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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

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