Re: [PATCH v2 01/26] rculist: introduce list_next_or_null_rr_rcu()

[Paul E. McKenney]

On Mon, May 21, 2018 at 08:16:59AM -0700, Linus Torvalds wrote:
> On Mon, May 21, 2018 at 6:51 AM Roman Penyaev <
> roman.penyaev@xxxxxxxxxxxxxxxx> wrote:
> 
> > No, I continue from the pointer, which I assigned on the previous IO
> > in order to send IO fairly and keep load balanced.
> 
> Right. And that's exactly what has both me and Paul nervous. You're no
> longer in the RCU domain. You're using a pointer where the lifetime has
> nothing to do with RCU any more.
> 
> Can it be done? Sure. But you need *other* locking for it (that you haven't
> explained), and it's fragile as hell.

He looks to actually have it right, but I would want to see a big comment
on the read side noting the leak of the pointer and documenting why it
is OK.

							Thanx, Paul

> It's probably best to not use RCU for it at all, but depend on that "other
> locking" that you have to have anyway, to keep the pointer valid over the
> non-RCU region.
> 
>                 Linus
>