CON00-J. Ensure visibility when accessing shared primitive variables

Reading a shared primitive variable in one thread may not yield the value of the latest write to the variable from another thread. It is important to ensure that a read of a shared variable sees the value of the most recent write to the variable. If this is not done, multiple threads may observe stale values of the shared variable and fail to act accordingly. Visibility of the latest value can be ensured by declaring the variable volatile or correctly synchronizing the reads and writes to the variable.

The use of volatile is safe under a very restrictive set of conditions, all of which must hold:

• A write to a variable does not depend on its current value
• The write is not involved with reads or writes of other variables
• Locking is not required for any other reason (all actions are atomic)

The first condition is sometimes relaxed when it can be ensured that only one thread ever updates the value of the variable [[Goetz 06]]. However, it is still possible for reader threads to see stale values of the variable while the writing thread is in the process of modifying its value, before writing it back. Furthermore, it is harder to maintain code that relies on such an invariant being true at all times.

Synchronizing the code makes it easier to reason about its behavior and is frequently, a more secure approach than using volatile. However, it is slightly more expensive and can cause deadlocks when used excessively.

Declaring a variable as volatile or correctly synchronizing the code guarantees that 64-bit primitive variables of type long and double are accessed atomically (see CON25-J. Ensure atomicity when reading and writing 64-bit values for information on sharing long and double variables amongst multiple threads).

Noncompliant Code Example

This noncompliant code example uses a shutdown() method to set a non-volatile done flag that is checked in the run() method.

final class ControlledStop implements Runnable {
  private boolean done = false;
 
  @Override public void run() {
    while (!done) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        // handle exception
        Thread.currentThread().interrupt(); // Reset interrupted status
      } 
    } 	 
  }

  public void shutdown() {
    done = true;
  }
}

If one thread invokes the shutdown() method to set the flag, it is possible that another thread might not observe this change. Consequently, the second thread may observe that done is still false and incorrectly invoke the sleep() method. In fact, a compiler is allowed to optimize the code if it determines that the value of done is never modified by the same thread, with the end result being an infinite loop.

Compliant Solution (volatile)

This compliant solution declares the done flag as volatile so that updates are visible to other threads.

final class ControlledStop implements Runnable {
  private volatile boolean done = false;
 
  public void run() {
    while (!done) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        // handle exception
        Thread.currentThread().interrupt(); // Reset interrupted status
      } 
    } 	 
  }

  public void shutdown() {
    done = true;
  }
}

Compliant Solution (java.util.concurrent.atomic.AtomicBoolean)

This compliant solution uses an AtomicBoolean flag to ensure that updates are visible to other threads.

final class ControlledStop implements Runnable {
  private final AtomicBoolean done = new AtomicBoolean(false);
 
  public void run() {
    while (!done.get()) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        // handle exception
        Thread.currentThread().interrupt(); // Reset interrupted status
      } 
    } 	 
  }

  public void shutdown() {
    done.set(true);
  }
}

Compliant Solution (synchronized)

This compliant solution uses the intrinsic lock of the Class object to ensure that updates are visible to other threads.

final class ControlledStop implements Runnable {
  private boolean done = false;
 
  public void run() {
    while (!isDone()) {
      try {
        // ...
        Thread.currentThread().sleep(1000); // Do something
      } catch(InterruptedException ie) { 
        // handle exception
        Thread.currentThread().interrupt(); // Reset interrupted status
      } 
    } 	 
  }

  public synchronized boolean isDone() {
    return done;
  }

  public synchronized void shutdown() {
    done = true;
  }
}

While this is an acceptable compliant solution, it has the following shortcomings as compared to the previously suggested ones:

• Performance: Intrinsic locks cause threads to block temporarily and may introduce some contention; volatile incurs no blocking
• Deadlock: Excessive synchronization can make the program deadlock prone.

However, synchronization is a more secure alternative in situations where the volatile keyword or a java.util.concurrent.atomic.Atomic* field is inappropriate, such as if a variable's new value depends on its old value. Refer to CON01-J. Ensure that compound operations on shared variables are atomic for more information.

Exceptions

EX1: Objects of type Class need not be made visible because they are created by the Virtual Machine and their initialization always precedes any subsequent use. JMM Mailing List

Risk Assessment

Failing to ensure visibility of shared primitive variables on accesses can lead to a thread seeing stale values of the variables.

Automated Detection

SureLogic Flashlight can detect violations of this guideline. It flags the noncompliant code example by specifying: "Instance fields with empty locksets". The unprotected field can be observed through its GUI.

Related Vulnerabilities

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

References

[[JLS 05]] Chapter 17, Threads and Locks, section 17.4.5 Happens-before Order, section 17.4.3 Programs and Program Order, section 17.4.8 Executions and Causality Requirements
[[Bloch 08]] Item 66: Synchronize access to shared mutable data
[[Goetz 06]] 3.4.2. "Example: Using Volatile to Publish Immutable Objects"
[[JPL 06]] 14.10.3. "The Happens-Before Relationship"
[[MITRE 09]] CWE ID 667 "Insufficient Locking", CWE ID 413 "Insufficient Resource Locking", CWE ID 366 "Race Condition within a Thread", CWE ID 567 "Unsynchronized Access to Shared Data"

11. Concurrency (CON)      11. Concurrency (CON)      CON02-J. Always synchronize on the appropriate object