Re: [PATCH v4] panic: Fixes the panic_print NMI backtrace setting

[Andrew Morton]

On Fri, 10 Feb 2023 17:35:10 -0300 "Guilherme G. Piccoli" <gpiccoli@xxxxxxxxxx> 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.
> 
> ...
> 
> 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?