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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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 "runtime/perfDataTypes.hpp"
35 #include "utilities/checkedCast.hpp"
36
37 class ObjectMonitor;
38 class ParkEvent;
39 class BasicLock;
40
41 // ObjectWaiter serves as a "proxy" or surrogate thread.
42 // TODO-FIXME: Eliminate ObjectWaiter and use the thread-specific
43 // ParkEvent instead. Beware, however, that the JVMTI code
44 // knows about ObjectWaiters, so we'll have to reconcile that code.
45 // See next_waiter(), first_waiter(), etc.
46
47 class ObjectWaiter : public CHeapObj<mtThread> {
48 public:
49 enum TStates : uint8_t { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER, TS_CXQ };
50 ObjectWaiter* volatile _next;
51 ObjectWaiter* volatile _prev;
52 JavaThread* _thread;
53 OopHandle _vthread;
54 ObjectMonitor* _monitor;
55 uint64_t _notifier_tid;
56 int _recursions;
57 volatile TStates TState;
58 volatile bool _notified;
59 bool _is_wait;
60 bool _at_reenter;
61 bool _interrupted;
62 bool _active; // Contention monitoring is enabled
63 public:
64 ObjectWaiter(JavaThread* current);
65 ObjectWaiter(oop vthread, ObjectMonitor* mon);
66 ~ObjectWaiter();
67 JavaThread* thread() { return _thread; }
68 bool is_vthread() { return _thread == nullptr; }
69 uint8_t state() { return TState; }
70 ObjectMonitor* monitor() { return _monitor; }
71 bool is_monitorenter() { return !_is_wait; }
72 bool is_wait() { return _is_wait; }
73 bool notified() { return _notified; }
74 bool at_reenter() { return _at_reenter; }
75 oop vthread();
76 void wait_reenter_begin(ObjectMonitor *mon);
77 void wait_reenter_end(ObjectMonitor *mon);
78 };
79
80 // The ObjectMonitor class implements the heavyweight version of a
81 // JavaMonitor. The lightweight BasicLock/stack lock version has been
82 // inflated into an ObjectMonitor. This inflation is typically due to
83 // contention or use of Object.wait().
84 //
85 // WARNING: This is a very sensitive and fragile class. DO NOT make any
86 // changes unless you are fully aware of the underlying semantics.
87 //
88 // ObjectMonitor Layout Overview/Highlights/Restrictions:
89 //
90 // - The _header field must be at offset 0 because the displaced header
91 // from markWord is stored there. We do not want markWord.hpp to include
92 // ObjectMonitor.hpp to avoid exposing ObjectMonitor everywhere. This
93 // means that ObjectMonitor cannot inherit from any other class nor can
94 // it use any virtual member functions. This restriction is critical to
95 // the proper functioning of the VM.
96 // - The _header and _owner fields should be separated by enough space
97 // to avoid false sharing due to parallel access by different threads.
98 // This is an advisory recommendation.
99 // - The general layout of the fields in ObjectMonitor is:
100 // _header
101 // <lightly_used_fields>
102 // <optional padding>
103 // _owner
104 // <remaining_fields>
105 // - The VM assumes write ordering and machine word alignment with
106 // respect to the _owner field and the <remaining_fields> that can
107 // be read in parallel by other threads.
108 // - Generally fields that are accessed closely together in time should
109 // be placed proximally in space to promote data cache locality. That
110 // is, temporal locality should condition spatial locality.
111 // - We have to balance avoiding false sharing with excessive invalidation
112 // from coherence traffic. As such, we try to cluster fields that tend
113 // to be _written_ at approximately the same time onto the same data
114 // cache line.
115 // - We also have to balance the natural tension between minimizing
116 // single threaded capacity misses with excessive multi-threaded
117 // coherency misses. There is no single optimal layout for both
118 // single-threaded and multi-threaded environments.
119 //
120 // - See TEST_VM(ObjectMonitor, sanity) gtest for how critical restrictions are
121 // enforced.
122 // - Adjacent ObjectMonitors should be separated by enough space to avoid
123 // false sharing. This is handled by the ObjectMonitor allocation code
124 // in synchronizer.cpp. Also see TEST_VM(SynchronizerTest, sanity) gtest.
