In software engineering, double-checked locking (also known as "double-checked locking optimization"[1]) is a software design pattern used to reduce the overhead of acquiring a lock by testing the locking criterion (the "lock hint") before acquiring the lock. Locking occurs only if the locking criterion check indicates that locking is required.
The original form of the pattern, appearing in Pattern Languages of Program Design 3,[2] has data races, depending on the memory model in use, and it is hard to get right. Some consider it to be an anti-pattern.[3] There are valid forms of the pattern, including the use of the keyword in Java and explicit memory barriers in C++.[4]
The pattern is typically used to reduce locking overhead when implementing "lazy initialization" in a multi-threaded environment, especially as part of the Singleton pattern. Lazy initialization avoids initializing a value until the first time it is accessed.
Consider, for example, this code segment in the Java programming language:
The problem is that this does not work when using multiple threads. A lock must be obtained in case two threads call getHelper simultaneously. Otherwise, either they may both try to create the object at the same time, or one may wind up getting a reference to an incompletely initialized object.
Synchronizing with a lock can fix this, as is shown in the following example:
This is correct and will most likely have sufficient performance. However, the first call to getHelper will create the object and only the few threads trying to access it during that time need to be synchronized; after that all calls just get a reference to the member variable. Since synchronizing a method could in some extreme cases decrease performance by a factor of 100 or higher,[5] the overhead of acquiring and releasing a lock every time this method is called seems unnecessary: once the initialization has been completed, acquiring and releasing the locks would appear unnecessary. Many programmers, including the authors of the double-checked locking design pattern, have attempted to optimize this situation in the following manner:
Intuitively, this algorithm is an efficient solution to the problem. But if the pattern is not written carefully, it will have a data race. For example, consider the following sequence of events:
Most runtimes have memory barriers or other methods for managing memory visibility across execution units. Without a detailed understanding of the language's behavior in this area, the algorithm is difficult to implement correctly. One of the dangers of using double-checked locking is that even a naive implementation will appear to work most of the time: it is not easy to distinguish between a correct implementation of the technique and one that has subtle problems. Depending on the compiler, the interleaving of threads by the scheduler and the nature of other concurrent system activity, failures resulting from an incorrect implementation of double-checked locking may only occur intermittently. Reproducing the failures can be difficult.
For the singleton pattern, double-checked locking is not needed:
C++11 and beyond also provide a built-in double-checked locking pattern in the form of std::once_flag and std::call_once:
// Singleton.hclass Singleton ;
// Singleton.cppstd::optional
Singleton* Singleton::GetInstance
If one truly wishes to use the double-checked idiom instead of the trivially working example above (for instance because Visual Studio before the 2015 release did not implement the C++11 standard's language about concurrent initialization quoted above [7]), one needs to use acquire and release fences:[8]
class Singleton ;
Singleton* Singleton::GetInstance
[https://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_once.html pthread_once]
must be usedto initialize library (or sub-module) code when its API does not have a dedicated initialization procedure required to be called in single-threaded mode.
import "sync"
var arrOnce sync.Oncevar arr []int
// getArr retrieves arr, lazily initializing on first call. Double-checked// locking is implemented with the sync.Once library function. The first// goroutine to win the race to call Do will initialize the array, while// others will block until Do has completed. After Do has run, only a// single atomic comparison will be required to get the array.func getArr []int
func main
As of J2SE 5.0, the volatile keyword is defined to create a memory barrier. This allows a solution that ensures that multiple threads handle the singleton instance correctly. This new idiom is described in http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html and http://www.oracle.com/technetwork/articles/javase/bloch-effective-08-qa-140880.html.
Note the local variable "", which seems unnecessary. The effect of this is that in cases where is already initialized (i.e., most of the time), the volatile field is only accessed once (due to "" instead of ""), which can improve the method's overall performance by as much as 40 percent.[9]
Java 9 introduced the class, which allows use of relaxed atomics to access fields, giving somewhat faster reads on machines with weak memory models, at the cost of more difficult mechanics and loss of sequential consistency (field accesses no longer participate in the synchronization order, the global order of accesses to volatile fields).[10]
If the helper object is static (one per class loader), an alternative is the initialization-on-demand holder idiom[11] (See Listing 16.6[12] from the previously cited text.)
This relies on the fact that nested classes are not loaded until they are referenced.
Semantics of field in Java 5 can be employed to safely publish the helper object without using :[13]
public class Foo
The local variable is required for correctness: simply using for both null checks and the return statement could fail due to read reordering allowed under the Java Memory Model.[14] Performance of this implementation is not necessarily better than the implementation.
In .NET Framework 4.0, the Lazy<T> class was introduced, which internally uses double-checked locking by default (ExecutionAndPublication mode) to store either the exception that was thrown during construction, or the result of the function that was passed to Lazy<T>:[15]
Lazy<T> actually implements […] double-checked locking. Double-checked locking performs an additional volatile read to avoid the cost of obtaining a lock if the object is already initialized..