natobject.cc

俄罗斯高人Mamaich的Pocket gcc编译器（运行在PocketPC上）的全部源代码。

  				// to acquire the lock, and hence request
  				// conversion to heavyweight status.
#   define FLAGS (LOCKED | HEAVY | REQUEST_CONVERSION)
  volatile _Jv_ThreadId_t light_thr_id;
				// Thr_id of holder of lightweight lock.
  				// Only updated by lightweight lock holder.
				// Must be recognizably invalid if the
				// lightweight lock is not held.
#   define INVALID_THREAD_ID 0  // Works for Linux?
				// If zero doesn't work, we have to
				// initialize lock table.
  volatile unsigned short light_count;
				// Number of times the lightweight lock
  				// is held minus one.  Zero if lightweight
  				// lock is not held.
  unsigned short heavy_count; 	// Total number of times heavyweight locks
  				// associated with this hash entry are held
  				// or waiting to be acquired.
  				// Threads in wait() are included eventhough
  				// they have temporarily released the lock.
  struct heavy_lock * heavy_locks;
  				// Chain of heavy locks.  Protected
  				// by lockbit for he.  Locks may
  				// remain allocated here even if HEAVY
  				// is not set and heavy_count is 0.
  				// If a lightweight and heavyweight lock
  				// correspond to the same address, the
  				// lightweight lock is the right one.
};

#ifndef JV_SYNC_TABLE_SZ
# define JV_SYNC_TABLE_SZ 2048
#endif

hash_entry light_locks[JV_SYNC_TABLE_SZ];

#define JV_SYNC_HASH(p) (((long)p ^ ((long)p >> 10)) % JV_SYNC_TABLE_SZ)

// Note that the light_locks table is scanned conservatively by the
// collector.  It is essential the the heavy_locks field is scanned.
// Currently the address field may or may not cause the associated object
// to be retained, depending on whether flag bits are set.
// This means that we can conceivable get an unexpected deadlock if
// 1) Object at address A is locked.
// 2) The client drops A without unlocking it.
// 3) Flag bits in the address entry are set, so the collector reclaims
//    the object at A.
// 4) A is reallocated, and an attempt is made to lock the result.
// This could be fixed by scanning light_locks in a more customized
// manner that ignores the flag bits.  But it can only happen with hand
// generated semi-illegal .class files, and then it doesn't present a
// security hole.

#ifdef LOCK_DEBUG
  void print_he(hash_entry *he)
  {
     fprintf(stderr, "lock hash entry = %p, index = %d, address = 0x%lx\n"
		     "\tlight_thr_id = 0x%lx, light_count = %d, "
		     "heavy_count = %d\n\theavy_locks:", he,
		     he - light_locks, he -> address, he -> light_thr_id,
		     he -> light_count, he -> heavy_count);
     print_hl_list(he -> heavy_locks);
     fprintf(stderr, "\n");
  }
#endif /* LOCK_DEBUG */

static bool mp = false; // Known multiprocesssor.

// Wait for roughly 2^n units, touching as little memory as possible.
static void
spin(unsigned n)
{
  const unsigned MP_SPINS = 10;
  const unsigned YIELDS = 4;
  const unsigned SPINS_PER_UNIT = 30;
  const unsigned MIN_SLEEP_USECS = 2001; // Shorter times spin under Linux.
  const unsigned MAX_SLEEP_USECS = 200000;
  static unsigned spin_limit = 0;
  static unsigned yield_limit = YIELDS;
  static bool spin_initialized = false;

  if (!spin_initialized)
    {
      mp = is_mp();
      if (mp)
	{
	  spin_limit = MP_SPINS;
	  yield_limit = MP_SPINS + YIELDS;
	}
      spin_initialized = true;
    }
  if (n < spin_limit)
    {
      unsigned i = SPINS_PER_UNIT << n;
      for (; i > 0; --i)
        __asm__ __volatile__("");
    }
  else if (n < yield_limit)
    {
      _Jv_ThreadYield();
    }
  else
    {
      unsigned duration = MIN_SLEEP_USECS << (n - yield_limit);
      if (n >= 15 + yield_limit || duration > MAX_SLEEP_USECS)
        duration = MAX_SLEEP_USECS;
      _Jv_platform_usleep(duration);
    }
}

