linux-kernel - Re: kexec crash kernel running with watchdog enabled

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Date:	Wed, 24 Jun 2015 12:06:03 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	dwalker@...o99.com
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org,
	david.daney@...ium.com, d.hatayama@...fujitsu.com,
	vgoyal@...hat.com, hidehiro.kawai.ez@...achi.com,
	linux-kernel@...r.kernel.org
Subject: Re: kexec crash kernel running with watchdog enabled

dwalker@...o99.com writes:

> On Wed, Jun 24, 2015 at 03:52:48PM +0900, Masami Hiramatsu wrote:
>> Hi,
>> 
>> On 2015/06/23 23:05, dwalker@...o99.com wrote:
>> > 
>> > Hi,
>> > 
>> > There was a commit in kernel/panic.c which altered when the kexec crash kernel is executed,
>> > 
>> > commit f06e5153f4ae2e2f3b0300f0e260e40cb7fefd45
>> > Author: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
>> > Date:   Fri Jun 6 14:37:07 2014 -0700
>> > 
>> >     kernel/panic.c: add "crash_kexec_post_notifiers" option for kdump after panic_notifers
>> > 
>> > 
>> > This made it possible for smp_send_stop() to be executed prior to calling the kexec crash
>> > kernel.
>> > 
>> > The issue is that smp_send_stop() offlines the cores, and other code depend on the cores being online.
>> > 
>> > In my case on Octeon here's an example,
>> > 
>> > panic()
>> >  crash_kexec()
>> >   machine_crash_shutdown()
>> >    octeon_generic_shutdown()
>> > 
>> > Inside octeon_generic_shutdown() the Octeon watchdog is shutdown for_each_online_cpu(), but since
>> > most of the cpu's already got offlined in smp_send_stop() it means the watchdog is still alive on
>> > those cores. This results in a reboot during the crash kernel execution.
>> 
>> Ah, I see.
>> 
>> > Another example seem to be in default_machine_crash_shutdown() where crash_kexec_prepare_cpus() depends
>> > on an IPI for saving the registers on different cores. However, the cpu's are all offlined with
>> > interrupts disabled so they won't be running those IPI's in this case.
>> > 
>> > I'm looking for any advice on how this should be fixed, or if it's already fixed. I'm not going to be
>> > submitting a patch so if anyone wants to submit one feel free to do so.
>> 
>> Hmm, IMHO, when the cpu goes to offline in appropriate way(smp_send_stop), it should stop
>> watchdog timer on the offlined cpu too.
>> Or, you can also register crash handler which stops all watchdogs, but it's a bit tricky.
>> 
>
> That doesn't really fix all the issue tho. As I was explaining generic MIPS code depends on the cpu's
> effectively being online for crash data collection (with an IPI). This issue may effect other architectures also,
> because smp_send_stop() offlines the cpu on other architectures also. I haven't surveyed the other architectures
> enough to know what issue could happen from this tho.
>
> Is it possible to move the smp_send_stop() below the notifiers ? I'm
> just throwing out ideas.

In general if you want reliability don't pass the kernel command line to
run the panic notifiers.

Eric

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