what is the reasoning behind volatile semantics in

问题:

Both C# and Java define that
* volatile reads have acquire semantics
* volatile writes have release semantics

My questions are:

1. Is this the only correct way to define volatile.
2. If not, will things be awfully different if the semantics were reversed, that is
   
   • volatile reads have release semantics
   • volatile writes have acquire semantics

回答1:

The reasoning behind the volatile semantic is rooted in the Java Memory Model, which is specified in terms of actions:

• reads and writes to variables
• locks and unlocks of monitors
• starting and joining with threads

The Java Memory Model defines a partial ordering called happens-before for the actions which can occur in a Java program. Normally there is no guarantee, that threads can see the results of each other actions.

Let's say you have two actions A and B. In order to guarantee, that a thread executing action B can see the results of action A, there must be a happens-before relationship between A and B. If not, the JVM is free to reorder them as it likes.

A program which is not correctly synchronized might have data races. A data race occurs, when a variable is read by > 1 threads and written by >= 1 thread(s), but the read and write actions are not ordered through the happens-before ordering.

Hence, a correctly synchronized program has no data races, and all actions within the program happen in a fixed order.

So actions are generally only partially ordered, but there is also a total order between:

• lock acquisition and release
• reads and writes to volatile variables

These actions are totally ordered.

This makes it sensible to describe happens-before in terms of "subsequent" lock acquisitions and reads of volatile variables.

Regarding your questions:

1. With the happen-before relationship you have an alternative definition of volatile
2. Reversing the order would not make sense to the definition above, especially since there is a total order involved.

This illustrates the happens-before relation when two threads synchronize using a common lock. All the actions within thread A are ordered by the program order rule, as are the actions within thread B. Because A releases lock M and B subsequently acquires M, all the actions in A before releasing the lock are therefore ordered before the actions in B after acquiring the lock. When two threads synchronize on different locks, we can't say anything about the ordering of actions between themthere is no happens-before relation between the actions in the two threads.

Source: Java Concurrency in Practice

回答2:

The power of the acquire/release semantics isn't so much about how soon other threads see the newly written value of the volatile field itself, but rather in the way volatile operations establish a happens-before relation across different threads. If a thread A reads a volatile field and sees a value that was written to that field in another thread B then thread A is also guaranteed to see values written to other (not necessarily volatile) variables by thread B before the point where it did the volatile write. This looks like cache flushing but only from the point of view of a thread that read the volatile, other threads that don't touch the volatile field have no ordering guarantees with respect to B and might see some of its earlier non-volatile writes but not others if the compiler/JIT is so inclined.

Monitor acquires/releases are similarly characterised by their induced happens-before relation - actions by one thread before a release of a monitor are guaranteed to be visible after a subsequent acquire of the same monitor by another thread. Volatiles give you the same ordering guarantees as monitor synchronisation but without blocking.