1 /* 2 * Copyright (c) 1998, 2018, 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 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<mtSynchronizer> { 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 // The rank 'tty' is also similar to 'special' and has the same restrictions. 91 // It is reserved for the tty_lock. 92 // Since memory accesses should be able to be performed pretty much anywhere 93 // in the code, that requires locks required for performing accesses being 94 // inherently a bit more special than even locks of the 'special' rank. 95 // NOTE: It is critical that the rank 'special' be the lowest (earliest) 96 // (except for "event" and "access") for the deadlock detection to work correctly. 97 // The rank native is only for use in Mutex's created by JVM_RawMonitorCreate, 98 // which being external to the VM are not subject to deadlock detection. 99 // The rank safepoint is used only for synchronization in reaching a 100 // safepoint and leaving a safepoint. It is only used for the Safepoint_lock 101 // currently. While at a safepoint no mutexes of rank safepoint are held 102 // by any thread. 103 // The rank named "leaf" is probably historical (and should 104 // be changed) -- mutexes of this rank aren't really leaf mutexes 105 // at all. 106 enum lock_types { 107 event, 108 access = event + 1, 109 tty = access + 2, 110 special = tty + 1, 111 suspend_resume = special + 1, 112 vmweak = suspend_resume + 2, 113 leaf = vmweak + 2, 114 safepoint = leaf + 10, 115 barrier = safepoint + 1, 116 nonleaf = barrier + 1, 117 max_nonleaf = nonleaf + 900, 118 native = max_nonleaf + 1 119 }; 120 121 // The WaitSet and EntryList linked lists are composed of ParkEvents. 122 // I use ParkEvent instead of threads as ParkEvents are immortal and 123 // type-stable, meaning we can safely unpark() a possibly stale 124 // list element in the unlock()-path. 125 126 protected: // Monitor-Mutex metadata 127 SplitWord _LockWord ; // Contention queue (cxq) colocated with Lock-byte 128 Thread * volatile _owner; // The owner of the lock 129 // Consider sequestering _owner on its own $line 130 // to aid future synchronization mechanisms. 131 ParkEvent * volatile _EntryList ; // List of threads waiting for entry 132 ParkEvent * volatile _OnDeck ; // heir-presumptive 133 volatile intptr_t _WaitLock [1] ; // Protects _WaitSet 134 ParkEvent * volatile _WaitSet ; // LL of ParkEvents 135 volatile bool _snuck; // Used for sneaky locking (evil). 136 char _name[MONITOR_NAME_LEN]; // Name of mutex 137 138 // Debugging fields for naming, deadlock detection, etc. (some only used in debug mode) 139 #ifndef PRODUCT 140 bool _allow_vm_block; 141 debug_only(int _rank;) // rank (to avoid/detect potential deadlocks) 142 debug_only(Monitor * _next;) // Used by a Thread to link up owned locks 143 debug_only(Thread* _last_owner;) // the last thread to own the lock 144 debug_only(static bool contains(Monitor * locks, Monitor * lock);) 145 debug_only(static Monitor * get_least_ranked_lock(Monitor * locks);) 146 debug_only(Monitor * get_least_ranked_lock_besides_this(Monitor * locks);) 147 #endif 148 149 void set_owner_implementation(Thread* owner) PRODUCT_RETURN; 150 void check_prelock_state (Thread* thread, bool safepoint_check) PRODUCT_RETURN; 151 void check_block_state (Thread* thread) PRODUCT_RETURN; 152 153 // platform-dependent support code can go here (in os_<os_family>.cpp) 154 public: 155 enum { 156 _no_safepoint_check_flag = true, 157 _allow_vm_block_flag = true, 158 _as_suspend_equivalent_flag = true 159 }; 160 161 // Locks can be acquired with or without safepoint check. 162 // Monitor::lock and Monitor::lock_without_safepoint_check 163 // checks these flags when acquiring a lock to ensure 164 // consistent checking for each lock. 165 // A few existing locks will sometimes have a safepoint check and 166 // sometimes not, but these locks are set up in such a way to avoid deadlocks. 167 enum SafepointCheckRequired { 168 _safepoint_check_never, // Monitors with this value will cause errors 169 // when acquired with a safepoint check. 170 _safepoint_check_sometimes, // Certain locks are called sometimes with and 171 // sometimes without safepoint checks. These 172 // locks will not produce errors when locked. 173 _safepoint_check_always // Causes error if locked without a safepoint 174 // check. 175 }; 176 177 NOT_PRODUCT(SafepointCheckRequired _safepoint_check_required;) 178 179 enum WaitResults { 180 CONDVAR_EVENT, // Wait returned because of condition variable notification 181 INTERRUPT_EVENT, // Wait returned because waiting thread was interrupted 182 NUMBER_WAIT_RESULTS 183 }; 184 185 private: 186 int TrySpin (Thread * Self) ; 187 int TryLock () ; 188 int TryFast () ; 189 int AcquireOrPush (ParkEvent * ev) ; 190 void IUnlock (bool RelaxAssert) ; 191 void ILock (Thread * Self) ; 192 int IWait (Thread * Self, jlong timo); 193 int ILocked () ; 194 195 protected: 196 static void ClearMonitor (Monitor * m, const char* name = NULL) ; 197 Monitor() ; 198 199 public: 200 Monitor(int rank, const char *name, bool allow_vm_block = false, 201 SafepointCheckRequired safepoint_check_required = _safepoint_check_always); 202 ~Monitor(); 203 204 // Wait until monitor is notified (or times out). 