linux-kernel - Re: Confused about hlist_unhashed

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Date:   Fri, 31 Jan 2020 10:55:01 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "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 10:52 AM Paul E. McKenney <paulmck@...nel.org> 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@...nel.org> wrote:
> > >
> > > Hi folks,
> > >
> > > 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 :
>
> 4d3d2ae81afd ("timer: Use hlist_unhashed_lockless() in timer_pending()")
> in -rcu, slated for not this merge window but the next one.  And
> including the changes in your later email, Eric.  Please see below
> and let me know whether you are OK with it.
>
>                                                         Thanx, Paul

Well, it seems we only have to wait for data_race() being available, right ?

Then push a patch using data_race() instead of READ_ONCE() thing.