CON00-J. Ensure visibility of shared primitive variables on accesses

Accesses of primitive variables are atomic, except for the 64-bit long and double variables. An atomic access of a shared variable has three characteristics:

• A write to a variable does not depend on its current value
• The write is not involved with writes of other variables
• There is no need to synchronize the code to access the variable

Variables should be declared as volatile when this criteria is met [[Goetz 06]]. If this is not done, multiple threads may observe stale values of the shared variables and fail to act accordingly.

Although 64-bit primitives such as long and double are not accessed atomically, ensuring their visibility by declaring them as volatile also ensures that they are accessed atomically (see CON25-J. Ensure atomicity when reading and writing 64-bit values).

Noncompliant Code Example (status flag)

This noncompliant code example uses a shutdown() method to set a non-volatile done flag that is checked in the run() method. 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 still observe that done is false and incorrectly invoke the sleep() method.

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

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

Compliant Solution (volatile status flag)

This compliant solution qualifies 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
      } 
    } 	 
  }

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

Exceptions

EX1: Correctly synchronized code that accesses a shared primitive variable always observes its latest value. In this case there is no requirement of explicitly ensuring visibility. However, excessive synchronization can lead to lock contention and deadlocks. Consequently, it should not be used unless there is some other need to use it, than using it to ensure the visibility of accesses to shared primitive variables. This exception performs operations that require synchronization (incrementing i) and as a side-effect, it ensures the visibility of the done flag.

final class ControlledStop implements Runnable {
  int i = 0;
  private boolean done = false;
 
  public synchronized void run() {
    while (!done) {
      // Do something 
      i++;
    } 	 
  }

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

Risk Assessment

Failing to ensure visibility of atomically modifiable shared variables can lead to a thread seeing stale values of a variable.

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, section 17.4.5 Happens-before Order, section 17.4.3 Programs and Program Order, section 17.4.8 Executions and Causality Requirements
[[Tutorials 08]] Java Concurrency Tutorial
[[Lea 00]] Sections, 2.2.7 The Java Memory Model, 2.2.5 Deadlock, 2.1.1.1 Objects and locks
[[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