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CWE-609 Individual Dictionary Definition (Draft 9)
Weakness ID
| Status: Draft 609 (Weakness Base) | | Description | Summary The program uses double-checked locking to access a resource without the overhead of explicit synchronization, but the locking is insufficient. Extended Description Double-checked locking refers to the situation where a programmer checks to see if a
resource has been initialized, grabs a lock, checks again to see if the resource has been
initialized, and then performs the initialization if it has not occurred yet. This should not be done, as is not guaranteed
to work in all languages and on all architectures.
In summary, other
threads may not be operating inside the synchronous block and are not guaranteed to see the
operations execute in the same order as they would appear inside the synchronous block. | | Potential Mitigations | While double-checked locking can be achieved in some languages, it is inherently flawed
in Java before 1.5, and cannot be achieved without compromising platform independence. Before Java 1.5, only use of the synchronized keyword is known to work. Beginning in Java 1.5, use of the "volatile" keyword allows double-checked locking to work successfully, although there is some debate as to whether it achieves sufficient performance gains. See references. | Demonstrative
Examples | It may seem that the following bit of code achieves thread safety while
avoiding unnecessary synchronization... Java Example: if (helper == null) {
synchronized (this) {
if (helper == null) {
helper = new Helper();
}
}
}
return helper; The programmer wants to guarantee that only one
<code>Helper()</code> object is ever allocated, but does
not want to pay the cost of synchronization every time this code is
called. Let's say helper is not initialized. Then, thread A comes along, sees
that helper==null, and enters the synchronized block and begins to execute: If a second thread, thread B, takes over in the middle of this call and
helper has not finished running the constructor, then thread B may make calls on
helper while its fields hold incorrect values. | | Context Notes | | | References | | | Relationships | | | Source Taxonomies | Anonymous Tool Vendor (under NDA) - | | Applicable Platforms | Java | | Time of Introduction | Implementation |
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