Skip to main content

Double Checked Locking Pattern, volatile keyword and memory barriers

Double Checked Locking Pattern (DCLP) is used to initialize singleton only once in a multithreaded application and also to avoid cost of acquiring lock everytime singleton is accessed. DCLP should be considered in two different dimensions. (1) CPU instruction reordering (2) Multi-processor machines.

In a single threaded application running on a modern CPU, use of volatile keyword in DCLP is really important. volatile keyword prevents any aggressive instruction reordering that modern CPUs might do. The static instance pointer of the singleton and the singleton by itself (both) should be volatile for the magic to work!

volatile exists for special purposes:
(1) the content of a volatile variable is “unstable” (can change by means unknown to the compiler),
(2) all writes to volatile data are “observable” so they must be executed religiously, and
(3) all operations on volatile data are executed in the sequence in which they appear in the source code.
The first two rules ensure proper reading and writing. The last one allows implementation of I/O protocols that mix input and output. This is informally what C and C++’s volatile guarantees.

In multiprocessor environment, DCLP should be used with memory barriers. This takes care of cache coherency issues between multiple CPUs and its effects on DCLP.

Source: C++ and the Perils of Double-Checked Locking

Comments

avitzur said…
Are the acquire and release barriers in Meyers and Alexandrescu's paper the same as read and write barriers?

- Ron
Wed Oct 04, 12:10:00 PM PDT
Post a Comment

Popular Content

Multi-dimensional arrays in C++11

What new can be said about multi-dimensional arrays in C++? As it turns out, quite a bit! With the advent of C++11, we get new standard library class std::array. We also get new language features, such as template aliases and variadic templates. So I'll talk about interesting ways in which they come together. It all started with a simple question of how to define a multi-dimensional std::array. It is a great example of deceptively simple things. Are the following the two arrays identical except that one is native and the other one is std::array? int native[3][4]; std::array<std::array<int, 3>, 4> arr; No! They are not. In fact, arr is more like an int[4][3]. Note the difference in the array subscripts. The native array is an array of 3 elements where every element is itself an array of 4 integers. 3 rows and 4 columns. If you want a std::array with the same layout, what you really need is: std::array<std::array<int, 4>, 3> arr; That's quite annoying for
24 comments
Read more

Covariance and Contravariance in C++ Standard Library

Covariance and Contravariance are concepts that come up often as you go deeper into generic programming. While designing a language that supports parametric polymorphism (e.g., templates in C++, generics in Java, C#), the language designer has a choice between Invariance, Covariance, and Contravariance when dealing with generic types. C++'s choice is "invariance". Let's look at an example. struct Vehicle {}; struct Car : Vehicle {}; std::vector<Vehicle *> vehicles; std::vector<Car *> cars; vehicles = cars; // Does not compile The above program does not compile because C++ templates are invariant. Of course, each time a C++ template is instantiated, the compiler creates a brand new type that uniquely represents that instantiation. Any other type to the same template creates another unique type that has nothing to do with the earlier one. Any two unrelated user-defined types in C++ can't be assigned to each-other by default. You have to provide a
17 comments
Read more

Unit Testing C++ Templates and Mock Injection Using Traits

Unit testing your template code comes up from time to time. (You test your templates, right?) Some templates are easy to test. No others. Sometimes it's not clear how to about injecting mock code into the template code that's under test. I've seen several reasons why code injection becomes challenging. Here I've outlined some examples below with roughly increasing code injection difficulty. Template accepts a type argument and an object of the same type by reference in constructor Template accepts a type argument. Makes a copy of the constructor argument or simply does not take one Template accepts a type argument and instantiates multiple interrelated templates without virtual functions Lets start with the easy ones. Template accepts a type argument and an object of the same type by reference in constructor This one appears straight-forward because the unit test simply instantiates the template under test with a mock type. Some assertion might be tested in
Post a Comment
Read more