CWE-609 双重检查的加锁机制

Double-Checked Locking

结构: Simple

Abstraction: Base

状态: Draft

被利用可能性: unkown

基本描述

The program uses double-checked locking to access a resource without the overhead of explicit synchronization, but the locking is insufficient.

扩展描述

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.

适用平台

Language: {'cwe_Name': 'Java', 'cwe_Prevalence': 'Undetermined'}

常见的影响

范围	影响	注释
['Integrity', 'Other']	['Modify Application Data', 'Alter Execution Logic']

可能的缓解方案

Implementation

策略:

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.

示例代码

例

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

bad 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.

Suppose that helper is not initialized. Then, thread A sees that helper==null and enters the synchronized block and begins to execute:

bad