1. setting _report_thread_active to false is not necessary protected by _report_thread_lock, because
_report_thread_active's type is bool, writing data is multi-threadly safety if size <= marchine word length
2. report_profile thread terminates early is possiable, in the function report_profile(), while (_report_thread_active) may
break if _report_thread_active is false, the thread of calling open() may be scheduled out between
_report_thread_started_cv.wait(l) and _report_thread_active = true, we should not assume that how long time elapsed
between a thread be scheduled twice
fcd4f0b5c2