linux-kernel - RE: [PATCH v3] vfio: fix deadlock between group lock and kvm lock

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Message-ID: <DS0PR11MB7529050661FCE4A5AC4B17C3C3D79@DS0PR11MB7529.namprd11.prod.outlook.com>
Date:   Fri, 3 Feb 2023 13:32:09 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>
CC:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "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 v3] vfio: fix deadlock between group lock and kvm lock

> From: Tian, Kevin <kevin.tian@...el.com>
> Sent: Friday, February 3, 2023 10:00 AM
> 
> > From: Alex Williamson <alex.williamson@...hat.com>
> > Sent: Friday, February 3, 2023 7:13 AM
> >
> > On Thu, 2 Feb 2023 23:04:10 +0000
> > "Tian, Kevin" <kevin.tian@...el.com> wrote:
> >
> > > > From: Alex Williamson <alex.williamson@...hat.com>
> > > > Sent: Friday, February 3, 2023 3:42 AM
> > > >
> > > >
> > > > LGTM.  I'm not sure moving the functions to vfio_main really buys us
> > > > anything since we're making so much use of group fields.  The cdev
> > > > approach will necessarily be different, so the bulk of the get code will
> > > > likely need to move back to group.c anyway.
> > > >
> > >
> > > well my last comment was based on Matthew's v2 where the get code
> > > gets a kvm passed in instead of implicitly retrieving group ref_lock
> > > internally. In that case the get/put helpers only contain device logic
> > > thus fit in vfio_main.c.
> > >
> > > with v3 then they have to be in group.c since we don't want to use
> > > group fields in vfio_main.c.
> > >
> > > but I still think v2 of the helpers is slightly better. The only difference
> > > between cdev and group when handling this race is using different
> > > ref_lock. the symbol get/put part is exactly same. So even if we
> > > merge v3 like this, very likely Yi has to change it back to v2 style
> > > to share the get/put helpers while just leaving the ref_lock part
> > > handled differently between the two path.
> >
> > I'm not really a fan of the asymmetry of the v2 version where the get
> > helper needs to be called under the new kvm_ref_lock, but the put
> > helper does not.  Having the get helper handle that makes the caller
> > much cleaner.  Thanks,
> >
> 
> What about passing the lock pointer into the helper? it's still slightly
> asymmetry as the put helper doesn't carry the lock pointer but it
> could also be interpreted as if the pointer has been saved in the get
> then if it needs to be referenced by the put there is no need to pass
> it in again.

For cdev, I may modify vfio_device_get_kvm_safe() to accept
struct kvm and let its caller hold a kvm_ref_lock (field within
struct vfio_device_file). Meanwhile, the group path holds
the group->kvm_ref_lock before invoking vfio_device_get_kvm_safe().
vfio_device_get_kvm_safe() just includes the symbol get/put and
the device->kvm and put_kvm set.

Regards,
Yi Liu