linux-kernel - Re: [PATCH v4 06/25] reboot: Warn if unregister_restart

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Message-ID: <41415df9-0d08-2944-5cf2-493468e21a22@gmail.com>
Date:   Fri, 10 Dec 2021 22:38:06 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
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 06/25] reboot: Warn if unregister_restart_handler()
 fails

10.12.2021 22:08, Rafael J. Wysocki пишет:
> On Fri, Dec 10, 2021 at 7:54 PM Dmitry Osipenko <digetx@...il.com> wrote:
>>
>> 10.12.2021 21:32, Rafael J. Wysocki пишет:
>>> On Fri, Nov 26, 2021 at 7:02 PM Dmitry Osipenko <digetx@...il.com> wrote:
>>>>
>>>> Emit warning if unregister_restart_handler() fails since it never should
>>>> fail. This will ease further API development by catching mistakes early.
>>>>
>>>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>>>> ---
>>>>  kernel/reboot.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/kernel/reboot.c b/kernel/reboot.c
>>>> index e6659ae329f1..f0e7b9c13f6b 100644
>>>> --- a/kernel/reboot.c
>>>> +++ b/kernel/reboot.c
>>>> @@ -210,7 +210,7 @@ EXPORT_SYMBOL(register_restart_handler);
>>>>   */
>>>>  int unregister_restart_handler(struct notifier_block *nb)
>>>>  {
>>>> -       return atomic_notifier_chain_unregister(&restart_handler_list, nb);
>>>> +       return WARN_ON(atomic_notifier_chain_unregister(&restart_handler_list, nb));
>>>
>>> The only reason why it can fail is if the object pointed to by nb is
>>> not in the chain.
>>
>> I had exactly this case where object wasn't in the chain due to a bug
>> and this warning was very helpful.
> 
> During the development.  In production it would be rather annoying.
> 
>>>  Why WARN() about this?  And what about systems with
>>> panic_on_warn set?
>>
>> That warning condition will never happen normally, only when something
>> is seriously wrong.
>>
>> Those systems with panic_on_warn will get what was they asked for.
> 
> They may not be asking for panicking on bugs in the reboot notifier
> code, though.  That's what your change is making them panic on.
> 

Alright, I'll drop the warnings and turn the warning about uniqueness
into error or warning message.