Re: Confused about hlist_unhashed_lockless()

[Eric Dumazet]

On Fri, Jan 31, 2020 at 8:57 AM Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Fri, Jan 31, 2020 at 08:48:05AM -0800, Eric Dumazet wrote:
> > On Fri, Jan 31, 2020 at 8:43 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > > I just ran into c54a2744497d ("list: Add hlist_unhashed_lockless()")
> > > but I'm a bit confused about what it's trying to achieve. It also seems
> > > to have been merged without any callers (even in -next) -- was that
> > > intentional?
> > >
> > > My main source of confusion is the lack of memory barriers. For example,
> > > if you look at the following pair of functions:
> > >
> > >
> > > static inline int hlist_unhashed_lockless(const struct hlist_node *h)
> > > {
> > >         return !READ_ONCE(h->pprev);
> > > }
> > >
> > > static inline void hlist_add_before(struct hlist_node *n,
> > >                                     struct hlist_node *next)
> > > {
> > >         WRITE_ONCE(n->pprev, next->pprev);
> > >         WRITE_ONCE(n->next, next);
> > >         WRITE_ONCE(next->pprev, &n->next);
> > >         WRITE_ONCE(*(n->pprev), n);
> > > }
> > >
> > >
> > > Then running these two concurrently on the same node means that
> > > hlist_unhashed_lockless() doesn't really tell you anything about whether
> > > or not the node is reachable in the list (i.e. there is another node
> > > with a next pointer pointing to it). In other words, I think all of
> > > these outcomes are permitted:
> > >
> > >         hlist_unhashed_lockless(n)      n reachable in list
> > >         0                               0 (No reordering)
> > >         0                               1 (No reordering)
> > >         1                               0 (No reordering)
> > >         1                               1 (Reorder first and last WRITE_ONCEs)
> > >
> > > So I must be missing some details about the use-case here. Please could
> > > you enlighten me? The RCU implementation permits only the first three
> > > outcomes afaict, why not use that and leave non-RCU hlist as it was?
> > >
> >
> > I guess the following has been lost :
>
> Thanks, although...
>
> > Author: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Date:   Thu Nov 7 11:23:14 2019 -0800
> >
> >     timer: use hlist_unhashed_lockless() in timer_pending()
> >
> >     timer_pending() is mostly used in lockless contexts.
>
> ... my point above still stands: the value returned by
> hlist_unhashed_lockless() doesn't tell you anything about whether or
> not the timer is reachable in the hlist or not. The comment above
> timer_pending() also states that:
>
>   | Callers must ensure serialization wrt. other operations done to
>   | this timer, e.g. interrupt contexts, or other CPUs on SMP.
>
> If that is intended to preclude list operations, shouldn't we use an
> RCU hlist instead of throwing {READ,WRITE}_ONCE() at the problem to
> shut the sanitiser up without actually fixing anything? :(


Sorry, but timer_pending() requires no serialization.

The only thing we need is a READ_ONCE() so that compiler is not allowed
to optimize out stuff like

loop() {
   if (timer_pending())
      something;
}