Date:   Mon, 29 Nov 2021 14:34:40 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     Thierry Reding <thierry.reding@...il.com>,
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Subject: Re: [PATCH v4 05/25] reboot: Warn if restart handler has duplicated
 priority

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 and some of current
users may rely on that order. I'm pretty sure that changing the order
will have unfortunate consequences.

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