Deadlock

A deadlock is when two or more threads are blocked waiting to obtain locks that some of the other threads in the deadlock are holding. Deadlock can occur when multiple threads need the same locks, at the same time, but obtain them in different order.

For instance, if thread 1 locks A, and tries to lock B, and thread 2 has already locked B, and tries to lock A, a deadlock arises. Thread 1 can never get B, and thread 2 can never get A. In addition, neither of them will ever know. They will remain blocked on each their object, A and B, forever. This situation is a deadlock.

The situation is illustrated below:

Thread 1  locks A, waits for B
Thread 2  locks B, waits for A

Here is an example of a TreeNode class that call synchronized methods in different instances:

public class TreeNode {

  TreeNode parent   = null;  
  List     children = new ArrayList();

  public synchronized void addChild(TreeNode child){
    if(!this.children.contains(child)) {
      this.children.add(child);
      child.setParentOnly(this);
    }
  }

  public synchronized void addChildOnly(TreeNode child){
    if(!this.children.contains(child){
      this.children.add(child);
    }
  }

  public synchronized void setParent(TreeNode parent){
    this.parent = parent;
    parent.addChildOnly(this);
  }

  public synchronized void setParentOnly(TreeNode parent){
    this.parent = parent;
  }
}

If a thread (1) calls the parent.addChild(child) method at the same time as another thread (2) calls the child.setParent(parent) method, on the same parent and child instances, a deadlock can occur. Here is some pseudo code that illustrates this:

Thread 1: parent.addChild(child); //locks parent
          --
>
 child.setParentOnly(parent);

Thread 2: child.setParent(parent); //locks child
          --
>
 parent.addChildOnly()

First thread 1 calls parent.addChild(child). Since addChild() is synchronized thread 1 effectively locks the parent object for access from other treads.

Then thread 2 calls child.setParent(parent). Since setParent() is synchronized thread 2 effectively locks the child object for acces from other threads.

Now both child and parent objects are locked by two different threads. Next thread 1 tries to call child.setParentOnly() method, but the child object is locked by thread 2, so the method call just blocks. Thread 2 also tries to call parent.addChildOnly() but the parent object is locked by thread 1, causing thread 2 to block on that method call. Now both threads are blocked waiting to obtain locks the other thread holds.

Note: The two threads must call parent.addChild(child) and child.setParent(parent) at the same time as described above, and on the same two parent and child instances for a deadlock to occur. The code above may execute fine for a long time until all of a sudden it deadlocks.

The threads really need to take the locks *at the same time*. For instance, if thread 1 is a bit ahead of thread2, and thus locks both A and B, then thread 2 will be blocked already when trying to lock B. Then no deadlock occurs. Since thread scheduling often is unpredictable there is no way to predict *when* a deadlock occurs. Only that it *can* occur.

More Complicated Deadlocks

Deadlock can also include more than two threads. This makes it harder to detect. Here is an example in which four threads have deadlocked:

Thread 1  locks A, waits for B
Thread 2  locks B, waits for C
Thread 3  locks C, waits for D
Thread 4  locks D, waits for A

Thread 1 waits for thread 2, thread 2 waits for thread 3, thread 3 waits for thread 4, and thread 4 waits for thread 1.

Database Deadlocks

A more complicated situation in which deadlocks can occur, is a database transaction. A database transaction may consist of many SQL update requests. When a record is updated during a transaction, that record is locked for updates from other transactions, until the first transaction completes. Each update request within the same transaction may therefore lock some records in the database.

If multiple transactions are running at the same time that need to update the same records, there is a risk of them ending up in a deadlock.

For example

Transaction 1, request 1, locks record 1 for update
Transaction 2, request 1, locks record 2 for update
Transaction 1, request 2, tries to lock record 2 for update.
Transaction 2, request 2, tries to lock record 1 for update.

Since the locks are taken in different requests, and not all locks needed for a given transaction are known ahead of time, it is hard to detect or prevent deadlocks in database transactions.