如何同步操作？

Question

我有用於兩階段鎖定的此實現。 問題在於，它在大多數情況下都可以正常工作，但並非所有情況下都完美。 我發現問題是由於我對Synchronized的使用而導致的，而這種使用方式卻有誤！ 我需要這樣的操作順序：

1. 解鎖任何鎖的線程應在其他任何線程開始鎖定同一鎖之前完全完成此階段（ul方法）。
2. 在喚醒時等待鎖的線程應該在開始執行其操作之前獲取該鎖的所有等待者。
3. 任何addEdge或removeEdge都需要同步，因為我對所有線程都使用相同的圖形對象。

這是代碼：

class LockTable{
     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;
    boolean getIt = false;
    synchronized(this) {
        try {
            lock = locks.get(oid);             // find the lock

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

               wait = true;
            }
            if(lock == null){
             getIt = true;
             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){
        synchronized(this){
            //System.out.println("Transaction " + tid + " is waiting..");
            lock.waiters.add(tid);
            lock.waitersType.add('s');
            Main.g.addEdge(tid, lock.tid);
            //System.out.println("Edge has been added "+tid + "==>" + lock.tid);
        }//sync
         if(Main.g.hasCycle()){
            //System.out.println("Detect Cycle in rl..Executing Restart");
            restart(tid);
        }
        //to exclude the restarted thread
        if(!Main.trans[tid].terminate){
            lock.lock.readLock().lock();
            lock.readers.add(tid);
          synchronized(this){
            lock.waitersType.remove(lock.waiters.indexOf(tid));
            lock.waiters.remove(lock.waiters.indexOf(tid));
            }//sync
            for(int i =0 ; i < lock.waiters.size();i++){
                if(lock.waitersType.get(i) == 'w'){
                    synchronized(Main.g){
                    Main.g.addEdge(lock.waiters.get(i), tid);
                    //System.out.println("Edge has been added "+lock.waiters.get(i) + "==>" + tid);
                    }//sync
                    if(Main.g.hasCycle())
                        restart(lock.waiters.get(i));
                }//if lock.waitersType
            }//for
        }//if terminate
        else
            return;
    }
   else
    if(!getIt){
              //System.out.println("Getting Shared Lock without Waiting");
              lock.lock.readLock().lock();
              lock.readers.add(tid);
              for(int i =0 ; i < lock.waiters.size();i++){
                if(lock.waitersType.get(i) == 'w'){
                    synchronized(Main.g){
                    Main.g.addEdge(lock.waiters.get(i), tid);
                    //System.out.println("Edge has been added "+lock.waiters.get(i) + "==>" + tid);
                    }
                    if(Main.g.hasCycle())
                        restart(lock.waiters.get(i));
                }//if lock.waitersType
              } 
    }

} // 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;
    boolean getIt = 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){
                getIt = true;
                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..");
           synchronized(this) {
            if(lock.lock.isWriteLocked()){
                Main.g.addEdge(tid, lock.tid);
                //System.out.println("Edge has been added "+tid + "==>" + lock.tid);
               }
            else{
                type = 1;
                for(int reader : lock.readers){
                     Main.g.addEdge(tid, reader);
                     //System.out.println("Edge has been added "+tid + "==>" + reader);
                }
                }//else
            if(Main.g.hasCycle()){
               //System.out.println("Detect Cycle");
               restart(tid);
             }//if
            //System.out.println("Graph: "+Main.g.toString());
         }//sync



