Double-Checked Locking
Weakness ID: 609 (Weakness Base)Status: Draft
+ Description

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.

+ Time of Introduction
  • Implementation
+ Applicable Platforms

Languages

Java

+ Demonstrative Examples

Example 1

It may seem that the following bit of code achieves thread safety while avoiding unnecessary synchronization...

(Bad Code)
Example Language: Java 
if (helper == null) {
synchronized (this) {
if (helper == null) {
helper = new Helper();
}
}
}
return helper;

The 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.

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:

(Bad Code)
 
helper = new Helper();

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.

+ 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.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory361Time and State
Development Concepts (primary)699
ChildOfWeakness BaseWeakness Base367Time-of-check Time-of-use (TOCTOU) Race Condition
Research Concepts (primary)1000
ChildOfWeakness BaseWeakness Base667Insufficient Locking
Research Concepts1000
CanPrecedeWeakness ClassWeakness Class362Race Condition
Development Concepts699
Research Concepts1000
+ References
David Bacon et al. "The "Double-Checked Locking is Broken" Declaration". <http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html>.
Jeremy Manson and Brian Goetz. "JSR 133 (Java Memory Model) FAQ". <http://www.cs.umd.edu/~pugh/java/memoryModel/jsr-133-faq.html#dcl>.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
Anonymous Tool Vendor (under NDA)Externally Mined
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Context Notes
2008-09-08CWE Content TeamMITREInternal
updated Description, Relationships, Taxonomy Mappings
2008-11-24CWE Content TeamMITREInternal
updated Demonstrative Examples
2009-01-12CWE Content TeamMITREInternal
updated Relationships
2009-05-27CWE Content TeamMITREInternal
updated Relationships
2009-10-29CWE Content TeamMITREInternal
updated Taxonomy Mappings
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Double Checked Locking
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