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 * 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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#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)] ;

} ;

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 was originally chosen to be 64 to avoid

// false sharing. Now, PaddedMonitor is available for this purpose.

// TODO: Check if _name[MONITOR_NAME_LEN] should better get replaced by const char*.

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 (blocking) locks, etc.

  // The rank 'access' is similar to 'special' and has the same restrictions on usage.

  // It is reserved for locks that may be required in order to perform memory accesses

  // that require special barriers, e.g. SATB GC barriers, that in turn uses locks.

  // The rank 'tty' is also similar to 'special' and has the same restrictions.

  // It is reserved for the tty_lock.

  // Since memory accesses should be able to be performed pretty much anywhere

  // in the code, that requires locks required for performing accesses being

  // inherently a bit more special than even locks of the 'special' rank.

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

  // (except for "event" and "access") for the deadlock detection 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,

       access         = event          +   1,

       tty            = access         +   2,

       special        = tty            +   1,

       suspend_resume = special        +   1,

       vmweak         = suspend_resume +   2,

       leaf           = vmweak         +   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

  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).

  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, bool safepoint_check) 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

  };

  // Locks can be acquired with or without safepoint check.

  // Monitor::lock and Monitor::lock_without_safepoint_check

  // checks these flags when acquiring a lock to ensure

  // consistent checking for each lock.

  // A few existing locks will sometimes have a safepoint check and

  // sometimes not, but these locks are set up in such a way to avoid deadlocks.

  enum SafepointCheckRequired {

    _safepoint_check_never,       // Monitors with this value will cause errors

                                  // when acquired with a safepoint check.

    _safepoint_check_sometimes,   // Certain locks are called sometimes with and

                                  // sometimes without safepoint checks. These

                                  // locks will not produce errors when locked.

    _safepoint_check_always       // Causes error if locked without a safepoint

                                  // check.

  };

  NOT_PRODUCT(SafepointCheckRequired _safepoint_check_required;)

  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,

          SafepointCheckRequired safepoint_check_required = _safepoint_check_always);

  ~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

  }

};

class PaddedMonitor : public Monitor {

  enum {

    CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Monitor),

    PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1

  };

  char _padding[PADDING_LEN];

 public:

  PaddedMonitor(int rank, const char *name, bool allow_vm_block = false,

               SafepointCheckRequired safepoint_check_required = _safepoint_check_always) :

    Monitor(rank, name, allow_vm_block, safepoint_check_required) {};

};

// 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

// visibility 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,

         SafepointCheckRequired safepoint_check_required = _safepoint_check_always);

  // default destructor

 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 ;

   }

};

class PaddedMutex : public Mutex {

  enum {

    CACHE_LINE_PADDING = (int)DEFAULT_CACHE_LINE_SIZE - (int)sizeof(Mutex),

    PADDING_LEN = CACHE_LINE_PADDING > 0 ? CACHE_LINE_PADDING : 1

  };

  char _padding[PADDING_LEN];

public:

  PaddedMutex(int rank, const char *name, bool allow_vm_block = false,

              SafepointCheckRequired safepoint_check_required = _safepoint_check_always) :

    Mutex(rank, name, allow_vm_block, safepoint_check_required) {};

};