1641     bool retry = false;

  1641     const hrtime_t now = gethrtime();

  1642     hrtime_t newtime = gethrtime();

  1642     const hrtime_t prev = max_hrtime;

  1643     hrtime_t oldmaxtime = max_hrtime;

  1643     if (now <= prev)  return prev;   // same or retrograde time;

  1644     hrtime_t retmaxtime = oldmaxtime;

  1644     const hrtime_t obsv = Atomic::cmpxchg(now, (volatile jlong*)&max_hrtime, prev);

  1645     while ((newtime > retmaxtime) && (retry == false || retmaxtime != oldmaxtime)) {

  1645     assert(obsv >= prev, "invariant");   // Monotonicity

  1646       oldmaxtime = retmaxtime;

  1646     // If the CAS succeeded then we're done and return "now".

  1647       retmaxtime = Atomic::cmpxchg(newtime, (volatile jlong *)&max_hrtime, oldmaxtime);

  1647     // If the CAS failed and the observed value "obs" is >= now then

  1648       retry = true;

  1648     // we should return "obs".  If the CAS failed and now > obs > prv then

  1649     }

  1649     // some other thread raced this thread and installed a new value, in which case

  1650     return (newtime > retmaxtime) ? newtime : retmaxtime;

  1650     // we could either (a) retry the entire operation, (b) retry trying to install now

  1651     // or (c) just return obs.  We use (c).   No loop is required although in some cases

  1652     // we might discard a higher "now" value in deference to a slightly lower but freshly

  1653     // installed obs value.   That's entirely benign -- it admits no new orderings compared

  1654     // to (a) or (b) -- and greatly reduces coherence traffic.

  1655     // We might also condition (c) on the magnitude of the delta between obs and now.

  1656     // Avoiding excessive CAS operations to hot RW locations is critical.

  1657     // See http://blogs.sun.com/dave/entry/cas_and_cache_trivia_invalidate

  1658     return (prev == obsv) ? now : obsv ;