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Date:   Mon, 7 Mar 2022 11:42:21 +0800
From:   "bhe@redhat.com" <bhe@redhat.com>
To:     "Guilherme G. Piccoli" <gpiccoli@igalia.com>
Cc:     Petr Mladek <pmladek@suse.com>,
        "d.hatayama@fujitsu.com" <d.hatayama@fujitsu.com>,
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        "dyoung@redhat.com" <dyoung@redhat.com>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
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        <andriy.shevchenko@linux.intel.com>,
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Subject: Re: [PATCH V4] notifier/panic: Introduce panic_notifier_filter
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On 03/06/22 at 11:21am, Guilherme G. Piccoli wrote:
> On 28/01/2022 10:38, Petr Mladek wrote:
> > [...] 
> > I think about the following solution:
> > 
> >     + split the notifiers into three lists:
> > 
> > 	+ info: stop watchdogs, provide extra info
> > 	+ hypervisor: poke hypervisor
> > 	+ reboot: actions needed only when crash dump did not happen
> > 
> >     + allow to call hypervisor notifiers before or after kdump
> > 
> >     + stop CPUs before kdump when either hypervisor notifiers or
> >       kmsg_dump is enabled
> > 
> > Note that it still allows to call kdump as the first action when
> > hypervisor notifiers are called after kdump and no kmsg dumper
> > is registered.
> > 
> > 
> > void panic(void)
> > {
> > 	[...]
> > 
> > 	if (crash_kexec_post_hypervisor || panic_print || enabled_kmsg_dump()) {
> > 		/*
> > 		 * Stop CPUs when some extra action is required before
> > 		 * crash dump. We will need architecture dependent extra
> > 		 * works in addition to stopping other CPUs.
> > 		 */
> > 		 crash_smp_send_stop();
> > 		 cpus_stopped = true;
> > 	}
> > 
> > 	if (crash_kexec_post_hypervisor) {
> > 		  /* Tell hypervisor about the panic */
> > 		  atomic_notifier_call_chain(&panic_hypervisor_notifier_list, 0, buf);
> > 	}
> > 
> > 	if (enabled_kmsg_dump) {
> > 		  /*
> > 		   * Print extra info by notifiers.
> > 		   * Prevent rumors, for example, by stopping watchdogs.
> > 		   */
> > 		  atomic_notifier_call_chain(&panic_info_notifier_list, 0, buf);
> > 	}
> > 
> > 	/* Optional extra info */
> > 	panic_printk_sys_info();
> > 
> > 	/* No dumper by default */
> > 	kmsg_dump();
> > 
> > 	/* Used only when crash kernel loaded */
> > 	__crash_kexec(NULL);
> > 
> > 	if (!cpus_stopped) {
> > 		/*
> > 		 * Note smp_send_stop is the usual smp shutdown function, which
> > 		 * unfortunately means it may not be hardened to work in a
> > 		 * panic situation.
> > 		 */
> > 		smp_send_stop();
> > 	}
> > 
> > 	if (!crash_kexec_post_hypervisor) {
> > 		  /* Tell hypervisor about the panic */
> > 		  atomic_notifier_call_chain(&panic_hypervisor_notifier_list, 0, buf);
> > 	}
> > 
> > 	if (!enabled_kmsg_dump) {
> > 		  /*
> > 		   * Print extra info by notifiers.
> > 		   * Prevent rumors, for example, by stopping watchdogs.
> > 		   */
> > 		  atomic_notifier_call_chain(&panic_info_notifier_list, 0, buf);
> > 	}
> > 
> > 	/*
> > 	 * Help to reboot a safe way.
> > 	 */
> > 	atomic_notifier_call_chain(&panic_reboot_notifier_list, 0, buf);
> > 
> > 	[...]
> > }
> > 
> > Any opinion?
> > Do the notifier list names make sense?
> > 
> > Best Regards,
> > Petr
> 
> 
> Hi folks, I'm working on this now, and while looking into it I've
> noticed that we have the concept of "priority" in the notifiers list.
> Basically, you can order the calls the way it fits best, priority is an
> integer and must the set in the moment of registration, it's up to the
> users of the notifiers to set it and enforce the ordering.
> 
> So what I'm thinking is: currently, only 3 or 4 panic notifiers make use
> of that. What if, since we're re-working this, we add a priority for
> *all* notifiers and enforce its usage? This way we guarantee
> consistency, it'd make debug easier and maybe even more important:
> having all the notifiers and their priorities in a list present in the
> header file would be great documentation about all the existing
> notifiers and how they are called - today this information is quite
> obscure and requires lots of code grepping!
> 
> Let me know your thoughts Petr / Baoquan - it would add slightly more
> code / complexity, but in my opinion the payback is very good.
> Cheers,

The ideal situation is each panic notifier has an order or index to
indicate its priority. Wondering how to make it. What I think of is
copying initcall. We have several priorities, at the same priority,
execution sequence is not important. Not sure if I get your point.

~~~~~~~
#define core_initcall(fn)               __define_initcall(fn, 1)
#define core_initcall_sync(fn)          __define_initcall(fn, 1s)
......
#define late_initcall(fn)               __define_initcall(fn, 7)
#define late_initcall_sync(fn)          __define_initcall(fn, 7s)