Here:
https://github.com/openjdk/jdk/blob/f61f520e699e3eb5104c9467ec8269b837da74db/src/hotspot/share/runtime/synchronizer.cpp#L79-L82

  size_t count = Atomic::add(&_count, 1u);
  if (count > max()) {
    Atomic::inc(&_max);
  }

This code is actually racy. Here is one of the awkward traces:

(assume count = 0, max = 0 initially)

Thread 1:
  size_t count = Atomic::add(&_count, 1u);
  // count is 1, _count is 1

Thread 2:
  size_t count = Atomic::add(&_count, 1u);
  // count is 2, _count is 2
  if (count > max()) { // TRUE
     Atomic::inc(&_max);
     // _max is now 1
  }

Thread 1: (resumes)
  // local count is still 1, but _max is now 1
  if (count > max()) { // FALSE
    ...
  }

End result: _count = 2, _max = 1