Re: [PATCH] kvm/vfio: Fix potential deadlock on vfio group_lock

Anthony Krowiak
2023-01-31T14:46:44Z

On 1/31/23 9:34 AM, Jason Gunthorpe wrote:
> On Tue, Jan 31, 2023 at 09:27:54AM -0500, Anthony Krowiak wrote:
>> I encountered a lockdep splat while running some regression tests today (see
>> below). I suspected it might be this patch so I reverted it, rebuilt the
>> kernel and ran the regression tests again; this time, the test ran cleanly.
>> It looks like this patch may not have fixed the problem for which it was
>> intended. Here is the relevant dmesg output:
> Well, it fixes the deadlock it intended to fix and created another one
> :)
>
> It means device drivers cannot obtain the kvm lock from their open
> functions in this new model.
>
> Why does ap need to touch kvm->lock? (via get_update_locks_for_kvm)

We need the kvm->lock because we take the vCPUs out of SIE in order to
dynamically change values in the APCB.

>
> Maybe you should split that lock and have a dedicated apcb lock?

I don't think that would suffice for taking the vCPUs out of SIE.

>
> Jason