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. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 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 #endif 144 145 void set_owner_implementation(Thread* owner) PRODUCT_RETURN; 146 void check_prelock_state (Thread* thread) PRODUCT_RETURN; 147 void check_block_state (Thread* thread) PRODUCT_RETURN; 148 149 // platform-dependent support code can go here (in os_<os_family>.cpp) 150 public: 151 enum { 152 _no_safepoint_check_flag = true, 153 _allow_vm_block_flag = true, 154 _as_suspend_equivalent_flag = true 155 }; 156 157 // Locks can be acquired with or without safepoint check. 158 // Monitor::lock and Monitor::lock_without_safepoint_check 159 // checks these flags when acquiring a lock to ensure 160 // consistent checking for each lock. 161 // A few existing locks will sometimes have a safepoint check and 162 // sometimes not, but these locks are set up in such a way to avoid deadlocks. 163 enum SafepointCheckRequired { 164 _safepoint_check_never, // Monitors with this value will cause errors 165 // when acquired with a safepoint check. 166 _safepoint_check_sometimes, // Certain locks are called sometimes with and 167 // sometimes without safepoint checks. These 168 // locks will not produce errors when locked. 169 _safepoint_check_always // Causes error if locked without a safepoint 170 // check. 171 }; 172 173 NOT_PRODUCT(SafepointCheckRequired _safepoint_check_required;) 174 175 enum WaitResults { 176 CONDVAR_EVENT, // Wait returned because of condition variable notification 177 INTERRUPT_EVENT, // Wait returned because waiting thread was interrupted 178 NUMBER_WAIT_RESULTS 179 }; 180 181 private: 182 int TrySpin (Thread * Self) ; 183 int TryLock () ; 184 int TryFast () ; 185 int AcquireOrPush (ParkEvent * ev) ; 186 void IUnlock (bool RelaxAssert) ; 187 void ILock (Thread * Self) ; 188 int IWait (Thread * Self, jlong timo); 189 int ILocked () ; 190 191 protected: 192 static void ClearMonitor (Monitor * m, const char* name = NULL) ; 193 Monitor() ; 194 195 public: 196 Monitor(int rank, const char *name, bool allow_vm_block = false, 197 SafepointCheckRequired safepoint_check_required = _safepoint_check_always); 198 ~Monitor(); 199 200 // Wait until monitor is notified (or times out). 201 // Defaults are to make safepoint checks, wait time is forever (i.e., 202 // zero), and not a suspend-equivalent condition. Returns true if wait 203 // times out; otherwise returns false. 204 bool wait(bool no_safepoint_check = !_no_safepoint_check_flag, 205 long timeout = 0, 206 bool as_suspend_equivalent = !_as_suspend_equivalent_flag); 207 bool notify(); 208 bool notify_all(); 209 210 211 void lock(); // prints out warning if VM thread blocks 212 void lock(Thread *thread); // overloaded with current thread 213 void unlock(); 214 bool is_locked() const { return _owner != NULL; } 215 216 bool try_lock(); // Like lock(), but unblocking. It returns false instead 217 218 // Lock without safepoint check. Should ONLY be used by safepoint code and other code 219 // that is guaranteed not to block while running inside the VM. 220 void lock_without_safepoint_check(); 221 void lock_without_safepoint_check (Thread * Self) ; 222 223 // Current owner - not not MT-safe. Can only be used to guarantee that 224 // the current running thread owns the lock 225 Thread* owner() const { return _owner; } 226 bool owned_by_self() const; 227 228 // Support for JVM_RawMonitorEnter & JVM_RawMonitorExit. These can be called by 229 // non-Java thread. (We should really have a RawMonitor abstraction) 230 void jvm_raw_lock(); 231 void jvm_raw_unlock(); 232 const char *name() const { return _name; } 233 234 void print_on_error(outputStream* st) const; 235 236 #ifndef PRODUCT 237 void print_on(outputStream* st) const; 238 void print() const { print_on(tty); } 239 debug_only(int rank() const { return _rank; }) 240 bool allow_vm_block() { return _allow_vm_block; } 241 242 debug_only(Monitor *next() const { return _next; }) 243 debug_only(void set_next(Monitor *next) { _next = next; }) 244 #endif 245 246 void set_owner(Thread* owner) { 247 #ifndef PRODUCT 248 set_owner_implementation(owner); 249 debug_only(void verify_Monitor(Thread* thr)); 250 #else 251 _owner = owner; 252 #endif 253 } 254 255 }; 256 257 // Normally we'd expect Monitor to extend Mutex in the sense that a monitor 258 // constructed from pthreads primitives might extend a mutex by adding 259 // a condvar and some extra metadata. In fact this was the case until J2SE7. 260 // 261 // Currently, however, the base object is a monitor. Monitor contains all the 262 // logic for wait(), notify(), etc. Mutex extends monitor and restricts the 263 // visibility of wait(), notify(), and notify_all(). 264 // 265 // Another viable alternative would have been to have Monitor extend Mutex and 266 // implement all the normal mutex and wait()-notify() logic in Mutex base class. 267 // The wait()-notify() facility would be exposed via special protected member functions 268 // (e.g., _Wait() and _Notify()) in Mutex. Monitor would extend Mutex and expose wait() 269 // as a call to _Wait(). That is, the public wait() would be a wrapper for the protected 270 // _Wait(). 271 // 272 // An even better alternative is to simply eliminate Mutex:: and use Monitor:: instead. 273 // After all, monitors are sufficient for Java-level synchronization. At one point in time 274 // there may have been some benefit to having distinct mutexes and monitors, but that time 275 // has past. 276 // 277 // The Mutex/Monitor design parallels that of Java-monitors, being based on 278 // thread-specific park-unpark platform-specific primitives. 279 280 281 class Mutex : public Monitor { // degenerate Monitor 282 public: 283 Mutex(int rank, const char *name, bool allow_vm_block = false, 284 SafepointCheckRequired safepoint_check_required = _safepoint_check_always); 285 ~Mutex () ; 286 private: 287 bool notify () { ShouldNotReachHere(); return false; } 288 bool notify_all() { ShouldNotReachHere(); return false; } 289 bool wait (bool no_safepoint_check, long timeout, bool as_suspend_equivalent) { 290 ShouldNotReachHere() ; 291 return false ; 292 } 293 }; 294 295 296 #endif // SHARE_VM_RUNTIME_MUTEX_HPP