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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13 * accompanied this code).
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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.inline.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 { 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 uint64_t _notifier_tid;
55 ParkEvent * _event;
56 volatile int _notified;
57 volatile TStates TState;
58 bool _active; // Contention monitoring is enabled
59 public:
60 ObjectWaiter(JavaThread* current);
61 ObjectWaiter(oop vthread);
62 ~ObjectWaiter() {
63 if (is_vthread()) {
64 assert(_vthread.resolve() != nullptr, "invariant");
65 _vthread.release(JavaThread::thread_oop_storage());
66 }
67 }
68 oop vthread() { return _vthread.resolve(); }
69 bool is_vthread() { return _thread == nullptr; }
70 void wait_reenter_begin(ObjectMonitor *mon);
71 void wait_reenter_end(ObjectMonitor *mon);
72 };
73
74 // The ObjectMonitor class implements the heavyweight version of a
75 // JavaMonitor. The lightweight BasicLock/stack lock version has been
76 // inflated into an ObjectMonitor. This inflation is typically due to
77 // contention or use of Object.wait().
78 //
79 // WARNING: This is a very sensitive and fragile class. DO NOT make any
80 // changes unless you are fully aware of the underlying semantics.
81 //
82 // ObjectMonitor Layout Overview/Highlights/Restrictions:
83 //
84 // - The _header field must be at offset 0 because the displaced header
85 // from markWord is stored there. We do not want markWord.hpp to include
86 // ObjectMonitor.hpp to avoid exposing ObjectMonitor everywhere. This
87 // means that ObjectMonitor cannot inherit from any other class nor can
88 // it use any virtual member functions. This restriction is critical to
89 // the proper functioning of the VM.
90 // - The _header and _owner fields should be separated by enough space
91 // to avoid false sharing due to parallel access by different threads.
92 // This is an advisory recommendation.
93 // - The general layout of the fields in ObjectMonitor is:
94 // _header
95 // <lightly_used_fields>
96 // <optional padding>
97 // _owner
98 // <remaining_fields>
99 // - The VM assumes write ordering and machine word alignment with
100 // respect to the _owner field and the <remaining_fields> that can
101 // be read in parallel by other threads.
102 // - Generally fields that are accessed closely together in time should
103 // be placed proximally in space to promote data cache locality. That
104 // is, temporal locality should condition spatial locality.
105 // - We have to balance avoiding false sharing with excessive invalidation
106 // from coherence traffic. As such, we try to cluster fields that tend
107 // to be _written_ at approximately the same time onto the same data
108 // cache line.
109 // - We also have to balance the natural tension between minimizing
110 // single threaded capacity misses with excessive multi-threaded
111 // coherency misses. There is no single optimal layout for both
112 // single-threaded and multi-threaded environments.
113 //
114 // - See TEST_VM(ObjectMonitor, sanity) gtest for how critical restrictions are
115 // enforced.
116 // - Adjacent ObjectMonitors should be separated by enough space to avoid
117 // false sharing. This is handled by the ObjectMonitor allocation code
118 // in synchronizer.cpp. Also see TEST_VM(SynchronizerTest, sanity) gtest.
119 //
120 // Futures notes:
121 // - Separating _owner from the <remaining_fields> by enough space to
122 // avoid false sharing might be profitable. Given
123 // http://blogs.oracle.com/dave/entry/cas_and_cache_trivia_invalidate
124 // we know that the CAS in monitorenter will invalidate the line
125 // underlying _owner. We want to avoid an L1 data cache miss on that
126 // same line for monitorexit. Putting these <remaining_fields>:
127 // _recursions, _EntryList, _cxq, and _succ, all of which may be
128 // fetched in the inflated unlock path, on a different cache line
129 // would make them immune to CAS-based invalidation from the _owner
130 // field.
131 //
132 // - The _recursions field should be of type int, or int32_t but not
133 // intptr_t. There's no reason to use a 64-bit type for this field
134 // in a 64-bit JVM.
