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 /*

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

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  * This code is free software; you can redistribute it and/or modify it

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  * published by the Free Software Foundation.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

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  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 #ifndef SHARE_VM_RUNTIME_MUTEX_HPP

 #define SHARE_VM_RUNTIME_MUTEX_HPP

 #include "memory/allocation.hpp"

 #include "runtime/os.hpp"

 #include "utilities/histogram.hpp"

 // The SplitWord construct allows us to colocate the contention queue

 // (cxq) with the lock-byte.  The queue elements are ParkEvents, which are

 // always aligned on 256-byte addresses - the least significant byte of

 // a ParkEvent is always 0.  Colocating the lock-byte with the queue

 // allows us to easily avoid what would otherwise be a race in lock()

 // if we were to use two completely separate fields for the contention queue

 // and the lock indicator.  Specifically, colocation renders us immune

 // from the race where a thread might enqueue itself in the lock() slow-path

 // immediately after the lock holder drops the outer lock in the unlock()

 // fast-path.

 //

 // Colocation allows us to use a fast-path unlock() form that uses

 // A MEMBAR instead of a CAS.  MEMBAR has lower local latency than CAS

 // on many platforms.

 //

 // See:

 // +  http://blogs.sun.com/dave/entry/biased_locking_in_hotspot

 // +  http://blogs.sun.com/dave/resource/synchronization-public2.pdf

 //

 // Note that we're *not* using word-tearing the classic sense.

 // The lock() fast-path will CAS the lockword and the unlock()

 // fast-path will store into the lock-byte colocated within the lockword.

 // We depend on the fact that all our reference platforms have

 // coherent and atomic byte accesses.  More precisely, byte stores

 // interoperate in a safe, sane, and expected manner with respect to

 // CAS, ST and LDs to the full-word containing the byte.

 // If you're porting HotSpot to a platform where that isn't the case

 // then you'll want change the unlock() fast path from:

 //    STB;MEMBAR #storeload; LDN

 // to a full-word CAS of the lockword.

 union SplitWord {   // full-word with separately addressable LSB

   volatile intptr_t FullWord ;

   volatile void * Address ;

   volatile jbyte Bytes [sizeof(intptr_t)] ;

 } ;

 // Endian-ness ... index of least-significant byte in SplitWord.Bytes[]

 #ifdef VM_LITTLE_ENDIAN

  #define _LSBINDEX 0

 #else

  #define _LSBINDEX (sizeof(intptr_t)-1)

 #endif

 class ParkEvent ;

 // See orderAccess.hpp.  We assume throughout the VM that mutex lock and

 // try_lock do fence-lock-acquire, and that unlock does a release-unlock,

 // *in that order*.  If their implementations change such that these

 // assumptions are violated, a whole lot of code will break.

 // The default length of monitor name is chosen to be 64 to avoid false sharing.

 static const int MONITOR_NAME_LEN = 64;

 class Monitor : public CHeapObj<mtInternal> {

  public:

   // A special lock: Is a lock where you are guaranteed not to block while you are

   // holding it, i.e., no vm operation can happen, taking other locks, etc.

   // NOTE: It is critical that the rank 'special' be the lowest (earliest)

   // (except for "event"?) for the deadlock dection to work correctly.

   // The rank native is only for use in Mutex's created by JVM_RawMonitorCreate,

   // which being external to the VM are not subject to deadlock detection.

   // The rank safepoint is used only for synchronization in reaching a

   // safepoint and leaving a safepoint.  It is only used for the Safepoint_lock

   // currently.  While at a safepoint no mutexes of rank safepoint are held

   // by any thread.

   // The rank named "leaf" is probably historical (and should

   // be changed) -- mutexes of this rank aren't really leaf mutexes

   // at all.

   enum lock_types {

        event,

        special,

        suspend_resume,

        leaf        = suspend_resume +   2,

        safepoint   = leaf           +  10,

        barrier     = safepoint      +   1,

        nonleaf     = barrier        +   1,

        max_nonleaf = nonleaf        + 900,

        native      = max_nonleaf    +   1

   };

   // The WaitSet and EntryList linked lists are composed of ParkEvents.

   // I use ParkEvent instead of threads as ParkEvents are immortal and

   // type-stable, meaning we can safely unpark() a possibly stale

   // list element in the unlock()-path.

  protected:                              // Monitor-Mutex metadata

   SplitWord _LockWord ;                  // Contention queue (cxq) colocated with Lock-byte

   enum LockWordBits { _LBIT=1 } ;

   Thread * volatile _owner;              // The owner of the lock

                                          // Consider sequestering _owner on its own $line

                                          // to aid future synchronization mechanisms.

