title: What happens if two threads read & write the same piece of memory
body: <p>It's my understanding that if two threads are reading from the same piece of memory, and no thread is writing to that memory, then the operation is safe. However, I'm not sure what happens if one thread is reading and the other is writing. What would happen? Is the result undefined? Or would the read just be stale? If a stale read is not a concern is it ok to have unsynchronized read-write to a variable? Or is it possible the data would be corrupted, and neither the read nor the write would be correct and one should always synchronize in this case?</p>
  
  <p>I want to say that I've learned it is the later case, that a race on memory access leaves the state undefined... but I don't remember where I may have learned that and I'm having a hard time finding the answer on google. My intuition is that a variable is operated on in registers, and that true (as in hardware) concurrency is impossible (or is it), so that the worst that could happen is stale data, i.e. the following:</p>
  
  <pre><code>WriteThread: copy value from memory to register
  WriteThread: update value in register
  ReadThread:  copy value of memory to register
  WriteThread: write new value to memory
  </code></pre>
  
  <p>At which point the read thread has stale data.</p>