Data structure where only one thread is allowed to write

MichaelsLock.rb

class MichaelsLock
  def initialize
    @mx = Mutex.new
  end

  def lock_writer
    @mx.synchronize do
      raise "Already locked" if @writer
      @writer = Thread.current
    end
  end

  def unlock_writer
    @mx.synchronize do
      ensure_this_writes
      @writer = nil
    end
  end

  # Transaction: do writer lock, yield, release writer lock
  def writer_work
    lock_writer
    begin
      yield
    ensure
      unlock_writer
    end
  end

  def data=(d)
    @mx.synchronize do
      ensure_this_writes
      @data = d
    end    
  end

  def data
    @mx.synchronize do
      @data
    end    
  end

private
  # lock must be held - otherwise we need a Monitor and have to lock
  # here as well
  def ensure_this_writes
    raise "Not writer thread" unless @writer == Thread.current
  end
end