// Wait for a hash entry to become unlocked.
static void
wait_unlocked (hash_entry *he)
{
  unsigned i = 0;
  while (he -> address & LOCKED)
    spin (i++);
}

// Return the heavy lock for addr if it was already allocated.
// The client passes in the appropriate hash_entry.
// We hold the lock for he.
static inline heavy_lock *
find_heavy (obj_addr_t addr, hash_entry *he)
{
  heavy_lock *hl = he -> heavy_locks;
  while (hl != 0 && hl -> address != addr) hl = hl -> next;
  return hl;
}

// Unlink the heavy lock for the given address from its hash table chain.
// Dies miserably and conspicuously if it's not there, since that should
// be impossible.
static inline void
unlink_heavy (obj_addr_t addr, hash_entry *he)
{
  heavy_lock **currentp = &(he -> heavy_locks);
  while ((*currentp) -> address != addr)
    currentp = &((*currentp) -> next);
  *currentp = (*currentp) -> next;
}

// Finalization procedure for objects that have associated heavy-weight
// locks.  This may replace the real finalization procedure.
static void
heavy_lock_obj_finalization_proc (void *obj, void *cd)
{
  heavy_lock *hl = (heavy_lock *)cd;

// This only addresses misalignment of statics, not heap objects.  It
// works only because registering statics for finalization is a noop,
// no matter what the least significant bits are.
#ifdef JV_LINKER_CANNOT_8BYTE_ALIGN_STATICS
  obj_addr_t addr = (obj_addr_t)obj & ~((obj_addr_t)0x7);
#else
  obj_addr_t addr = (obj_addr_t)obj;
#endif
  hash_entry *he = light_locks + JV_SYNC_HASH(addr);
  obj_addr_t he_address = (he -> address & ~LOCKED);

  // Acquire lock bit immediately.  It's possible that the hl was already
  // destroyed while we were waiting for the finalizer to run.  If it
  // was, the address field was set to zero.  The address filed access is
  // protected by the lock bit to ensure that we do this exactly once.
  // The lock bit also protects updates to the objects finalizer.
  while (!compare_and_swap(&(he -> address), he_address, he_address|LOCKED ))
    {
      // Hash table entry is currently locked.  We can't safely 
      // touch the list of heavy locks.  
      wait_unlocked(he);
      he_address = (he -> address & ~LOCKED);
    }
  if (0 == hl -> address)
    {
      // remove_all_heavy destroyed hl, and took care of the real finalizer.
      release_set(&(he -> address), he_address);
      return;
    }
  assert(hl -> address == addr);
  GC_finalization_proc old_finalization_proc = hl -> old_finalization_proc;
  if (old_finalization_proc != 0)
    {
      // We still need to run a real finalizer.  In an idealized
      // world, in which people write thread-safe finalizers, that is
      // likely to require synchronization.  Thus we reregister
      // ourselves as the only finalizer, and simply run the real one.
      // Thus we don't clean up the lock yet, but we're likely to do so
      // on the next GC cycle.
      // It's OK if remove_all_heavy actually destroys the heavy lock,
      // since we've updated old_finalization_proc, and thus the user's
      // finalizer won't be rerun.
      void * old_client_data = hl -> old_client_data;
      hl -> old_finalization_proc = 0;
      hl -> old_client_data = 0;
#     ifdef HAVE_BOEHM_GC
        GC_REGISTER_FINALIZER_NO_ORDER(obj, heavy_lock_obj_finalization_proc, cd, 0, 0);
#     endif
      release_set(&(he -> address), he_address);
      old_finalization_proc(obj, old_client_data);
    }
  else
    {
      // The object is really dead, although it's conceivable that
      // some thread may still be in the process of releasing the
      // heavy lock.  Unlink it and, if necessary, register a finalizer
      // to destroy sync_info.
      unlink_heavy(addr, he);
      hl -> address = 0; 	// Don't destroy it again.
      release_set(&(he -> address), he_address);
#     if defined (_Jv_HaveCondDestroy) || defined (_Jv_HaveMutexDestroy)
        // Make sure lock is not held and then destroy condvar and mutex.
        _Jv_MutexLock(&(hl->si.mutex));
        _Jv_MutexUnlock(&(hl->si.mutex));
        heavy_lock_finalization_proc (hl);
#     endif
    }
}

