linux-kernel - Re: Confused about hlist_unhashed

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Date:   Fri, 31 Jan 2020 17:20:58 +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 09:06:27AM -0800, Eric Dumazet wrote:
> On Fri, Jan 31, 2020 at 8:57 AM Will Deacon <will@...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:
> > > > 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? :(
> 
> 
> Sorry, but timer_pending() requires no serialization.

Then we should update the comment!

Without serialisation, timer_pending() as currently implemented does
not reliably tell you whether the timer is in the hlist. Is that not a
problem? Using an RCU hlist does not introduce serialisation, but does
at least rule out the case where timer_pending() returns false for a
timer that /is/ reachable in the list by another CPU.

> 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;

If that was the case, then you wouldn't need to touch hlist_add_before()
at all so there's got to be more to it than that or we can revert that
part of the patch.

Will