Re: [RFC PATCH v1] panic: Flush unsafe consoles before panic reboot

[Ryan Roberts <ryan.roberts@arm.com>]

On 15/09/2026 11:01, Ryan Roberts wrote:
> Netconsole registers with CON_NBCON_ATOMIC_UNSAFE, so its write_atomic()
> callback is only usable during nbcon_atomic_flush_unsafe(). Previously
> this was only called by vpanic() if panic_timeout=0 - i.e. if the system
> was configured not to reboot on panic. So if the system was configured
> to reboot on panic, netconsole would never receive the panic logs.
> 
> Move vpanic()'s emergency_restart() call to after the call to
> nbcon_atomic_flush_unsafe() to solve this problem. The downside is that
> potentially unsafe operations are now performed prior to
> emergency_restart() which could theoretically reduce the chances of the
> restart succeeding. But we are already in a panic situation so it could
> be argued that everything is best effort already.
> 
> In older kernels (v6.19 and earlier) netconsole is able to print these
> panic logs while the system is configured to reboot on panic. So from a
> user perspective this is a regression caused by commit 7eab73b18630
> ("netconsole: convert to NBCON console infrastructure").
> 
> We have automated test systems without BMC access, which rely on these
> two features working together.
> 
> Fixes: 7eab73b18630 ("netconsole: convert to NBCON console infrastructure")
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---

Oops forgot to add commentary to the bottom:

I've set this as RFC because I'm not sure if the current behaviour is a
deliberate design decision? I guess if we flush the unsafe console, there is a
possibility it deadlocks, preventing the subsequent reboot. If people are
concerned about this, perhaps this needs a cmdline flag?

Thanks,
Ryan



>  kernel/panic.c | 35 ++++++++++++++++++-----------------
>  1 file changed, 18 insertions(+), 17 deletions(-)
> 
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 213725b612aa..473dc7d13b23 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -732,32 +732,22 @@ void vpanic(const char *fmt, va_list args)
>  			mdelay(PANIC_TIMER_STEP);
>  		}
>  	}
> -	if (panic_timeout != 0) {
> -		/*
> -		 * This will not be a clean reboot, with everything
> -		 * shutting down.  But if there is a chance of
> -		 * rebooting the system it will be rebooted.
> -		 */
> -		if (panic_reboot_mode != REBOOT_UNDEFINED)
> -			reboot_mode = panic_reboot_mode;
> -		emergency_restart();
> -	}
> +	if (panic_timeout == 0) {
>  #ifdef __sparc__
> -	{
>  		extern int stop_a_enabled;
>  		/* Make sure the user can actually press Stop-A (L1-A) */
>  		stop_a_enabled = 1;
>  		pr_emerg("Press Stop-A (L1-A) from sun keyboard or send break\n"
> -			 "twice on console to return to the boot prom\n");
> -	}
> +			"twice on console to return to the boot prom\n");
>  #endif
>  #if defined(CONFIG_S390)
> -	disabled_wait();
> +		disabled_wait();
>  #endif
> -	pr_emerg("---[ end Kernel panic - not syncing: %s ]---\n", buf);
> +		pr_emerg("---[ end Kernel panic - not syncing: %s ]---\n", buf);
> 
> -	/* Do not scroll important messages printed above */
> -	suppress_printk = 1;
> +		/* Do not scroll important messages printed above */
> +		suppress_printk = 1;
> +	}
> 
>  	/*
>  	 * The final messages may not have been printed if in a context that
> @@ -767,6 +757,17 @@ void vpanic(const char *fmt, va_list args)
>  	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
>  	nbcon_atomic_flush_unsafe();
> 
> +	if (panic_timeout != 0) {
> +		/*
> +		 * This will not be a clean reboot, with everything
> +		 * shutting down.  But if there is a chance of
> +		 * rebooting the system it will be rebooted.
> +		 */
> +		if (panic_reboot_mode != REBOOT_UNDEFINED)
> +			reboot_mode = panic_reboot_mode;
> +		emergency_restart();
> +	}
> +
>  	local_irq_enable();
>  	for (i = 0; ; i += PANIC_TIMER_STEP) {
>  		touch_softlockup_watchdog();
> --
> 2.43.0
>