Device loses barrier support (was: Fixed patch for simple barriers.)

[Andi Kleen]

> the 1st write request ends with success
> the 2nd write request ends with -EOPNOTSUPP
> the 3rd write request ends with success
> 
> --- when you first see -EOPNOTSUPP, you have already corrupted filesystem 
> (the 3rd write passed while the filesystem expected that it would be 

There's no passing of requests during pvmove. It's a really strong
barrier.

> finished after the 2nd write) and you are in an interrupt context, where 
> you can't reissue -EOPNOTSUPP request. So what do you want to do?

The barrier aware file systems I know of just resubmit synchronously when 
a barrier fails.

-Andi