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The happens-before order subsumes the program order; ... The Java memory model describes how threads in the Java programming language interact through memory.
In computer science, the happened-before relation (denoted: ) is a relation between the result of two events, such that if one event should happen before another event, the result must reflect that, even if those events are in reality executed out of order (usually to optimize program flow).
Modern programming languages like Java therefore implement a memory model. The memory model specifies synchronization barriers that are established via special, well-defined synchronization operations such as acquiring a lock by entering a synchronized block or method. The memory model stipulates that changes to the values of shared variables ...
(In Java 5 or later) Volatile reads and writes establish a happens-before relationship, much like acquiring and releasing a mutex. [16] This relationship is simply a guarantee that memory writes by one specific statement are visible to another specific statement. A volatile fields are linearizable.
A memory barrier must be inserted before thread #2's assignment to f to ensure that the new value of x is visible to other processors at or prior to the change in the value of f. Another important point is a memory barrier must also be inserted before thread #1's access to x to ensure the value of x is not read prior to seeing the change in the ...
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The hb1 relation is defined elsewhere in the paper, and is an example of a typical "happens-before" relation; intuitively, if we can prove that we are in a situation where one memory operation X is guaranteed to be executed to completion before another memory operation Y begins, then we say that "X happens-before Y". If neither "X happens ...