Re: [PATCH v3 4/4] panic: use sys_info_with_filter() to avoid duplicate backtraces

[Bradley Morgan]

On July 2, 2026 10:09:41 AM GMT+01:00, Petr Mladek <pmladek@xxxxxxxx>
wrote:
>On Mon 2026-06-29 13:54:18, Bradley Morgan wrote:
>> On 29 June 2026 12:40:52 BST, Feng Tang <feng.tang@xxxxxxxxxxxxxxxxx>
>> wrote:
>> >On Fri, Jun 26, 2026 at 02:14:14PM +0200, Petr Mladek wrote:
>> >> On Fri 2026-06-26 12:23:50, Petr Mladek wrote:
>> >> > On Thu 2026-06-25 15:25:58, Bradley Morgan wrote:
>> >> > > panic_other_cpus_shutdown() handles SYS_INFO_ALL_BT before
>stopping
>> >the
>> >> > > other CPUs. Do not ask sys_info() to handle that bit again later
>in
>> >the
>> >> > > panic path.
>> >> > > 
>> >> > > Use sys_info_with_filter() so panic_print=all_bt does not request
>> >more
>> >> > > output after the CPUs are stopped.
>> >> > > 
>> >> > > Fixes: a9af76a78760 ("watchdog: add sys_info sysctls to dump sys
>> >info on system lockup")
>> >> > > Cc: stable@xxxxxxxxxxxxxxx
>> >> > > Signed-off-by: Bradley Morgan <include@xxxxxxxxx>
>> >> > > ---
>> >> > >  kernel/panic.c | 2 +-
>> >> > >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >> > > 
>> >> > > diff --git a/kernel/panic.c b/kernel/panic.c
>> >> > > index 213725b612aa..eb842823df61 100644
>> >> > > --- a/kernel/panic.c
>> >> > > +++ b/kernel/panic.c
>> >> > > @@ -680,7 +680,7 @@ void vpanic(const char *fmt, va_list args)
>> >> > >  	 */
>> >> > >  	atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
>> >> > >  
>> >> > > -	sys_info(panic_print);
>> >> > > +	sys_info_with_filter(panic_print, SYS_INFO_ALL_BT);
>> >> > 
>> >> > Hmm, this prevents printing backtraces from all CPUs completely.
>> >> > But what if they were not printed?
>> >> > 
>> >> > They might be printed by:
>> >> > 
>> >> > static void panic_other_cpus_shutdown(bool crash_kexec)
>> >> > {
>> >> > 	if (panic_print & SYS_INFO_ALL_BT)
>> >> > 		panic_trigger_all_cpu_backtrace();
>> >> > 
>> >> > [...]
>> >> > }
>> >> > 
>> >> > But it checks only "panic_print" variable. It won't do anything
>> >> > when (panic_print == 0).
>> >> > 
>> >> > In this case, we might still want to print the backraces when
>> >> > SYS_INFO_ALL_BT is set in kernel_si_info.
>> >> > 
>> >> > >  	kmsg_dump_desc(KMSG_DUMP_PANIC, buf);
>> >> > 
>> >> > Of course, we might fix panic_other_cpus_shutdown() to check also
>> >> > kernel_si_info.
>> >> > 
>> >> > But it all becomes very hairy. We have several levels:
>> >> > 
>> >> >    + watchdog-all_bt-specific option, e.g.
>> >sysctl_hardlockup_all_cpu_backtrace
>> >> > 
>> >> >    + watchdog-specific si_info preferences, e.g. hardlockup_si_mask
>> >> > 
>> >> >    + panic-specific si_info: panic_print
>> >> > 
>> >> >    + universal fallback for any layer: kernel_si_info
>> >> > 
>> >> > Now, we try to check all these variables back and forth to
>> >> > trigger all backtraces or to avoid triggering them.
>> >> > And it clearly does not work well and the code is more and more
>> >> > hairy.
>> >> > 
>> >> > I think about another approach. The word "waterfall" comes to my
>mind.
>> >> > Instead of checking all the settings back and forth, let's process
>> >> > each setting one by one and just remember what has been done and
>> >> > skip this in the next level.
>> >> > 
>> >> > All the si_info actions seems to dump a global system state.
>> >> > So, it would make sense to remember the state in a global variable
>> >> > even when it might be modified by more CPUs in parallel.
>> >> > 
>> >> > I am going to think more about it.
>> >> 
>> >> I have created a POC using Gemini. I haven't tested it.
>> >> But it looks acceptable. And the logic seems to be more
>> >> straightforward.
>> >> 
>> >> One drawback is that it requires adding the _reset()
>> >> call for all sys_info() callers. It is fine in principle
>> >> but it might complicate back-porting because all changes
>> >> have to be done in one patch.
>> >> 
>> >> But honestly, this is a nice to have fix. Most people could
>> >> live happily without it.
>> >> 
>> >> From 3c66436d9978030845a96bfaedd6b914536e2ac4 Mon Sep 17 00:00:00
>2001
>> >> From: Petr Mladek <pmladek@xxxxxxxx>
>> >> Date: Fri, 26 Jun 2026 13:55:41 +0200
>> >> Subject: [POC] sys_info: Introduce state-tracking APIs to prevent
>> >duplicate
>> >>  backtraces
>> >> 
>> >> In watchdog, panic, and hung task detection scenarios, sys_info() can
>> >> be called multiple times or alongside direct backtrace triggers like
>> >> trigger_allbutcpu_cpu_backtrace(). This results in identical
>backtraces
>> >> being dumped repeatedly from all CPUs, cluttering the kernel log and
>> >> delaying or obscuring critical debug details.
>> >> 
>> >> Introduce a state tracking bitmask and associated helpers:
>> >> - sys_info_done(mask): Marks specific sys_info bits as already
>printed.
>> >> - sys_info_reset(): Resets the tracking state.
>> >> - sys_info_is_done(mask): Checks if all bits in the mask have been
>> >printed.
>> >> 
>> >> Update sys_info() to automatically filter out already printed bits
>> >> using this state. Integrate these APIs with the generic hardlockup
>> >> and softlockup watchdogs, the PowerPC watchdog, the hung task
>detector,
>> >> and the panic core. This ensures that each piece of system
>information
>> >> and backtrace output is printed at most once per lockup/panic event,
>> >> and the state is reset cleanly when a lockup does not trigger a
>panic.
>> >> 
>> >> Races between sys_info() callers are ignored. It should be acceptable
>> >> because the output from various watchdogs has never been
>synchronized.
>> >> And panic() never returns.
>> >> 
>> >> Assisted-by: gemini-1.5-flash
>> >> Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
>> >
>> >Yep. There are cases that people want panic on task-hung or sw/hw
>lockup,
>> >and this could remove much duplication of sys info dump, thanks!
>> >
>> >Reviewed-by: Feng Tang <feng.tang@xxxxxxxxxxxxxxxxx>
>> 
>> Thanks,
>> 
>> im feeling a new file to do all the force panic jazz, but putting tape
>> on sys_info.c isn't bd either.
>
>I wonder how to move forward with this.
>
>Honestly, I am not sure what exactly you mean by creating another
>API for tracking the reports so I could not judge it. Feel free
>to sent some POC.
>
>Otherwise, I would go with my proposal to remember the printed states
>by the sys_info API. I am not sure whether I should send a proper
>patch or you would like to somehow improve it.
>
>Best Regards,
>Petr
>


sup petr, here's my poc


This should make my entire thing make sense