thread.rb

class A
  attr_accessor :foo

  def initialize
    @mutex = Mutex.new
  end

  def perform
    t1 = Thread.new do
      @foo ||= 10
    end
    
    t2 = Thread.new do
      @foo ||= 20
    end
    
    t1.join
    t2.join
  end
end

10.times do
  a = A.new
  a.perform
  puts a.foo
end