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

[Mikulas Patocka]

On Thu, 4 Dec 2008, Andi Kleen wrote:

> > 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.

You start pvmove. The filesystem doesn't know about pvmove.

The next time filesystem does somethig, it submits these 3 requests and 
the 2nd fill unexpectedly fail.

So the fact that pvmove drains the request queue won't help you.

> > 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.

... and produce structure corruption for certain period in time, because 
the writes meant to be ordered are submitted unordered.

Mikulas

> -Andi
>