CON00-J. Declare shared variables as volatile to ensure visibility and prevent reordering of statements

Memory that can be shared between threads is called shared memory or heap memory. The term variable is used in the context of this guideline, to refer to both fields and array elements [[JLS 05]].

All instance fields, static fields, and array elements are stored in heap memory. Local variables, formal method parameters, or exception handler parameters are never shared between threads and are not affected by the memory model.

The Java Language Specification defines the Java Memory Model (JMM) which describes possible behaviors of a multi-threaded Java program. Concurrent executions are typically interleaved but the situation is complicated by statements that may be reordered by the compiler or runtime system. This results in execution orders that are not immediately obvious from an examination of the source-code.

There are two requirements for implementing synchronization correctly:

1. Happens-before consistency: If two accesses follow the happens-before relationship, data races cannot occur. However, this is necessary but not sufficient for acceptable program behavior. In addition, often the particular execution order of a program must be sequential consistent.

Consider the following example in which a and b are (shared) global variables or instance fields but r1 and r2 are local variables not accessible by other threads.

Initially, let a = 0 and b = 0.

Because, in Thread 1, the two assignments a = 10; and r1 = b; are not related, the compiler or runtime system is free to reorder them. Similarly in Thread 2, the statements may be freely reordered. Although it may seem counter-intuitive, the Java memory model allows a read to see a write that occurs later in the execution order.

Two possible execution orders and actual assignments are:

In this ordering, r1 and r2 read the original values of the variables a and b even though they are expected to see the updated values.

In this ordering, r1 and r2 read the values of a and b written from step 3 and 4, before the steps are executed.

"The fact that we allow a read to see a write that comes later in the execution order can sometimes thus result in unacceptable behaviors." [[JLS 05]].

Program order is the execution order that is expected when a single thread is running the statements sequentially, as written in a method. Even if statements execute in the expected order (program order), caching can prevent the latest values from being reflected in the main memory.

2. [Sequential consistency]: This property provides a very strong guarantee that the compiler will not optimize away or reorder any statements. It guarantees that the program is free from data races. It also ensures that each access is atomic and immediately visible to other threads. However, "Sequential consistency and/or freedom from data races still allows errors arising from groups of operations that need to be perceived atomically and are not." [[JLS 05]].

The use of sequential consistency as the sole memory model mechanism makes it easy for a programmer to follow the logic, however, introduces a performance penalty. Implementing synchronization correctly guarantees that all executions of the program are sequentially consistent. Consequently, synchronizing code is more expensive than using volatile variables which guarantee a weaker form of sequential consistency.

Volatile read and write operations cannot be reordered with respect to each other and in addition, as required by the JMM, volatile read and write operations are also not reordered with respect to operations on non-volatile variables.

Using volatile variables guarantees a weaker form of sequential consistency because statements prior to a volatile access appear to occur in program order with respect to the statement that writes to the volatile variable, to a thread that is attempting to read the same volatile variable. The previous statements may, however, be freely reordered and may not be sequential consistent with respect to each other. This makes the cost of using volatile cheaper than synchronizing every piece of code.

"Finally, note that the actual execution order of instructions and memory accesses can be in any order as long as the actions of the thread appear to that thread as if program order were followed, and provided all values read are allowed for by the memory model. This allows the programmer to fully understand the semantics of the programs they write, and it allows compiler writers and virtual machine implementors to perform complex optimizations that a simpler memory model would not permit." [[JPL 06]].

A write to a volatile field happens-before every subsequent read of that field. Statements that occur before the write to the volatile field also happen-before the read of the volatile field, from possibly another thread. Consequently, declaring a variable volatile guarantees that writes are always visible to subsequent reads of the variable from any thread. When reading the volatile variable, the other thread will also see statements occurring before the write to the volatile variable, to have already executed with prior occurrences of volatile and non-volatile fields assuming the assigned values.

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. 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

This compliant solution qualifies the done flag as volatile so that updates by one thread are immediately visible to another thread.

final class ControlledStop implements Runnable {
  private volatile boolean done = false;
  // ...
}

Noncompliant Code Example

This noncompliant code example declares a non-volatile int variable that is initialized in the constructor depending on a security check. In a multi-threading scenario, it is possible that the statements will be reordered so that the boolean flag initialized is set to true before the initialization has concluded. If it is possible to obtain a partially initialized instance of the class in a subclass using a finalizer attack (OBJ04-J. Do not allow partially initialized objects to be accessed), a race condition can be exploited by invoking the getBalance() method to obtain the balance even though initialization is still underway.

class BankOperation {
  private int balance = 0;
  private boolean initialized = false;
 
  public BankOperation() {
    if (!performAccountVerification()) {
      throw new SecurityException("Invalid Account"); 
    }
    balance = 1000;   
    initialized = true; 
  }
  
  private int getBalance() {
    if (initialized == true) {
      return balance;
    }
    else {
      return -1;
    }
  }
}

Compliant Solution

This compliant solution declares the initialized flag as volatile to ensure that the initialization statements are not reordered.

class BankOperation {
  private int balance = 0;
  private volatile boolean initialized = false; // Declared volatile
  // ...
}

The use of the volatile keyword is inappropriate for composite operations on shared variables (CON01-J. Design APIs that ensure atomicity of composite operations and visibility of results).

Noncompliant Code Example

This noncompliant code example consists of two classes, an immutable ImmutablePoint class and a mutable Holder class. Holder is mutable because a new ImmutablePoint instance can be assigned to it using the setPoint() method. If one thread updates the value of the ipoint field, another thread may still see the reference of the old value.

class Holder {
  ImmutablePoint ipoint;
  
  Holder(ImmutablePoint ip) {
   ipoint = ip;
  }
  
  void getPoint() {
    return ipoint();
  }

  void setPoint(ImmutablePoint ip) {
    this.ipoint = ip;
  }
}

public class ImmutablePoint {
  final int x = 20;
  final int y = 10;
}

Compliant Solution

This compliant solution declares the ipoint field as volatile so that updates are immediately visible to other threads.

class Holder {
  volatile ImmutablePoint ipoint;
  
  Holder(ImmutablePoint ip) {
    ipoint = ip;
  }
  
  void getPoint() {
    return ipoint();
  }

  void setPoint(ImmutablePoint ip) {
    this.ipoint = ip;
  }
}

Note that no synchronization is necessary for the setPoint() method because it operates atomically on immutable data, that is, on an instance of ImmutablePoint.

Declaring immutable fields as volatile enables their safe publication, in that, once published, it is impossible to change the state of the sub-object.

Noncompliant Code Example

Thread-safe objects (which may not be strictly immutable) must declare their nonfinal fields as volatile to ensure that no thread sees any field references before the sub-objects' initialization has concluded. This noncompliant code example does not declare the map field as volatile.

public class Container<K,V> {
  Map<K,V> map;

  public Container() {
    map = new HashMap<K,V>();	
    // Put values in HashMap
  }

  public V get(Object k) {
    return map.get(k);
  }
}

Compliant Solution

This compliant solution declares the map field as volatile to ensure other threads see an up-to-date HashMap reference and object state.

public class Container<K,V> {
  volatile Map<K,V> map;
  // ...
}

Risk Assessment

Failing to use volatile to guarantee visibility of shared values across multiple thread and prevent reordering of statements can result in unpredictable control flow.

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