   ParkEvent * volatile _EntryList ;      // List of threads waiting for entry

   ParkEvent * volatile _OnDeck ;         // heir-presumptive

   volatile intptr_t _WaitLock [1] ;      // Protects _WaitSet

   ParkEvent * volatile  _WaitSet ;       // LL of ParkEvents

   volatile bool     _snuck;              // Used for sneaky locking (evil).

   int NotifyCount ;                      // diagnostic assist

   char _name[MONITOR_NAME_LEN];          // Name of mutex

   // Debugging fields for naming, deadlock detection, etc. (some only used in debug mode)

 #ifndef PRODUCT

   bool      _allow_vm_block;

   debug_only(int _rank;)                 // rank (to avoid/detect potential deadlocks)

   debug_only(Monitor * _next;)           // Used by a Thread to link up owned locks

   debug_only(Thread* _last_owner;)       // the last thread to own the lock

   debug_only(static bool contains(Monitor * locks, Monitor * lock);)

   debug_only(static Monitor * get_least_ranked_lock(Monitor * locks);)

   debug_only(Monitor * get_least_ranked_lock_besides_this(Monitor * locks);)

 #endif

   void set_owner_implementation(Thread* owner)                        PRODUCT_RETURN;

   void check_prelock_state     (Thread* thread)                       PRODUCT_RETURN;

   void check_block_state       (Thread* thread)                       PRODUCT_RETURN;

   // platform-dependent support code can go here (in os_<os_family>.cpp)

  public:

   enum {

     _no_safepoint_check_flag    = true,

     _allow_vm_block_flag        = true,

     _as_suspend_equivalent_flag = true

   };

   enum WaitResults {

     CONDVAR_EVENT,         // Wait returned because of condition variable notification

     INTERRUPT_EVENT,       // Wait returned because waiting thread was interrupted

     NUMBER_WAIT_RESULTS

   };

  private:

    int  TrySpin (Thread * Self) ;

    int  TryLock () ;

    int  TryFast () ;

    int  AcquireOrPush (ParkEvent * ev) ;

    void IUnlock (bool RelaxAssert) ;

    void ILock (Thread * Self) ;

    int  IWait (Thread * Self, jlong timo);

    int  ILocked () ;

  protected:

    static void ClearMonitor (Monitor * m, const char* name = NULL) ;

    Monitor() ;

  public:

   Monitor(int rank, const char *name, bool allow_vm_block=false);

   ~Monitor();

   // Wait until monitor is notified (or times out).

   // Defaults are to make safepoint checks, wait time is forever (i.e.,

   // zero), and not a suspend-equivalent condition. Returns true if wait

   // times out; otherwise returns false.

   bool wait(bool no_safepoint_check = !_no_safepoint_check_flag,

             long timeout = 0,

             bool as_suspend_equivalent = !_as_suspend_equivalent_flag);

   bool notify();

   bool notify_all();

   void lock(); // prints out warning if VM thread blocks

   void lock(Thread *thread); // overloaded with current thread

   void unlock();

   bool is_locked() const                     { return _owner != NULL; }

   bool try_lock(); // Like lock(), but unblocking. It returns false instead

   // Lock without safepoint check. Should ONLY be used by safepoint code and other code

   // that is guaranteed not to block while running inside the VM.

   void lock_without_safepoint_check();

   void lock_without_safepoint_check (Thread * Self) ;

   // Current owner - not not MT-safe. Can only be used to guarantee that

   // the current running thread owns the lock

   Thread* owner() const         { return _owner; }

   bool owned_by_self() const;

   // Support for JVM_RawMonitorEnter & JVM_RawMonitorExit. These can be called by

   // non-Java thread. (We should really have a RawMonitor abstraction)

   void jvm_raw_lock();

   void jvm_raw_unlock();

   const char *name() const                  { return _name; }

   void print_on_error(outputStream* st) const;

   #ifndef PRODUCT

     void print_on(outputStream* st) const;

     void print() const                      { print_on(tty); }

     debug_only(int    rank() const          { return _rank; })

     bool   allow_vm_block()                 { return _allow_vm_block; }

     debug_only(Monitor *next()  const         { return _next; })

     debug_only(void   set_next(Monitor *next) { _next = next; })

   #endif

   void set_owner(Thread* owner) {

   #ifndef PRODUCT

     set_owner_implementation(owner);

     debug_only(void verify_Monitor(Thread* thr));

   #else

     _owner = owner;

   #endif

   }

 };

 // Normally we'd expect Monitor to extend Mutex in the sense that a monitor

 // constructed from pthreads primitives might extend a mutex by adding

 // a condvar and some extra metadata.  In fact this was the case until J2SE7.

 //

 // Currently, however, the base object is a monitor.  Monitor contains all the

 // logic for wait(), notify(), etc.   Mutex extends monitor and restricts the

 // visiblity of wait(), notify(), and notify_all().

 //

 // Another viable alternative would have been to have Monitor extend Mutex and

 // implement all the normal mutex and wait()-notify() logic in Mutex base class.

 // The wait()-notify() facility would be exposed via special protected member functions

 // (e.g., _Wait() and _Notify()) in Mutex.  Monitor would extend Mutex and expose wait()

 // as a call to _Wait().  That is, the public wait() would be a wrapper for the protected

 // _Wait().

 //

 // An even better alternative is to simply eliminate Mutex:: and use Monitor:: instead.

 // After all, monitors are sufficient for Java-level synchronization.   At one point in time

 // there may have been some benefit to having distinct mutexes and monitors, but that time

 // has past.

 //

 // The Mutex/Monitor design parallels that of Java-monitors, being based on

 // thread-specific park-unpark platform-specific primitives.

 class Mutex : public Monitor {      // degenerate Monitor

  public:

    Mutex (int rank, const char *name, bool allow_vm_block=false);

    ~Mutex () ;

  private:

    bool notify ()    { ShouldNotReachHere(); return false; }

    bool notify_all() { ShouldNotReachHere(); return false; }

    bool wait (bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {

      ShouldNotReachHere() ;

      return false ;

    }

 };

 #endif // SHARE_VM_RUNTIME_MUTEX_HPP