Message-ID: <45228c88-4d51-591e-5da5-9ec468e71684@gmail.com>
Date:   Fri, 10 Dec 2021 22:42:43 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Michał Mirosław <mirq-linux@...e.qmqm.pl>,
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Subject: Re: [PATCH v4 05/25] reboot: Warn if restart handler has duplicated
 priority

10.12.2021 22:14, Rafael J. Wysocki пишет:
> 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 purpose is to let developer know that driver needs to be corrected.

>> 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.
> 
>> We can make it a strong requirement, but only after converting and
>> testing all kernel drivers.
> 
> Right.
> 
>> I'll consider to add patches for that.
> 
> But can you avoid adding more patches to this series?

I won't add more patches since such patches can be added only after
completion of transition to the new API of the whole kernel.

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