linux-kernel - Re: [PATCH V3 11/11] panic: Fixes the panic

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Date:   Sun, 18 Sep 2022 11:13:28 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To:     akpm@...ux-foundation.org, kexec@...ts.infradead.org
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Subject: Re: [PATCH V3 11/11] panic: Fixes the panic_print NMI backtrace
 setting

On 19/08/2022 19:17, Guilherme G. Piccoli wrote:
> Commit 8d470a45d1a6 ("panic: add option to dump all CPUs backtraces in panic_print")
> introduced a setting for the "panic_print" kernel parameter to allow
> users to request a NMI backtrace on panic. Problem is that the panic_print
> handling happens after the secondary CPUs are already disabled, hence
> this option ended-up being kind of a no-op - kernel skips the NMI trace
> in idling CPUs, which is the case of offline CPUs.
> 
> Fix it by checking the NMI backtrace bit in the panic_print prior to
> the CPU disabling function.
> 
> Fixes: 8d470a45d1a6 ("panic: add option to dump all CPUs backtraces in panic_print")
> Cc: Feng Tang <feng.tang@...el.com>
> Cc: Petr Mladek <pmladek@...e.com>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.com>
> 
> ---
> 
> V3:
> - No changes.
> 
> V2:
> - new patch, there was no V1 of this one.
> 
> Hi folks, thanks upfront for reviews. This is a new patch, fixing an issue
> I found in my tests, so I shoved it into this fixes series.
> 
> 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.
> 
> Let me know your thoughts! Cheers,
> 
> Guilherme
> 
> 
>  kernel/panic.c | 47 +++++++++++++++++++++++++++--------------------
>  1 file changed, 27 insertions(+), 20 deletions(-)
> [...]

Hi Andrew, sorry for the ping.

Does the patch makes sense for you? Any comments are much appreciated!
Tnx in advance,


Guilherme