In G1RootProcessor::process_java_roots there is this comment:

  // We need to make make sure that the "strong" nmethods are processed first
  // using the strong closure. Only after that we process the weakly reachable
  // nmethods.
  // We need to strictly separate the strong and weak nmethod processing because
  // any processing claims that nmethod, i.e. will not be iterated again.
  // Which means if an nmethod is processed first and claimed, the strong processing
  // will not happen, and the oops reachable by that nmethod will not be marked
  // properly.
  //
  // That is why we process strong nmethods first, synchronize all threads via a
  // barrier, and only then allow weak processing. To minimize the wait time at
  // that barrier we do the strong nmethod processing first, and immediately after-
  // wards indicate that that thread is done. Hopefully other root processing after
  // nmethod processing is enough so there is no need to wait.
  //
  // This is only required in the concurrent start pause with class unloading enabled.

After JDK-8230706 the process is quite different and this comment only confusing the reader.