This method is kind of awkward because existence of `@mutex` determines whether it should be using `IO.pread` or not. Personally, I would add another ivar called `@use_pread` in `initialize` to separate these concerns. Then I would modify this code to read something like this:
```ruby
  def atomic_read(length, offset) #:nodoc:
    if @mutex
      @mutex.synchronize { atomic_read_unguarded(length, offset) }
    else
      atomic_read_unguarded(length, offset)
    end
  end

  def atomic_read_unguarded(length, offset)
    unless @use_pread
      io = @contents || @file
      io.seek(offset)
      io.read(length)
    else
      IO.pread(@file.fileno, length, offset)
    end
  end
```