java - 等待锁操作时线程永远阻塞

Question

我正在编写一个两阶段锁定的java实现。所以，我使用可重入锁(读写锁)。问题是，当线程执行 lock.readLock.lock() 或 lock.writeLock().lock() 并且锁已经被锁定时，即使使用 lock.readLock().unlock 解锁了锁，它也会永远卡住() 或 lock.writeLock().unlock()。所以，看起来解锁并没有吵醒服务员!这是导致问题的代码:

class LockTable
{
//    /*******************************************************************************
//     * This class is used to represent an individual lock.
//     * @param tid     the id of the transaction holding the lock
//     * @param shared  whether the lock is shared (true) or exclusive (false)
//     */
//    void Lock (int tid, boolean shared)
//    {
//        Semaphore sem = new Semaphore (0);
//    } // Lock class

    /** Associative map of locks held by transactions of the form (key = oid, value = lock)
     */
    private HashMap<Integer,MyLock> locks;

    public LockTable(){
        locks= new HashMap<Integer,MyLock>();

    }

    /*******************************************************************************
     * Acquire a shared/read lock on data object oid.
     * @param tid  the transaction id
     * @param oid  the data object id
     */
    void rl (int tid, int oid) throws InterruptedException
    {

        MyLock lock=null;
        boolean wait = false;
        synchronized(this) {
            try {
                lock = locks.get(oid);             // find the lock

                if((lock != null) && (lock.lock.isWriteLocked())){

                   wait = true;

                  // System.out.println(locks.get(oid).shared);
                }

                if(lock == null){
                 lock = new MyLock(tid, true);
                 lock.lock.readLock().lock();
                 lock.readers.add(tid);
                 locks.put(oid, lock);
              }

            } catch(Exception e) {
                System.out.println(e.getStackTrace());        // lock not found, so oid is not locked;
            } // try
        }//synch


        if (wait){

            System.out.println("Transaction " + tid + " is waiting..");
            Main.g.addEdge(tid, lock.tid);
                    if(Main.g.hasCycle())
                        restart(tid);

            //to exclude the restarted thread
            if(!Main.trans[tid].terminate){

                lock.lock.readLock().lock();
                Main.g.removeEdge(tid, lock.tid);
                synchronized(this){
                lock.readers.add(tid);
                }//synchronized
            }//if isInturrupted
            else
                return;
        }
       else
        synchronized(this) {
              lock.lock.readLock().lock();
              lock.readers.add(tid);
      } // synchronized

    } // rl

    /*******************************************************************************
     * Acquire an exclusive/write lock on data object oid.
     * @param tid  the transaction id
     * @param oid  the data object id
     */
    void wl (int tid, int oid) throws InterruptedException
    {
         //type to determine the last lock type in order
         //to be able to remove the edges from waitfor graph
        int type = 0;
        MyLock lock = null;
        boolean wait = false;

        synchronized(this) {
            try {
                lock = locks.get(oid);             // find the lock
                if(lock != null && (lock.lock.isWriteLocked() || lock.readers.size() > 0))
                {
                    wait = true;
                }
                if(lock == null){
                    lock = new MyLock(tid);
                    lock.lock.writeLock().lock();
                    locks.put(oid,lock);
                }
            } catch(Exception e) {
                System.out.println(e.getStackTrace());        // lock not found, so oid is not locked;
            } // try
      }
         if (wait){
                System.out.println("Transaction " + tid + " is waiting..");
                if(lock.lock.isWriteLocked())
                    Main.g.addEdge(tid, lock.tid);
                else{
                    type = 1;
                    for(int reader : lock.readers)
                         Main.g.addEdge(tid, reader);
                    }//else

                            if(Main.g.hasCycle())
                            {
                                restart(tid);
                            }//if
           if(!Main.trans[tid].terminate){
                System.out.println("I'm waiting here in wl");
                lock.lock.writeLock().lock();
                System.out.println("Wakeup..");
                if(type == 0)
                    Main.g.removeEdge(tid, lock.tid);
                else
                    for(int reader : lock.readers)
                        Main.g.removeEdge(tid, reader);
                lock.tid = tid;
             }
            else
                return;

        }// if(wait) ==> for the lock to be released
        else 
            lock.lock.writeLock().lock();

