Problem with spotlight attenuation?

Hi, i want to know, how to create a long light source with shadow, without any artifacts…

By artifacts i mean situation, when i setting up attenuation radius, for example 30000, spotlight looking something like that


And, when i turning off cast shadows, light seems to be ok


Is that some king of a bug? And, is there a way to fix it?

ps: sorry for bad english =)