Re: [PATCH v2 2/4] watchdog: export watchdog_mutex and lockup_detector_reconfigure

[Laurent Dufour <ldufour@linux.ibm.com>]

On 24/06/2022, 08:31:55, Michael Ellerman wrote:
> Laurent Dufour <ldufour@linux.ibm.com> writes:
>> In some cricunstances it may be interesting to reconfigure the watchdog
>> from inside the kernel.
>>
>> On PowerPC, this may helpful before and after a LPAR migration (LPM) is
>> initiated, because it implies some latencies, watchdog, and especially NMI
>> watchdog is expected to be triggered during this operation. Reconfiguring
>> the watchdog, would prevent it to happen too frequently during LPM.
>>
>> The watchdog_mutex is exported to allow some variable to be changed under
>> its protection and prevent any conflict.
>> The lockup_detector_reconfigure() function is exported and is expected to
>> be called under the protection of watchdog_mutex.
>>
>> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
>> ---
>>  include/linux/nmi.h | 3 +++
>>  kernel/watchdog.c   | 6 +++---
>>  2 files changed, 6 insertions(+), 3 deletions(-)
> 
> Is there a maintainer for kernel/watchdog.c ?

Nothing clearly identified AFAICT.

I'll add the commit signers reported by get_maintainer.pl.

> There's Wim & Guenter at linux-watchdog@vger but I think that's only for
> drivers/watchdog?
> 
> Maybe we should Cc that list anyway?

Yes, that's a good idea.

> 
> 
>> diff --git a/include/linux/nmi.h b/include/linux/nmi.h
>> index 750c7f395ca9..84300fb0f90a 100644
>> --- a/include/linux/nmi.h
>> +++ b/include/linux/nmi.h
>> @@ -122,6 +122,9 @@ int watchdog_nmi_probe(void);
>>  int watchdog_nmi_enable(unsigned int cpu);
>>  void watchdog_nmi_disable(unsigned int cpu);
>>  
>> +extern struct mutex watchdog_mutex;
>> +void lockup_detector_reconfigure(void);
> 
> It would be preferable if we didn't export the mutex.
> 
> I think you could arrange that by ...
> 
> Renaming lockup_detector_configure() to __lockup_detector_configure()
> and then adding a new lockup_detector_configure() that is non-static and
> takes the lock around __lockup_detector_configure().

Unfortunately, that will not be enough, because this mutex is also used to
protect wd_watchdog, to ensure it is not changed while another operation is
in progress.

I may try finding another way to protect that value, may be using
WRITE/READ_ONCE(). Indeed, the only requirement is to read a stable value
in watchdog_calc_timeouts().

Thanks,
Laurent.