An atomic operation 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
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Variables should be declared as {{volatile}} when this criteria is met \[[Goetz 06|AA. Java References#Goetz 06]\]. If this is not done, multiple threads may observe stale values of the shared variables and fail to act accordingly. |
volatile, multiple threads may observe stale values of the flags and fail to act accordingly. 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.
| Code Block | ||
|---|---|---|
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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.
| Code Block | ||
|---|---|---|
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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;
}
}
|
Risk Assessment
Failing to declare status flags as volatile can result in ensure visibility of atomically modifiable shared variables can lead to a thread seeing stale values of the flagsa variable.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
CON00- J | medium | probable | medium | P8 | L2 |
Automated Detection
TODO
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
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\[[JLS 05|AA. Java References#JLS 05]\] [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 \[[Goetz 06|AA. Java References#Goetz 06]\] 3.4.2. "Example: Using Volatile to Publish Immutable Objects" \[[JPL 06|AA. Java References#JPL 06]\] 14.10.3. "The Happens-Before Relationship" \[[MITRE 09|AA. Java References#MITRE 09]\] [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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