linux-kernel - Re: [PATCH 3/3] panic: Allow printing extra panic information on kdump

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Message-ID: <fbadf45f-e21f-6f23-8165-e3568f119ebb@igalia.com>
Date:   Thu, 13 Jan 2022 10:00:49 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-doc@...r.kernel.org, mcgrof@...nel.org,
        keescook@...omium.org, yzaikin@...gle.com,
        akpm@...ux-foundation.org, feng.tang@...el.com,
        siglesias@...lia.com, kernel@...ccoli.net
Subject: Re: [PATCH 3/3] panic: Allow printing extra panic information on
 kdump

On 13/01/2022 06:02, Petr Mladek wrote:
> [...]
> Is anyone really using this approach? kmsg_dump() looks like a better
> choice when there are memory constrains. It does not need to reserve
> memory for booting the crash kernel.
> 
> I would not mind much but this change depends on a not fully reliable
> assumption, see below.
> 
> Also it will also complicate the solution for the kmsg_dump() code path.
> It would be better to discuss this togeter with the other patch
> https://lore.kernel.org/r/20220106212835.119409-1-gpiccoli@igalia.com

Hi Petr, thanks for your analysis here. Indeed, our use case benefits
from both this and the other patch in the thread you mentioned above -
see [0].


> [...]
>> (b) We assume that the code path won't return from __crash_kexec()
>> so we didn't guard against double execution of panic_print_sys_info().
> 
> This sounds suspiciously. There is small race window but it actually works.
> __crash_kexec() really never returns when @kexec_crash_image is
> loaded. Well, it might break in the future if the code is modified.
> 
> Best Regards,
> Petr

OK, so since this patch is already on linux-next and is relevant for our
use case, how about if we explicitly guard against the double print, as
I suggested in [0]?

Cheers,

Guilherme


[0]
https://lore.kernel.org/lkml/ba0e29ba-0e08-df6e-ade5-eb58ae2495e3@igalia.com/