Double-Checked Locking
Draft Base
Structure: Simple
Description
The product uses double-checked locking to access a resource without the overhead of explicit synchronization, but the locking is insufficient.
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 it 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.
Common Consequences 1
Scope: IntegrityOther
Impact: Modify Application DataAlter Execution Logic
Potential Mitigations 1
Phase: Implementation
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 1
ID : DX-70
It may seem that the following bit of code achieves thread safety while avoiding unnecessary synchronization...Code Example:
Bad
Java
javaThe programmer wants to guarantee that only one Helper() object is ever allocated, but does not want to pay the cost of synchronization every time this code is called.
Suppose that helper is not initialized. Then, thread A sees that helper==null and enters the synchronized block and begins to execute:
Code Example:
Bad
Java
javaIf 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.
References 3
The "Double-Checked Locking is Broken" Declaration
David Bacon et al
ID: REF-490
JSR 133 (Java Memory Model) FAQ
Jeremy Manson and Brian Goetz
ID: REF-491
The Art of Software Security Assessment
Mark Dowd, John McDonald, and Justin Schuh
Addison Wesley
2006
ID: REF-62
Applicable Platforms
Languages:
Java : Undetermined
Modes of Introduction
Implementation
Related Weaknesses
ChildOf:
Improper Locking (CWE-667)
Taxonomy Mapping
- The CERT Oracle Secure Coding Standard for Java (2011)
- Software Fault Patterns