Hello,

It generally looks good to me.  The only worry I have is around
wb_shutdown() synchronization and if that is actually an issue it
shouldn't be too difficult to fix.  The other thing which came to mind
is that the congested->__bdi sever semantics.  IIRC, that one was also
to support the "bdi must go away now" behavior.  As bdi is refcnted
now, I think we can probably just let cong hold onto the bdi rather
than try to sever the ref there.

Thanks a lot for working on this!

-- 
tejun