Date:   Thu, 13 Jan 2022 10:31:39 +0100
From:   Petr Mladek <pmladek@suse.com>
To:     "Guilherme G. Piccoli" <gpiccoli@igalia.com>
Cc:     linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-doc@vger.kernel.org, mcgrof@kernel.org,
        keescook@chromium.org, yzaikin@google.com,
        akpm@linux-foundation.org, feng.tang@intel.com,
        siglesias@igalia.com, kernel@gpiccoli.net
Subject: Re: [PATCH 2/3] panic: Add option to dump all CPUs backtraces in
 panic_print
Message-ID: <Yd/xe5c2HfhwqWwk@alley>
References: <20211109202848.610874-1-gpiccoli@igalia.com>
 <20211109202848.610874-3-gpiccoli@igalia.com>
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On Tue 2021-11-09 17:28:47, Guilherme G. Piccoli wrote:
> Currently the "panic_print" parameter/sysctl allows some interesting debug
> information to be printed during a panic event. This is useful for example
> in cases the user cannot kdump due to resource limits, or if the user
> collects panic logs in a serial output (or pstore) and prefers a fast
> reboot instead of a kdump.

Yes, I have missed this possibility many times.

> Happens that currently there's no way to see all CPUs backtraces in
> a panic using "panic_print" on architectures that support that. We do
> have "oops_all_cpu_backtrace" sysctl, but although partially overlapping
> in the functionality, they are orthogonal in nature: "panic_print" is
> a panic tuning (and we have panics without oopses, like direct calls to
> panic() or maybe other paths that don't go through oops_enter()
> function), and the original purpose of "oops_all_cpu_backtrace" is to
> provide more information on oopses for cases in which the users desire
> to continue running the kernel even after an oops, i.e., used in
> non-panic scenarios.

panic() already prevents double backtrace of the CPU that Oopsed, see:

#ifdef CONFIG_DEBUG_BUGVERBOSE
	/*
	 * Avoid nested stack-dumping if a panic occurs during oops processing
	 */
	if (!test_taint(TAINT_DIE) && oops_in_progress <= 1)
		dump_stack();
#endif

It should be possible to do something similar also for backtraces
on all CPUs.

There are more situation when the backtraces are printed and panic()
is called, for example: softlockup_panic and
softlockup_all_cpu_backtrace.

Well, it is just nice to have. People probably will not use these
options together. And it is better to have the backtraces twice
than do not have them at all.

> So, we hereby introduce an additional bit for "panic_print" to allow
> dumping the CPUs backtraces during a panic event.
>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>

Feel free to use:

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr