c.rb

require 'fcntl'
require 'thread'
class ConcurrentFile
  def initialize
    @out = File.open 't', 'w'
    @mutex = Mutex.new
  end

  def lock
    @out.flock(File::LOCK_EX)
    @mutex.synchronize do
      @data = File.read('t').to_i || 0
      @out.pos = 0
      @out.write @data + 1
      @out.flush
    end
  ensure
    @out.flock(File::LOCK_UN)
  end
end

a = ConcurrentFile.new
b = ConcurrentFile.new

c = Thread.new {
  1000.times { a.lock }
}

d = Thread.new {
  1000.times { b.lock }
}

c.join
d.join

p File.read('t')