// We hold the lock on he, and heavy_count is 0.
// Release the lock by replacing the address with new_address_val.
// Remove all heavy locks on the list.  Note that the only possible way
// in which a lock may still be in use is if it's in the process of
// being unlocked.
static void
remove_all_heavy (hash_entry *he, obj_addr_t new_address_val)
{
  assert(he -> heavy_count == 0);
  assert(he -> address & LOCKED);
  heavy_lock *hl = he -> heavy_locks;
  he -> heavy_locks = 0;
  // We would really like to release the lock bit here.  Unfortunately, that
  // Creates a race between or finalizer removal, and the potential
  // reinstallation of a new finalizer as a new heavy lock is created.
  // This may need to be revisited.
  for(; 0 != hl; hl = hl->next)
    {
      obj_addr_t obj = hl -> address;
      assert(0 != obj);	// If this was previously finalized, it should no
      			// longer appear on our list.
      hl -> address = 0; // Finalization proc might still see it after we
      			 // finish.
      GC_finalization_proc old_finalization_proc = hl -> old_finalization_proc;
      void * old_client_data = hl -> old_client_data;
#     ifdef HAVE_BOEHM_GC
	// Remove our finalization procedure.
        // Reregister the clients if applicable.
          GC_REGISTER_FINALIZER_NO_ORDER((GC_PTR)obj, old_finalization_proc,
			  		 old_client_data, 0, 0);
      	  // Note that our old finalization procedure may have been
          // previously determined to be runnable, and may still run.
      	  // FIXME - direct dependency on boehm GC.
#     endif
#     if defined (_Jv_HaveCondDestroy) || defined (_Jv_HaveMutexDestroy)
        // Wait for a possible lock holder to finish unlocking it.
        // This is only an issue if we have to explicitly destroy the mutex
        // or possibly if we have to destroy a condition variable that is
        // still being notified.
          _Jv_MutexLock(&(hl->si.mutex));
          _Jv_MutexUnlock(&(hl->si.mutex));
          heavy_lock_finalization_proc (hl);
#     endif
    }
  release_set(&(he -> address), new_address_val);
}

// We hold the lock on he and heavy_count is 0.
// We release it by replacing the address field with new_address_val.
// Remove all heavy locks on the list if the list is sufficiently long.
// This is called periodically to avoid very long lists of heavy locks.
// This seems to otherwise become an issue with SPECjbb, for example.
static inline void
maybe_remove_all_heavy (hash_entry *he, obj_addr_t new_address_val)
{
  static const int max_len = 5;
  heavy_lock *hl = he -> heavy_locks;

  for (int i = 0; i < max_len; ++i)
    {
      if (0 == hl) 
	{
  	  release_set(&(he -> address), new_address_val);
	  return;
	}
      hl = hl -> next;
    }
  remove_all_heavy(he, new_address_val);
}

// Allocate a new heavy lock for addr, returning its address.
// Assumes we already have the hash_entry locked, and there
// is currently no lightweight or allocated lock for addr.
// We register a finalizer for addr, which is responsible for
// removing the heavy lock when addr goes away, in addition
// to the responsibilities of any prior finalizer.
// This unfortunately holds the lock bit for the hash entry while it
// allocates two objects (on for the finalizer).
// It would be nice to avoid that somehow ...
static heavy_lock *
alloc_heavy(obj_addr_t addr, hash_entry *he)
{
  heavy_lock * hl = (heavy_lock *) _Jv_AllocTraceTwo(sizeof (heavy_lock));
  
  hl -> address = addr;
  _Jv_MutexInit (&(hl -> si.mutex));
  _Jv_CondInit (&(hl -> si.condition));
# if defined (_Jv_HaveCondDestroy) || defined (_Jv_HaveMutexDestroy)
    hl->si.init = true;  // needed ?
# endif
  hl -> next = he -> heavy_locks;
  he -> heavy_locks = hl;
  // FIXME: The only call that cheats and goes directly to the GC interface.
# ifdef HAVE_BOEHM_GC
    GC_REGISTER_FINALIZER_NO_ORDER(
		    	  (void *)addr, heavy_lock_obj_finalization_proc,
			  hl, &hl->old_finalization_proc,
			  &hl->old_client_data);
# endif /* HAVE_BOEHM_GC */
  return hl;
}

