1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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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 was originally chosen to be 64 to avoid
85 // false sharing. Now, PaddedMonitor is available for this purpose.
86 // TODO: Check if _name[MONITOR_NAME_LEN] should better get replaced by const char*.
87 static const int MONITOR_NAME_LEN = 64;
88
89 class Monitor : public CHeapObj<mtInternal> {
90
91 public:
92 // A special lock: Is a lock where you are guaranteed not to block while you are
93 // holding it, i.e., no vm operation can happen, taking other locks, etc.
94 // NOTE: It is critical that the rank 'special' be the lowest (earliest)
95 // (except for "event"?) for the deadlock detection to work correctly.
96 // The rank access is reserved for locks that may be required to perform
97 // memory accesses that require special GC barriers, such as SATB barriers.
98 // Since memory accesses should be able to be performed pretty much anywhere
99 // in the code, that wannts being more special than the "special" rank.
100 // The rank native is only for use in Mutex's created by JVM_RawMonitorCreate,
101 // which being external to the VM are not subject to deadlock detection.
102 // The rank safepoint is used only for synchronization in reaching a
103 // safepoint and leaving a safepoint. It is only used for the Safepoint_lock
104 // currently. While at a safepoint no mutexes of rank safepoint are held
105 // by any thread.
106 // The rank named "leaf" is probably historical (and should
107 // be changed) -- mutexes of this rank aren't really leaf mutexes
108 // at all.
109 enum lock_types {
110 event,
111 access = event + 1,
112 special = access + 3,
113 suspend_resume,
114 leaf = suspend_resume + 2,
115 safepoint = leaf + 10,
116 barrier = safepoint + 1,
117 nonleaf = barrier + 1,
118 max_nonleaf = nonleaf + 900,
119 native = max_nonleaf + 1
120 };
121
122 // The WaitSet and EntryList linked lists are composed of ParkEvents.
123 // I use ParkEvent instead of threads as ParkEvents are immortal and
124 // type-stable, meaning we can safely unpark() a possibly stale
125 // list element in the unlock()-path.
126
127 protected: // Monitor-Mutex metadata
128 SplitWord _LockWord ; // Contention queue (cxq) colocated with Lock-byte
129 enum LockWordBits { _LBIT=1 } ;
130 Thread * volatile _owner; // The owner of the lock
131 // Consider sequestering _owner on its own $line
132 // to aid future synchronization mechanisms.
133 ParkEvent * volatile _EntryList ; // List of threads waiting for entry
134 ParkEvent * volatile _OnDeck ; // heir-presumptive
135 volatile intptr_t _WaitLock [1] ; // Protects _WaitSet
136 ParkEvent * volatile _WaitSet ; // LL of ParkEvents
137 volatile bool _snuck; // Used for sneaky locking (evil).
138 int NotifyCount ; // diagnostic assist
139 char _name[MONITOR_NAME_LEN]; // Name of mutex
140
141 // Debugging fields for naming, deadlock detection, etc. (some only used in debug mode)
142 #ifndef PRODUCT
143 bool _allow_vm_block;
144 debug_only(int _rank;) // rank (to avoid/detect potential deadlocks)
145 debug_only(Monitor * _next;) // Used by a Thread to link up owned locks
146 debug_only(Thread* _last_owner;) // the last thread to own the lock
147 debug_only(static bool contains(Monitor * locks, Monitor * lock);)
148 debug_only(static Monitor * get_least_ranked_lock(Monitor * locks);)
149 debug_only(Monitor * get_least_ranked_lock_besides_this(Monitor * locks);)
150 #endif
151
152 void set_owner_implementation(Thread* owner) PRODUCT_RETURN;
153 void check_prelock_state (Thread* thread) PRODUCT_RETURN;
154 void check_block_state (Thread* thread) PRODUCT_RETURN;
155
156 // platform-dependent support code can go here (in os_<os_family>.cpp)
157 public:
158 enum {
159 _no_safepoint_check_flag = true,
160 _allow_vm_block_flag = true,
161 _as_suspend_equivalent_flag = true
162 };
163
164 // Locks can be acquired with or without safepoint check.
165 // Monitor::lock and Monitor::lock_without_safepoint_check
166 // checks these flags when acquiring a lock to ensure
167 // consistent checking for each lock.
168 // A few existing locks will sometimes have a safepoint check and
169 // sometimes not, but these locks are set up in such a way to avoid deadlocks.
170 enum SafepointCheckRequired {
171 _safepoint_check_never, // Monitors with this value will cause errors
172 // when acquired with a safepoint check.
173 _safepoint_check_sometimes, // Certain locks are called sometimes with and
174 // sometimes without safepoint checks. These
175 // locks will not produce errors when locked.
176 _safepoint_check_always // Causes error if locked without a safepoint
177 // check.
178 };
179
180 NOT_PRODUCT(SafepointCheckRequired _safepoint_check_required;)
181
182 enum WaitResults {
183 CONDVAR_EVENT, // Wait returned because of condition variable notification
184 INTERRUPT_EVENT, // Wait returned because waiting thread was interrupted
185 NUMBER_WAIT_RESULTS
186 };
187
188 private:
189 int TrySpin (Thread * Self) ;
190 int TryLock () ;
191 int TryFast () ;
192 int AcquireOrPush (ParkEvent * ev) ;
193 void IUnlock (bool RelaxAssert) ;
194 void ILock (Thread * Self) ;
195 int IWait (Thread * Self, jlong timo);
196 int ILocked () ;
197
198 protected:
199 static void ClearMonitor (Monitor * m, const char* name = NULL) ;
200 Monitor() ;
201
202 public:
203 Monitor(int rank, const char *name, bool allow_vm_block = false,
204 SafepointCheckRequired safepoint_check_required = _safepoint_check_always);
205 ~Monitor();
206
207 // Wait until monitor is notified (or times out).
