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

[Ryo Takakura]

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@xxxxxxxxxxxxxxxxxxx>
>---
> 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 -----

Sincerely,
Ryo Takakura

[0] https://lore.kernel.org/all/20240820063001.36405-30-john.ogness@xxxxxxxxxxxxx/