linux-kernel - Re: Confused about hlist_unhashed

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Date:   Fri, 31 Jan 2020 16:57:18 +0000
From:   Will Deacon <will@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: Confused about hlist_unhashed_lockless()

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@...nel.org> 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@...gle.com>
> 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? :(

Will