succ, all of which may be
141 // fetched in the inflated unlock path, on a different cache line
142 // would make them immune to CAS-based invalidation from the _owner
143 // field.
144 //
145 // - TODO: The _recursions field should be of type int, or int32_t but not
146 // intptr_t. There's no reason to use a 64-bit type for this field
147 // in a 64-bit JVM.
148
149 #define OM_CACHE_LINE_SIZE DEFAULT_CACHE_LINE_SIZE
150
151 class ObjectMonitor : public CHeapObj<mtObjectMonitor> {
152 friend class VMStructs;
153 JVMCI_ONLY(friend class JVMCIVMStructs;)
154
155 static OopStorage* _oop_storage;
156
157 // List of j.l.VirtualThread waiting to be unblocked by unblocker thread.
158 static OopHandle _vthread_list_head;
159 // ParkEvent of unblocker thread.
160 static ParkEvent* _vthread_unparker_ParkEvent;
161
162 // Because of frequent access, the metadata field is at offset zero (0).
163 // Enforced by the assert() in metadata_addr().
164 // * LM_LIGHTWEIGHT with UseObjectMonitorTable:
165 // Contains the _object's hashCode.
166 // * LM_LEGACY, LM_MONITOR, LM_LIGHTWEIGHT without UseObjectMonitorTable:
167 // Contains the displaced object header word - mark
168 volatile uintptr_t _metadata; // metadata
169 WeakHandle _object; // backward object pointer
170 // Separate _metadata and _owner on different cache lines since both can
171 // have busy multi-threaded access. _metadata and _object are set at initial
172 // inflation. The _object does not change, so it is a good choice to share
173 // its cache line with _metadata.
174 DEFINE_PAD_MINUS_SIZE(0, OM_CACHE_LINE_SIZE, sizeof(_metadata) +
175 sizeof(WeakHandle));
176
177 static const int64_t NO_OWNER = 0;
178 static const int64_t ANONYMOUS_OWNER = 1;
179 static const int64_t DEFLATER_MARKER = 2;
180
181 int64_t volatile _owner; // Either owner_id of owner, NO_OWNER, ANONYMOUS_OWNER or DEFLATER_MARKER.
182 volatile uint64_t _previous_owner_tid; // thread id of the previous owner of the monitor
183 // Separate _owner and _next_om on different cache lines since
184 // both can have busy multi-threaded access. _previous_owner_tid is only
185 // changed by ObjectMonitor::exit() so it is a good choice to share the
186 // cache line with _owner.
187 DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(void* volatile) +
188 sizeof(volatile uint64_t));
189 ObjectMonitor* _next_om; // Next ObjectMonitor* linkage
190 volatile intx _recursions; // recursion count, 0 for first entry
191 ObjectWaiter* volatile _entry_list; // Threads blocked on entry or reentry.
192 // The list is actually composed of wait-nodes,
193 // acting as proxies for Threads.
194 ObjectWaiter* volatile _entry_list_tail; // _entry_list is the head, this is the tail.
195 int64_t volatile _succ; // Heir presumptive thread - used for futile wakeup throttling
196
197 volatile int _SpinDuration;
198
199 int _contentions; // Number of active contentions in enter(). It is used by is_busy()
200 // along with other fields to determine if an ObjectMonitor can be
201 // deflated. It is also used by the async deflation protocol. See
202 // ObjectMonitor::deflate_monitor().
203 int64_t _unmounted_vthreads; // Number of nodes in the _entry_list associated with unmounted vthreads.
204 // It might be temporarily more than the actual number but never less.
205
206 ObjectWaiter* volatile _wait_set; // LL of threads waiting on the monitor - wait()
207 volatile int _waiters; // number of waiting threads
208 volatile int _wait_set_lock; // protects wait set queue - simple spinlock
209
210 // Used in LM_LEGACY mode to store BasicLock* in case of inflation by contending thread.
