1 /*
2 * Copyright (c) 1998, 2014, Oracle and/or its affiliates. All rights reserved.
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 class MonitorBase : public CHeapObj<mtInternal> {
85 public:
86 // A special lock: Is a lock where you are guaranteed not to block while you are
87 // holding it, i.e., no vm operation can happen, taking other locks, etc.
88 // NOTE: It is critical that the rank 'special' be the lowest (earliest)
89 // (except for "event"?) for the deadlock detection to work correctly.
90 // The rank native is only for use in Mutex's created by JVM_RawMonitorCreate,
91 // which being external to the VM are not subject to deadlock detection.
92 // The rank safepoint is used only for synchronization in reaching a
93 // safepoint and leaving a safepoint. It is only used for the Safepoint_lock
94 // currently. While at a safepoint no mutexes of rank safepoint are held
95 // by any thread.
96 // The rank named "leaf" is probably historical (and should
97 // be changed) -- mutexes of this rank aren't really leaf mutexes
98 // at all.
99 enum lock_types {
100 event,
101 special,
102 suspend_resume,
103 leaf = suspend_resume + 2,
104 safepoint = leaf + 10,
105 barrier = safepoint + 1,
106 nonleaf = barrier + 1,
107 max_nonleaf = nonleaf + 900,
108 native = max_nonleaf + 1
109 };
110
111 // The WaitSet and EntryList linked lists are composed of ParkEvents.
112 // I use ParkEvent instead of threads as ParkEvents are immortal and
113 // type-stable, meaning we can safely unpark() a possibly stale
114 // list element in the unlock()-path.
115
116 protected: // Monitor-Mutex metadata
117 SplitWord _LockWord ; // Contention queue (cxq) colocated with Lock-byte
118 enum LockWordBits { _LBIT=1 } ;
119 Thread * volatile _owner; // The owner of the lock
120 // Consider sequestering _owner on its own $line
121 // to aid future synchronization mechanisms.
122 ParkEvent * volatile _EntryList ; // List of threads waiting for entry
123 ParkEvent * volatile _OnDeck ; // heir-presumptive
124 volatile intptr_t _WaitLock [1] ; // Protects _WaitSet
125 ParkEvent * volatile _WaitSet ; // LL of ParkEvents
126 volatile bool _snuck; // Used for sneaky locking (evil).
127 int NotifyCount ; // diagnostic assist
128
129 // Debugging fields for naming, deadlock detection, etc. (some only used in debug mode)
130 #ifndef PRODUCT
131 bool _allow_vm_block;
132 debug_only(int _rank;) // rank (to avoid/detect potential deadlocks)
133 debug_only(Monitor * _next;) // Used by a Thread to link up owned locks
134 debug_only(Thread* _last_owner;) // the last thread to own the lock
135 #endif
136
137 // platform-dependent support code can go here (in os_<os_family>.cpp)
138 public:
139 enum {
140 _no_safepoint_check_flag = true,
141 _allow_vm_block_flag = true,
142 _as_suspend_equivalent_flag = true
143 };
144
145 // Locks can be acquired with or without safepoint check.
146 // Monitor::lock and Monitor::lock_without_safepoint_check
147 // checks these flags when acquiring a lock to ensure
148 // consistent checking for each lock.
149 // A few existing locks will sometimes have a safepoint check and
150 // sometimes not, but these locks are set up in such a way to avoid deadlocks.
151 enum SafepointCheckRequired {
152 _safepoint_check_never, // Monitors with this value will cause errors
153 // when acquired with a safepoint check.
154 _safepoint_check_sometimes, // Certain locks are called sometimes with and
155 // sometimes without safepoint checks. These
156 // locks will not produce errors when locked.
157 _safepoint_check_always // Causes error if locked without a safepoint
158 // check.
159 };
160
161 NOT_PRODUCT(SafepointCheckRequired _safepoint_check_required;)
162
163 enum WaitResults {
164 CONDVAR_EVENT, // Wait returned because of condition variable notification
165 INTERRUPT_EVENT, // Wait returned because waiting thread was interrupted
166 NUMBER_WAIT_RESULTS
167 };
168 };
169
170 class Monitor : public MonitorBase {
171 protected:
172 // The default length of monitor name is chosen to avoid false sharing.
173 enum {
174 CACHE_LINE_PADDING = DEFAULT_CACHE_LINE_SIZE - sizeof(MonitorBase),
175 MONITOR_NAME_LEN = CACHE_LINE_PADDING > 64 ? CACHE_LINE_PADDING : 64
176 };
177 char _name[MONITOR_NAME_LEN]; // Name of mutex
178 // Other fields should be declared in MonitorBase.