125 //
126 // Futures notes:
127 // - Separating _owner from the <remaining_fields> by enough space to
128 // avoid false sharing might be profitable. Given
129 // http://blogs.oracle.com/dave/entry/cas_and_cache_trivia_invalidate
130 // we know that the CAS in monitorenter will invalidate the line
131 // underlying _owner. We want to avoid an L1 data cache miss on that
132 // same line for monitorexit. Putting these <remaining_fields>:
133 // _recursions, _EntryList, _cxq, and _succ, all of which may be
134 // fetched in the inflated unlock path, on a different cache line
135 // would make them immune to CAS-based invalidation from the _owner
136 // field.
137 //
138 // - The _recursions field should be of type int, or int32_t but not
139 // intptr_t. There's no reason to use a 64-bit type for this field
140 // in a 64-bit JVM.
141
142 #define OM_CACHE_LINE_SIZE DEFAULT_CACHE_LINE_SIZE
143
144 class ObjectMonitor : public CHeapObj<mtObjectMonitor> {
145 friend class ObjectSynchronizer;
146 friend class ObjectWaiter;
147 friend class VMStructs;
148 friend class MonitorList;
149 JVMCI_ONLY(friend class JVMCIVMStructs;)
150
151 static OopStorage* _oop_storage;
152
153 static OopHandle _vthread_cxq_head;
154 static ParkEvent* _vthread_unparker_ParkEvent;
155
156 // The sync code expects the header field to be at offset zero (0).
157 // Enforced by the assert() in header_addr().
158 volatile markWord _header; // displaced object header word - mark
159 WeakHandle _object; // backward object pointer
160 // Separate _header and _owner on different cache lines since both can
161 // have busy multi-threaded access. _header and _object are set at initial
162 // inflation. The _object does not change, so it is a good choice to share
163 // its cache line with _header.
164 DEFINE_PAD_MINUS_SIZE(0, OM_CACHE_LINE_SIZE, sizeof(volatile markWord) +
165 sizeof(WeakHandle));
166 // Used by async deflation as a marker in the _owner field.
167 // Note that the choice of the two markers is peculiar:
168 // - They need to represent values that cannot be pointers. In particular,
169 // we achieve this by using the lowest two bits.
170 // - ANONYMOUS_OWNER should be a small value, it is used in generated code
171 // and small values encode much better.
172 // - We test for anonymous owner by testing for the lowest bit, therefore
173 // DEFLATER_MARKER must *not* have that bit set.
174 #define DEFLATER_MARKER reinterpret_cast<void*>(2)
175 public:
176 // NOTE: Typed as uintptr_t so that we can pick it up in SA, via vmStructs.
177 static const uintptr_t ANONYMOUS_OWNER = 1;
178
179 private:
180 static void* anon_owner_ptr() { return reinterpret_cast<void*>(ANONYMOUS_OWNER); }
181
182 void* volatile _owner; // pointer to owning thread OR BasicLock
183 BasicLock* volatile _stack_locker; // can this share a cache line with owner? they're used together
184 volatile uint64_t _previous_owner_tid; // thread id of the previous owner of the monitor
185 // Separate _owner and _next_om on different cache lines since
186 // both can have busy multi-threaded access. _previous_owner_tid is only
187 // changed by ObjectMonitor::exit() so it is a good choice to share the
188 // cache line with _owner.
189 DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, 2 * sizeof(void* volatile) +
190 sizeof(volatile uint64_t));
191 ObjectMonitor* _next_om; // Next ObjectMonitor* linkage
192 volatile intx _recursions; // recursion count, 0 for first entry
193 ObjectWaiter* volatile _EntryList; // Threads blocked on entry or reentry.
194 // The list is actually composed of WaitNodes,
195 // acting as proxies for Threads.
196
197 ObjectWaiter* volatile _cxq; // LL of recently-arrived threads blocked on entry.
198 JavaThread* volatile _succ; // Heir presumptive thread - used for futile wakeup throttling
199 JavaThread* volatile _Responsible;
200
201 volatile int _SpinDuration;
202
203 int _contentions; // Number of active contentions in enter(). It is used by is_busy()
204 // along with other fields to determine if an ObjectMonitor can be
205 // deflated. It is also used by the async deflation protocol. See
206 // ObjectMonitor::deflate_monitor().