135
136 #define OM_CACHE_LINE_SIZE DEFAULT_CACHE_LINE_SIZE
137
138 class ObjectMonitor : public CHeapObj<mtObjectMonitor> {
139 friend class ObjectSynchronizer;
140 friend class ObjectWaiter;
141 friend class VMStructs;
142 friend class MonitorList;
143 JVMCI_ONLY(friend class JVMCIVMStructs;)
144
145 static OopStorage* _oop_storage;
146
147 static OopHandle _vthread_cxq_head;
148 static ParkEvent* _vthread_unparker_ParkEvent;
149
150 // The sync code expects the header field to be at offset zero (0).
151 // Enforced by the assert() in header_addr().
152 volatile markWord _header; // displaced object header word - mark
153 WeakHandle _object; // backward object pointer
154 // Separate _header and _owner on different cache lines since both can
155 // have busy multi-threaded access. _header and _object are set at initial
156 // inflation. The _object does not change, so it is a good choice to share
157 // its cache line with _header.
158 DEFINE_PAD_MINUS_SIZE(0, OM_CACHE_LINE_SIZE, sizeof(volatile markWord) +
159 sizeof(WeakHandle));
160 // Used by async deflation as a marker in the _owner field.
161 // Note that the choice of the two markers is peculiar:
162 // - They need to represent values that cannot be pointers. In particular,
163 // we achieve this by using the lowest two bits.
164 // - ANONYMOUS_OWNER should be a small value, it is used in generated code
165 // and small values encode much better.
166 // - We test for anonymous owner by testing for the lowest bit, therefore
167 // DEFLATER_MARKER must *not* have that bit set.
168 #define DEFLATER_MARKER reinterpret_cast<void*>(2)
169 public:
170 // NOTE: Typed as uintptr_t so that we can pick it up in SA, via vmStructs.
171 static const uintptr_t ANONYMOUS_OWNER = 1;
172
173 private:
174 static void* anon_owner_ptr() { return reinterpret_cast<void*>(ANONYMOUS_OWNER); }
175
176 void* volatile _owner; // pointer to owning thread OR BasicLock
177 BasicLock* volatile _stack_locker; // can this share a cache line with owner? they're used together
178 volatile uint64_t _previous_owner_tid; // thread id of the previous owner of the monitor
179 // Separate _owner and _next_om on different cache lines since
180 // both can have busy multi-threaded access. _previous_owner_tid is only
181 // changed by ObjectMonitor::exit() so it is a good choice to share the
182 // cache line with _owner.
183 DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, 2 * sizeof(void* volatile) +
184 sizeof(volatile uint64_t));
185 ObjectMonitor* _next_om; // Next ObjectMonitor* linkage
186 volatile intx _recursions; // recursion count, 0 for first entry
187 ObjectWaiter* volatile _EntryList; // Threads blocked on entry or reentry.
188 // The list is actually composed of WaitNodes,
189 // acting as proxies for Threads.
190
191 ObjectWaiter* volatile _cxq; // LL of recently-arrived threads blocked on entry.
192 JavaThread* volatile _succ; // Heir presumptive thread - used for futile wakeup throttling
193 JavaThread* volatile _Responsible;
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 protected:
202 ObjectWaiter* volatile _WaitSet; // LL of threads wait()ing on the monitor
203 volatile int _waiters; // number of waiting threads
204 private:
205 volatile int _WaitSetLock; // protects Wait Queue - simple spinlock
206
207 public:
208
209 static void Initialize();
210 static void Initialize2();
211
212 static OopHandle& vthread_cxq_head() { return _vthread_cxq_head; }
213 static ParkEvent* vthread_unparker_ParkEvent() { return _vthread_unparker_ParkEvent; }
214
215 // Only perform a PerfData operation if the PerfData object has been
216 // allocated and if the PerfDataManager has not freed the PerfData
217 // objects which can happen at normal VM shutdown.
