CHAPTER 17: Threads and Locks

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17.7 Rules for Volatile Variables

If a variable is declared volatile , then additional constraints apply to the actions of each thread. Let T be a thread and let V and W be volatile variables.

• An use action by T on V is permitted only if the previous action by T on V was load, and a load action by T on V is permitted only if the next action by T on V is use. The use action is said to be "associated" with the read action that corresponds to the load.
• A store action by T on V is permitted only if the previous action by T on V was assign, and an assign action by T on V is permitted only if the next action by T on V is store. The assign action is said to be "associated" with the write action that corresponds to the store.
• Let action A be a use or assign by thread T on variable V, let action F be the load or store associated with A, and let action P be the read or write of V that corresponds to F. Similarly, let action B be a use or assign by thread T on variable W, let action G be the load or store associated with B, and let action Q be the read or write of V that corresponds to G. If A precedes B, then P must precede Q. (Less formally: actions on the master copies of volatile variables on behalf of a thread are performed by the main memory in exactly the order that the thread requested.)