atomic_test.rb

require 'thread'

class Foo
  attr_reader :a
  attr_reader :assignment_count, :evaluation_count

  def initialize
    @a = nil
    @evaluation_lock = Mutex.new
    @assignment_lock = Mutex.new
    @evaluation_count = 0
    @assignment_count = 0
  end

  def a=(arg)
    Thread.pass
    @a = arg
    @assignment_lock.synchronize do
      @assignment_count += 1
    end
  end

  def b
    @evaluation_lock.synchronize do
      @evaluation_count += 1
    end
    1
  end
end

5.times do
  obj = Foo.new
  1000.times.map {
    Thread. new {
      obj.a ||= obj.b
    }
  }.each(&:join)
  p [obj.assignment_count, obj.evaluation_count]
end