linux-kernel - Re: [PATCH v4] panic: Fixes the panic

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Date:   Sun, 26 Feb 2023 13:11:09 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     bhe@...hat.com, pmladek@...e.com, linux-kernel@...r.kernel.org,
        kexec@...ts.infradead.org, dyoung@...hat.com,
        d.hatayama@...fujitsu.com, feng.tang@...el.com,
        hidehiro.kawai.ez@...achi.com, keescook@...omium.org,
        mikelley@...rosoft.com, vgoyal@...hat.com, kernel-dev@...lia.com,
        kernel@...ccoli.net, stable@...r.kernel.org
Subject: Re: [PATCH v4] panic: Fixes the panic_print NMI backtrace setting

On 26/02/2023 02:44, Andrew Morton wrote:
> On Fri, 10 Feb 2023 17:35:10 -0300 "Guilherme G. Piccoli" <gpiccoli@...lia.com> wrote:
> [...] 
>> Notice that while at it, I got rid of the "crash_kexec_post_notifiers"
>> local copy in panic(). This was introduced by commit b26e27ddfd2a
>> ("kexec: use core_param for crash_kexec_post_notifiers boot option"),
>> but it is not clear from comments or commit message why this local copy
>> is required.
>>
>> My understanding is that it's a mechanism to prevent some concurrency,
>> in case some other CPU modify this variable while panic() is running.
>> I find it very unlikely, hence I removed it - but if people consider
>> this copy needed, I can respin this patch and keep it, even providing a
>> comment about that, in order to be explict about its need.
> 
> Only two sites change crash_kexec_post_notifiers, in
> arch/powerpc/kernel/fadump.c and drivers/hv/hv_common.c.  Yes, it's
> very unlikely that this will be altered while panic() is running and
> the consequences will be slight anyway.
> 
> But formally, we shouldn't do this, especially in a -stable
> backportable patch.  So please, let's have the minimal bugfix for now
> and we can look at removing that local at a later time?
> 

Thanks Andrew, I agree with you! I just sent a V5 with the bugfix alone,
not changing this local/global variable behavior.

Cheers,


Guilherme