# File lib/log4r/MDC.rb, line 19
    def self.check_thread_instance()
      # need to interlock here, so that if
      # another thread is entering this section
      # of code before the main thread does,
      # then the main thread copy of the MDC
      # is setup before then attempting to clone
      # it off
      if ( Thread.current[MDCNAME] == nil ) then
        $globalMDCLock.synchronize do 
          if ( Thread.main[MDCNAME] == nil ) then
            Thread.main[MDCNAME] = Hash.new
          end
          if ( Thread.current != Thread.main ) then
            Thread.current[MDCNAME] = Hash.new
            Thread.main[MDCNAME].each{ |k,v| Thread.current[MDCNAME][k] = v }
          end
        end
      end
    end