211 BasicLock* volatile _stack_locker;
212
213 public:
214
215 static void Initialize();
216 static void Initialize2();
217
218 static OopHandle& vthread_list_head() { return _vthread_list_head; }
219 static ParkEvent* vthread_unparker_ParkEvent() { return _vthread_unparker_ParkEvent; }
220
221 static int Knob_SpinLimit;
222
223 static ByteSize metadata_offset() { return byte_offset_of(ObjectMonitor, _metadata); }
224 static ByteSize owner_offset() { return byte_offset_of(ObjectMonitor, _owner); }
225 static ByteSize recursions_offset() { return byte_offset_of(ObjectMonitor, _recursions); }
226 static ByteSize succ_offset() { return byte_offset_of(ObjectMonitor, _succ); }
227 static ByteSize entry_list_offset() { return byte_offset_of(ObjectMonitor, _entry_list); }
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 == NO_OWNER implies _recursions = 0
255 intptr_t ret_code = intptr_t(_waiters) | intptr_t(_entry_list);
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 int64_t owner() const; // Returns NO_OWNER if DEFLATER_MARKER is observed.
272 int64_t owner_raw() const;
273
274 // These methods return the value we set in _owner when acquiring
275 // the monitor with the given thread/vthread, AKA owner_id.
276 static int64_t owner_id_from(JavaThread* thread);
277 static int64_t owner_id_from(oop vthread);
278
279 // Returns true if owner field == DEFLATER_MARKER and false otherwise.
280 bool owner_is_DEFLATER_MARKER() const;
281 // Returns true if 'this' is being async deflated and false otherwise.
282 bool is_being_async_deflated();
283 // Clear _owner field; current value must match thread's owner_id.
284 void release_clear_owner(JavaThread* thread);
285 // Simply set _owner field to new_value; current value must match old_value.
286 void 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 void set_owner_from(int64_t old_value, JavaThread* current);
289 // Try to set _owner field to new_value if the current value matches
290 // old_value, using Atomic::cmpxchg(). Otherwise, does not change the
291 // _owner field. Returns the prior value of the _owner field.
292 int64_t try_set_owner_from_raw(int64_t old_value, int64_t new_value);
293 // Same as above but uses owner_id of current as new_value.
294 int64_t try_set_owner_from(int64_t old_value, JavaThread* current);
295
296 // Methods to check and set _succ. The successor is the thread selected
297 // from _entry_list by the current owner when releasing the monitor,
298 // to run again and re-try acquiring the monitor. It is used to avoid
299 // unnecessary wake-ups if there is already a successor set.
300 bool has_successor() const;
301 bool has_successor(JavaThread* thread) const;
302 void set_successor(JavaThread* thread);
303 void set_successor(oop vthread);
304 void clear_successor();
305 int64_t successor() const;
306
307 // Returns true if _owner field == owner_id of thread, false otherwise.
308 bool has_owner(JavaThread* thread) const { return owner() == owner_id_from(thread); }
309 // Set _owner field to owner_id of thread; current value must be NO_OWNER.
310 void set_owner(JavaThread* thread) { set_owner_from(NO_OWNER, thread); }
311 // Try to set _owner field from NO_OWNER to owner_id of thread.
312 bool try_set_owner(JavaThread* thread) {
313 return try_set_owner_from(NO_OWNER, thread) == NO_OWNER;
314 }
315
316 bool has_anonymous_owner() const { return owner_raw() == ANONYMOUS_OWNER; }
317 void set_anonymous_owner() {
318 set_owner_from_raw(NO_OWNER, ANONYMOUS_OWNER);
319 }
320 void set_owner_from_anonymous(JavaThread* owner) {
321 set_owner_from(ANONYMOUS_OWNER, owner);
322 }
323
324 // Get and set _stack_locker.
325 BasicLock* stack_locker() const;
326 void set_stack_locker(BasicLock* locker);
327
328 // Simply get _next_om field.
329 ObjectMonitor* next_om() const;
330 // Simply set _next_om field to new_value.
331 void set_next_om(ObjectMonitor* new_value);
332
333 int contentions() const;
334 void add_to_contentions(int value);
335 intx recursions() const { return _recursions; }
336 void set_recursions(size_t recursions);
337 void increment_recursions(JavaThread* current);
338 void inc_unmounted_vthreads();
339 void dec_unmounted_vthreads();
340 bool has_unmounted_vthreads() const;
341
342 // JVM/TI GetObjectMonitorUsage() needs this:
343 int waiters() const;
344 ObjectWaiter* first_waiter() { return _wait_set; }
345 ObjectWaiter* next_waiter(ObjectWaiter* o) { return o->_next; }
346 JavaThread* thread_of_waiter(ObjectWaiter* o) { return o->_thread; }
347
348 ObjectMonitor(oop object);
349 ~ObjectMonitor();
350
351 oop object() const;
352 oop object_peek() const;
353 bool object_is_dead() const;
354 bool object_refers_to(oop obj) const;
355
356 // Returns true if the specified thread owns the ObjectMonitor. Otherwise
357 // returns false and throws IllegalMonitorStateException (IMSE).
