On Thu, Apr 8, 2021 at 2:04 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
>
> On Tue, Apr 06, 2021 at 10:55:59AM +0200, Gioh Kim wrote:
> > On Thu, Apr 1, 2021 at 8:44 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Mar 25, 2021 at 04:32:58PM +0100, Gioh Kim wrote:
> > > > diff --git a/drivers/infiniband/ulp/rtrs/rtrs-clt.c b/drivers/infiniband/ulp/rtrs/rtrs-clt.c
> > > > index 42f49208b8f7..1519191d7154 100644
> > > > +++ b/drivers/infiniband/ulp/rtrs/rtrs-clt.c
> > > > @@ -808,6 +808,9 @@ static struct rtrs_clt_sess *get_next_path_min_inflight(struct path_it *it)
> > > > int inflight;
> > > >
> > > > list_for_each_entry_rcu(sess, &clt->paths_list, s.entry) {
> > > > + if (unlikely(READ_ONCE(sess->state) != RTRS_CLT_CONNECTED))
> > > > + continue;
> > >
> > > There is no way this could be right, a READ_ONCE can't guarentee that
> > > a following load is not going to happen without races.
> > >
> > > You need locking.
> >
> > Hi Jason,
> >
> > rtrs_clt_request() calls rcu_read_lock() before calling
> > get_next_path_min_inflight().
> > And rtrs_clt_change_state_from_to(), that changes the sess->state,
> > calls spin_lock_irq() before changing it.
> > I think that is enough, isn't it?
>
> Why would that be enough?
>
> Under RCU this check is racy and effetively does nothing.
Thank you for your review.
I will have a discussion with my colleagues and let you know the result.
>
> This is an OK usage of RCU:
>
> list_del_rcu(&sess->s.entry);
>
> /* Make sure everybody observes path removal. */
> synchronize_rcu();
>
> And you could say that observing the sess in the list is required, but
> checking state is pointless.
>
> Jason