207 protected:
208 ObjectWaiter* volatile _WaitSet; // LL of threads wait()ing on the monitor
209 volatile int _waiters; // number of waiting threads
210 private:
211 volatile int _WaitSetLock; // protects Wait Queue - simple spinlock
212
213 public:
214
215 static void Initialize();
216 static void Initialize2();
217
218 static OopHandle& vthread_cxq_head() { return _vthread_cxq_head; }
219 static ParkEvent* vthread_unparker_ParkEvent() { return _vthread_unparker_ParkEvent; }
220
221 // Only perform a PerfData operation if the PerfData object has been
222 // allocated and if the PerfDataManager has not freed the PerfData
223 // objects which can happen at normal VM shutdown.
224 //
225 #define OM_PERFDATA_OP(f, op_str) \
226 do { \
227 if (ObjectMonitor::_sync_ ## f != nullptr && \
228 PerfDataManager::has_PerfData()) { \
229 ObjectMonitor::_sync_ ## f->op_str; \
230 } \
231 } while (0)
232
233 static PerfCounter * _sync_ContendedLockAttempts;
234 static PerfCounter * _sync_FutileWakeups;
235 static PerfCounter * _sync_Parks;
236 static PerfCounter * _sync_Notifications;
237 static PerfCounter * _sync_Inflations;
238 static PerfCounter * _sync_Deflations;
239 static PerfLongVariable * _sync_MonExtant;
240
241 static int Knob_SpinLimit;
242
243 static ByteSize owner_offset() { return byte_offset_of(ObjectMonitor, _owner); }
244 static ByteSize recursions_offset() { return byte_offset_of(ObjectMonitor, _recursions); }
245 static ByteSize cxq_offset() { return byte_offset_of(ObjectMonitor, _cxq); }
246 static ByteSize succ_offset() { return byte_offset_of(ObjectMonitor, _succ); }
247 static ByteSize EntryList_offset() { return byte_offset_of(ObjectMonitor, _EntryList); }
248 static ByteSize stack_locker_offset(){ return byte_offset_of(ObjectMonitor, _stack_locker); }
249
250 // ObjectMonitor references can be ORed with markWord::monitor_value
251 // as part of the ObjectMonitor tagging mechanism. When we combine an
252 // ObjectMonitor reference with an offset, we need to remove the tag
253 // value in order to generate the proper address.
254 //
255 // We can either adjust the ObjectMonitor reference and then add the
256 // offset or we can adjust the offset that is added to the ObjectMonitor
257 // reference. The latter avoids an AGI (Address Generation Interlock)
258 // stall so the helper macro adjusts the offset value that is returned
259 // to the ObjectMonitor reference manipulation code:
260 //
261 #define OM_OFFSET_NO_MONITOR_VALUE_TAG(f) \
262 ((in_bytes(ObjectMonitor::f ## _offset())) - checked_cast<int>(markWord::monitor_value))
263
264 markWord header() const;
265 volatile markWord* header_addr();
266 void set_header(markWord hdr);
267
268 bool is_busy() const {
269 // TODO-FIXME: assert _owner == null implies _recursions = 0
270 intptr_t ret_code = intptr_t(_waiters) | intptr_t(_cxq) | intptr_t(_EntryList);
271 int cnts = contentions(); // read once
272 if (cnts > 0) {
273 ret_code |= intptr_t(cnts);
274 }
275 if (!owner_is_DEFLATER_MARKER()) {
276 ret_code |= intptr_t(owner_raw());
277 }
278 return ret_code != 0;
279 }
280 const char* is_busy_to_string(stringStream* ss);
281
282 bool is_entered(JavaThread* current) const;
283 int contentions() const;
284
285 // Returns true if this OM has an owner, false otherwise.
286 bool has_owner() const;
287 void* owner() const; // Returns null if DEFLATER_MARKER is observed.
288 bool is_owner(JavaThread* thread) const { return owner() == owner_for(thread); }
289 bool is_owner_anonymous() const { return owner_raw() == anon_owner_ptr(); }
290 bool is_stack_locker(JavaThread* current);
291 BasicLock* stack_locker() const;
292
293 private:
294 void* owner_raw() const;
295 void* owner_for(JavaThread* thread) const;
296 // Returns true if owner field == DEFLATER_MARKER and false otherwise.
297 bool owner_is_DEFLATER_MARKER() const;
298 // Returns true if 'this' is being async deflated and false otherwise.
299 bool is_being_async_deflated();
300 // Clear _owner field; current value must match old_value.
