netdev - Re: [PATCH 19/30] panic: Add the panic hypervisor notifier list

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Message-ID: <YoOi9PFK/JnNwH+D@alley>
Date:   Tue, 17 May 2022 15:28:20 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Evan Green <evgreen@...omium.org>
Cc:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        David Gow <davidgow@...gle.com>,
        Julius Werner <jwerner@...omium.org>,
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Subject: Re: [PATCH 19/30] panic: Add the panic hypervisor notifier list

On Mon 2022-05-16 09:02:10, Evan Green wrote:
> On Mon, May 16, 2022 at 8:07 AM Guilherme G. Piccoli
> <gpiccoli@...lia.com> wrote:
> >
> > Thanks for the review!
> >
> > I agree with the blinking stuff, I can rework and add all LED/blinking
> > stuff into the loop list, it does make sense. I'll comment a bit in the
> > others below...
> >
> > On 16/05/2022 11:01, Petr Mladek wrote:
> > > [...]
> > >> --- a/arch/mips/sgi-ip22/ip22-reset.c
> > >> +++ b/arch/mips/sgi-ip22/ip22-reset.c
> > >> @@ -195,7 +195,7 @@ static int __init reboot_setup(void)
> > >>      }
> > >>
> > >>      timer_setup(&blink_timer, blink_timeout, 0);
> > >> -    atomic_notifier_chain_register(&panic_notifier_list, &panic_block);
> > >> +    atomic_notifier_chain_register(&panic_hypervisor_list, &panic_block);
> > >
> > > This notifier enables blinking. It is not much safe. It calls
> > > mod_timer() that takes a lock internally.
> > >
> > > This kind of functionality should go into the last list called
> > > before panic() enters the infinite loop. IMHO, all the blinking
> > > stuff should go there.
> > > [...]
> > >> --- a/arch/mips/sgi-ip32/ip32-reset.c
> > >> +++ b/arch/mips/sgi-ip32/ip32-reset.c
> > >> @@ -145,7 +144,7 @@ static __init int ip32_reboot_setup(void)
> > >>      pm_power_off = ip32_machine_halt;
> > >>
> > >>      timer_setup(&blink_timer, blink_timeout, 0);
> > >> -    atomic_notifier_chain_register(&panic_notifier_list, &panic_block);
> > >> +    atomic_notifier_chain_register(&panic_hypervisor_list, &panic_block);
> > >
> > > Same here. Should be done only before the "loop".
> > > [...]
> >
> > Ack.
> >
> >
> > >> --- a/drivers/firmware/google/gsmi.c
> > >> +++ b/drivers/firmware/google/gsmi.c
> > >> @@ -1034,7 +1034,7 @@ static __init int gsmi_init(void)
> > >>
> > >>      register_reboot_notifier(&gsmi_reboot_notifier);
> > >>      register_die_notifier(&gsmi_die_notifier);
> > >> -    atomic_notifier_chain_register(&panic_notifier_list,
> > >> +    atomic_notifier_chain_register(&panic_hypervisor_list,
> > >>                                     &gsmi_panic_notifier);
> > >
> > > I am not sure about this one. It looks like some logging or
> > > pre_reboot stuff.
> > >
> >
> > Disagree here. I'm looping Google maintainers, so they can comment.
> > (CCed Evan, David, Julius)
> >
> > This notifier is clearly a hypervisor notification mechanism. I've fixed
> > a locking stuff there (in previous patch), I feel it's low-risk but even
> > if it's mid-risk, the class of such callback remains a perfect fit with
> > the hypervisor list IMHO.
>
> This logs a panic to our "eventlog", a tiny logging area in SPI flash
> for critical and power-related events. In some cases this ends up
> being the only clue we get in a Chromebook feedback report that a
> panic occurred, so from my perspective moving it to the front of the
> line seems like a good idea.

IMHO, this would really better fit into the pre-reboot notifier list:

   + the callback stores the log so it is similar to kmsg_dump()
     or console_flush_on_panic()

   + the callback should be proceed after "info" notifiers
     that might add some other useful information.

Honestly, I am not sure what exactly hypervisor callbacks do. But I
think that they do not try to extract the kernel log because they
would need to handle the internal format.

Best Regards,
Petr