179
180 public:
181 #ifndef PRODUCT
182 debug_only(static bool contains(Monitor * locks, Monitor * lock);)
183 debug_only(static Monitor * get_least_ranked_lock(Monitor * locks);)
184 debug_only(Monitor * get_least_ranked_lock_besides_this(Monitor * locks);)
185 #endif
186
187 void set_owner_implementation(Thread* owner) PRODUCT_RETURN;
188 void check_prelock_state (Thread* thread) PRODUCT_RETURN;
189 void check_block_state (Thread* thread) PRODUCT_RETURN;
190
191 private:
192 int TrySpin (Thread * Self) ;
193 int TryLock () ;
194 int TryFast () ;
195 int AcquireOrPush (ParkEvent * ev) ;
196 void IUnlock (bool RelaxAssert) ;
197 void ILock (Thread * Self) ;
198 int IWait (Thread * Self, jlong timo);
199 int ILocked () ;
200
201 protected:
202 static void ClearMonitor (Monitor * m, const char* name = NULL) ;
203 Monitor() ;
204
205 public:
206 Monitor(int rank, const char *name, bool allow_vm_block = false,
207 SafepointCheckRequired safepoint_check_required = _safepoint_check_always);
208 ~Monitor();
209
210 // Wait until monitor is notified (or times out).
211 // Defaults are to make safepoint checks, wait time is forever (i.e.,
212 // zero), and not a suspend-equivalent condition. Returns true if wait
213 // times out; otherwise returns false.
214 bool wait(bool no_safepoint_check = !_no_safepoint_check_flag,
215 long timeout = 0,
216 bool as_suspend_equivalent = !_as_suspend_equivalent_flag);
217 bool notify();
218 bool notify_all();
219
220
221 void lock(); // prints out warning if VM thread blocks
222 void lock(Thread *thread); // overloaded with current thread
223 void unlock();
224 bool is_locked() const { return _owner != NULL; }
225
226 bool try_lock(); // Like lock(), but unblocking. It returns false instead
227
228 // Lock without safepoint check. Should ONLY be used by safepoint code and other code
229 // that is guaranteed not to block while running inside the VM.
230 void lock_without_safepoint_check();
231 void lock_without_safepoint_check (Thread * Self) ;
232
233 // Current owner - not not MT-safe. Can only be used to guarantee that
234 // the current running thread owns the lock
235 Thread* owner() const { return _owner; }
236 bool owned_by_self() const;
237
238 // Support for JVM_RawMonitorEnter & JVM_RawMonitorExit. These can be called by
239 // non-Java thread. (We should really have a RawMonitor abstraction)
240 void jvm_raw_lock();
241 void jvm_raw_unlock();
242 const char *name() const { return _name; }
243
244 void print_on_error(outputStream* st) const;
245
246 #ifndef PRODUCT
247 void print_on(outputStream* st) const;
248 void print() const { print_on(tty); }
249 debug_only(int rank() const { return _rank; })
250 bool allow_vm_block() { return _allow_vm_block; }
251
252 debug_only(Monitor *next() const { return _next; })
253 debug_only(void set_next(Monitor *next) { _next = next; })
254 #endif
255
256 void set_owner(Thread* owner) {
257 #ifndef PRODUCT
258 set_owner_implementation(owner);
259 debug_only(void verify_Monitor(Thread* thr));
260 #else
261 _owner = owner;
262 #endif
263 }
264
265 };
266
267 // Normally we'd expect Monitor to extend Mutex in the sense that a monitor
268 // constructed from pthreads primitives might extend a mutex by adding
269 // a condvar and some extra metadata. In fact this was the case until J2SE7.
270 //
271 // Currently, however, the base object is a monitor. Monitor contains all the
272 // logic for wait(), notify(), etc. Mutex extends monitor and restricts the
273 // visibility of wait(), notify(), and notify_all().
274 //
275 // Another viable alternative would have been to have Monitor extend Mutex and
276 // implement all the normal mutex and wait()-notify() logic in Mutex base class.
277 // The wait()-notify() facility would be exposed via special protected member functions
278 // (e.g., _Wait() and _Notify()) in Mutex. Monitor would extend Mutex and expose wait()
279 // as a call to _Wait(). That is, the public wait() would be a wrapper for the protected
280 // _Wait().
281 //
282 // An even better alternative is to simply eliminate Mutex:: and use Monitor:: instead.
283 // After all, monitors are sufficient for Java-level synchronization. At one point in time
284 // there may have been some benefit to having distinct mutexes and monitors, but that time
285 // has past.
286 //
287 // The Mutex/Monitor design parallels that of Java-monitors, being based on
288 // thread-specific park-unpark platform-specific primitives.
289
290
291 class Mutex : public Monitor { // degenerate Monitor
292 public:
293 Mutex(int rank, const char *name, bool allow_vm_block = false,
294 SafepointCheckRequired safepoint_check_required = _safepoint_check_always);
295 ~Mutex () ;
296 private:
297 bool notify () { ShouldNotReachHere(); return false; }
298 bool notify_all() { ShouldNotReachHere(); return false; }
299 bool wait (bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {
300 ShouldNotReachHere() ;
301 return false ;
302 }
303 };
304
305
306 #endif // SHARE_VM_RUNTIME_MUTEX_HPP