Simple Deadlock Examples

Question

Maybe a simple bank situation.

class Account {
  double balance;

  void withdraw(double amount){
     balance -= amount;
  } 

  void deposit(double amount){
     balance += amount;
  } 

   void transfer(Account from, Account to, double amount){
        sync(from);
        sync(to);

        from.withdraw(amount);
        to.deposit(amount);

        release(to);
        release(from);
    }

}

Obviously, should there be two threads which attempt to run transfer(a, b) and transfer(b, a) at the same time, then a deadlock is going to occur because they try to acquire the resources in reverse order.

This code is also great for looking at solutions to the deadlock as well. Hope this helps!

Here's a code example from the computer science department of a university in Taiwan showing a simple java example with resource locking. That's very "real-life" relevant to me. Code below:

/**
 * Adapted from The Java Tutorial
 * Second Edition by Campione, M. and
 * Walrath, K.Addison-Wesley 1998
 */

/**
 * This is a demonstration of how NOT to write multi-threaded programs.
 * It is a program that purposely causes deadlock between two threads that
 * are both trying to acquire locks for the same two resources.
 * To avoid this sort of deadlock when locking multiple resources, all threads
 * should always acquire their locks in the same order.
 **/
public class Deadlock {
  public static void main(String[] args){
    //These are the two resource objects 
    //we'll try to get locks for
    final Object resource1 = "resource1";
    final Object resource2 = "resource2";
    //Here's the first thread.
    //It tries to lock resource1 then resource2
    Thread t1 = new Thread() {
      public void run() {
        //Lock resource 1
        synchronized(resource1){
          System.out.println("Thread 1: locked resource 1");
          //Pause for a bit, simulating some file I/O or 
          //something. Basically, we just want to give the 
          //other thread a chance to run. Threads and deadlock
          //are asynchronous things, but we're trying to force 
          //deadlock to happen here...
          try{ 
            Thread.sleep(50); 
          } catch (InterruptedException e) {}

          //Now wait 'till we can get a lock on resource 2
          synchronized(resource2){
            System.out.println("Thread 1: locked resource 2");
          }
        }
      }
    };

    //Here's the second thread.  
    //It tries to lock resource2 then resource1
    Thread t2 = new Thread(){
      public void run(){
        //This thread locks resource 2 right away
        synchronized(resource2){
          System.out.println("Thread 2: locked resource 2");
          //Then it pauses, for the same reason as the first 
          //thread does
          try{
            Thread.sleep(50); 
          } catch (InterruptedException e){}

          //Then it tries to lock resource1.  
          //But wait!  Thread 1 locked resource1, and 
          //won't release it till it gets a lock on resource2.  
          //This thread holds the lock on resource2, and won't
          //release it till it gets resource1.  
          //We're at an impasse. Neither thread can run, 
          //and the program freezes up.
          synchronized(resource1){
            System.out.println("Thread 2: locked resource 1");
          }
        }
      }
    };

    //Start the two threads. 
    //If all goes as planned, deadlock will occur, 
    //and the program will never exit.
    t1.start(); 
    t2.start();
  }
}

Let nature explain deadlock,

Deadlock: Frog vs. Snake

"I would love to have seen them go their separate ways, but I was exhausted," the photographer said. "The frog was all the time trying to pull the snake off, but the snake just wouldn't let go".

If method1() and method2() both will be called by two or many threads, there is a good chance of deadlock because if thread 1 acquires lock on String object while executing method1() and thread 2 acquires lock on Integer object while executing method2() both will be waiting for each other to release lock on Integer and String to proceed further, which will never happen.

public void method1() {
    synchronized (String.class) {
        System.out.println("Acquired lock on String.class object");

        synchronized (Integer.class) {
            System.out.println("Acquired lock on Integer.class object");
        }
    }
}

public void method2() {
    synchronized (Integer.class) {
        System.out.println("Acquired lock on Integer.class object");

        synchronized (String.class) {
            System.out.println("Acquired lock on String.class object");
        }
    }
}

One of the simple deadlock example I have come across.

public class SimpleDeadLock {
   public static Object l1 = new Object();
   public static Object l2 = new Object();
   private int index;
   public static void main(String[] a) {
      Thread t1 = new Thread1();
      Thread t2 = new Thread2();
      t1.start();
      t2.start();
   }
   private static class Thread1 extends Thread {
      public void run() {
         synchronized (l1) {
            System.out.println("Thread 1: Holding lock 1...");
            try { Thread.sleep(10); }
            catch (InterruptedException e) {}
            System.out.println("Thread 1: Waiting for lock 2...");
            synchronized (l2) {
               System.out.println("Thread 2: Holding lock 1 & 2...");
            }
         }
      }
   }
   private static class Thread2 extends Thread {
      public void run() {
         synchronized (l2) {
            System.out.println("Thread 2: Holding lock 2...");
            try { Thread.sleep(10); }
            catch (InterruptedException e) {}
            System.out.println("Thread 2: Waiting for lock 1...");
            synchronized (l1) {
               System.out.println("Thread 2: Holding lock 2 & 1...");
            }
         }
      }
   }
}

Here's a simple example in C++11.

#include <mutex>    // mutex
#include <iostream> // cout 
#include <cstdio>   // getchar
#include <thread>   // this_thread, yield
#include <future>   // async
#include <chrono>   // seconds

using namespace std;
mutex _m1;
mutex _m2;

// Deadlock will occur because func12 and func21 acquires the two locks in reverse order

void func12()
{
    unique_lock<mutex> l1(_m1);
    this_thread::yield(); // hint to reschedule
    this_thread::sleep_for( chrono::seconds(1) );
    unique_lock<mutex> l2(_m2 );
}

void func21()
{
    unique_lock<mutex> l2(_m2);
    this_thread::yield(); // hint to reschedule
    this_thread::sleep_for( chrono::seconds(1) );
    unique_lock<mutex> l1(_m1);
}

int main( int argc, char* argv[] )
{
    async(func12);
    func21();
    cout << "All done!"; // this won't be executed because of deadlock
    getchar();
}