linux-kernel - Re: [PATCH v4 05/25] reboot: Warn if restart handler has duplicated priority

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Date:   Mon, 29 Nov 2021 00:06:19 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
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Subject: Re: [PATCH v4 05/25] reboot: Warn if restart handler has duplicated
 priority

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.