301 void release_clear_owner(JavaThread* old_value);
302 // Simply set _owner field to new_value; current value must match old_value.
303 void set_owner_from_raw(void* old_value, void* new_value);
304 void set_owner_from(void* old_value, JavaThread* current);
305 // Simply set _owner field to current; current value must match basic_lock_p.
306 void set_owner_from_BasicLock(JavaThread* current);
307 // Try to set _owner field to new_value if the current value matches
308 // old_value, using Atomic::cmpxchg(). Otherwise, does not change the
309 // _owner field. Returns the prior value of the _owner field.
310 void* try_set_owner_from_raw(void* old_value, void* new_value);
311 void* try_set_owner_from(void* old_value, JavaThread* current);
312
313 void set_stack_locker(BasicLock* locker);
314
315 void set_owner_anonymous() {
316 set_owner_from_raw(nullptr, anon_owner_ptr());
317 }
318
319 void set_owner_from_anonymous(JavaThread* owner) {
320 set_owner_from(anon_owner_ptr(), owner);
321 }
322
323 // Simply get _next_om field.
324 ObjectMonitor* next_om() const;
325 // Simply set _next_om field to new_value.
326 void set_next_om(ObjectMonitor* new_value);
327
328 void add_to_contentions(int value);
329 intx recursions() const { return _recursions; }
330 void set_recursions(size_t recursions);
331
332 public:
333 // JVM/TI GetObjectMonitorUsage() needs this:
334 int waiters() const;
335 ObjectWaiter* first_waiter() { return _WaitSet; }
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
345 // Returns true if the specified thread owns the ObjectMonitor. Otherwise
346 // returns false and throws IllegalMonitorStateException (IMSE).
347 bool check_owner(TRAPS);
348
349 private:
350 class ExitOnSuspend {
351 protected:
352 ObjectMonitor* _om;
353 bool _om_exited;
354 public:
355 ExitOnSuspend(ObjectMonitor* om) : _om(om), _om_exited(false) {}
356 void operator()(JavaThread* current);
357 bool exited() { return _om_exited; }
358 };
359 class ClearSuccOnSuspend {
360 protected:
361 ObjectMonitor* _om;
362 public:
363 ClearSuccOnSuspend(ObjectMonitor* om) : _om(om) {}
364 void operator()(JavaThread* current);
365 };
366 public:
367 bool enter_for(JavaThread* locking_thread);
368 bool enter(JavaThread* current);
369 void resume_operation(JavaThread* current, ObjectWaiter* node);
370 void exit(JavaThread* current, bool not_suspended = true);
371 void wait(jlong millis, bool interruptible, TRAPS);
372 void notify(TRAPS);
373 void notifyAll(TRAPS);
374
375 void print() const;
376 #ifdef ASSERT
377 void print_debug_style_on(outputStream* st) const;
378 #endif
379 void print_on(outputStream* st) const;
380
381 // Use the following at your own risk
382 intx complete_exit(JavaThread* current);
383
384 private:
385 void AddWaiter(ObjectWaiter* waiter);
386 void INotify(JavaThread* current);
387 ObjectWaiter* DequeueWaiter();
388 void DequeueSpecificWaiter(ObjectWaiter* waiter);
389 void UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* current_node);
390 void EnterI(JavaThread* current);
391 void ReenterI(JavaThread* current, ObjectWaiter* current_node);
392
393 bool VThreadMonitorEnter(JavaThread* current, ObjectWaiter* node = nullptr);
394 void VThreadWait(JavaThread* current, jlong millis);
395 bool VThreadWaitReenter(JavaThread* current, ObjectWaiter* node);
396 void VThreadEpilog(JavaThread* current, ObjectWaiter* node);
397
398 enum class TryLockResult { Interference = -1, HasOwner = 0, Success = 1 };
399
400 TryLockResult TryLock(JavaThread* current);
401
402 bool TrySpin(JavaThread* current);
403 bool short_fixed_spin(JavaThread* current, int spin_count, bool adapt);
404 void ExitEpilog(JavaThread* current, ObjectWaiter* Wakee);
405
406 // Deflation support
407 bool deflate_monitor();
408 void install_displaced_markword_in_object(const oop obj);
409 };
410
411 #endif // SHARE_RUNTIME_OBJECTMONITOR_HPP