linux-kernel - RE: [PATCH v4 1/2] vfio: fix deadlock between group lock and kvm lock

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Date:   Mon, 6 Feb 2023 03:52:25 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "jgg@...dia.com" <jgg@...dia.com>
CC:     "cohuck@...hat.com" <cohuck@...hat.com>,
        "farman@...ux.ibm.com" <farman@...ux.ibm.com>,
        "pmorel@...ux.ibm.com" <pmorel@...ux.ibm.com>,
        "borntraeger@...ux.ibm.com" <borntraeger@...ux.ibm.com>,
        "frankja@...ux.ibm.com" <frankja@...ux.ibm.com>,
        "imbrenda@...ux.ibm.com" <imbrenda@...ux.ibm.com>,
        "david@...hat.com" <david@...hat.com>,
        "akrowiak@...ux.ibm.com" <akrowiak@...ux.ibm.com>,
        "jjherne@...ux.ibm.com" <jjherne@...ux.ibm.com>,
        "pasic@...ux.ibm.com" <pasic@...ux.ibm.com>,
        "zhenyuw@...ux.intel.com" <zhenyuw@...ux.intel.com>,
        "Wang, Zhi A" <zhi.a.wang@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "intel-gvt-dev@...ts.freedesktop.org" 
        <intel-gvt-dev@...ts.freedesktop.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v4 1/2] vfio: fix deadlock between group lock and kvm lock

> From: Matthew Rosato <mjrosato@...ux.ibm.com>
> Sent: Saturday, February 4, 2023 5:50 AM
> 
> After 51cdc8bc120e, we have another deadlock scenario between the
> kvm->lock and the vfio group_lock with two different codepaths acquiring
> the locks in different order.  Specifically in vfio_open_device, vfio
> holds the vfio group_lock when issuing device->ops->open_device but some
> drivers (like vfio-ap) need to acquire kvm->lock during their open_device
> routine;  Meanwhile, kvm_vfio_release will acquire the kvm->lock first
> before calling vfio_file_set_kvm which will acquire the vfio group_lock.
> 
> To resolve this, let's remove the need for the vfio group_lock from the
> kvm_vfio_release codepath.  This is done by introducing a new spinlock to
> protect modifications to the vfio group kvm pointer, and acquiring a kvm
> ref from within vfio while holding this spinlock, with the reference held
> until the last close for the device in question.
> 
> Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
> Reported-by: Anthony Krowiak <akrowiak@...ux.ibm.com>
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Matthew Rosato <mjrosato@...ux.ibm.com>

Reviewed-by: Kevin Tian <kevin.tian@...el.com>