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

Any thread that invokes wait() expects to wake up and resume execution when some condition predicate becomes true. As recommended by CON18-J. Always invoke wait() and await() methods inside a loop, waiting threads should test their condition predicates upon receiving notifications and resume waiting if they are false (that is, when the condition expression in the loop evaluates to true).

The methods notify() and notifyAll() of package java.lang.Object are used to waken waiting thread(s). These methods must be invoked from code that holds the same object lock as the waiting thread(s). The notifyAll() method wakes up all threads and allows ones whose condition predicate is true (loop expression is false), to resume execution. Furthermore, if all the threads whose condition predicate evaluates to true previously held a specific lock before going into the wait state, only one of them will reacquire the lock upon being notified, and the others may, presumably, resume waiting. The notify() method wakes up only one thread, and makes no guarantees as to which thread is notified. If the thread's condition predicate doesn't allow, the chosen thread may not awaken, defeating the purpose of the notification.

The notify() method may only be invoked if all the following conditions are met:

• Every condition predicate in every waiting thread would be true (condition expressions in loops will be false) if a notification were received by each, independently. Furthermore, all these threads must perform the same set of operations after waking up. In other words, any one thread can be selected to wake up and resume for a single invocation of notify().
• Only one thread is required to 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. Multiple condition predicates in the same statement should be avoided.
• No untrusted code has access to the object being waited on. If untrusted code has access to this object, it can wait() on the object and intercept a notify() call.

The java.util.concurrent utilities (interface Condition) provide the signal() and signalAll() methods to awaken waiting threads that are blocked on an await() call. Much 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. It is always safer to use signalAll() albeit a small performance penalty. Similarly, any thread that is blocked on a wait() method invocation on a Java object which is being used as a condition queue, should be notified using notifyAll().

Noncompliant Code Example (notify())

This noncompliant code example shows a complex multi-step process being undertaken by several threads. Each thread executes one step of the process; the step being currently performed is enumerated by the step field. Each thread waits for the step field to indicate that it is time to perform the corresponding thread's step. After performing the step, each thread increments step to transfer control to the next thread, notifies it, and exits.

public final class ProcessStep implements Runnable {
  private static final Object lock = new Object();
  private static int time = 0;
  private final int step; // Do operations when the step reaches this value

  public ProcessStep(int step) {
    this.step = step;
  }

  public void run() {
    try {
      synchronized (lock) {
        while (time != step) { 
          lock.wait();  
        }

        // ... Do operations

        time++;
        lock.notify();
      }
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    }    
  }

  public static void main(String[] args) {
    for (int i = 4; i >= 0; i--) {
      new Thread(new ProcessStep(i)).start();
    }
  }
}

This noncompliant code example violates the liveness property. Each thread has a different condition predicate, as each requires step to have a different value before proceeding. The Object.notify() method wakes up only one thread at a time. Unless it happens to wake up the thread that is required to perform the next step, the program stalls.

Compliant Solution (notifyAll())

In this compliant solution, all threads that have performed their own step use notifyAll() to notify other waiting threads. Consequently, threads that are ready can perform the task, while all other threads whose condition predicate is false (loop condition expression is true), promptly go back to sleep.

  // ...
  public void run() {
    try {
      synchronized (lock) {
        while (time != step) { 
          lock.wait();  
        }

        // ... Do operations

        time++;
        lock.notifyAll();
      }
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    }    
  }

Noncompliant Code Example (Condition interface)

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

public class ProcessStep implements Runnable {
  private static final Lock lock = new ReentrantLock();
  private static final Condition condition = lock.newCondition();
  private static int time = 0;
  private final int step; // Do operations when the step reaches this value

  public ProcessStep(int step) {
    this.step = step;
  }

  public void run() {
    lock.lock();
    try {
      while (time != step) { 
        condition.await();  
      }

      // ... Do operations

      time++;
      condition.signal();
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    } finally {
      lock.unlock();
    }
  }

  public static void main(String[] args) {
    for (int i = 4; i >= 0; i--) {
      new Thread(new ProcessStep(i)).start();
    }
  }
}

Similar to Object.notify(), the signal() method may choose any one thread 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.

  // ...
  public void run() {
    lock.lock();
    try {
      while (time != step) { 
        condition.await();  
      }

      // ... Do operations

      time++;
      condition.signalAll();
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    } finally {
      lock.unlock();
    }
  }

Compliant Solution (unique Condition per thread)

This compliant solution assigns each thread its own condition, and makes all the Condition objects accessible to all the threads.

// Declare class as final because its constructor throws an exception 
public final class ProcessStep implements Runnable { 
  private static final Lock lock = new ReentrantLock();
  private static int time = 0;
  private final int step; // Do operations when the step reaches this value
  private static final int MAX_STEPS = 5;
  private static final Condition[] conditions = new Condition[MAX_STEPS];

  public ProcessStep(int step) {
    if (step <= MAX_STEPS) {
      this.step = step;
      conditions[step] = lock.newCondition();
    } else {
      throw new IllegalArgumentException("Too many threads");
    }
  }

  public void run() {
    lock.lock();
    try {
      while (time != step) { 
        conditions[step].await();  
      }

      // ... Do operations

      time++;
      if (step + 1 < conditions.length) {
        conditions[step + 1].signal();
      }
    } catch (InterruptedException ie) {
      Thread.currentThread().interrupt(); // Reset interrupted status
    } finally {
      lock.unlock();
    }
  }

  public static void main(String[] args) {
    for (int i = MAX_STEPS - 1; i >= 0; i--) {
      ProcessStep ms = new ProcessStep(i);
      new Thread(ms).start();
    }
  }
}

Even though signal() is used, it is guaranteed that only one thread will awaken because each condition predicate corresponds to a unique Condition variable. All threads perform the same operations. This compliant solution is only safe if untrusted code cannot create a thread with an instance of this class.

Risk Assessment

Notifying a single thread instead of all waiting threads can pose 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