面试中关于多线程同步，你必须要思考的问题

  static final class FairSync extends Sync {
      
        final void lock() {
            acquire(1);
        }

        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }

   public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        UNSAFE.park(false, 0L);
        setBlocker(t, null);
    }

 Unsafe_Park(JNIEnv *env, jobject unsafe, jboolean isAbsolute, jlong time){
 ...
 thread->parker()->park(isAbsolute != 0, time);
 ...
 }
 

void Parker::park(bool isAbsolute, jlong time) {
  
  ...
  //获得当前线程
  Thread* thread = Thread::current();
  assert(thread->is_Java_thread(), "Must be JavaThread");
  JavaThread *jt = (JavaThread *)thread;

 //如果当前线程被设置了interrupted标记，则直接返回
  if (Thread::is_interrupted(thread, false)) {
    return;
  }
 
  if (time > 0) {
  //unpacktime中根据isAbsolute的值来填充absTime结构体，isAbsolute为true时，time代表绝对时间且单位是毫秒，否则time是相对时间且单位是纳秒
  //absTime.tvsec代表了对于时间的秒
  //absTime.tv_nsec代表对应时间的纳秒
    unpackTime(&absTime, isAbsolute, time);
  }

	//调用mutex trylock方法
    if (Thread::is_interrupted(thread, false) || pthread_mutex_trylock(_mutex) != 0) {
    return;
  }
	
 	//_counter是一个许可的数量，跟ReentrantLock里定义的许可变量基本都是一个原理。unpack方法调用时会将_counter赋值为1。
 	//_counter>0代表已经有人调用了unpark，所以不用阻塞
  int status ;
  if (_counter > 0)  { // no wait needed
    _counter = 0;
    //释放mutex锁
    status = pthread_mutex_unlock(_mutex);
    return;
  }

//设置线程状态为CONDVAR_WAIT
  OSThreadWaitState osts(thread->osthread(), false /* not Object.wait() */);
 ...
 //等待
 _cur_index = isAbsolute ? ABS_INDEX : REL_INDEX;
 pthread_cond_timedwait(&_cond[_cur_index], _mutex,  &absTime);
 
 ...
  //释放mutex锁
  status = pthread_mutex_unlock(_mutex) ;
  
  
}

  enum ThreadState {
  ALLOCATED,                    // Memory has been allocated but not initialized
  INITIALIZED,                  // The thread has been initialized but yet started
  RUNNABLE,                     // Has been started and is runnable, but not necessarily running
  MONITOR_WAIT,                 // Waiting on a contended monitor lock
  CONDVAR_WAIT,                 // Waiting on a condition variable
  OBJECT_WAIT,                  // Waiting on an Object.wait() call
  BREAKPOINTED,                 // Suspended at breakpoint
  SLEEPING,                     // Thread.sleep()
  ZOMBIE                        // All done, but not reclaimed yet
};

//pthread_mutex_t是posix中的互斥锁结构体
int
__pthread_mutex_trylock (mutex)
     pthread_mutex_t *mutex;
{
  int oldval;
  pid_t id = THREAD_GETMEM (THREAD_SELF, tid);
switch (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex),
			    PTHREAD_MUTEX_TIMED_NP))
    {
    
    case PTHREAD_MUTEX_ERRORCHECK_NP:
    case PTHREAD_MUTEX_TIMED_NP:
    case PTHREAD_MUTEX_ADAPTIVE_NP:
      /* Normal mutex.  */
      if (lll_trylock (mutex->__data.__lock) != 0)
	break;

      /* Record the ownership.  */
      mutex->__data.__owner = id;
      ++mutex->__data.__nusers;

      return 0;
    }
    
}
 //以下代码在lowlevellock.h中
   #define __lll_trylock(futex) \
  (atomic_compare_and_exchange_val_acq (futex, 1, 0) != 0)
  #define lll_trylock(futex) __lll_trylock (&(futex))

int
internal_function attribute_hidden
__pthread_mutex_unlock_usercnt (mutex, decr)
     pthread_mutex_t *mutex;
     int decr;
{
    if (__builtin_expect (type, PTHREAD_MUTEX_TIMED_NP)
      == PTHREAD_MUTEX_TIMED_NP)
    {
      /* Always reset the owner field.  */
    normal:
      mutex->__data.__owner = 0;
      if (decr)
	/* One less user.  */
	--mutex->__data.__nusers;

      /* Unlock.  */
      lll_unlock (mutex->__data.__lock, PTHREAD_MUTEX_PSHARED (mutex));
      return 0;
    }
 }

 

 #define __lll_unlock(futex, private)					      \
  ((void) ({								      \
    int *__futex = (futex);						      \
    int __val = atomic_exchange_rel (__futex, 0);			      \
									      \
    if (__builtin_expect (__val > 1, 0))				      \
      lll_futex_wake (__futex, 1, private);				      \
  }))
#define lll_unlock(futex, private) __lll_unlock(&(futex), private)


