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POsynchronizes-with, happens-before relation and acquire-release semantics
 

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    <p>I need help in understanding synchronizes-with relation. The more I'm reading it an trying to understand example, the more I feel that I understand nothing. Sometimes I feel that here is it, I've got it, but after looking at another example I get confused again. Please help me to get it right.</p> <p>It's said that an operation A synchronizes-with an operation B if A is a store to some atomic variable m, with release semantics, B is a load from the same variable m, with acquire semantics, and B reads the value stored by A. It's also said that an operation A happens-before an operation B if</p> <ul> <li>A is performed on the same thread as B, and A is before B in program order, or</li> <li>A synchronizes-with B, or</li> <li>A happens-before some other operation C, and C happens-before B</li> </ul> <p>OK. If we look at this example</p> <p>thread0 performs | thread1 performs</p> <hr> <p>store x (release) | load x (acquire)</p> <p>does store to x here synchronize-with load from x? If we do have synchronizes-with relationship here, then store to x happens before load from x, so everything what is sequenced before store to x in thread 0 happens-before load from x in thread 1. It means there is enforced ordering here. Is it right? But in this case I don't understand what does the "and B reads the value stored by A" part of definition mean? If thread 1 is faster then thread 0 it may read the old value. So what is the relationship here and is there any relationship at all? If there is no, how can I provide that relationship?</p> <p>Thanks in advance.</p>
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