[PATCH v2 1/6] panic: Add a notifier chain for pre-kdump callbacks

[Guilherme G. Piccoli]

On 28/09/2026 14:55, Zack Rusin wrote:
> Selected crash callbacks should not require running every panic notifier
> before kdump. Add a separate atomic chain with an at-most-once dispatch
> helper, and invoke it before panic reaches the kmsg dumpers.
> 
> The helper permits another call from the crash path without repeating
> callbacks or recursing into them. Document that callbacks may run in NMI
> context, with other CPUs running and the kexec lock held.
> 
> Link: https://lore.kernel.org/r/aquttpMVKX8e6zGB@pathway.suse.cz
> Signed-off-by: Zack Rusin <zack.rusin at broadcom.com>
> ---
> New in v2: separate pre-kdump chain, with a set-once dispatch guard.
> 
>  include/linux/panic_notifier.h |  3 +++
>  kernel/panic.c                 | 26 ++++++++++++++++++++++++++
>  2 files changed, 29 insertions(+)
> 
> diff --git a/include/linux/panic_notifier.h b/include/linux/panic_notifier.h
> index 41e32483d7a7..3ad390764a41 100644
> --- a/include/linux/panic_notifier.h
> +++ b/include/linux/panic_notifier.h
> @@ -6,6 +6,9 @@
>  #include <linux/types.h>
>  
>  extern struct atomic_notifier_head panic_notifier_list;
> +extern struct atomic_notifier_head panic_pre_kdump_list;
> +
> +void panic_notify_pre_kdump(char *msg);
>  
>  extern bool crash_kexec_post_notifiers;
>  
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 213725b612aa..d78e90abb192 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -82,6 +82,10 @@ ATOMIC_NOTIFIER_HEAD(panic_notifier_list);
>  
>  EXPORT_SYMBOL(panic_notifier_list);
>  
> +struct atomic_notifier_head panic_pre_kdump_list =
> +	ATOMIC_NOTIFIER_INIT(panic_pre_kdump_list);
> +EXPORT_SYMBOL_GPL(panic_pre_kdump_list);
> +
>  static void panic_print_deprecated(void)
>  {
>  	pr_info_once("Kernel: The 'panic_print' parameter is now deprecated. Please use 'panic_sys_info' and 'panic_console_replay' instead.\n");
> @@ -567,6 +571,27 @@ static void panic_other_cpus_shutdown(bool crash_kexec)
>  		crash_smp_send_stop();
>  }
>  
> +/**
> + * panic_notify_pre_kdump - invoke selected callbacks before kdump
> + * @msg: Panic message, or NULL for direct crash-kexec entry
> + *
> + * Dispatch at most once per boot, including recursive entry. Callbacks run
> + * with interrupts disabled, possibly in NMI context, before other CPUs have
> + * necessarily stopped. The kexec lock may be held. They must not sleep,
> + * allocate, wait for other CPUs, or call into kexec management operations.
> + * The message is read-only and may only be used during the callback.
> + */
> +void panic_notify_pre_kdump(char *msg)
> +{
> +	static atomic_t started = ATOMIC_INIT(0);
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
> +	if (!atomic_xchg(&started, 1))
> +		atomic_notifier_call_chain(&panic_pre_kdump_list, 0, msg);
> +	local_irq_restore(flags);
> +}
> +
>  /**
>   * vpanic - halt the system
>   * @fmt: The text string to print
> @@ -682,6 +707,7 @@ void vpanic(const char *fmt, va_list args)
>  
>  	sys_info(panic_print);
>  
> +	panic_notify_pre_kdump(buf);
>  	kmsg_dump_desc(KMSG_DUMP_PANIC, buf);
>  
>  	/*

Hi Zack, thanks for trying to improve this conundrum called panic
notifiers heh

I confess at first sight I got a bit confused, and only after reading
patch 1 and 2 twice, things made sense to me: you're hooking
__crash_kexec() directly!

So, in my head, 2 things that would make sense upfront: merge patches #1
and #2, and comment a bit about it in the code.


But more than this, IIUC (and please correct me if I'm wrong): in case
__crash_kexec() is called earlier, THAT call invokes
panic_notify_pre_kdump(). Now, if that call doesn't happen (for example,
'cause crash_kexec_post_notifiers=Y), then panic_notify_pre_kdump() is
called from vpanic(), *after* the regular panic notifiers!!? Is it
intentional?

If it is, why even call it here, if __crash_exec() will be called again,
in next line, in such case of crash_kexec_post_notifiers=Y? It could be
easily done outside of `if(kexec_crash_image)` [at __crash_kexec()].

Is it really correct/expected that such "early" notifiers will end-up
being the latest notifiers if crash_kexec_post_notifiers=Y? Maybe this
is aimed at collecting also the potential extra info dumped in some
regular panic notifier?
Thanks,


Guilherme