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我正在为两阶段锁定编写一个 java 实现。所以,我正在使用可重入锁(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 解锁锁,它也会永远卡住它们() 或 lock.writeLock().unlock()。所以,看起来解锁并没有唤醒服务员!!!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
4

2 回答 2

4
Method A:
...snip...
synchronized(this) {
    ...snip...
    lock.lock.readLock().lock();
    ...snip...
}

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

您遇到的问题是在持有另一个同步器的同时基本上阻塞一个同步器的死锁。当您在代码块Lock.lock()内部调用时,将进入一个状态,同时继续保持内部锁定。由于实际持有锁以获得内在锁的线程和内在锁永远不会被释放,所以你有一个死锁。synchronized(this)ThreadBLOCKINGthisthis

例如:
Thread1 进入Method A,获得一个锁this
Thread2 进入Method A,无法获得thisso 阻塞Thread1 获得
Readlock Thread1
释放this
Thread2 获得this
Thread2 阻塞等待Readlock Thread1
进入Method B,无法获得thisso 阻塞
死锁

于 2011-04-06T03:20:12.463 回答
0

also, don't forget about the non-happy path. make sure to put your unlock() inside of a finally, otherwise if an exception gets thrown, unlock() is never called and you end up in deadlock.

于 2011-04-06T05:02:46.717 回答