       if(!Main.trans[tid].terminate){
            synchronized(this){
            lock.waiters.add(tid);
            lock.waitersType.add('w');
           }
            //System.out.println("I'm waiting here in wl");
            lock.lock.writeLock().lock();
            lock.tid = tid;
            //System.out.println("Wakeup..");
            synchronized(this){
                lock.waitersType.remove(lock.waiters.indexOf(tid));
                lock.waiters.remove(lock.waiters.indexOf(tid));
                //System.out.println("the number of waiters after wakeup: " + lock.waiters.size());
           }//sync
                for(int i =0 ; i < lock.waiters.size();i++){
                    synchronized(Main.g){
                    Main.g.addEdge(lock.waiters.get(i), tid);
                   // System.out.println("Edge has been added "+lock.waiters.get(i) + "==>" + tid);
                    //System.out.println("Graph: "+Main.g.toString());
                    }//sync
                    if(Main.g.hasCycle())
                        restart(lock.waiters.get(i));
                }//for
         }
        else
            return; 
    }// if(wait) ==> for the lock to be released
    else
        if(!getIt){
            lock.lock.writeLock().lock();
            lock.tid = tid;
            for(int i =0 ; i < lock.waiters.size();i++){
                synchronized(Main.g){
                Main.g.addEdge(lock.waiters.get(i), tid);
                //System.out.println("Edge has been added "+lock.waiters.get(i) + "==>" + tid);
                }
                if(Main.g.hasCycle())
                    restart(lock.waiters.get(i));
            }//for

     }

} // wl

void restart(int tid){
 synchronized(this) {
    Main.rollBack++;
    MyLock lock;
    List<Integer> toRemove = new ArrayList();
    for(int i : locks.keySet()){
       lock = locks.get(i);
       //for all the locks in the lock table delete the restarted thread from the waiters list
       if(lock.waiters.contains(tid)){
           lock.waitersType.remove(lock.waiters.indexOf(tid));
           lock.waiters.remove(lock.waiters.indexOf(tid));
        }

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


               //remove the edges between the waiters of this lock and the thread that unlocked it
                  for(int j=0;j<lock.waiters.size();j++)
                      Main.g.removeEdge(lock.waiters.get(j), lock.tid);
               //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.lock.getReadLockCount()>0){
                  //remove the edges between the waiters of this lock and the thread that unlocked it
                  for(int j=0;j<lock.waiters.size();j++)
                      Main.g.removeEdge(lock.waiters.get(j), 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
    Main.g.removeEdges(tid);
    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
    }
        if((lock != null) && (lock.lock.isWriteLockedByCurrentThread())){
                  //remove the edges between the waiters of this lock and the thread that unlocked it
                  for(int i=0;i<lock.waiters.size();i++)
                      synchronized(Main.g){
                      Main.g.removeEdge(lock.waiters.get(i), lock.tid);
                      }//sync
                   //System.out.println("tid: " + tid + " unlock object: " + oid);
                   lock.lock.writeLock().unlock();
                  //print out 
                  //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)){
              synchronized(this){
              if(lock.readers.contains(tid)){
                lock.readers.remove(lock.readers.indexOf(tid));
                }
                //remove the edges between the waiters of this lock and the thread that unlocked it
                  for(int i=0;i<lock.waiters.size();i++)
                      synchronized(Main.g){
                      Main.g.removeEdge(lock.waiters.get(i), 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

有幾個與你的使用問題synchronized 。

1. 除極少數情況外，對共享庫的所有讀/寫（特別是寫）訪問必須在同一對象上同步。 當您在某些地方寫入Main.g時，您正在同步它，但是在某些地方卻沒有同步它（例如，在wl方法的if(wait)內部）。

2. 據我了解您的代碼目標， lock.{waiters|readers}的狀態lock.{waiters|readers}必須與圖的狀態（ Main.g ）保持同步 。 這意味着，每當您更改例如waiters ，然后相應地更新Main.g ，這兩個操作都必須自動執行。 為此，您需要將它們包裝在一個不間斷的synchronized語句中。 我可以看到您了解此概念，因為您在某些地方正在執行此操作，但隨后似乎在其他地方錯過了它。 例如：在rl方法，內部if(!getIt){您更新lock.readers然后Main.g沒有任何同步。

總體而言，我無法提供任何具體信息，因為代碼非常復雜，因此很難說出它的意圖。 但是我認為您可以通過鎖定代碼的較大部分來解決一些鎖定問題。 例如，你可以只同步整個方法rl ， wl和ul在this通過增加synchronized關鍵字的方法聲明，並擺脫這些方法中的所有細粒度鎖的。 細粒度的鎖定可以為您提供更好的性能，但代價是復雜性高。 我建議您先從更簡單的鎖定方案開始，然后逐步改進它們（如果確實需要！）。