    } // wl

    void restart(int tid){
     synchronized(this) {
        MyLock lock;
        List<Integer> toRemove = new ArrayList();
        for(int i : locks.keySet()){
           lock = locks.get(i);

               //lock.sem.release();
               if(lock.lock.isWriteLockedByCurrentThread()){

                   System.out.println("Transaction"+tid+" unlock object "+ i +" in order to restart");
                   lock.lock.writeLock().unlock();
                    System.out.println("number of write holders: " + lock.lock.writeLock().getHoldCount());
                    System.out.println("number of read holders: " + lock.lock.getReadHoldCount());
                    System.out.println("number of waiters: " + lock.lock.getQueueLength());
                   toRemove.add(i);

               }
           if(!lock.lock.isWriteLocked())
               if(lock.readers.contains(tid)){
                   // lock.numberOfReaders --;

                      System.out.println("Transaction"+tid+" unlock object "+ i +" in order to restart");
                      lock.readers.remove(lock.readers.indexOf(tid));
                      lock.lock.readLock().unlock();
                      System.out.println("number of write holders: " + lock.lock.getWriteHoldCount());
                      System.out.println("number of read holders: " + lock.lock.getReadHoldCount());
                      System.out.println("number of waiters: " + lock.lock.getQueueLength());
                      toRemove.add(i);  

                  }//if
        }//for
        for(int i = 0; i < toRemove.size() ; i ++)
            locks.remove(toRemove.get(i));
        Main.g.removeEdges(tid);

       // Thread.currentThread().interrupt();
        Main.trans[tid].terminate = true;

        System.out.println("Transaction" + tid + " restarted");

        }//sync
    }

    /*******************************************************************************
     * Unlock/release the lock on data object oid.
     * @param tid  the transaction id
     * @param oid  the data object id
     */
    void ul (int tid, int oid)
    {
       MyLock lock = null;
        boolean error = false;
        synchronized(this) {
            try {
                lock = locks.get(oid);                    // find the lock
                if( lock == null)
                    System.out.println("println: lock not found");

            } catch(Exception e) {
                System.out.println("lock not found");   // lock not found
            } // try
        }//sync
            if((lock != null) && (lock.lock.isWriteLockedByCurrentThread())){

                      System.out.println("tid: " + tid + " unlock object: " + oid);
                      lock.lock.writeLock().unlock();
                      System.out.println("done with unlock");
                      System.out.println("number of write holders: " + lock.lock.writeLock().getHoldCount());
                      System.out.println("number of read holders: " + lock.lock.getReadHoldCount());
                      System.out.println("number of waiters: " + lock.lock.getQueueLength());
          }// if lock != null
            else
              if((lock != null) && (lock.readers.size()>0)){
                  if(lock.readers.contains(tid)){
                    lock.readers.remove(lock.readers.indexOf(tid));
                    lock.lock.readLock().unlock();
                    System.out.println("Transaction"+tid+" unlocked shared lock on object "+oid);
                    //System.out.println("number of write holders: " + lock.lock.readLock().);
                    System.out.println("number of read holders: " + lock.lock.getReadHoldCount());
                    System.out.println("number of waiters: " + lock.lock.getQueueLength());

                  }//if lock.readers
              }//if


        if (error) 
            System.out.println ("Error: ul: no lock for oid = " + oid + " found/owned");
    } // ul

Answer 1

Method A:
...snip...
synchronized(this) {
    ...snip...
    lock.lock.readLock().lock();
    ...snip...
}

Method B:
...snip...
synchronized(this) {
    ...snip...
    lock.lock.readLock().unlock();
    ...snip...
}

您遇到的问题是由于一个同步器基本上阻塞而同时持有另一个同步器而导致死锁。当您在 synchronized(this) 代码块内调用 Lock.lock() 时，Thread 将进入 BLOCKING 状态，同时继续持有 this 上的内在锁。由于实际持有锁以获取 this 上的内在锁的线程并且内在锁永远不会被释放，因此出现了死锁。

例如:
Thread1进入方法A，获得this上的锁
Thread2进入方法A，无法获取this因此阻塞
Thread1获得Readlock
Thread1 发布this
线程2获取this
Thread2 阻塞等待 Readlock
线程1进入方法B，无法获取this因此阻塞
陷入僵局