[PATCH] powerpc/rtas: Fix hang in race against concurrent cpu offline

[Juliet Kim]

On 6/25/19 12:29 PM, Nathan Lynch wrote:
> Juliet Kim <julietk at linux.vnet.ibm.com> writes:
>> The commit
>> (“powerpc/rtas: Fix a potential race between CPU-Offline & Migration)
>> attempted to fix a hang in Live Partition Mobility(LPM) by abandoning
>> the LPM attempt if a race between LPM and concurrent CPU offline was
>> detected.
>>
>> However, that fix failed to notify Hypervisor that the LPM attempted
>> had been abandoned which results in a system hang.
> It is surprising to me that leaving a migration unterminated would cause
> Linux to hang. Can you explain more about how that happens?
>
PHYP will block further requests(next partition migration, dlpar etc) while
it's in suspending state. That would have a follow-on effect on the HMC and
potentially this and other partitions.
>> Fix this by sending a signal PHYP to cancel the migration, so that PHYP
>> can stop waiting, and clean up the migration.
> This is well-spotted and rtas_ibm_suspend_me() needs to signal
> cancellation in several error paths. But I don't agree that this is one
> of them: this race is going to be a temporary condition in any
> production setting, and retrying would allow the migration to succeed.
If LPM and CPU offine requests conflict with one another, it might be better
to let them fail and let the customer decide which he prefers. IBM i cancels
migration if the other OS components/operations veto migration. It’s consistent
with other OS behavior for LPM. I think all the IBM products should have a
consistent customer experience. Even if the race can be temporary, it still
could happen and can cause livelock.