Re: [linux-next:master] [serial] d353934702: BUG:kernel_reboot-without-warning_in_test_stage

[Petr Mladek <pmladek@suse.com>]

On Wed 2026-09-23 12:47:22, John Ogness wrote:
> (Added printk folks To/Cc.)
> 
> Hi Oliver,
> 
> On 2026-08-06, kernel test robot <oliver.sang@intel.com> wrote:
> > similar to
> > https://lore.kernel.org/all/202602271552.c972ef9e-lkp@intel.com/
> > we still found we have kernel_reboot-without-warning_in_test_stage by this
> > change in our tests.
> >
> > hard for us to understand the connection, and we cannot capture more useful
> > information from serail. just report FYI what we observed in our tests.
> >
> >
> > kernel test robot noticed "BUG:kernel_reboot-without-warning_in_test_stage" on:
> >
> > commit: d3539347022ad4eeb9dbd29c50bfca17b9d8a146 ("serial: 8250: Switch to nbcon console, take 2")
> > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> >
> > in testcase: boot
> >
> > config: i386-randconfig-2006-20250825
> > compiler: gcc-14
> > test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 32G
> >
> > (please refer to attached dmesg/kmsg for entire log/backtrace)
> >
> >
> > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > the same patch/commit), kindly add following tags
> > | Reported-by: kernel test robot <oliver.sang@intel.com>
> > | Closes: https://lore.kernel.org/oe-lkp/202608061008.48a1e76e-lkp@intel.com
> >
> >
> >
> > [   65.107802][  T272] INFO: lkp CACHE_DIR is /tmp/cache
> > [   65.107809][  T272]
> > BUG: kernel reboot-without-warning in test stage
> >
> >
> >
> > The kernel config and materials to reproduce are available at:
> > https://download.01.org/0day-ci/archive/20260806/202608061008.48a1e76e-lkp@intel.com
> 
> This problem is because the kernel buffer is not flushed before
> performing the sysrq-triggered emergency restart. The following patch
> sort of addresses this:
> 
> ----- BEGIN RFC PATCH -----
> diff --git a/kernel/reboot.c b/kernel/reboot.c
> index 695c33e75efd9..5d1ff39b21f90 100644
> --- a/kernel/reboot.c
> +++ b/kernel/reboot.c
> @@ -8,6 +8,7 @@
>  #define pr_fmt(fmt)	"reboot: " fmt
>  
>  #include <linux/atomic.h>
> +#include <linux/console.h>
>  #include <linux/ctype.h>
>  #include <linux/export.h>
>  #include <linux/kexec.h>
> @@ -92,8 +93,11 @@ static BLOCKING_NOTIFIER_HEAD(reboot_notifier_list);
>  void emergency_restart(void)
>  {
>  	kmsg_dump(KMSG_DUMP_EMERG);
> +	nbcon_cpu_emergency_enter();
> +	printk_trigger_flush();
>  	system_state = SYSTEM_RESTART;
>  	machine_emergency_restart();
> +	nbcon_cpu_emergency_exit();
>  }
>  EXPORT_SYMBOL_GPL(emergency_restart);
>  
> ----- END RFC PATCH -----
> 
> @pmladek:
> 
> It is necessary to put the CPU into an emergency state, otherwise atomic
> printing will not be used. This is kind of a best effort, as opposed to
> panic() where eventually unsafe flushes are attempted. So if the sysrq-b
> is triggered while another CPU is printing to the console, the user
> still might not see any of the pending messages on reset.
> 
> Any comments from your side on this?

I think that it is a reasonable approach in principle.

The above patch handles only emergency_restart(). The similar problem
would be even in other code paths where the system is going down.
I think about using NBCON_EMERGENCY_PRIO automatically in all
these states, something like:

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index d17704fe93ae..48446926dc50 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1446,6 +1446,10 @@ enum nbcon_prio nbcon_get_default_prio(void)
 	if (panic_on_this_cpu())
 		return NBCON_PRIO_PANIC;
 
+	/* Do not rely on kthreads when the system is going down. */
+	if (system_state > SYSTEM_RUNNING)
+		return NBCON_PRIO_EMERGENCY;
+
 	cpu_emergency_nesting = nbcon_get_cpu_emergency_nesting();
 	if (*cpu_emergency_nesting)
 		return NBCON_PRIO_EMERGENCY;
diff --git a/kernel/reboot.c b/kernel/reboot.c
index d177d89fcc33..776784a82499 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -8,6 +8,7 @@
 #define pr_fmt(fmt)	"reboot: " fmt
 
 #include <linux/atomic.h>
+#include <linux/console.h>
 #include <linux/ctype.h>
 #include <linux/export.h>
 #include <linux/kexec.h>
@@ -94,6 +95,7 @@ void emergency_restart(void)
 {
 	kmsg_dump(KMSG_DUMP_EMERG);
 	system_state = SYSTEM_RESTART;
+	printk_trigger_flush();
 	machine_emergency_restart();
 }
 EXPORT_SYMBOL_GPL(emergency_restart);
@@ -102,6 +104,7 @@ void kernel_restart_prepare(char *cmd)
 {
 	blocking_notifier_call_chain(&reboot_notifier_list, SYS_RESTART, cmd);
 	system_state = SYSTEM_RESTART;
+	printk_trigger_flush();
 	usermodehelper_disable();
 	device_shutdown();
 }
@@ -305,6 +308,7 @@ static void kernel_shutdown_prepare(enum system_states state)
 	blocking_notifier_call_chain(&reboot_notifier_list,
 		(state == SYSTEM_HALT) ? SYS_HALT : SYS_POWER_OFF, NULL);
 	system_state = state;
+	printk_trigger_flush();
 	usermodehelper_disable();
 	device_shutdown();
 }

Note that it would use NBCON_EMERGENCY_PRIO even in SYSTEM_SUSPEND
state. But I think that it does not have any real effect because
it seems to be done after the consoles are suspended. at least
in hibernation_platform_enter().

Best Regards,
Petr