# two threads, one mutex require 'thread' m = Mutex.new r = 0 max = 2000 lmax = (max * max)/2 (1..2).map{ Thread.new{ i = 0 while i<lmax i += 1 m.synchronize{ r += 1 } end } }.each{|e| e.join } raise r.to_s if r != max * max