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Date:   Mon, 24 Jan 2022 21:59:02 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     "Guilherme G. Piccoli" <gpiccoli@igalia.com>,
        HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>,
        Petr Mladek <pmladek@suse.com>, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org, dyoung@redhat.com,
        linux-doc@vger.kernel.org, vgoyal@redhat.com,
        stern@rowland.harvard.edu, akpm@linux-foundation.org,
        andriy.shevchenko@linux.intel.com, corbet@lwn.net,
        halves@canonical.com, kernel@gpiccoli.net,
        Will Deacon <will@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        John Ogness <john.ogness@linutronix.de>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Juergen Gross <jgross@suse.com>, mikelley@microsoft.com
Subject: Re: [PATCH V4] notifier/panic: Introduce panic_notifier_filter
Message-ID: <20220124135902.GB8305@MiWiFi-R3L-srv>
References: <20220108153451.195121-1-gpiccoli@igalia.com>
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 <20220123220711.44f1484c9b510eea8cda9c47@kernel.org>
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On 01/23/22 at 10:07pm, Masami Hiramatsu wrote:
> On Sat, 22 Jan 2022 18:55:14 +0800
> Baoquan He <bhe@redhat.com> wrote:
> 
> > On 01/21/22 at 05:31pm, Guilherme G. Piccoli wrote:
> > ......
> > > > IMHO, the right solution is to split the callbacks into 2 or more
> > > > notifier list. Then we might rework panic() to do:
> > > > 
> > > > void panic(void)
> > > > {
> > > > 	[...]
> > > > 
> > > > 	/* stop watchdogs + extra info */
> > > > 	atomic_notifier_call_chain(&panic_disable_watchdogs_notifier_list, 0, buf);
> > > > 	atomic_notifier_call_chain(&panic_info_notifier_list, 0, buf);
> > > > 	panic_print_sys_info();
> > > > 
> > > > 	/* crash_kexec + kmsg_dump in configurable order */
> > > > 	if (!_crash_kexec_post_kmsg_dump) {
> > > > 		__crash_kexec(NULL);
> > > > 		smp_send_stop();
> > > > 	} else {
> > > > 		crash_smp_send_stop();
> > > > 	}
> > > > 
> > > > 	kmsg_dump();
> > > > 	if (_crash_kexec_post_kmsg_dump)
> > > > 		__crash_kexec(NULL);
> > > > 
> > > > 	/* infinite loop or reboot */
> > > > 	atomic_notifier_call_chain(&panic_hypervisor_notifier_list, 0, buf);
> > > > 	atomic_notifier_call_chain(&panic_rest_notifier_list, 0, buf);
> > > > 
> > > > 	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
> > > > [...] 
> > > > Two notifier lists might be enough in the above scenario. I would call
> > > > them:
> > > > 
> > > > 	panic_pre_dump_notifier_list
> > > > 	panic_post_dump_notifier_list
> > > > 
> > > > 
> > > > It is a real solution that will help everyone. It is more complicated now
> > > > but it will makes things much easier in the long term. And it might be done
> > > > step by step:
> > > > 
> > > >      1. introduce the two notifier lists
> > > >      2. convert all users: one by one
> > > >      3. remove the original notifier list when there is no user
> > > 
> > > That's a great idea! I'm into it, if we have a consensus. The thing that
> > > scares me most here is that this is a big change and consumes time to
> > > implement - I'd not risk such time if somebody is really against that.
> > > So, let's see more opinions, maybe the kdump maintainers have good input.
> > 
> > I am fine with it. As long as thing is made clear, glad to see code is
> > refactored to be more understandable and improved. Earlier, during several
> > rounds of discussion between you and Petr, seveal pitfalls have been
> > pointed out and avoided.
> > 
> > Meanwhile, I would suggest Masa and HATAYAMA to help give input about
> > panic_notifier usage and refactory. AFAIK, they contributed code and use
> > panic_notifier in their product or environment a lot, that will be very
> > helpful to get the first hand information from them.
> > 
> > Hi Masa, HATAYANA,
> > 
> > Any comment on this? (Please ignore this if it's not in your care.)
> 
> No, that looks good idea to me. BTW, the 'dump' in the new notifieers
> means both kmsg_dump and crash dump, right?

Thanks for quick response, Masa.

I guess it's crash dump, namely kdump.

About pre_dump, if the dump is crash dump, hope those pre_dump notifiers
will be executed under conditional check, e.g only if 'crash_kexec_post_notifiers'
is specified in kernel cmdline.