1 /*
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   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   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.
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   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  *
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  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
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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