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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 class ParkEvent ;
71
72 // See orderAccess.hpp. We assume throughout the VM that mutex lock and
73 // try_lock do fence-lock-acquire, and that unlock does a release-unlock,
74 // *in that order*. If their implementations change such that these
75 // assumptions are violated, a whole lot of code will break.
76
77 // The default length of monitor name was originally chosen to be 64 to avoid
78 // false sharing. Now, PaddedMonitor is available for this purpose.
79 // TODO: Check if _name[MONITOR_NAME_LEN] should better get replaced by const char*.
80 static const int MONITOR_NAME_LEN = 64;
81
82 class Monitor : public CHeapObj<mtInternal> {
83
84 public:
85 // A special lock: Is a lock where you are guaranteed not to block while you are
86 // holding it, i.e., no vm operation can happen, taking other (blocking) locks, etc.
87 // The rank 'access' is similar to 'special' and has the same restrictions on usage.
88 // It is reserved for locks that may be required in order to perform memory accesses
89 // that require special barriers, e.g. SATB GC barriers, that in turn uses locks.
90 // Since memory accesses should be able to be performed pretty much anywhere
91 // in the code, that requires locks required for performing accesses being
92 // inherently a bit more special than even locks of the 'special' rank.
93 // NOTE: It is critical that the rank 'special' be the lowest (earliest)
94 // (except for "event" and "access") for the deadlock detection to work correctly.
95 // The rank native is only for use in Mutex's created by JVM_RawMonitorCreate,
96 // which being external to the VM are not subject to deadlock detection.
97 // The rank safepoint is used only for synchronization in reaching a
98 // safepoint and leaving a safepoint. It is only used for the Safepoint_lock
99 // currently. While at a safepoint no mutexes of rank safepoint are held
100 // by any thread.
101 // The rank named "leaf" is probably historical (and should
102 // be changed) -- mutexes of this rank aren't really leaf mutexes
103 // at all.
104 enum lock_types {
105 event,
106 access = event + 1,
107 special = access + 2,
108 suspend_resume = special + 1,
109 leaf = suspend_resume + 2,
110 safepoint = leaf + 10,
111 barrier = safepoint + 1,
112 nonleaf = barrier + 1,
113 max_nonleaf = nonleaf + 900,
114 native = max_nonleaf + 1
115 };
116
117 // The WaitSet and EntryList linked lists are composed of ParkEvents.
118 // I use ParkEvent instead of threads as ParkEvents are immortal and
119 // type-stable, meaning we can safely unpark() a possibly stale
120 // list element in the unlock()-path.
121
122 protected: // Monitor-Mutex metadata
123 SplitWord _LockWord ; // Contention queue (cxq) colocated with Lock-byte
124 Thread * volatile _owner; // The owner of the lock
125 // Consider sequestering _owner on its own $line
126 // to aid future synchronization mechanisms.
127 ParkEvent * volatile _EntryList ; // List of threads waiting for entry
128 ParkEvent * volatile _OnDeck ; // heir-presumptive
129 volatile intptr_t _WaitLock [1] ; // Protects _WaitSet
130 ParkEvent * volatile _WaitSet ; // LL of ParkEvents
131 volatile bool _snuck; // Used for sneaky locking (evil).
132 int NotifyCount ; // diagnostic assist
133 char _name[MONITOR_NAME_LEN]; // Name of mutex
134
135 // Debugging fields for naming, deadlock detection, etc. (some only used in debug mode)
136 #ifndef PRODUCT
137 bool _allow_vm_block;
138 debug_only(int _rank;) // rank (to avoid/detect potential deadlocks)
139 debug_only(Monitor * _next;) // Used by a Thread to link up owned locks
140 debug_only(Thread* _last_owner;) // the last thread to own the lock
141 debug_only(static bool contains(Monitor * locks, Monitor * lock);)
142 debug_only(static Monitor * get_least_ranked_lock(Monitor * locks);)
143 debug_only(Monitor * get_least_ranked_lock_besides_this(Monitor * locks);)
144 #endif
145
146 void set_owner_implementation(Thread* owner) PRODUCT_RETURN;
147 void check_prelock_state (Thread* thread) PRODUCT_RETURN;
148 void check_block_state (Thread* thread) PRODUCT_RETURN;
149
150 // platform-dependent support code can go here (in os_<os_family>.cpp)
151 public:
152 enum {
153 _no_safepoint_check_flag = true,
154 _allow_vm_block_flag = true,
155 _as_suspend_equivalent_flag = true
156 };
157
158 // Locks can be acquired with or without safepoint check.
159 // Monitor::lock and Monitor::lock_without_safepoint_check
160 // checks these flags when acquiring a lock to ensure
161 // consistent checking for each lock.
162 // A few existing locks will sometimes have a safepoint check and
163 // sometimes not, but these locks are set up in such a way to avoid deadlocks.
164 enum SafepointCheckRequired {
165 _safepoint_check_never, // Monitors with this value will cause errors
166 // when acquired with a safepoint check.
167 _safepoint_check_sometimes, // Certain locks are called sometimes with and
168 // sometimes without safepoint checks. These
169 // locks will not produce errors when locked.
170 _safepoint_check_always // Causes error if locked without a safepoint
171 // check.
