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

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Date:   Fri, 10 Dec 2021 21:54:17 +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 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.

>  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.