```ruby
module ServiceMixin
  extend ActiveSupport::Concern

  included do
    # Hide new in favor of using .call with block to force synchronization
    private_class_method :new
  end

  module ClassMethods
    def call(*args)
      raise "no block given" unless block_given?
      synchronize { yield new(*args) }
      nil
    end

    private

    def mutex
      @mutex ||= Mutex.new
    end

    def synchronize
      mutex.synchronize { yield }
    end
  end

  def method_missing(method_name, *args)
    service.send(method_name, *args)
  end

  def respond_to_missing?(*args)
    service.respond_to?(*args)
  end
end
```

Change ServiceMixin.call to return the yield result for better chaining.
user
```ruby
module ServiceMixin
  extend ActiveSupport::Concern

  included do
    # Hide new in favor of using .call with block to force synchronization
    private_class_method :new
  end

  module ClassMethods
    def call(*args)
      raise "no block given" unless block_given?
      synchronize { yield new(*args) }
    end

    private

    def mutex
      @mutex ||= Mutex.new
    end

    def synchronize
      mutex.synchronize { yield }
    end
  end

  def method_missing(method_name, *args)
    service.send(method_name, *args)
  end

  def respond_to_missing?(*args)
    service.respond_to?(*args)
  end
end
```
assistant