// Return the heavy lock for addr, allocating if necessary.
// Assumes we have the cache entry locked, and there is no lightweight
// lock for addr.
static heavy_lock *
get_heavy(obj_addr_t addr, hash_entry *he)
{
  heavy_lock *hl = find_heavy(addr, he);
  if (0 == hl)
    hl = alloc_heavy(addr, he);
  return hl;
}

void
_Jv_MonitorEnter (jobject obj)
{
#ifdef JV_LINKER_CANNOT_8BYTE_ALIGN_STATICS
  obj_addr_t addr = (obj_addr_t)obj & ~((obj_addr_t)FLAGS);
#else
  obj_addr_t addr = (obj_addr_t)obj;
#endif
  obj_addr_t address;
  unsigned hash = JV_SYNC_HASH(addr);
  hash_entry * he = light_locks + hash;
  _Jv_ThreadId_t self = _Jv_ThreadSelf();
  unsigned count;
  const unsigned N_SPINS = 18;

  // We need to somehow check that addr is not NULL on the fast path.
  // A very predictable
  // branch on a register value is probably cheaper than dereferencing addr.
  // We could also permanently lock the NULL entry in the hash table.
  // But it's not clear that's cheaper either.
  if (__builtin_expect(!addr, false))
    throw new java::lang::NullPointerException;
   
  assert(!(addr & FLAGS));
retry:
  if (__builtin_expect(compare_and_swap(&(he -> address),
					0, addr),true))
    {
      assert(he -> light_thr_id == INVALID_THREAD_ID);
      assert(he -> light_count == 0);
      he -> light_thr_id = self;
      // Count fields are set correctly.  Heavy_count was also zero,
      // but can change asynchronously.
      // This path is hopefully both fast and the most common.
      return;
    }
  address = he -> address;
  if ((address & ~(HEAVY | REQUEST_CONVERSION)) == addr)
    {
      if (he -> light_thr_id == self)
	{
	  // We hold the lightweight lock, and it's for the right
	  // address.
	  count = he -> light_count;
	  if (count == USHRT_MAX)
	    {
	      // I think most JVMs don't check for this.
	      // But I'm not convinced I couldn't turn this into a security
	      // hole, even with a 32 bit counter.
	      throw new java::lang::IllegalMonitorStateException(
		JvNewStringLatin1("maximum monitor nesting level exceeded")); 
	    }
	  he -> light_count = count + 1;
	  return;
	}
      else
	{
	  // Lightweight lock is held, but by somone else.
          // Spin a few times.  This avoids turning this into a heavyweight
    	  // lock if the current holder is about to release it.
          for (unsigned int i = 0; i < N_SPINS; ++i)
	    {
	      if ((he -> address & ~LOCKED) != (address & ~LOCKED)) goto retry;
	      spin(i);
            }
	  address &= ~LOCKED;
	  if (!compare_and_swap(&(he -> address), address, address | LOCKED ))
	    {
	      wait_unlocked(he);      
	      goto retry;
	    }
	  heavy_lock *hl = get_heavy(addr, he);
	  ++ (he -> heavy_count);
	  // The hl lock acquisition can't block for long, since it can
	  // only be held by other threads waiting for conversion, and
	  // they, like us, drop it quickly without blocking.
	  _Jv_MutexLock(&(hl->si.mutex));
	  assert(he -> address == address | LOCKED );
	  release_set(&(he -> address), (address | REQUEST_CONVERSION | HEAVY));
				// release lock on he
	  while ((he -> address & ~FLAGS) == (address & ~FLAGS))
	    {
	      // Once converted, the lock has to retain heavyweight
	      // status, since heavy_count > 0 . 
	      _Jv_CondWait (&(hl->si.condition), &(hl->si.mutex), 0, 0);
	    }
	  keep_live(addr);
		// Guarantee that hl doesn't get unlinked by finalizer.
		// This is only an issue if the client fails to release
		// the lock, which is unlikely.
	  assert(he -> address & HEAVY);
	  // Lock has been converted, we hold the heavyweight lock,
	  // heavy_count has been incremented.
	  return;
        }
    }
  obj_addr_t was_heavy = (address & HEAVY);