#define lll_futex_wake(ftx, nr, private)				\
({									\
   DO_INLINE_SYSCALL(futex, 3, (long) (ftx),				\
		     __lll_private_flag (FUTEX_WAKE, private),		\
		     (int) (nr));					\
   _r10 == -1 ? -_retval : _retval;					\
})

#define __lll_lock(futex, private)					      \
  ((void) ({								      \
    int *__futex = (futex);						      \
    if (__builtin_expect (atomic_compare_and_exchange_bool_acq (__futex,      \
								1, 0), 0))    \
      {									      \
	if (__builtin_constant_p (private) && (private) == LLL_PRIVATE)	      \
	  __lll_lock_wait_private (__futex);				      \
	else								      \
	  __lll_lock_wait (__futex, private);				      \
      }									      \
  }))
#define lll_lock(futex, private) __lll_lock (&(futex), private)

void
__lll_lock_wait_private (int *futex)
{
//第一次进来的时候futex==1,所以不会走这个if
  if (*futex == 2)
    lll_futex_wait (futex, 2, LLL_PRIVATE);
//在这里会把futex设置成2，并调用futex_wait让当前线程等待
  while (atomic_exchange_acq (futex, 2) != 0)
    lll_futex_wait (futex, 2, LLL_PRIVATE);
}

int
int
__pthread_cond_timedwait (cond, mutex, abstime)
     pthread_cond_t *cond;
     pthread_mutex_t *mutex;
     const struct timespec *abstime;
{
  struct _pthread_cleanup_buffer buffer;
  struct _condvar_cleanup_buffer cbuffer;
  int result = 0;

  /* Catch invalid parameters.  */
  if (abstime->tv_nsec < 0 || abstime->tv_nsec >= 1000000000)
    return EINVAL;

  int pshared = (cond->__data.__mutex == (void *) ~0l)
		? LLL_SHARED : LLL_PRIVATE;

  //1.获得cond锁
  lll_lock (cond->__data.__lock, pshared);

  //2.释放mutex锁
  int err = __pthread_mutex_unlock_usercnt (mutex, 0);
  if (err)
    {
      lll_unlock (cond->__data.__lock, pshared);
      return err;
    }

  /* We have one new user of the condvar.  */
  //每执行一次wait(pthread_cond_timedwait/pthread_cond_wait)，__total_seq就会+1
  ++cond->__data.__total_seq;
  //用来执行futex_wait的变量
  ++cond->__data.__futex;
  //标识该cond还有多少线程在使用，pthread_cond_destroy需要等待所有的操作完成
  cond->__data.__nwaiters += 1 << COND_NWAITERS_SHIFT;

  /* Remember the mutex we are using here.  If there is already a
     different address store this is a bad user bug.  Do not store
     anything for pshared condvars.  */
  //保存mutex锁
  if (cond->__data.__mutex != (void *) ~0l)
    cond->__data.__mutex = mutex;

  /* Prepare structure passed to cancellation handler.  */
  cbuffer.cond = cond;
  cbuffer.mutex = mutex;

  /* Before we block we enable cancellation.  Therefore we have to
     install a cancellation handler.  */
  __pthread_cleanup_push (&buffer, __condvar_cleanup, &cbuffer);

  /* The current values of the wakeup counter.  The "woken" counter
     must exceed this value.  */
  //记录futex_wait前的__wakeup_seq（为该cond上执行了多少次sign操作+timeout次数）和__broadcast_seq（代表在该cond上执行了多少次broadcast）
  unsigned long long int val;
  unsigned long long int seq;
  val = seq = cond->__data.__wakeup_seq;
  /* Remember the broadcast counter.  */
  cbuffer.bc_seq = cond->__data.__broadcast_seq;

  while (1)
    {
      //3.计算要wait的相对时间
      struct timespec rt;
      {
#ifdef __NR_clock_gettime
	INTERNAL_SYSCALL_DECL (err);
	int ret;
	ret = INTERNAL_VSYSCALL (clock_gettime, err, 2,
				(cond->__data.__nwaiters
				 & ((1 << COND_NWAITERS_SHIFT) - 1)),
				&rt);
# ifndef __ASSUME_POSIX_TIMERS
	if (__builtin_expect (INTERNAL_SYSCALL_ERROR_P (ret, err), 0))
	  {
	    struct timeval tv;
	    (void) gettimeofday (&tv, NULL);

	    /* Convert the absolute timeout value to a relative timeout.  */
	    rt.tv_sec = abstime->tv_sec - tv.tv_sec;
	    rt.tv_nsec = abstime->tv_nsec - tv.tv_usec * 1000;
	  }
	else
# endif
	  {
	    /* Convert the absolute timeout value to a relative timeout.  */
	    rt.tv_sec = abstime->tv_sec - rt.tv_sec;
	    rt.tv_nsec = abstime->tv_nsec - rt.tv_nsec;
	  }
#else
	/* Get the current time.  So far we support only one clock.  */
	struct timeval tv;
	(void) gettimeofday (&tv, NULL);