208 // Defaults are to make safepoint checks, wait time is forever (i.e.,
209 // zero), and not a suspend-equivalent condition. Returns true if wait
210 // times out; otherwise returns false.
211 bool wait(bool no_safepoint_check = !_no_safepoint_check_flag,
212 long timeout = 0,
213 bool as_suspend_equivalent = !_as_suspend_equivalent_flag);
214 bool notify();
215 bool notify_all();
216
217
218 void lock(); // prints out warning if VM thread blocks
219 void lock(Thread *thread); // overloaded with current thread
220 void unlock();
221 bool is_locked() const { return _owner != NULL; }
222
223 bool try_lock(); // Like lock(), but unblocking. It returns false instead
224
225 // Lock without safepoint check. Should ONLY be used by safepoint code and other code
226 // that is guaranteed not to block while running inside the VM.
227 void lock_without_safepoint_check();
228 void lock_without_safepoint_check (Thread * Self) ;
229
230 // Current owner - not not MT-safe. Can only be used to guarantee that
231 // the current running thread owns the lock
232 Thread* owner() const { return _owner; }
233 bool owned_by_self() const;
234
235 // Support for JVM_RawMonitorEnter & JVM_RawMonitorExit. These can be called by
236 // non-Java thread. (We should really have a RawMonitor abstraction)
237 void jvm_raw_lock();
238 void jvm_raw_unlock();
239 const char *name() const { return _name; }
240
241 void print_on_error(outputStream* st) const;
242
243 #ifndef PRODUCT
244 void print_on(outputStream* st) const;
245 void print() const { print_on(tty); }
246 debug_only(int rank() const { return _rank; })
247 bool allow_vm_block() { return _allow_vm_block; }
248
249 debug_only(Monitor *next() const { return _next; })
250 debug_only(void set_next(Monitor *next) { _next = next; })
251 #endif
252
253 void set_owner(Thread* owner) {
254 #ifndef PRODUCT
255 set_owner_implementation(owner);
256 debug_only(void verify_Monitor(Thread* thr));
257 #else
258 _owner = owner;
259 #endif
260 }
261
262 };
263
264 class PaddedMonitor : public Monitor {
265 enum {
266 CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Monitor),
267 PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1
268 };
269 char _padding[PADDING_LEN];
270 public:
271 PaddedMonitor(int rank, const char *name, bool allow_vm_block = false,
272 SafepointCheckRequired safepoint_check_required = _safepoint_check_always) :
273 Monitor(rank, name, allow_vm_block, safepoint_check_required) {};
274 };
275
276 // Normally we'd expect Monitor to extend Mutex in the sense that a monitor
277 // constructed from pthreads primitives might extend a mutex by adding
278 // a condvar and some extra metadata. In fact this was the case until J2SE7.
279 //
280 // Currently, however, the base object is a monitor. Monitor contains all the
281 // logic for wait(), notify(), etc. Mutex extends monitor and restricts the
282 // visibility of wait(), notify(), and notify_all().
283 //
284 // Another viable alternative would have been to have Monitor extend Mutex and
285 // implement all the normal mutex and wait()-notify() logic in Mutex base class.
286 // The wait()-notify() facility would be exposed via special protected member functions
287 // (e.g., _Wait() and _Notify()) in Mutex. Monitor would extend Mutex and expose wait()
288 // as a call to _Wait(). That is, the public wait() would be a wrapper for the protected
289 // _Wait().
290 //
291 // An even better alternative is to simply eliminate Mutex:: and use Monitor:: instead.
292 // After all, monitors are sufficient for Java-level synchronization. At one point in time
293 // there may have been some benefit to having distinct mutexes and monitors, but that time
294 // has past.
295 //
296 // The Mutex/Monitor design parallels that of Java-monitors, being based on
297 // thread-specific park-unpark platform-specific primitives.
298
299
300 class Mutex : public Monitor { // degenerate Monitor
301 public:
302 Mutex(int rank, const char *name, bool allow_vm_block = false,
303 SafepointCheckRequired safepoint_check_required = _safepoint_check_always);
304 ~Mutex () ;
305 private:
306 bool notify () { ShouldNotReachHere(); return false; }
307 bool notify_all() { ShouldNotReachHere(); return false; }
308 bool wait (bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {
309 ShouldNotReachHere() ;
310 return false ;
311 }
312 };
313
314 class PaddedMutex : public Mutex {
315 enum {
316 CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Mutex),
317 PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1
318 };
319 char _padding[PADDING_LEN];
320 public:
321 PaddedMutex(int rank, const char *name, bool allow_vm_block = false,
322 SafepointCheckRequired safepoint_check_required = _safepoint_check_always) :
323 Mutex(rank, name, allow_vm_block, safepoint_check_required) {};
324 };
325
326 #endif // SHARE_VM_RUNTIME_MUTEX_HPP