linux-kernel - Re: [PATCH v4 03/25] notifier: Add atomic/blocking_notifier_has_unique

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Message-ID: <CAJZ5v0gwnY07vg71_NB8RDWyv84FtMsmx7UTDd8TkUd7vFzc6A@mail.gmail.com>
Date:   Fri, 10 Dec 2021 20:05:32 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
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Subject: Re: [PATCH v4 03/25] notifier: Add atomic/blocking_notifier_has_unique_priority()

On Fri, Dec 10, 2021 at 7:52 PM Dmitry Osipenko <digetx@...il.com> wrote:
>
> 10.12.2021 21:19, Rafael J. Wysocki пишет:
> ...
> >> +bool atomic_notifier_has_unique_priority(struct atomic_notifier_head *nh,
> >> +               struct notifier_block *n)
> >> +{
> >> +       unsigned long flags;
> >> +       bool ret;
> >> +
> >> +       spin_lock_irqsave(&nh->lock, flags);
> >> +       ret = notifier_has_unique_priority(&nh->head, n);
> >> +       spin_unlock_irqrestore(&nh->lock, flags);
> >
> > This only works if the caller can prevent new entries from being added
> > to the list at this point or if the caller knows that they cannot be
> > added for some reason, but the kerneldoc doesn't mention this
> > limitation.
>
> I'll update the comment.
>
> ..
> >> +bool blocking_notifier_has_unique_priority(struct blocking_notifier_head *nh,
> >> +               struct notifier_block *n)
> >> +{
> >> +       bool ret;
> >> +
> >> +       /*
> >> +        * This code gets used during boot-up, when task switching is
> >> +        * not yet working and interrupts must remain disabled. At such
> >> +        * times we must not call down_read().
> >> +        */
> >> +       if (system_state != SYSTEM_BOOTING)
> >
> > No, please don't do this, it makes the whole thing error-prone.
>
> What should I do then?

First of all, do you know of any users who may want to call this
during early initialization?  If so, then why may they want to do
that?

Depending on the above, I would consider adding a special mechanism for them.

> >> +               down_read(&nh->rwsem);
> >> +
> >> +       ret = notifier_has_unique_priority(&nh->head, n);
> >> +
> >> +       if (system_state != SYSTEM_BOOTING)
> >> +               up_read(&nh->rwsem);
> >
> > And still what if a new entry with a non-unique priority is added to
> > the chain at this point?
>
> If entry with a non-unique priority is added after the check, then
> obviously it won't be detected.

Why isn't this a problem?

> I don't understand the question. These
> down/up_read() are the locks that prevent the race, if that's the question.

Not really, they only prevent the race from occurring while
notifier_has_unique_priority() is running.

If anyone depends on this check for correctness, they need to lock the
rwsem, do the check, do the thing depending on the check while holding
the rwsem and then release the rwsem.  Otherwise it is racy.