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From:   "d.hatayama@fujitsu.com" <d.hatayama@fujitsu.com>
To:     Baoquan He <bhe@redhat.com>,
        "Guilherme G. Piccoli" <gpiccoli@igalia.com>,
        Masami Hiramatsu <mhiramat@kernel.org>
CC:     Petr Mladek <pmladek@suse.com>,
        "kexec@lists.infradead.org" <kexec@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "dyoung@redhat.com" <dyoung@redhat.com>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "vgoyal@redhat.com" <vgoyal@redhat.com>,
        "stern@rowland.harvard.edu" <stern@rowland.harvard.edu>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "andriy.shevchenko@linux.intel.com" 
        <andriy.shevchenko@linux.intel.com>,
        "corbet@lwn.net" <corbet@lwn.net>,
        "halves@canonical.com" <halves@canonical.com>,
        "kernel@gpiccoli.net" <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" <mikelley@microsoft.com>,
        "d.hatayama@fujitsu.com" <d.hatayama@fujitsu.com>
Subject: Re: [PATCH V4] notifier/panic: Introduce panic_notifier_filter
Thread-Topic: [PATCH V4] notifier/panic: Introduce panic_notifier_filter
Thread-Index: AQHYBKVl3QVmCewphUe4SBvooFofu6xsF5CAgAHq+wCAAPE7AIADMVso
Date:   Mon, 24 Jan 2022 11:43:34 +0000
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References: <20220108153451.195121-1-gpiccoli@igalia.com>
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In-Reply-To: <20220122105514.GA18258@MiWiFi-R3L-srv>
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=0A=
> On 01/21/22 at 05:31pm, Guilherme G. Piccoli wrote:=0A=
> ......=0A=
> > > IMHO, the right solution is to split the callbacks into 2 or more=0A=
> > > notifier list. Then we might rework panic() to do:=0A=
> > >=0A=
> > > void panic(void)=0A=
> > > {=0A=
> > >     [...]=0A=
> > >=0A=
> > >     /* stop watchdogs + extra info */=0A=
> > >     atomic_notifier_call_chain(&panic_disable_watchdogs_notifier_list=
, 0, buf);=0A=
> > >     atomic_notifier_call_chain(&panic_info_notifier_list, 0, buf);=0A=
> > >     panic_print_sys_info();=0A=
> > >=0A=
> > >     /* crash_kexec + kmsg_dump in configurable order */=0A=
> > >     if (!_crash_kexec_post_kmsg_dump) {=0A=
> > >             __crash_kexec(NULL);=0A=
> > >             smp_send_stop();=0A=
> > >     } else {=0A=
> > >             crash_smp_send_stop();=0A=
> > >     }=0A=
> > >=0A=
> > >     kmsg_dump();=0A=
> > >     if (_crash_kexec_post_kmsg_dump)=0A=
> > >             __crash_kexec(NULL);=0A=
> > >=0A=
> > >     /* infinite loop or reboot */=0A=
> > >     atomic_notifier_call_chain(&panic_hypervisor_notifier_list, 0, bu=
f);=0A=
> > >     atomic_notifier_call_chain(&panic_rest_notifier_list, 0, buf);=0A=
> > >=0A=
> > >     console_flush_on_panic(CONSOLE_FLUSH_PENDING);=0A=
> > > [...]=0A=
> > > Two notifier lists might be enough in the above scenario. I would cal=
l=0A=
> > > them:=0A=
> > >=0A=
> > >     panic_pre_dump_notifier_list=0A=
> > >     panic_post_dump_notifier_list=0A=
> > >=0A=
> > >=0A=
> > > It is a real solution that will help everyone. It is more complicated=
 now=0A=
> > > but it will makes things much easier in the long term. And it might b=
e done=0A=
> > > step by step:=0A=
> > >=0A=
> > >      1. introduce the two notifier lists=0A=
> > >      2. convert all users: one by one=0A=
> > >      3. remove the original notifier list when there is no user=0A=
> >=0A=
> > That's a great idea! I'm into it, if we have a consensus. The thing tha=
t=0A=
> > scares me most here is that this is a big change and consumes time to=
=0A=
> > implement - I'd not risk such time if somebody is really against that.=
=0A=
> > So, let's see more opinions, maybe the kdump maintainers have good inpu=
t.=0A=
> =0A=
> I am fine with it. As long as thing is made clear, glad to see code is=0A=
> refactored to be more understandable and improved. Earlier, during severa=
l=0A=
> rounds of discussion between you and Petr, seveal pitfalls have been=0A=
> pointed out and avoided.=0A=
> =0A=
> Meanwhile, I would suggest Masa and HATAYAMA to help give input about=0A=
> panic_notifier usage and refactory. AFAIK, they contributed code and use=
=0A=
> panic_notifier in their product or environment a lot, that will be very=
=0A=
> helpful to get the first hand information from them.=0A=
> =0A=
> Hi Masa, HATAYANA,=0A=
> =0A=
> Any comment on this? (Please ignore this if it's not in your care.)=0A=
> =0A=
=0A=
Thanks for CCing to me. I like this patch set. I have same motivation.=0A=
=0A=
For example, when I used crash_kexec_post_notifiers, I sometimes ran into=
=0A=
deadlock in printk's exclusion logic during the call of panic notifiers sin=
ce=0A=
kaslr outputs kernel offset at panic by dump_kernel_offset() via panic noti=
fers=0A=
(although this might never happen now thanks to lockless implementation).=
=0A=
=0A=
The problem is that in the current design, we have to run all the=0A=
tasks registered, although most of them are actually unnecessary for=0A=
other users' requirements. Each user wants to call only their own handlers=
=0A=
in order to keep kdump as reliable as possible.=0A=
=0A=
I've just started reading this patch set and have no other comments for now=
.=0A=
=0A=
Thanks.=0A=
HATAYAMA, Daisuke=0A=