Date:   Tue, 31 Jan 2023 10:34:35 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Anthony Krowiak <akrowiak@linux.ibm.com>
Cc:     Yi Liu <yi.l.liu@intel.com>, alex.williamson@redhat.com,
        pbonzini@redhat.com, mjrosato@linux.ibm.com, kevin.tian@intel.com,
        cohuck@redhat.com, farman@linux.ibm.com, pmorel@linux.ibm.com,
        borntraeger@linux.ibm.com, frankja@linux.ibm.com,
        imbrenda@linux.ibm.com, david@redhat.com, jjherne@linux.ibm.com,
        pasic@linux.ibm.com, zhenyuw@linux.intel.com, zhi.a.wang@intel.com,
        seanjc@google.com, linux-s390@vger.kernel.org, kvm@vger.kernel.org,
        intel-gvt-dev@lists.freedesktop.org,
        intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kvm/vfio: Fix potential deadlock on vfio group_lock
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References: <20230120150528.471752-1-yi.l.liu@intel.com>
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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)

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

Jason