1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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13 * accompanied this code).
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24
25 #ifndef SHARE_VM_RUNTIME_MUTEX_HPP
26 #define SHARE_VM_RUNTIME_MUTEX_HPP
27
28 #include "memory/allocation.hpp"
29 #include "runtime/os.hpp"
30 #include "utilities/histogram.hpp"
31
32 // The SplitWord construct allows us to colocate the contention queue
33 // (cxq) with the lock-byte. The queue elements are ParkEvents, which are
34 // always aligned on 256-byte addresses - the least significant byte of
35 // a ParkEvent is always 0. Colocating the lock-byte with the queue
36 // allows us to easily avoid what would otherwise be a race in lock()
37 // if we were to use two completely separate fields for the contention queue
38 // and the lock indicator. Specifically, colocation renders us immune
39 // from the race where a thread might enqueue itself in the lock() slow-path
40 // immediately after the lock holder drops the outer lock in the unlock()
41 // fast-path.
42 //
43 // Colocation allows us to use a fast-path unlock() form that uses
44 // A MEMBAR instead of a CAS. MEMBAR has lower local latency than CAS
45 // on many platforms.
46 //
47 // See:
48 // + http://blogs.sun.com/dave/entry/biased_locking_in_hotspot
49 // + http://blogs.sun.com/dave/resource/synchronization-public2.pdf
50 //
51 // Note that we're *not* using word-tearing the classic sense.
52 // The lock() fast-path will CAS the lockword and the unlock()
53 // fast-path will store into the lock-byte colocated within the lockword.
54 // We depend on the fact that all our reference platforms have
55 // coherent and atomic byte accesses. More precisely, byte stores
56 // interoperate in a safe, sane, and expected manner with respect to
57 // CAS, ST and LDs to the full-word containing the byte.
58 // If you're porting HotSpot to a platform where that isn't the case
59 // then you'll want change the unlock() fast path from:
60 // STB;MEMBAR #storeload; LDN
61 // to a full-word CAS of the lockword.
62
63
64 union SplitWord { // full-word with separately addressable LSB
65 volatile intptr_t FullWord ;
66 volatile void * Address ;
67 volatile jbyte Bytes [sizeof(intptr_t)] ;
68 } ;
69
70 // Endian-ness ... index of least-significant byte in SplitWord.Bytes[]
71 #ifdef VM_LITTLE_ENDIAN
72 #define _LSBINDEX 0
73 #else
74 #define _LSBINDEX (sizeof(intptr_t)-1)
75 #endif
76
77 class ParkEvent ;
78
79 // See orderAccess.hpp. We assume throughout the VM that mutex lock and
80 // try_lock do fence-lock-acquire, and that unlock does a release-unlock,
81 // *in that order*. If their implementations change such that these
82 // assumptions are violated, a whole lot of code will break.
83
84 // The default length of monitor name is chosen to be 64 to avoid false sharing.
85 static const int MONITOR_NAME_LEN = 64;
86
87 class Monitor : public CHeapObj<mtInternal> {
88
89 public:
90 // A special lock: Is a lock where you are guaranteed not to block while you are
91 // holding it, i.e., no vm operation can happen, taking other locks, etc.
92 // NOTE: It is critical that the rank 'special' be the lowest (earliest)
93 // (except for "event"?) for the deadlock detection to work correctly.
94 // The rank native is only for use in Mutex's created by JVM_RawMonitorCreate,
95 // which being external to the VM are not subject to deadlock detection.
96 // The rank safepoint is used only for synchronization in reaching a
97 // safepoint and leaving a safepoint. It is only used for the Safepoint_lock
98 // currently. While at a safepoint no mutexes of rank safepoint are held
99 // by any thread.
100 // The rank named "leaf" is probably historical (and should
101 // be changed) -- mutexes of this rank aren't really leaf mutexes
102 // at all.
103 enum lock_types {
104 event,
105 special,
106 suspend_resume,
107 leaf = suspend_resume + 2,
108 safepoint = leaf + 10,
109 barrier = safepoint + 1,
110 nonleaf = barrier + 1,
111 max_nonleaf = nonleaf + 900,
112 native = max_nonleaf + 1
113 };
114
115 // The WaitSet and EntryList linked lists are composed of ParkEvents.
116 // I use ParkEvent instead of threads as ParkEvents are immortal and
117 // type-stable, meaning we can safely unpark() a possibly stale
118 // list element in the unlock()-path.
119
120 protected: // Monitor-Mutex metadata
121 SplitWord _LockWord ; // Contention queue (cxq) colocated with Lock-byte
122 enum LockWordBits { _LBIT=1 } ;
123 Thread * volatile _owner; // The owner of the lock
124 // Consider sequestering _owner on its own $line
125 // to aid future synchronization mechanisms.
126 ParkEvent * volatile _EntryList ; // List of threads waiting for entry
127 ParkEvent * volatile _OnDeck ; // heir-presumptive
128 volatile intptr_t _WaitLock [1] ; // Protects _WaitSet
129 ParkEvent * volatile _WaitSet ; // LL of ParkEvents
130 volatile bool _snuck; // Used for sneaky locking (evil).