218 //
219 #define OM_PERFDATA_OP(f, op_str) \
220 do { \
221 if (ObjectMonitor::_sync_ ## f != nullptr && \
222 PerfDataManager::has_PerfData()) { \
223 ObjectMonitor::_sync_ ## f->op_str; \
224 } \
225 } while (0)
226
227 static PerfCounter * _sync_ContendedLockAttempts;
228 static PerfCounter * _sync_FutileWakeups;
229 static PerfCounter * _sync_Parks;
230 static PerfCounter * _sync_Notifications;
231 static PerfCounter * _sync_Inflations;
232 static PerfCounter * _sync_Deflations;
233 static PerfLongVariable * _sync_MonExtant;
234
235 static int Knob_SpinLimit;
236
237 static ByteSize owner_offset() { return byte_offset_of(ObjectMonitor, _owner); }
238 static ByteSize recursions_offset() { return byte_offset_of(ObjectMonitor, _recursions); }
239 static ByteSize cxq_offset() { return byte_offset_of(ObjectMonitor, _cxq); }
240 static ByteSize succ_offset() { return byte_offset_of(ObjectMonitor, _succ); }
241 static ByteSize EntryList_offset() { return byte_offset_of(ObjectMonitor, _EntryList); }
242 static ByteSize stack_locker_offset(){ return byte_offset_of(ObjectMonitor, _stack_locker); }
243
244 // ObjectMonitor references can be ORed with markWord::monitor_value
245 // as part of the ObjectMonitor tagging mechanism. When we combine an
246 // ObjectMonitor reference with an offset, we need to remove the tag
247 // value in order to generate the proper address.
248 //
249 // We can either adjust the ObjectMonitor reference and then add the
250 // offset or we can adjust the offset that is added to the ObjectMonitor
251 // reference. The latter avoids an AGI (Address Generation Interlock)
252 // stall so the helper macro adjusts the offset value that is returned
253 // to the ObjectMonitor reference manipulation code:
254 //
255 #define OM_OFFSET_NO_MONITOR_VALUE_TAG(f) \
256 ((in_bytes(ObjectMonitor::f ## _offset())) - checked_cast<int>(markWord::monitor_value))
257
258 markWord header() const;
259 volatile markWord* header_addr();
260 void set_header(markWord hdr);
261
262 bool is_busy() const {
263 // TODO-FIXME: assert _owner == null implies _recursions = 0
264 intptr_t ret_code = intptr_t(_waiters) | intptr_t(_cxq) | intptr_t(_EntryList);
265 int cnts = contentions(); // read once
266 if (cnts > 0) {
267 ret_code |= intptr_t(cnts);
268 }
269 if (!owner_is_DEFLATER_MARKER()) {
270 ret_code |= intptr_t(owner_raw());
271 }
272 return ret_code != 0;
273 }
274 const char* is_busy_to_string(stringStream* ss);
275
276 bool is_entered(JavaThread* current) const;
277 int contentions() const;
278
279 // Returns true if this OM has an owner, false otherwise.
280 bool has_owner() const;
281 void* owner() const; // Returns null if DEFLATER_MARKER is observed.
282 bool is_owner(JavaThread* thread) const { return owner() == owner_for(thread); }
283 bool is_owner_anonymous() const { return owner_raw() == anon_owner_ptr(); }
284 bool is_stack_locker(JavaThread* current);
285 BasicLock* stack_locker() const;
286
287 private:
288 void* owner_raw() const;
289 void* owner_for(JavaThread* thread) const;
290 // Returns true if owner field == DEFLATER_MARKER and false otherwise.
291 bool owner_is_DEFLATER_MARKER() const;
292 // Returns true if 'this' is being async deflated and false otherwise.
293 bool is_being_async_deflated();
294 // Clear _owner field; current value must match old_value.
295 void release_clear_owner(JavaThread* old_value);
296 // Simply set _owner field to new_value; current value must match old_value.
297 void set_owner_from_raw(void* old_value, void* new_value);
298 void set_owner_from(void* old_value, JavaThread* current);
299 // Simply set _owner field to current; current value must match basic_lock_p.
