in line 7 sometimes will be [...true, false...] values because that is not within synchronize block (line 8)

mutex_sync_reading.rb

mutex = Mutex.new
flags = [false, false, false, false, false, false, false, false, false, false]

threads = 50.times.map do
  Thread.new do
    100000.times do
      puts flags.to_s
      mutex.synchronize do
        flags.map! { |f| !f }
      end
    end
  end
end
threads.each(&:join)