131 int NotifyCount ; // diagnostic assist
132 char _name[MONITOR_NAME_LEN]; // Name of mutex
133
134 // Debugging fields for naming, deadlock detection, etc. (some only used in debug mode)
135 #ifndef PRODUCT
136 bool _allow_vm_block;
137 debug_only(int _rank;) // rank (to avoid/detect potential deadlocks)
138 debug_only(Monitor * _next;) // Used by a Thread to link up owned locks
139 debug_only(Thread* _last_owner;) // the last thread to own the lock
140 debug_only(static bool contains(Monitor * locks, Monitor * lock);)
141 debug_only(static Monitor * get_least_ranked_lock(Monitor * locks);)
142 debug_only(Monitor * get_least_ranked_lock_besides_this(Monitor * locks);)
143 debug_only(void ensure_lock_acquisition_consistency(const Thread* new_owner) const;)
144 debug_only(bool acquired_with_no_safepoint_check(void) const;)
145 debug_only(bool _acquired_with_no_safepoint_check;)
146 #endif
147
148 void set_owner_implementation(Thread* owner) PRODUCT_RETURN;
149 void check_prelock_state (Thread* thread) PRODUCT_RETURN;
150 void check_block_state (Thread* thread) PRODUCT_RETURN;
151
152 // platform-dependent support code can go here (in os_<os_family>.cpp)
153 public:
154 enum {
155 _no_safepoint_check_flag = true,
156 _allow_vm_block_flag = true,
157 _as_suspend_equivalent_flag = true
158 };
159
160 enum WaitResults {
161 CONDVAR_EVENT, // Wait returned because of condition variable notification
162 INTERRUPT_EVENT, // Wait returned because waiting thread was interrupted
163 NUMBER_WAIT_RESULTS
164 };
165
166 private:
167 int TrySpin (Thread * Self) ;
168 int TryLock () ;
169 int TryFast () ;
170 int AcquireOrPush (ParkEvent * ev) ;
171 void IUnlock (bool RelaxAssert) ;
172 void ILock (Thread * Self) ;
173 int IWait (Thread * Self, jlong timo);
174 int ILocked () ;
175
176 protected:
177 static void ClearMonitor (Monitor * m, const char* name = NULL) ;
178 Monitor() ;
179
180 public:
181 Monitor(int rank, const char *name, bool allow_vm_block=false);
182 ~Monitor();
183
184 // Wait until monitor is notified (or times out).
185 // Defaults are to make safepoint checks, wait time is forever (i.e.,
186 // zero), and not a suspend-equivalent condition. Returns true if wait
187 // times out; otherwise returns false.
188 bool wait(bool no_safepoint_check = !_no_safepoint_check_flag,
189 long timeout = 0,
190 bool as_suspend_equivalent = !_as_suspend_equivalent_flag);
191 bool notify();
192 bool notify_all();
193
194
195 void lock(); // prints out warning if VM thread blocks
196 void lock(Thread *thread); // overloaded with current thread
197 void unlock();
198 bool is_locked() const { return _owner != NULL; }
199
200 bool try_lock(); // Like lock(), but unblocking. It returns false instead
201
202 // Lock without safepoint check. Should ONLY be used by safepoint code and other code
203 // that is guaranteed not to block while running inside the VM.
204 void lock_without_safepoint_check();
205 void lock_without_safepoint_check (Thread * Self) ;
206
207 // Current owner - not not MT-safe. Can only be used to guarantee that
208 // the current running thread owns the lock
209 Thread* owner() const { return _owner; }
210 bool owned_by_self() const;
211
212 // Support for JVM_RawMonitorEnter & JVM_RawMonitorExit. These can be called by
213 // non-Java thread. (We should really have a RawMonitor abstraction)
214 void jvm_raw_lock();
215 void jvm_raw_unlock();
216 const char *name() const { return _name; }
217
218 void print_on_error(outputStream* st) const;
219
220 #ifndef PRODUCT
221 void print_on(outputStream* st) const;
222 void print() const { print_on(tty); }
223 debug_only(int rank() const { return _rank; })
224 bool allow_vm_block() { return _allow_vm_block; }
225
226 debug_only(Monitor *next() const { return _next; })
227 debug_only(void set_next(Monitor *next) { _next = next; })
228 #endif
229
230 void set_owner(Thread* owner) {
231 #ifndef PRODUCT
232 set_owner_implementation(owner);
233 debug_only(void verify_Monitor(Thread* thr));
234 #else
235 _owner = owner;
236 #endif
237 }
238
239 };
240
241 // Normally we'd expect Monitor to extend Mutex in the sense that a monitor
242 // constructed from pthreads primitives might extend a mutex by adding
243 // a condvar and some extra metadata. In fact this was the case until J2SE7.
244 //
245 // Currently, however, the base object is a monitor. Monitor contains all the
246 // logic for wait(), notify(), etc. Mutex extends monitor and restricts the
247 // visibility of wait(), notify(), and notify_all().
248 //
249 // Another viable alternative would have been to have Monitor extend Mutex and
250 // implement all the normal mutex and wait()-notify() logic in Mutex base class.
251 // The wait()-notify() facility would be exposed via special protected member functions
252 // (e.g., _Wait() and _Notify()) in Mutex. Monitor would extend Mutex and expose wait()
253 // as a call to _Wait(). That is, the public wait() would be a wrapper for the protected
254 // _Wait().
255 //
256 // An even better alternative is to simply eliminate Mutex:: and use Monitor:: instead.
257 // After all, monitors are sufficient for Java-level synchronization. At one point in time
258 // there may have been some benefit to having distinct mutexes and monitors, but that time
259 // has past.
260 //
261 // The Mutex/Monitor design parallels that of Java-monitors, being based on
262 // thread-specific park-unpark platform-specific primitives.
263
264
265 class Mutex : public Monitor { // degenerate Monitor
266 public:
267 Mutex (int rank, const char *name, bool allow_vm_block=false);
268 ~Mutex () ;
269 private:
270 bool notify () { ShouldNotReachHere(); return false; }
271 bool notify_all() { ShouldNotReachHere(); return false; }
272 bool wait (bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {
273 ShouldNotReachHere() ;
274 return false ;
275 }
276 };
277
278
279 #endif // SHARE_VM_RUNTIME_MUTEX_HPP