Re: [PATCH RFC] panic: call panic handlers before panic_other_cpus_shutdown()

[Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>]

Hey Ryo,

On Fri, Feb 21, 2025 at 11:23:28AM +0900, Ryo Takakura wrote:
> Hi Hamza!
> 
> On Thu, 20 Feb 2025 17:53:00 -0500, Hamza Mahfooz wrote:
> >Since, the panic handlers may require certain cpus to be online to panic
> >gracefully, we should call them before turning off SMP. Without this
> >re-ordering, on Hyper-V hv_panic_vmbus_unload() times out, because the
> >vmbus channel is bound to VMBUS_CONNECT_CPU and unless the crashing cpu
> >is the same as VMBUS_CONNECT_CPU, VMBUS_CONNECT_CPU will be offlined by
> >crash_smp_send_stop() before the vmbus channel can be deconstructed.
> >
> >Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
> >---
> > kernel/panic.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> >diff --git a/kernel/panic.c b/kernel/panic.c
> >index fbc59b3b64d0..9712a46dfe27 100644
> >--- a/kernel/panic.c
> >+++ b/kernel/panic.c
> >@@ -372,8 +372,6 @@ void panic(const char *fmt, ...)
> > 	if (!_crash_kexec_post_notifiers)
> > 		__crash_kexec(NULL);
> > 
> >-	panic_other_cpus_shutdown(_crash_kexec_post_notifiers);
> >-
> > 	printk_legacy_allow_panic_sync();
> 
> I think printk_legacy_allow_panic_sync() is placed after 
> panic_other_cpus_shutdown() so that it flushes the stored 
> cpus backtraces as described [0].
> 
> > 	/*
> >@@ -382,6 +380,8 @@ void panic(const char *fmt, ...)
> > 	 */
> > 	atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
> > 
> >+	panic_other_cpus_shutdown(_crash_kexec_post_notifiers);
> >+
> 
> So maybe panic_other_cpus_shutdown() should be palced after 
> atomic_notifier_call_chain() along with printk_legacy_allow_panic_sync()
> like below?
> 
> ----- BEGIN -----
> diff --git a/kernel/panic.c b/kernel/panic.c
> index d8635d5cecb2..7ac40e85ee27 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -372,16 +372,16 @@ void panic(const char *fmt, ...)
>         if (!_crash_kexec_post_notifiers)
>                 __crash_kexec(NULL);
> 
> -       panic_other_cpus_shutdown(_crash_kexec_post_notifiers);
> -
> -       printk_legacy_allow_panic_sync();
> -
>         /*
>          * Run any panic handlers, including those that might need to
>          * add information to the kmsg dump output.
>          */
>         atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
> 
> +       panic_other_cpus_shutdown(_crash_kexec_post_notifiers);
> +
> +       printk_legacy_allow_panic_sync();
> +
>         panic_print_sys_info(false);
> 
>         kmsg_dump_desc(KMSG_DUMP_PANIC, buf);
> ----- END -----

Ya, that looks fine to me, that's actually how I had it initally, but I
wasn't sure if it had to go before the panic handlers. So, I erred on
the side of caution.

BR,
Hamza

> 
> Sincerely,
> Ryo Takakura
> 
> [0] https://lore.kernel.org/all/20240820063001.36405-30-john.ogness@linutronix.de/