title: atomic variable also require lock on read-modify-store operation?
body: <p>I know atomic variable is lock-free!!<br />
  It doesn't lock thread, but I have one question..<br />
  Read-Modify-Store operation like std::atomic::fetch_add is also executed atomically???<br />
  I think this operation isn't just a one instruction.<br />
  It need multiple cycle... So If i doesn't lock memory bus ( Actually i don't know if mutex locking contain memory bus lock), Other thread can make memory operation between Read and Store.<br />
  So I think it require locking even if atomic variable...</p>
  <p>Am i knowing well???</p>