Re: [PATCH] powerpc/fadump: re-register firmware-assisted dump if already registered

[Hari Bathini <hbathini@linux.vnet.ibm.com>]

On Friday 14 September 2018 07:58 PM, Petr Tesarik wrote:
> On Fri, 14 Sep 2018 19:36:02 +0530
> Hari Bathini <hbathini@linux.ibm.com> wrote:
>
>> Firmware-Assisted Dump (FADump) needs to be registered again after any
>> memory hot add/remove operation to update the crash memory ranges. But
>> currently, the kernel returns '-EEXIST' if we try to register without
>> uregistering it first. This could expose the system to racing issues
>> while unregistering and registering FADump from userspace during udev
>> events. Spare the userspace of this and let it be taken care of in the
>> kernel space for a simpler interface.
>>
>> Since this change, running 'echo 1 > /sys/kernel/fadump_registered'
>> would result in re-regisering (unregistering and registering) FADump,
>> if it was already registered.
> Great improvement to the API!
>
> Any suggestions what should be done in a client which tries to be
> compatible with kernels before this change and after this change?

If `echo 1 > /sys/kernel/fadump_registered` fails, check for the output
of  `cat /sys/kernel/fadump_registered` and if it is still `1`, that 
indicates
old kernel and we are already registered. Treat it as success if being
registered is what we care about or unregister/register (if re-register
is the intention)..

Hope that helps..

Thanks
Hari