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Composite operations on shared variables (consisting of more than one discrete operation) are,must bybe definition, non-atomicperformed atomically. ForErrors example,can thearise Javafrom expression {{x+\+}} is non-atomic because it is a composite operation consisting of three discrete operations: reading the current value of {{x}}, adding one to it, and writing the new, incremented value back to {{x}}.  Errors can arise from composite operations that need to be perceived atomically but are not. \[[JLS 05|AA. Java References#JLS 05]\].composite operations that need to be perceived atomically but are not. \[[JLS 05|AA. Java References#JLS 05]\].

For atomicity of a grouping of calls to independently atomic methods of the existing Java API, see CON07For atomicity of a grouping of calls to independently atomic methods of the existing Java API, see CON07-J. Do not assume that a grouping 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 though this is not an issue with most modern JVMs (see CON25-J. Ensure atomicity when reading and writing 64-bit values).

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reads and writes of 64-bit values to be non-atomic though this is not an issue with most modern JVMs (see CON25-J. Ensure atomicity when reading and writing 64-bit values).

Noncompliant Code Example (increment/decrement)

Pre- and post-increment and pre- and post-decrement operations in Java are non-atomic in that value written depends upon the value initially read from the operand. For example, x++ is non-atomic because it is a composite operation consisting of three discrete operations: reading the current value of x, adding one to it, and writing the new, incremented value back to x.

This noncompliant code example contains a data race that may result in the itemsInInventory field being incorrectly decremented.

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Similarly, the returnItem() method that increments itemsInInventory is also non-atomic.

Noncompliant Code Example (volatile)

This noncompliant code example attempts to resolve the problem by declaring itemsInInventory volatile.

private volatile int itemsInInventory = 100;

public final intvoid removeItem() {
  if (itemsInInventory > 0 MIN_INVENTORY) {
    return itemsInInventory--;  // Returns new count of items in inventory
  }
  else {
   return -1underStocked();
 // Error code}
}

public final intvoid returnItem() {
  if (itemsInInventory ==> Integer.MAX_VALUEINVENTORY) { //
 Check for integer overflowoverStocked();
  }
  return -1; // Error Codeelse {
  }
  return itemsInInventory++;
  }
}

Volatile variables are unsuitable when more than one read/write operation needs to be atomic. The use of a volatile variable in this noncompliant code example guarantees that once itemsInInventory has been updated, the new value is visible to all threads that read the field. However, because the post decrement operator is nonatomic, even when volatile is used, the interleaving described in the previous noncompliant code example is still possible.

Similarly, the post increment composite operation in the returnItem() method is non-atomic.

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

The java.util.concurrent utilities can be used to atomically manipulate a shared variable. This compliant solution uses a java.util.concurrent.atomic.AtomicInteger variable which allows composite operations to be performed atomically.

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Notably, this functionality could also be implemented by using the compareAndSet() method. The getAndIncrement() alternative is useful when control over setting the returned value must lie in the hands of the caller instead of the invoked method (returnItem()).

Compliant Solution (method synchronization)

Synchronization provides a way to safely share object state across multiple threads without the need to reason about reorderings, compiler optimizations, and hardware specific behavior.

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If code is synchronized correctly, updates to shared variables are instantly made visible to other threads. Synchronization is more expensive than using the optimized java.util.concurrent utilities and should generally be preferred when it is sufficiently complex to carry out the operation atomically using the utilities. When synchronizing, care must be taken to avoid deadlocks (see CON12-J. Avoid deadlock by requesting and releasing locks in the same order).

Compliant Solution (block synchronization)

Constructors and methods can use an alternative representation called block synchronization which synchronizes a block of code rather than a method, as highlighted in this compliant solution.

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When the number of items is 0 most of the time, the synchronized block may be moved inside the if condition to reduce the performance cost associated with synchronization. In that case, the variable itemsInInventory must be declared as volatile because the check to determine whether it is greater than 0 should rely on the latest value of itemsInInventory.

Compliant Solution (ReentrantLock)

This compliant solution uses a java.util.concurrent.locks.ReentrantLock to atomically perform the post-decrement operation.

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Code that uses this lock behaves similar to synchronized code that uses the traditional monitor lock. ReentrantLock provides several other capabilities, for instance, the tryLock() method does not block waiting if another thread is already holding the lock. The class java.util.concurrent.locks.ReentrantReadWriteLock can be used when some thread requires a lock to write information while other threads require the lock to concurrently read the information.

Noncompliant Code Example (addition)

In this noncompliant code example, the two fields a and b may be set by multiple threads, using the setValues() method.

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The getSum() method may return a different sum every time it is invoked from different threads. For instance, if a and b currently have the value 0, and one thread calls getSum() while another calls setValues(1, 1), then getSum() might return 0, 1, or 2. Of these, the value 1 is unacceptable; it is returned when the first thread reads a and b, after the second thread has set the value of a but before it has set the value of b.

Compliant Solution (addition)

This compliant solution synchronizes the setValues() method so that the entire operation is atomic.

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Unlike the noncompliant code example, if a and b currently have the value 0, and one thread calls getSum() while another calls setValues(1, 1), getSum() may return return 0, or 2, depending on which thread obtains the intrinsic lock first. The locking guarantees that getSum() will never return the unacceptable value 1.

Risk Assessment

If operations on shared variables are not atomic, unexpected results may be produced. For example, there can be inadvertent information disclosure as one user may be able to receive information about other users.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[API 06|AA. Java References#API 06]\] Class AtomicInteger
\[[JLS 05|AA. Java References#JLS 05]\] [Chapter 17, Threads and Locks|http://java.sun.com/docs/books/jls/third_edition/html/memory.html], section 17.4.5 Happens-before Order, section 17.4.3 Programs and Program Order, section 17.4.8 Executions and Causality Requirements
\[[Tutorials 08|AA. Java References#Tutorials 08]\] [Java Concurrency Tutorial|http://java.sun.com/docs/books/tutorial/essential/concurrency/index.html]
\[[Lea 00|AA. Java References#Lea 00]\] Sections, 2.2.7 The Java Memory Model, 2.2.5 Deadlock, 2.1.1.1 Objects and locks
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 66: Synchronize access to shared mutable data
\[[Daconta 03|AA. Java References#Daconta 03]\] Item 31: Instance Variables in Servlets
\[[JavaThreads 04|AA. Java References#JavaThreads 04]\] Section 5.2 Atomic Variables
\[[Goetz 06|AA. Java References#Goetz 06]\] 2.3. "Locking"
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 667|http://cwe.mitre.org/data/definitions/667.html] "Insufficient Locking", [CWE ID 413|http://cwe.mitre.org/data/definitions/413.html] "Insufficient Resource Locking", [CWE ID 366|http://cwe.mitre.org/data/definitions/366.html] "Race Condition within a Thread", [CWE ID 567|http://cwe.mitre.org/data/definitions/567.html] "Unsynchronized Access to Shared Data"

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