linux-kernel - Re: [PATCH] kvm/vfio: Fix potential deadlock on vfio group

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Message-ID: <b0ed98f6-3586-3151-47e7-9ec86d6c716d@linux.ibm.com>
Date:   Tue, 31 Jan 2023 09:46:18 -0500
From:   Anthony Krowiak <akrowiak@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Yi Liu <yi.l.liu@...el.com>, alex.williamson@...hat.com,
        pbonzini@...hat.com, mjrosato@...ux.ibm.com, kevin.tian@...el.com,
        cohuck@...hat.com, farman@...ux.ibm.com, pmorel@...ux.ibm.com,
        borntraeger@...ux.ibm.com, frankja@...ux.ibm.com,
        imbrenda@...ux.ibm.com, david@...hat.com, jjherne@...ux.ibm.com,
        pasic@...ux.ibm.com, zhenyuw@...ux.intel.com, zhi.a.wang@...el.com,
        seanjc@...gle.com, linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm/vfio: Fix potential deadlock on vfio group_lock


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