Is it possible for a thread to Deadlock itself?

Question

Is it technically possible for a thread in Java to deadlock itself?

I was asked this at an interview a while back and responded that it wasn't possible but the interviewer told me that it is. Unfortunately I wasn't able to get his method on how to achieve this deadlock.

This got me thinking and the only situation that I can think of is where you can have this happen is where you have an RMI server process which contained a method that calls itself. The line of code that calls the method is placed in a synchronized block.

Is that even possible or was the interviewer incorrect?

The source code I was thinking about was along these lines (where testDeadlock is running in an RMI server process)

public boolean testDeadlock () throws RemoteException {
    synchronized (this) {
        //Call testDeadlock via RMI loopback            
    }
}

Answer 1

Although the comments here are being pedantic about "deadlock" happening if at least two threads/actions are competing for the same resource...I think the spirit of this question was to discuss a need for Reentrant lock - especially in context of "recursive" locking

Here's an example in python (I am certain the concept stays the same in Java): If you change the RLock to Lock (i.e. reentrant lock to lock, the thread will hang)

import threading

"""
Change RLock to Lock to make it "hang"
"""
lock = threading.Condition(threading.RLock())


def print_list(list):
    lock.acquire()
    if not list:
        lock.release()
        return
    print(list[0])
    print_list(list[1:])
    lock.release()


print_list([1, 2, 3, 4, 5])

Answer 2

I know this is a old post. Here is another example how it could happen if your code has interaction with external resources:

I have a thread, that open a database connection, start a transactionA, and begin update. The same thread, open another connection, start another transactionB. However, because transactionA hasn't committed yet, and it has database table locked, transactionB happens to access this locked table, so it has to wait

In the end the same thread is block by itself because it opened up more than one database connection.

---

This happened a lot in the application that I work with because there are many modules in the application, and a thread can run though many methods. These methods opens their own connections. Since we had different developers write their code, they may not see the how their code are begin called, and therefore couldn't see the overall database transactions that opened by the application.

Answer 3

While I haven't used Java I have deadlocked a single-thread app before. IIRC: Routine A locked a piece of data to update it. Routine B also locked the same piece of data to update it. Due to requirements changes A ended up calling B. Oops.

Of course this was just an ordinary development bug that I caught the first time I tried to run the code but it did deadlock itself. I would think deadlocks of this type would be possible in any language that supports a filesystem.

Answer 4

Here's a way for a thread to deadlock itself.

public class DeadlockMe
{
    public static void main(String[] args)
    {
        DeadlockThing foo = new DeadlockThing();
        synchronized(foo)
        {
            try
            {
                foo.wait();
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
        }
    }
}

The thread creates an instance of a class - any class and waits on it. Because the thread created an object with local scope, there's no possible way for any other thread to notify the object to wake up the thread.

Answer 5

It depends on what you mean by "deadlock" exactly. For example, you could easily wait() on a monitor which nothing would ever pulse... but I don't think I'd call that deadlock, as such.

Thinking along your "method that calls itself" lines, if your server only ran a certain number of threads, they could all be busy waiting from responses from the same server, if that counts. (Simplest example: the server only uses one thread for processing. If you write a request handler which calls into the same server, it will be waiting for the blocked thread to finish handling the request before it can serve the same request...) This isn't really a "synchronized block" sort of deadlock, but it's certainly a danger to be aware of.

EDIT: To apply this answer to the definition in the others, the competing actions here would be "complete the current request" and "handle the new request". Each action is waiting for the other to occur.

Answer 6

Upgrading from a read lock to a write lock (trying to acquire a write lock while holding a read lock) will result in the thread getting completely blocked. Is that a deadlock? You be the judge... But that's the easiest way to create the effect with a single thread.

http://download.oracle.com/javase/6/docs/api/java/util/concurrent/locks/ReentrantReadWriteLock.html