VNA02-J. Ensure that compound operations on shared variables are atomic

Compound operations are operations that consist of more than one discrete operation. Expressions that include postfix or prefix increment (++), postfix or prefix decrement (--), or compound assignment operators always result in compound operations. Compound assignment expressions use operators such as *=, /=, %=, +=, -=, <<=, >>=, >>>=, ^= and |= [[JLS 2005]]. Compound operations on shared variables must be performed atomically to prevent [data races] and [race conditions].

For information about the atomicity of a grouping of calls to independently atomic methods that belong to thread-safe classes, see guideline VNA03-J. Do not assume that a group of calls to independently atomic methods is atomic.

The Java Language Specification also permits reads and writes of 64-bit values to be non-atomic. For more information, see guideline VNA05-J. Ensure atomicity when reading and writing 64-bit values.

Noncompliant Code Example (Logical Negation)

This noncompliant code example declares a shared boolean flag variable and provides a toggle() method that negates the current value of flag.

final class Flag {
  private boolean flag = true;

  public void toggle() {  // Unsafe
    flag = !flag;
  }

  public boolean getFlag() { // Unsafe
    return flag;
  }
}

Execution of this code may result in a data race because the value of flag is read, negated, and written back.

Consider, for example, two threads that call toggle(). The expected effect of toggling flag twice is that it is restored to its original value. However, the following scenario leaves flag in the incorrect state:

As a result, the effect of the call by t₂ is not reflected in flag; the program behaves as if toggle() was called only once, not twice.

Noncompliant Code Example (Bitwise Negation)

Similarly, the toggle() method can use the compound assignment operator ^= to negate the current value of flag.

final class Flag {
  private boolean flag = true;

  public void toggle() {  // Unsafe
    flag ^= true;  // Same as flag = !flag;
  }

  public boolean getFlag() { // Unsafe
    return flag;
  }
}

This code is also not thread-safe. A data race exists because ^= is a non-atomic compound operation.

Noncompliant Code Example (volatile)

Declaring flag volatile does not help either:

final class Flag {
  private volatile boolean flag = true;

  public void toggle() {  // Unsafe
    flag ^= true;
  }

  public boolean getFlag() { // Safe
    return flag;
  }
}

This code remains unsuitable for multithreaded use because declaring a variable as volatile does not guarantee the atomicity of compound operations on it.

Compliant Solution (Synchronization)

This compliant solution declares both the toggle() and getFlag() methods as synchronized.

final class Flag {
  private boolean flag = true;

  public synchronized void toggle() {
    flag ^= true; // Same as flag = !flag;
  }

  public synchronized boolean getFlag() {
    return flag;
  }
}

This guards reads and writes to the flag field with a lock on the instance, that is, this. This compliant solution ensures that changes are visible to all the threads. Now, only two execution orders are possible, one of which is shown below.

The second execution order involves the same operations, but t₂ starts and finishes before t₁.

Compliance with guideline LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code can reduce the likelihood of misuse by ensuring that untrusted callers cannot access the lock object.

Compliant Solution (Volatile-Read, Synchronized-Write)

In this compliant solution, the getFlag() method is not synchronized, and flag is declared as volatile. This solution is compliant because the read of flag in the getFlag() method is an atomic operation and the volatile qualification assures visibility. The toggle() method still requires synchronization because it performs a non-atomic operation.

final class Flag {
  private volatile boolean flag = true;

  public synchronized void toggle() {
    flag ^= true; // Same as flag = !flag;
  }

  public boolean getFlag() {
    return flag;
  }
}

This approach may not be used when a getter method performs operations other than just returning the value of a volatile field without having to use any synchronization. Unless read performance is critical, this technique may not offer significant advantages over synchronization [[Goetz 2006]].

Guideline VNA06-J. Do not assume that declaring an object reference volatile guarantees visibility of its members also addresses the volatile-read, synchronized-write pattern.

Compliant Solution (Read-Write Lock)

This compliant solution uses a read-write lock to ensure atomicity and visibility.

final class Flag {
  private boolean flag = true;
  private final ReadWriteLock lock = new ReentrantReadWriteLock();
  private final Lock readLock = lock.readLock();
  private final Lock writeLock = lock.writeLock();

  public void toggle() {
    writeLock.lock();
    try {
      flag ^= true; // Same as flag = !flag;
    } finally {
      writeLock.unlock();
    }
  }

  public boolean getFlag() {
    readLock.lock();
    try {
      return flag;
    } finally {
      readLock.unlock();
    }
  }
}

Read-write locks allow shared state to be accessed by multiple readers or a single writer but never both. According to Goetz [[Goetz 2006]]

In practice, read-write locks can improve performance for frequently accessed read-mostly data structures on multiprocessor systems; under other conditions they perform slightly worse than exclusive locks due to their greater complexity.