300 void set_owner_from_BasicLock(JavaThread* current);
301 // Try to set _owner field to new_value if the current value matches
302 // old_value, using Atomic::cmpxchg(). Otherwise, does not change the
303 // _owner field. Returns the prior value of the _owner field.
304 void* try_set_owner_from_raw(void* old_value, void* new_value);
305 void* try_set_owner_from(void* old_value, JavaThread* current);
306
307 void set_stack_locker(BasicLock* locker);
308
309 void set_owner_anonymous() {
310 set_owner_from_raw(nullptr, anon_owner_ptr());
311 }
312
313 void set_owner_from_anonymous(JavaThread* owner) {
314 set_owner_from(anon_owner_ptr(), owner);
315 }
316
317 // Simply get _next_om field.
318 ObjectMonitor* next_om() const;
319 // Simply set _next_om field to new_value.
320 void set_next_om(ObjectMonitor* new_value);
321
322 void add_to_contentions(int value);
323 intx recursions() const { return _recursions; }
324 void set_recursions(size_t recursions);
325
326 public:
327 // JVM/TI GetObjectMonitorUsage() needs this:
328 int waiters() const;
329 ObjectWaiter* first_waiter() { return _WaitSet; }
330 ObjectWaiter* next_waiter(ObjectWaiter* o) { return o->_next; }
331 JavaThread* thread_of_waiter(ObjectWaiter* o) { return o->_thread; }
332
333 ObjectMonitor(oop object);
334 ~ObjectMonitor();
335
336 oop object() const;
337 oop object_peek() const;
338
339 // Returns true if the specified thread owns the ObjectMonitor. Otherwise
340 // returns false and throws IllegalMonitorStateException (IMSE).
341 bool check_owner(TRAPS);
342
343 private:
344 class ExitOnSuspend {
345 protected:
346 ObjectMonitor* _om;
347 bool _om_exited;
348 public:
349 ExitOnSuspend(ObjectMonitor* om) : _om(om), _om_exited(false) {}
350 void operator()(JavaThread* current);
351 bool exited() { return _om_exited; }
352 };
353 class ClearSuccOnSuspend {
354 protected:
355 ObjectMonitor* _om;
356 public:
357 ClearSuccOnSuspend(ObjectMonitor* om) : _om(om) {}
358 void operator()(JavaThread* current);
359 };
360 public:
361 bool enter_for(JavaThread* locking_thread);
362 bool enter(JavaThread* current);
363 void redo_enter(JavaThread* current);
364 void exit(JavaThread* current, bool not_suspended = true);
365 void wait(jlong millis, bool interruptible, TRAPS);
366 void notify(TRAPS);
367 void notifyAll(TRAPS);
368
369 void print() const;
370 #ifdef ASSERT
371 void print_debug_style_on(outputStream* st) const;
372 #endif
373 void print_on(outputStream* st) const;
374
375 // Use the following at your own risk
376 intx complete_exit(JavaThread* current);
377
378 private:
379 void AddWaiter(ObjectWaiter* waiter);
380 void INotify(JavaThread* current);
381 ObjectWaiter* DequeueWaiter();
382 void DequeueSpecificWaiter(ObjectWaiter* waiter);
383 void EnterI(JavaThread* current);
384 void ReenterI(JavaThread* current, ObjectWaiter* current_node);
385 bool HandlePreemptedVThread(JavaThread* current);
386 void VThreadEpilog(JavaThread* current);
387 void UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* current_node, oop vthread = nullptr);
388 ObjectWaiter* LookupWaiter(int64_t threadid);
389
390
391 enum class TryLockResult { Interference = -1, HasOwner = 0, Success = 1 };
392
393 TryLockResult TryLock(JavaThread* current);
394
395 bool TrySpin(JavaThread* current);
396 bool short_fixed_spin(JavaThread* current, int spin_count, bool adapt);
397 void ExitEpilog(JavaThread* current, ObjectWaiter* Wakee);
398
399 // Deflation support
400 bool deflate_monitor();
401 void install_displaced_markword_in_object(const oop obj);
402 };
403
404 #endif // SHARE_RUNTIME_OBJECTMONITOR_HPP