358 bool check_owner(TRAPS);
359
360 private:
361 class ExitOnSuspend {
362 protected:
363 ObjectMonitor* _om;
364 bool _om_exited;
365 public:
366 ExitOnSuspend(ObjectMonitor* om) : _om(om), _om_exited(false) {}
367 void operator()(JavaThread* current);
368 bool exited() { return _om_exited; }
369 };
370 class ClearSuccOnSuspend {
371 protected:
372 ObjectMonitor* _om;
373 public:
374 ClearSuccOnSuspend(ObjectMonitor* om) : _om(om) {}
375 void operator()(JavaThread* current);
376 };
377
378 bool enter_is_async_deflating();
379 void notify_contended_enter(JavaThread *current);
380 public:
381 void enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark);
382 bool enter_for(JavaThread* locking_thread);
383 bool enter(JavaThread* current);
384 bool try_enter(JavaThread* current, bool check_for_recursion = true);
385 bool spin_enter(JavaThread* current);
386 void enter_with_contention_mark(JavaThread* current, ObjectMonitorContentionMark& contention_mark);
387 void exit(JavaThread* current, bool not_suspended = true);
388 bool resume_operation(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont);
389 void wait(jlong millis, bool interruptible, TRAPS);
390 void notify(TRAPS);
391 void notifyAll(TRAPS);
392 void quick_notify(JavaThread* current);
393 void quick_notifyAll(JavaThread* current);
394
395 void print() const;
396 #ifdef ASSERT
397 void print_debug_style_on(outputStream* st) const;
398 #endif
399 void print_on(outputStream* st) const;
400
401 // Use the following at your own risk
402 intx complete_exit(JavaThread* current);
403
404 private:
405 void add_to_entry_list(JavaThread* current, ObjectWaiter* node);
406 void add_waiter(ObjectWaiter* waiter);
407 bool notify_internal(JavaThread* current);
408 ObjectWaiter* dequeue_waiter();
409 void dequeue_specific_waiter(ObjectWaiter* waiter);
410 void enter_internal(JavaThread* current);
411 void reenter_internal(JavaThread* current, ObjectWaiter* current_node);
412 void entry_list_build_dll(JavaThread* current);
413 void unlink_after_acquire(JavaThread* current, ObjectWaiter* current_node);
414 ObjectWaiter* entry_list_tail(JavaThread* current);
415
416 bool vthread_monitor_enter(JavaThread* current, ObjectWaiter* node = nullptr);
417 void vthread_wait(JavaThread* current, jlong millis, bool interruptible);
418 bool vthread_wait_reenter(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont);
419 void vthread_epilog(JavaThread* current, ObjectWaiter* node);
420
421 enum class TryLockResult { Interference = -1, HasOwner = 0, Success = 1 };
422
423 bool try_lock_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark);
424 bool try_lock_or_add_to_entry_list(JavaThread* current, ObjectWaiter* node);
425 TryLockResult try_lock(JavaThread* current);
426
427 bool try_spin(JavaThread* current);
428 bool short_fixed_spin(JavaThread* current, int spin_count, bool adapt);
429 void exit_epilog(JavaThread* current, ObjectWaiter* Wakee);
430
431 public:
432 // Deflation support
433 bool deflate_monitor(Thread* current);
434 void install_displaced_markword_in_object(const oop obj);
435
436 // JFR support
437 static bool is_jfr_excluded(const Klass* monitor_klass);
438 };
439
440 // RAII object to ensure that ObjectMonitor::is_being_async_deflated() is
441 // stable within the context of this mark.
442 class ObjectMonitorContentionMark : StackObj {
443 DEBUG_ONLY(friend class ObjectMonitor;)
444
445 ObjectMonitor* _monitor;
446 bool _extended;
447
448 NONCOPYABLE(ObjectMonitorContentionMark);
449
450 public:
451 explicit ObjectMonitorContentionMark(ObjectMonitor* monitor);
452 ~ObjectMonitorContentionMark();
453
454 // Extends the contention scope beyond this objects lifetime.
455 // Requires manual decrement of the contentions counter.
456 void extend();
457 };
458
459 #endif // SHARE_RUNTIME_OBJECTMONITOR_HPP