Message-ID: <CAMuHMdXW1bCLkJhC1Jnf2rkS1rBnXsMX=4LMVdXDvMV5HOzrLw@mail.gmail.com>
Date:   Mon, 13 Dec 2021 10:23:12 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Dmitry Osipenko <digetx@...il.com>,
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Subject: Re: [PATCH v4 05/25] reboot: Warn if restart handler has duplicated priority

On Fri, Dec 10, 2021 at 8:14 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> On Fri, Dec 10, 2021 at 8:04 PM Dmitry Osipenko <digetx@...il.com> wrote:
> > 10.12.2021 21:27, Rafael J. Wysocki пишет:
> > > On Mon, Nov 29, 2021 at 12:34 PM Dmitry Osipenko <digetx@...il.com> wrote:
> > >> 29.11.2021 03:26, Michał Mirosław пишет:
> > >>> On Mon, Nov 29, 2021 at 12:06:19AM +0300, Dmitry Osipenko wrote:
> > >>>> 28.11.2021 03:28, Michał Mirosław пишет:
> > >>>>> On Fri, Nov 26, 2021 at 09:00:41PM +0300, Dmitry Osipenko wrote:
> > >>>>>> Add sanity check which ensures that there are no two restart handlers
> > >>>>>> registered with the same priority. Normally it's a direct sign of a
> > >>>>>> problem if two handlers use the same priority.
> > >>>>>
> > >>>>> The patch doesn't ensure the property that there are no duplicated-priority
> > >>>>> entries on the chain.
> > >>>>
> > >>>> It's not the exact point of this patch.
> > >>>>
> > >>>>> I'd rather see a atomic_notifier_chain_register_unique() that returns
> > >>>>> -EBUSY or something istead of adding an entry with duplicate priority.
> > >>>>> That way it would need only one list traversal unless you want to
> > >>>>> register the duplicate anyway (then you would call the older
> > >>>>> atomic_notifier_chain_register() after reporting the error).
> > >>>>
> > >>>> The point of this patch is to warn developers about the problem that
> > >>>> needs to be fixed. We already have such troubling drivers in mainline.
> > >>>>
> > >>>> It's not critical to register different handlers with a duplicated
> > >>>> priorities, but such cases really need to be corrected. We shouldn't
> > >>>> break users' machines during transition to the new API, meanwhile
> > >>>> developers should take action of fixing theirs drivers.
> > >>>>
> > >>>>> (Or you could return > 0 when a duplicate is registered in
> > >>>>> atomic_notifier_chain_register() if the callers are prepared
> > >>>>> for that. I don't really like this way, though.)
> > >>>>
> > >>>> I had a similar thought at some point before and decided that I'm not in
> > >>>> favor of this approach. It's nicer to have a dedicated function that
> > >>>> verifies the uniqueness, IMO.
> > >>>
> > >>> I don't like the part that it traverses the list second time to check
> > >>> the uniqueness. But actually you could avoid that if
> > >>> notifier_chain_register() would always add equal-priority entries in
> > >>> reverse order:
> > >>>
> > >>>  static int notifier_chain_register(struct notifier_block **nl,
> > >>>               struct notifier_block *n)
> > >>>  {
> > >>>       while ((*nl) != NULL) {
> > >>>               if (unlikely((*nl) == n)) {
> > >>>                       WARN(1, "double register detected");
> > >>>                       return 0;
> > >>>               }
> > >>> -             if (n->priority > (*nl)->priority)
> > >>> +             if (n->priority >= (*nl)->priority)
> > >>>                       break;
> > >>>               nl = &((*nl)->next);
> > >>>       }
> > >>>       n->next = *nl;
> > >>>       rcu_assign_pointer(*nl, n);
> > >>>       return 0;
> > >>>  }
> > >>>
> > >>> Then the check for uniqueness after adding would be:
> > >>>
> > >>>  WARN(nb->next && nb->priority == nb->next->priority);
> > >>
> > >> We can't just change the registration order because invocation order of
> > >> the call chain depends on the registration order
> > >
> > > It doesn't if unique priorities are required and isn't that what you want?
> > >
> > >> and some of current
> > >> users may rely on that order. I'm pretty sure that changing the order
> > >> will have unfortunate consequences.
> > >
> > > Well, the WARN() doesn't help much then.
> > >
> > > Either you can make all of the users register with unique priorities,
> > > and then you can make the registration reject non-unique ones, or you
> > > cannot assume them to be unique.
> >
> > There is no strong requirement for priorities to be unique, the reboot.c
> > code will work properly.
>
> In which case adding the WARN() is not appropriate IMV.
>
> Also I've looked at the existing code and at least in some cases the
> order in which the notifiers run doesn't matter.  I'm not sure what
> the purpose of this patch is TBH.
>
> > The potential problem is on the user's side and the warning is intended
> > to aid the user.
>
> Unless somebody has the panic_on_warn mentioned previously set and
> really the user need not understand what the WARN() is about.  IOW,
> WARN() helps developers, not users.

Do panic_on_warn and reboot_on_panic play well with having a WARN()
in the reboot notifier handling?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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