linux-kernel - Re: [PATCH RFC] panic: call panic handlers before panic_other_cpus

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Message-Id: <20250222054405.298294-1-ryotkkr98@gmail.com>
Date: Sat, 22 Feb 2025 14:44:05 +0900
From: Ryo Takakura <ryotkkr98@...il.com>
To: hamzamahfooz@...ux.microsoft.com
Cc: akpm@...ux-foundation.org,
	bhe@...hat.com,
	decui@...rosoft.com,
	gregkh@...uxfoundation.org,
	haiyangz@...rosoft.com,
	jani.nikula@...el.com,
	jfalempe@...hat.com,
	joel.granados@...nel.org,
	john.ogness@...utronix.de,
	linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	pmladek@...e.com,
	ryotkkr98@...il.com,
	wei.liu@...nel.org
Subject: Re: [PATCH RFC] panic: call panic handlers before panic_other_cpus_shutdown()

On Fri, 21 Feb 2025 16:23:07 -0500, Hamza Mahfooz wrote:
>On Fri, Feb 21, 2025 at 11:23:28AM +0900, Ryo Takakura wrote:
>> 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@...ux.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.

I see, sorry that I was only speaking in relation to stored backtraces.
It seems that printk_legacy_allow_panic_sync() is placed before 
atomic_notifier_call_chain() so that it can handle flushing before calling
any panic handlers as described [0].

I'm not really familar with the problems associated with panic handlers
so I hope maybe John and Petr can help on this matter...

Sincerely,
Ryo Takakura

>BR,
>Hamza

[0] https://lore.kernel.org/lkml/ZeHSgZs9I3Ihvpye@alley/