Profiling the application can determine the suitability of read-write locks.

Compliant Solution (AtomicBoolean)

This compliant solution declares flag as an AtomicBoolean type.

import java.util.concurrent.atomic.AtomicBoolean;

final class Flag {
  private AtomicBoolean flag = new AtomicBoolean(true);

  public void toggle() {
    boolean temp;
    do {
      temp = flag.get();
    } while (!flag.compareAndSet(temp, !temp));
  }

  public AtomicBoolean getFlag() {
    return flag;
  }
}

The flag variable is updated using the compareAndSet() method of the AtomicBoolean class. All updates are visible to other threads.

Noncompliant Code Example (Addition of Primitives)

In this noncompliant code example, multiple threads can invoke the setValues() method to set the a and b fields. Because this class does not test for integer overflow, a user of the Adder class must ensure that the arguments to the setValues() method can be added without overflow. (For more information, see guideline NUM00-J. Detect or prevent integer overflow.)

final class Adder {
  private int a;
  private int b;

  public int getSum() {
    return a + b;
  }

  public void setValues(int a, int b) {
    this.a = a;
    this.b = b;
  }
}

The getSum() method contains a race condition. For example, if a and b currently have the values 0 and Integer.MAX_VALUE, respectively, and one thread calls getSum() while another calls setValues(Integer.MAX_VALUE, 0), the getSum() method might return 0 or Integer.MAX_VALUE, or it might overflow and wrap. Overflow will occur when the first thread reads a and b after the second thread has set the value of a to Integer.MAX_VALUE, but before it has set the value of b to 0.

Note that declaring the variables as volatile does not resolve the issue because these compound operations involve reads and writes of multiple variables.

Noncompliant Code Example (Addition of Atomic Integers)

In this noncompliant code example, a and b are replaced with atomic integers.

final class Adder {
  private final AtomicInteger a = new AtomicInteger();
  private final AtomicInteger b = new AtomicInteger();

  public int getSum() {
    return a.get() + b.get();
  }

  public void setValues(int a, int b) {
    this.a.set(a);
    this.b.set(b);
  }
}

The simple replacement of the two int fields with atomic integers, in this example, does not eliminate the race condition because the compound operation a.get() + b.get() is still non-atomic.

Compliant Solution (Addition)

This compliant solution synchronizes the setValues() and getSum() methods to ensure atomicity.

final class Adder {
  private int a;
  private int b;

  public synchronized int getSum() {
    return a + b;
  }

  public synchronized void setValues(int a, int b) {
    this.a = a;
    this.b = b;
  }
}

Any operations within the synchronized methods are now atomic, with respect to other synchronized methods that lock on that object's monitor (intrinsic lock). It is now possible, for example, to add overflow checking to the synchronized getSum() method without introducing the possibility of a race condition.

Risk Assessment

If operations on shared variables are not atomic, unexpected results can be produced. For example, information can be disclosed inadvertently because one user can receive information about other users.

Automated Detection

The SureLogic Flashlight tool can diagnose violations of this guideline as instance fields with empty locksets.

Automated Detection

The Coverity Prevent Version 5.0 ATOMICITY checker can detect the instances of non-atomic update of a concurrently shared value. The result of the update will be determined by the interleaving of thread execution. The GUARDED_BY_VIOLATION checker can detect the instances where thread shared data is accessed without holding an appropriate lock, possibly causing a race condition.

Related Vulnerabilities

Any vulnerabilities resulting from the violation of this guideline are listed on the CERT website.

Related Guidelines

MITRE CWE: CWE-667 "Insufficient Locking"

MITRE CWE: CWE-413 "Insufficient Resource Locking"

MITRE CWE: CWE-366 "Race Condition within a Thread"

MITRE CWE: CWE-567 "Unsynchronized Access to Shared Data"

Bibliography

[[API 2006]] Class AtomicInteger
[[JLS 2005]] 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 2008]] Java Concurrency Tutorial
[[Lea 2000]] Section 2.2.7 The Java Memory Model, Section 2.1.1.1 Objects and Locks
[[Bloch 2008]] Item 66: Synchronize access to shared mutable data
[[Goetz 2006]] 2.3. "Locking"

Issue Tracking

      07. Visibility and Atomicity (VNA)