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Programmers sometimes assume that composite operations on primitive data are atomic. However, this is not true in a multithreaded environment. Even code that does not involve a composite operation can be unsafe for use, for instance:

x = 3;

This code can be fixed by declaring the variable x as volatile. Declaring a shared mutable variable volatile ensures the visibility of the latest updates to it across other threads but does not guarantee the atomicity of composite operations. For example, the post-increment operation consisting of the sequence read-modify-write is not atomic even when the variable is declared volatile.

The statement shown below increments the value of x and is nonatomic because it encapsulates several operations - Composite operations consisting of more than one discrete operation are, by definition, non-atomic. For example, the Java 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 setting x to writing the new, incremented value .

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back to x.

"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." \[[JLS 05|AA. Java References#JLS 05]\]. In such cases, the {{java.util.concurrent}} utilities can be used to atomically manipulate a shared variable. If these utilities do not provide the required functionality in the form of atomic methods, operations that use the variable should be correctly synchronized. 

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The Java Language Specification also permits reads and writes of 64-bit values to be nonatomic 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 (post-decrement composite operation)

In this noncompliant code example, the field itemsInInventory can be accessed by multiple threadsThis noncompliant code example contains a data race that may result in the itemsInInventory field being incorrectly decremented.

private int itemsInInventory = 100;

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

For example, if the remoteItem() method is concurrently invoked by two threads, the execution of these threads may be interleaved so that:

1. The first thread reads the current value of itemsInInventory (100).
2. The second thread reads the current value of itemsInInventory (100).
3. The first thread decrements the locally cached value (99).
4. The second thread decrements the locally cached value (99).
5. The first thread writes the locally cached value to itemsInInventory (99).
6. The second thread writes the locally cached value to itemsInInventory (99).

As a result, a decrement operation is "lost" and the itemsInInventory value is now incorrectHowever, when a thread is updating the value of itemsInInventory, it is possible for other threads to read the original value (that is, the value before the update). Furthermore, it is possible for two threads to perform the -- operator simultaneously. If this happens, both will read the same value, decrement it, and write the decremented value. This causes itemsInInventory to be decremented by 1 even though it was decremented twice! This is because the post decrement operator is nonatomic.

Noncompliant Code Example (volatile)

This noncompliant code example attempts to fix repair the visibility issue problem by declaring the itemsInInventory as volatile.

private volatile int itemsInInventory = 100;

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

This guarantees that once the update has taken place, it is immediately visible to all threads that read the field. However, when a thread is in the process of updating the value of itemsInInventory (after the read and modification, but before the write), it is still possible for other threads to read the original value (that is, the value before the update). Furthermore, two threads may still decrement itemsInInventory at the same time, resulting in an incorrect value. This is 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 immediately visible to all threads that read the field. However, because the post decrement operator is nonatomic non-atomic, even when {{volatile is used, the interleaving described in the previous noncompliant code exampleis still possible.

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

Volatile variables are unsuitable when more than one load/store operation needs to be atomic. This compliant solution uses a java.util.concurrent.atomic.AtomicInteger variable which allows several certain composite operations to be performed atomically.

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The {{compareAndSet()}} method takes two arguments, the expected value of a variable when the method is invoked and the updated value. This compliant solution uses this method to atomically set the value of {{itemsInInventory}} to the updated value if and only if the current value equals the expected value. \[[API 06|AA. Java References#API 06]\] 

Compliant Solution (method synchronization)

This compliant solution uses method synchronization to synchronize access to itemsInInventory. Consequently, access to itemsInInventory is mutually exclusive and its state consistent across all threads.

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Synchronization is more expensive than using the optimized java.util.concurrent utilities and should only be used when the utilities do not contain the facilities (methods) required to carry out the atomic operation. 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 simultaneously 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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