CON19-J. Notify all waiting threads instead of a single thread

The methods java.lang.Object.notify() and java.lang.Object.notifyAll() are used to waken waiting thread(s). These methods must be called from code that holds the same object lock as the waiting thread(s). The method notify() is deceptive in most cases unless all of the following conditions hold: [[Goetz 06]]

• Only one condition predicate is used with the locked object. Also, each thread must execute the same code after waking up from a wait.
• Only one thread must wake up on the notify signal. This is contingent on the condition predicate, in that, only one predicate must fulfill the condition and allow the thread to proceed.

These requirements are typically true when only one thread is waiting. Otherwise, if either condition does not hold, incorrect program behavior may result.

The java.util.concurrent utilities (interface Condition) provide the signal() and signalAll() methods to awaken waiting threads that are blocked on an await() call. Like the notify() method, the signal() method wakes up any one of the threads that is waiting on the condition and consequently, may be insecure.

Noncompliant Code Example (notify())

This noncompliant code example violates the liveness property. A lock is obtained using a raw Object lock and three threads are started. Two condition predicates are used. One checks whether the buffer has zero elements and the other checks if the buffer is full with ten elements (buffer is not shown for brevity, only the count of the number of elements in the buffer at any time is shown). Initially the buffer is neither full nor empty. Conditions are created so that the buffer becomes empty and thread 1 goes into wait state, followed by thread 2, when the buffer becomes full.

public class MissedSignal implements Runnable {
  private static final Object lock = new Object();
  private static int buffer_count = 5;
  private static int number; // Selects function based on thread number	
  
  public void run (){
    synchronized(lock) {
      try {    	  
        if(number == 1) {	  
          while(buffer_count == 0) { 
      	    lock.wait();		  
      	  }  
      	} else if(number == 2) {  	    		  
      	  while(buffer_count == 10) {	
      	    lock.wait();   	    		    	        	    
      	  }
      	} else if(number == 3) {	   
          lock.notify();      	   		  
        } 	    	  
      } catch (InterruptedException ie) {
        // Handle the exception
        Thread.currentThread().interrupt(); // Reset interrupted status
      }
    }	  
  }

  public static void setThreadParameters(int threadNumber, int bufferCount) {
    number = threadNumber;
    buffer_count = bufferCount;
  }
                
  public static void main(String[] args) throws IOException, InterruptedException {    
    MissedSignal ms = new MissedSignal();
     
    for(int i = 0; i < 3; i++) {          // Buffer count does not matter for third thread
      setThreadParameters(i + 1, i * 10); // Sets buffer count for threads in the sequence: 0, 10, 20
      new Thread(ms).start();
      Thread.sleep(1000);
    }
  }
}

Note that when thread 2 goes into the wait state, the condition predicate of thread 1 becomes false. When notify() is invoked by thread 3, it can be delivered to either thread 1 or thread 2 depending on the particular Java Virtual Machine (JVM). If thread 1 is chosen to be notified, its condition turns out to be false, which terminates it. This is the required functionality, that is, any thread whose condition predicate is false must be terminated. However, if the notification is delivered to thread 2, it has no effect because its condition predicate is still true, and consequently, it goes into the wait state once again. Thread 1 continues to wait despite its condition predicate being false and is not terminated in this case.

Compliant Solution (notify all threads)

This compliant solution uses the notifyAll() method which sends notifications to all threads that wait on the same lock object. As a result, liveness is not affected unlike the noncompliant code example. The condition predicate controls which threads can resume their operations. Ensure that the lock is released promptly after the call to notifyAll().

else if(number == 3) {
  list.notifyAll();      	   		  
} 	    	  

Noncompliant Code Example (Condition interface)

This noncompliant code example derives from the previous noncompliant code example but uses the Condition interface. Field cond is used to let threads wait on different condition predicates.

final Condition cond = lock.newCondition();
// ...

public void run (){
  lock.lock();  
  try {    	  
    if(number == 1) {	  
      while(buffer_count == 0) { 
        cond.await();		  
      }  
    } else if(number == 2) {  	    		  
      while(buffer_count == 10) {	
        cond.await();   	    		    	        	    
      }      
    } else if(number == 3) {
      cond.signal();	
    } 	   
  } catch (InterruptedException ie) {
    // Handle the exception
    Thread.currentThread().interrupt(); // Reset interrupted status
  } finally {
    lock.unlock();
  }
}	  

Similar to Object.notify(), the cond.signal() method may choose either of the threads and awaken it.

Compliant Solution (signalAll())

This compliant solution uses the signalAll() method to resume all the waiting threads whose condition predicate allows doing so.

} else if(number == 3) {
  cond.signalAll();
} 

Compliant Solution (unique Condition variable per thread)

This compliant solution uses two different condition variables full and empty to indicate whether the the buffer is full or empty, respectively.

final Lock lock = new ReentrantLock();
final Condition full = lock.newCondition();
final Condition empty = lock.newCondition();
// ...

public void run (){
  lock.lock();  
  try {    	  
    if(number == 1) {	  
      while(buffer_count == 0) { 
        empty.await();		  
      }  
    } else if(number == 2) {  	    		  
      while(buffer_count == 10) {	
        full.await();   	    		    	        	    
      }
    } else if(number == 3) {
      empty.signal();	
      full.signal();      	   		  
    } 	   
  } catch (InterruptedException ie) {
    // Handle the exception
    Thread.currentThread().interrupt(); // Reset interrupted status
  } finally {
    lock.unlock();
  }
}	  

Even though signal() is used, it is guaranteed that only one thread will awaken because each condition predicate corresponds to a unique Condition variable.

Exceptions

EX1: If there are several similar threads waiting for a notification, and it is permissible to invoke any of them, notify() may be used. The criteria for liveness is relaxed in this case.

Risk Assessment

Invoking the notify() method instead of notifyAll() can be a threat to the liveness property of the system.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[JLS 05]] Chapter 17, Threads and Locks
[[Goetz 06]] Section 14.2.4, Notification
[[Bloch 01]] Item 50: Never invoke wait outside a loop

CON18-J. Always invoke wait() and await() methods inside a loop      11. Concurrency (CON)      CON20-J. Do not perform operations that may block while holding a lock