linux-kernel - Re: [PATCH v2 2/2] x86/panic: Add x86_panic_handler as default post-panic behavior

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Message-ID: <781ef1d7-f9c6-4a15-b94d-c735e83e7d01@gmail.com>
Date: Tue, 29 Apr 2025 10:25:04 -0500
From: Carlos Bilbao <carlos.bilbao.osdev@...il.com>
To: John Ogness <john.ogness@...utronix.de>, carlos.bilbao@...nel.org,
 tglx@...utronix.de, seanjc@...gle.com, jan.glauber@...il.com
Cc: bilbao@...edu, pmladek@...e.com, akpm@...ux-foundation.org,
 jani.nikula@...el.com, linux-kernel@...r.kernel.org,
 gregkh@...uxfoundation.org, takakura@...inux.co.jp
Subject: Re: [PATCH v2 2/2] x86/panic: Add x86_panic_handler as default
 post-panic behavior

Hello John,

On 4/29/25 11:53, John Ogness wrote:
> On 2025-04-28, carlos.bilbao@...nel.org wrote:
>> From: Carlos Bilbao <carlos.bilbao@...nel.org>
>>
>> Add function x86_panic_handler() as the default behavior for x86 for
>> post-panic stage via panic_set_handling(). Instead of busy-wait loop, it
>> will halt if there's no console to save CPU cycles.
>>
>> Signed-off-by: Carlos Bilbao (DigitalOcean) <carlos.bilbao@...nel.org>
>> ---
>>  arch/x86/kernel/setup.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
>> index 9d2a13b37833..3bfef55e9adb 100644
>> --- a/arch/x86/kernel/setup.c
>> +++ b/arch/x86/kernel/setup.c
>> @@ -16,6 +16,7 @@
>>  #include <linux/initrd.h>
>>  #include <linux/iscsi_ibft.h>
>>  #include <linux/memblock.h>
>> +#include <linux/panic.h>
>>  #include <linux/panic_notifier.h>
>>  #include <linux/pci.h>
>>  #include <linux/root_dev.h>
>> @@ -837,6 +838,15 @@ static void __init x86_report_nx(void)
>>  	}
>>  }
>>  
>> +
>> +static void x86_panic_handler(void)
>> +{
>> +	if (console_trylock()) {
>> +		console_unlock();
>> +		safe_halt();
>> +	}
> I do not understand what you are trying to accomplish with the
> console_trylock(). At this point in the panic, all the messages are
> already output. The console lock is totally irrelevant.
>
> Also, the console lock is only valid for legacy consoles.
>
> I see no reason why you don't just use safe_halt() as your panic
> handler.


Yes, in my original implementation I simply used halt, but I was trying to
be cautious in case any remaining messages hadn't flushed. I wonder, can we
be certain that, as you said, all output (e.g., a backtrace) has already
been displayed on screen at this point? I'm not sure.


>
> John Ogness


Thanks,

Carlos