Re: [PATCH v4 03/25] notifier: Add atomic/blocking_notifier_has_unique_priority()

Dmitry Osipenko
2021-12-10T19:33:40Z

10.12.2021 22:05, Rafael J. Wysocki пишет:
> On Fri, Dec 10, 2021 at 7:52 PM Dmitry Osipenko <digetx@gmail.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?

I'll need to carefully review all those dozens of platform restart
handlers to answer this question.

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

Please notice that every blocking_notifier_*() function has this
SYSTEM_BOOTING check, it's not my invention. Notifier API needs to be
generic.

>>>> + 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.
>

It's fine that it's a bit "racy" since in the context of this series. We
always do the check after adding new entry, so it's not a problem.

There are two options:

1. Use blocking_notifier_has_unique_priority() like it's done in this
patchset. Remove it after all drivers are converted to the new API and
add blocking_notifier_chain_register_unique().

2. Add blocking_notifier_chain_register_unique(), but don't let it fail
the registration of non-unique entries until all drivers are converted
to the new API.