	/* Convert the absolute timeout value to a relative timeout.  */
	rt.tv_sec = abstime->tv_sec - tv.tv_sec;
	rt.tv_nsec = abstime->tv_nsec - tv.tv_usec * 1000;
#endif
      }
      if (rt.tv_nsec < 0)
	{
	  rt.tv_nsec += 1000000000;
	  --rt.tv_sec;
	}
   /*---计算要wait的相对时间 end---- */

  //是否超时
      /* Did we already time out?  */
      if (__builtin_expect (rt.tv_sec < 0, 0))
	{
    //被broadcast唤醒，这里疑问的是，为什么不需要判断__wakeup_seq？
	  if (cbuffer.bc_seq != cond->__data.__broadcast_seq)
	    goto bc_out;

	  goto timeout;
	}

      unsigned int futex_val = cond->__data.__futex;

      //4.释放cond锁，准备wait
      lll_unlock (cond->__data.__lock, pshared);

      /* Enable asynchronous cancellation.  Required by the standard.  */
      cbuffer.oldtype = __pthread_enable_asynccancel ();

      //5.调用futex_wait
      /* Wait until woken by signal or broadcast.  */
      err = lll_futex_timed_wait (&cond->__data.__futex,
				  futex_val, &rt, pshared);

      /* Disable asynchronous cancellation.  */
      __pthread_disable_asynccancel (cbuffer.oldtype);


      //6.重新获得cond锁，因为又要访问&修改cond的数据了
      lll_lock (cond->__data.__lock, pshared);

      //__broadcast_seq值发生改变，代表发生了有线程调用了广播
      if (cbuffer.bc_seq != cond->__data.__broadcast_seq)
	goto bc_out;

     //判断是否是被sign唤醒的，sign会增加__wakeup_seq
     //第二个条件cond->__data.__woken_seq != val的意义在于
    //可能两个线程A、B在wait，一个线程调用了sign导致A被唤醒，这时B因为超时被唤醒
    //对于B线程来说，执行到这里时第一个条件也是满足的，从而导致上层拿到的result不是超时
    //所以这里需要判断下__woken_seq（即该cond已经被唤醒的线程数）是否等于__wakeup_seq（sign执行次数+timeout次数）
      val = cond->__data.__wakeup_seq;
      if (val != seq && cond->__data.__woken_seq != val)
	break;

      /* Not woken yet.  Maybe the time expired?  */
      if (__builtin_expect (err == -ETIMEDOUT, 0))
	{
	timeout:
	  /* Yep.  Adjust the counters.  */
	  ++cond->__data.__wakeup_seq;
	  ++cond->__data.__futex;

	  /* The error value.  */
	  result = ETIMEDOUT;
	  break;
	}
    }

  //一个线程已经醒了所以这里__woken_seq +1
  ++cond->__data.__woken_seq;

 bc_out:
  //
  cond->__data.__nwaiters -= 1 << COND_NWAITERS_SHIFT;

  /* If pthread_cond_destroy was called on this variable already,
     notify the pthread_cond_destroy caller all waiters have left
     and it can be successfully destroyed.  */
  if (cond->__data.__total_seq == -1ULL
      && cond->__data.__nwaiters < (1 << COND_NWAITERS_SHIFT))
    lll_futex_wake (&cond->__data.__nwaiters, 1, pshared);

 //9.cond数据修改完毕，释放锁
  lll_unlock (cond->__data.__lock, pshared);

  /* The cancellation handling is back to normal, remove the handler.  */
  __pthread_cleanup_pop (&buffer, 0);

 //10.重新获得mutex锁
  err = __pthread_mutex_cond_lock (mutex);

  return err ?: result;
}

Object condObj=new Object();
voilate int flag = 0;
public void waitTest(){
	if(flag == 0){
		condObj.wait();
	}
}
public void notifyTest(){
	flag=1;
	condObj.notify();
}

Object condObj=new Object();
voilate int flag = 0;
public void waitTest(){
	synchronized(condObj){
		if(flag == 0){
			condObj.wait();
		}
	}
	
}
public void notifyTest(){
	synchronized(condObj){
		flag=1;
		condObj.notify();
	}
}

int
__pthread_cond_signal (cond)
     pthread_cond_t *cond;
{
  int pshared = (cond->__data.__mutex == (void *) ~0l)
		? LLL_SHARED : LLL_PRIVATE;

  //因为要操作cond的数据，所以要加锁
  lll_lock (cond->__data.__lock, pshared);

  /* Are there any waiters to be woken?  */
  if (cond->__data.__total_seq > cond->__data.__wakeup_seq)
    {
      //__wakeup_seq为执行sign与timeout次数的和
      ++cond->__data.__wakeup_seq;
      ++cond->__data.__futex;

       ...
		//唤醒wait的线程
      lll_futex_wake (&cond->__data.__futex, 1, pshared);
    }

  /* We are done.  */
  lll_unlock (cond->__data.__lock, pshared);

  return 0;
}