Re: [PATCH] printk: Allow direct printing for PREEMPT_RT during panic

[takakura]

Hi, John!

On 2024-10-10, john.ogness@xxxxxxxxxxxxx wrote:
>On 2024-10-10, takakura@xxxxxxxxxxxxx wrote:
>> If PREEMPT_RT was enabled, printing for legacy consoles are deferred
>> by default, including after printk_legacy_allow_panic_sync() during 
>> panic which allows direct printing afterwards in case of !PREEMPT_RT.
>> As a result, printing of messages during panic for PREEMPT_RT
>> is handled by the console_flush_on_panic() called at the end.
>
>This is on purpose because legacy consoles are not safe in many contexts
>under PREEMPT_RT. console_flush_on_panic() is used as a final "hope and
>pray" excercise even though it is not safe on PREEMPT_RT, which is why
>it is at the end of panic(). printk_legacy_allow_panic_sync() only
>exists for !PREEMPT_RT.
>
>> In case if kexec was loaded, console_flush_on_panic() will not be
>> called and starts booting into the second kernel without printing
>> the messages.
>
>If legacy printing is allowed before, the kexec may never happen because
>the unsafe legacy printers can hang the system.

Thanks for clarifying, I see that it was on purpose.

>> Allow direct printing for PREEMPT_RT during panic so that messages
>> before kexec gets printed.
>
>Sorry, NACK. This goes against everything we have worked for with the
>rework. The solution is to update your console drivers to NBCON.

Got it, I will look into it!

>John Ogness

Sincerely,
Ryo Takakura