205 // Defaults are to make safepoint checks, wait time is forever (i.e., 206 // zero), and not a suspend-equivalent condition. Returns true if wait 207 // times out; otherwise returns false. 208 bool wait(bool no_safepoint_check = !_no_safepoint_check_flag, 209 long timeout = 0, 210 bool as_suspend_equivalent = !_as_suspend_equivalent_flag); 211 bool notify(); 212 bool notify_all(); 213 214 215 void lock(); // prints out warning if VM thread blocks 216 void lock(Thread *thread); // overloaded with current thread 217 void unlock(); is_locked() const218 bool is_locked() const { return _owner != NULL; } 219 220 bool try_lock(); // Like lock(), but unblocking. It returns false instead 221 222 // Lock without safepoint check. Should ONLY be used by safepoint code and other code 223 // that is guaranteed not to block while running inside the VM. 224 void lock_without_safepoint_check(); 225 void lock_without_safepoint_check (Thread * Self) ; 226 227 // Current owner - not not MT-safe. Can only be used to guarantee that 228 // the current running thread owns the lock owner() const229 Thread* owner() const { return _owner; } 230 bool owned_by_self() const; 231 232 // Support for JVM_RawMonitorEnter & JVM_RawMonitorExit. These can be called by 233 // non-Java thread. (We should really have a RawMonitor abstraction) 234 void jvm_raw_lock(); 235 void jvm_raw_unlock(); name() const236 const char *name() const { return _name; } 237 238 void print_on_error(outputStream* st) const; 239 240 #ifndef PRODUCT 241 void print_on(outputStream* st) const; print() const242 void print() const { print_on(::tty); } debug_only(int rank ()const{ return _rank; })243 debug_only(int rank() const { return _rank; }) 244 bool allow_vm_block() { return _allow_vm_block; } 245 246 debug_only(Monitor *next() const { return _next; }) debug_only(void set_next (Monitor * next){ _next = next; })247 debug_only(void set_next(Monitor *next) { _next = next; }) 248 #endif 249 250 void set_owner(Thread* owner) { 251 #ifndef PRODUCT 252 set_owner_implementation(owner); 253 debug_only(void verify_Monitor(Thread* thr)); 254 #else 255 _owner = owner; 256 #endif 257 } 258 259 }; 260 261 class PaddedMonitor : public Monitor { 262 enum { 263 CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Monitor), 264 PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1 265 }; 266 char _padding[PADDING_LEN]; 267 public: PaddedMonitor(int rank,const char * name,bool allow_vm_block=false,SafepointCheckRequired safepoint_check_required=_safepoint_check_always)268 PaddedMonitor(int rank, const char *name, bool allow_vm_block = false, 269 SafepointCheckRequired safepoint_check_required = _safepoint_check_always) : 270 Monitor(rank, name, allow_vm_block, safepoint_check_required) {}; 271 }; 272 273 // Normally we'd expect Monitor to extend Mutex in the sense that a monitor 274 // constructed from pthreads primitives might extend a mutex by adding 275 // a condvar and some extra metadata. In fact this was the case until J2SE7. 276 // 277 // Currently, however, the base object is a monitor. Monitor contains all the 278 // logic for wait(), notify(), etc. Mutex extends monitor and restricts the 279 // visibility of wait(), notify(), and notify_all(). 280 // 281 // Another viable alternative would have been to have Monitor extend Mutex and 282 // implement all the normal mutex and wait()-notify() logic in Mutex base class. 283 // The wait()-notify() facility would be exposed via special protected member functions 284 // (e.g., _Wait() and _Notify()) in Mutex. Monitor would extend Mutex and expose wait() 285 // as a call to _Wait(). That is, the public wait() would be a wrapper for the protected 286 // _Wait(). 287 // 288 // An even better alternative is to simply eliminate Mutex:: and use Monitor:: instead. 289 // After all, monitors are sufficient for Java-level synchronization. At one point in time 290 // there may have been some benefit to having distinct mutexes and monitors, but that time 291 // has past. 292 // 293 // The Mutex/Monitor design parallels that of Java-monitors, being based on 294 // thread-specific park-unpark platform-specific primitives. 295 296 297 class Mutex : public Monitor { // degenerate Monitor 298 public: 299 Mutex(int rank, const char *name, bool allow_vm_block = false, 300 SafepointCheckRequired safepoint_check_required = _safepoint_check_always); 301 // default destructor 302 private: notify()303 bool notify () { ShouldNotReachHere(); return false; } notify_all()304 bool notify_all() { ShouldNotReachHere(); return false; } wait(bool no_safepoint_check,long timeout,bool as_suspend_equivalent)305 bool wait (bool no_safepoint_check, long timeout, bool as_suspend_equivalent) { 306 ShouldNotReachHere() ; 307 return false ; 308 } 309 }; 310 311 class PaddedMutex : public Mutex { 312 enum { 313 CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Mutex), 314 PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1 315 }; 316 char _padding[PADDING_LEN]; 317 public: PaddedMutex(int rank,const char * name,bool allow_vm_block=false,SafepointCheckRequired safepoint_check_required=_safepoint_check_always)318 PaddedMutex(int rank, const char *name, bool allow_vm_block = false, 319 SafepointCheckRequired safepoint_check_required = _safepoint_check_always) : 320 Mutex(rank, name, allow_vm_block, safepoint_check_required) {}; 321 }; 322 323 #endif // SHARE_VM_RUNTIME_MUTEX_HPP 324