172 };
173
174 NOT_PRODUCT(SafepointCheckRequired _safepoint_check_required;)
175
176 enum WaitResults {
177 CONDVAR_EVENT, // Wait returned because of condition variable notification
178 INTERRUPT_EVENT, // Wait returned because waiting thread was interrupted
179 NUMBER_WAIT_RESULTS
180 };
181
182 private:
183 int TrySpin (Thread * Self) ;
184 int TryLock () ;
185 int TryFast () ;
186 int AcquireOrPush (ParkEvent * ev) ;
187 void IUnlock (bool RelaxAssert) ;
188 void ILock (Thread * Self) ;
189 int IWait (Thread * Self, jlong timo);
190 int ILocked () ;
191
192 protected:
193 static void ClearMonitor (Monitor * m, const char* name = NULL) ;
194 Monitor() ;
195
196 public:
197 Monitor(int rank, const char *name, bool allow_vm_block = false,
198 SafepointCheckRequired safepoint_check_required = _safepoint_check_always);
199 ~Monitor();
200
201 // Wait until monitor is notified (or times out).
202 // Defaults are to make safepoint checks, wait time is forever (i.e.,
203 // zero), and not a suspend-equivalent condition. Returns true if wait
204 // times out; otherwise returns false.
205 bool wait(bool no_safepoint_check = !_no_safepoint_check_flag,
206 long timeout = 0,
207 bool as_suspend_equivalent = !_as_suspend_equivalent_flag);
208 bool notify();
209 bool notify_all();
210
211
212 void lock(); // prints out warning if VM thread blocks
213 void lock(Thread *thread); // overloaded with current thread
214 void unlock();
215 bool is_locked() const { return _owner != NULL; }
216
217 bool try_lock(); // Like lock(), but unblocking. It returns false instead
218
219 // Lock without safepoint check. Should ONLY be used by safepoint code and other code
220 // that is guaranteed not to block while running inside the VM.
221 void lock_without_safepoint_check();
222 void lock_without_safepoint_check (Thread * Self) ;
223
224 // Current owner - not not MT-safe. Can only be used to guarantee that
225 // the current running thread owns the lock
226 Thread* owner() const { return _owner; }
227 bool owned_by_self() const;
228
229 // Support for JVM_RawMonitorEnter & JVM_RawMonitorExit. These can be called by
230 // non-Java thread. (We should really have a RawMonitor abstraction)
231 void jvm_raw_lock();
232 void jvm_raw_unlock();
233 const char *name() const { return _name; }
234
235 void print_on_error(outputStream* st) const;
236
237 #ifndef PRODUCT
238 void print_on(outputStream* st) const;
239 void print() const { print_on(tty); }
240 debug_only(int rank() const { return _rank; })
241 bool allow_vm_block() { return _allow_vm_block; }
242
243 debug_only(Monitor *next() const { return _next; })
244 debug_only(void set_next(Monitor *next) { _next = next; })
245 #endif
246
247 void set_owner(Thread* owner) {
248 #ifndef PRODUCT
249 set_owner_implementation(owner);
250 debug_only(void verify_Monitor(Thread* thr));
251 #else
252 _owner = owner;
253 #endif
254 }
255
256 };
257
258 class PaddedMonitor : public Monitor {
259 enum {
260 CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Monitor),
261 PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1
262 };
263 char _padding[PADDING_LEN];
264 public:
265 PaddedMonitor(int rank, const char *name, bool allow_vm_block = false,
266 SafepointCheckRequired safepoint_check_required = _safepoint_check_always) :
267 Monitor(rank, name, allow_vm_block, safepoint_check_required) {};
268 };
269
270 // Normally we'd expect Monitor to extend Mutex in the sense that a monitor
271 // constructed from pthreads primitives might extend a mutex by adding
272 // a condvar and some extra metadata. In fact this was the case until J2SE7.
273 //
274 // Currently, however, the base object is a monitor. Monitor contains all the
275 // logic for wait(), notify(), etc. Mutex extends monitor and restricts the
276 // visibility of wait(), notify(), and notify_all().
277 //
278 // Another viable alternative would have been to have Monitor extend Mutex and
279 // implement all the normal mutex and wait()-notify() logic in Mutex base class.
280 // The wait()-notify() facility would be exposed via special protected member functions
281 // (e.g., _Wait() and _Notify()) in Mutex. Monitor would extend Mutex and expose wait()
282 // as a call to _Wait(). That is, the public wait() would be a wrapper for the protected
283 // _Wait().
284 //
285 // An even better alternative is to simply eliminate Mutex:: and use Monitor:: instead.
286 // After all, monitors are sufficient for Java-level synchronization. At one point in time
287 // there may have been some benefit to having distinct mutexes and monitors, but that time
288 // has past.
289 //
290 // The Mutex/Monitor design parallels that of Java-monitors, being based on
291 // thread-specific park-unpark platform-specific primitives.
292
293
294 class Mutex : public Monitor { // degenerate Monitor
295 public:
296 Mutex(int rank, const char *name, bool allow_vm_block = false,
297 SafepointCheckRequired safepoint_check_required = _safepoint_check_always);
298 // default destructor
299 private:
300 bool notify () { ShouldNotReachHere(); return false; }
301 bool notify_all() { ShouldNotReachHere(); return false; }
302 bool wait (bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {
303 ShouldNotReachHere() ;
304 return false ;
305 }
306 };
307
308 class PaddedMutex : public Mutex {
309 enum {
310 CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Mutex),
311 PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1
312 };
313 char _padding[PADDING_LEN];
314 public:
315 PaddedMutex(int rank, const char *name, bool allow_vm_block = false,
316 SafepointCheckRequired safepoint_check_required = _safepoint_check_always) :
317 Mutex(rank, name, allow_vm_block, safepoint_check_required) {};
318 };
319
320 #endif // SHARE_VM_RUNTIME_MUTEX_HPP