Re: [RFC] watchdog: pretimeout: add a notifier call chain governor

[Guenter Roeck <linux@roeck-us.net>]

On 4/13/21 9:56 AM, Stefan Eichenberger wrote:
> Hi Guenter,
> 
> Thanks a lot for the fast feedback!
> 
> On Tue, Apr 13, 2021 at 09:15:35AM -0700, Guenter Roeck wrote:
>> On 4/13/21 8:45 AM, Stefan Eichenberger wrote:
>>> Add a governor which calls a notifier call chain. This allows the
>>> registration of custom functions to the watchdog pretimeout. This is
>>> useful when writing out-of-tree kernel modules and it is more similar to
>>> the register_reboot_notifier or register_oom_notifier API.
>>>
>>> Signed-off-by: Stefan Eichenberger <eichest@gmail.com>
>>
>> It seems questionable to implement such a notification as governor.
>> A governor is configurable, and letting userspace override notifications
>> seems very odd and unusual. Please provide a use case.
> 
> What I would like to do is to have an out-of-tree module which will
> write a pattern into a reserved memory region when a watchdog occurs.
> After a reboot the module then read this reserved memory region and can
> differentiate between different reset reasons. Here the example kernel
> module:
> https://github.com/embear-engineering/sample-kernel-modules/blob/use-watchdog-register/reset-reason/reset-reason.c
> 
> Registering to the watchdog happens on line 180.
> 
> I think I could just implement a governor in the module but it is not
> really flexible because then I would have to compile the kernel with
> e.g. the noop governor and then switch to my governor after the module
> loaded. Then I thought why not using a notifier chain similar to reboot,
> panic, or oom.
> 

I have two problems with this: First, we do not support in-kernel code purely
for out-of-tree drivers. Second, the idea of using a configurable notifier
(ie one that can be disabled by userspace action) seems odd. If anything,
that would have to be an unconditional notifier. However, again, that would
require a use case in the upstream kernel.

Guenter